Small is the new big. Pluto’s family of small satellites sparks big discussions and new ideas.

Continuing this series of talks from the Pluto Science Conference being held July 22-26, 2013 at the Johns Hopkins University Applied Physics Lab (APL) in Laurel, MD. This blog entry highlights a selection of talks on Small Satellites the afternoon of July 23rd.

Hal Weaver (APL) gave us a hearty introduction to “Pluto’s Small Satellites.” The Pluto system is rich. It has five confirmed moons, Charon (1978), Nix (2005), Hydra (2005), Kerberos (2011, formerly know as P4)  and Styx (2012, formerly known as P5).

Pluto satellite parameters

The Pluto system at a glance. Key top-level parameters of the satellites a=semimajor axis (from the Pluto-Charon barycenter/center of mass) in kilometers, P=orbital period in days. The moons appear to be in orbital resonances Hydra:Kerberos:Nix:Styx:Charon = 6:5:4:3:1.

What about their albedo? Albedo is a measurement of a body’s reflectance, a reflection coefficient, where an albedo equal to 1 is “white” and an albedo equal to 0 is essentially “black” (e.g., dirty snowballs like comet nuclei have albedos ~0.04). It should be noted that albedo values can be functions of color (wavelength of light). We know that Pluto has an albedo ~0.5 and Charon has albedo ~0.35. Regolith exchange and dynamics agreements favor albedo ~0.35 for these small satellites, and assuming that density=1 (icy body).

What are implications of these small satellite discoveries? These questions were posed: (1) Pluto system is highly compact and rich, so are there more satellites not yet discovered? (2) Was there a giant impact origin of Pluto System? (3) Could rings also form? (4) Could other large KBOs have multiple satellites? (We know Haumea has 2 companions. Could there be others?).

What role will New Horizons bring? New Horizons will play a key role for small satellites, measuring their size and their shapes. Note: Additional occultation observations from Earth could reveal additional satellites and also provide measurements of their sizes, but not shapes.

New Horizons best spatial resolution of the small satellites is: 0.46 km/pix (Nix), 1.14 km/pix (Hydra), 3.2 km/pix (Kerberos), and 3.2 km/pix (Styx). Best estimates right now for the sizes of these bodies, assuming albedo 0.35, are Hydra 50 km, Nix 40 km, Kerberos 10 km, Styx 4 km. That translates to roughly ~44, ~37, ~3, and ~1 pixels across Hydra, Nix, Kerberos, and Styx, respectively.

At the time of Kerberos & Styx’ discovery, the New Horizons Mission Ops team had already designed the Pluto science sequence of observations to run aboard the spacecraft.  In the spirit of exploration,  the team had wisely reserved a few TBD (to be determined) observations that they now have placed observations of Kerberos and Styx that fit within the constraints. Firm flexibility at its finest.

Scott Kenyon (Harvard SAO, by phone) “Formation of Pluto’s Low Mass Satellites.” He and his team looked at both the giant impact (Canup) and capture (Roskol) formation paths for Pluto and Charon. They model a debris disk where viscous diffusion expands the disk, collisions circularize the orbits, particles experience migration, and satellites eventually grow. They found that lower mask disks take longer to reach equilibrium, do produce more satellites, and also produce the smaller satellites. Calculations with large seed planetesimals produce less satellites. Calculations also do predict 1-km size objects in large orbits (orbits beyond Hydra) in a diffuse debris disk.

For more details about their paper on the formation of Pluto’s low mass satellites is found here

What role will New Horizons bring? New Horizons can test these predictions if they discover more satellites when they look at the Pluto system on approach and departure.

Peter Thomas (Cornell University) and Keith Noll (NASA GSFC) provided a talk about “Pluto’s Small Satellites: What to Expect, What They Might Tell Us.”

Small satellites of planets: variety and dynamics role. We have a small selection of satellites of 20-100 km range (e.g. Metis, Amalthea, Thebe, Atlas, Prometheus, Pandora, Epimetheus, Janus, Hyperion, Phoebe and asteroids Mathilde, Eros, Ida). Best “comparatives” come from the Saturn family from amazing Cassini images, but these divided into two groups whether they are located within the ring arcs or not. Small satellites are irregular in shape, have high porosity (40-70% void space), weak (tidally fractured), crater morphology varies, regolith depths & distribution over surface, icy & rocky, and some have albedo markings.

Saturns Moons

Saturn’s moons may be useful “comparatives” for describing Pluto’s small satellites.

Predictions for New Horizons. Peter Thomas is excited to see New Horizons’ images of the small satellites. He predicts they will not look like egg-shaped. Thomas’ Best Guess: A Deimos/Hyperion hybrid morphology.

KBOs and their satellites: variety and collision role. There are three multiple systems known in the Kuiper Belt: Pluto (6 components), Haumea (3 components) and 47171 1999 Tc36 (3 components). There are also 74 binary systems to date. The Pluto system is collisional. Unfortunately most of the KBO binaries have too low angular momentum to imply a collisional origin, but there is a subset of TNO binaries that could be a comparative set. Multiple collision systems in the Kuiper Belt could serve as possible analogs of the Pluto system.

Plutino Binaries

Plutino binaries (above) are also “comparatives” images for describing Pluto’s small satellites. Other comparative bodies, which may have collisional origin could be Quaoar, 1998 SM165, Salacia, and Eris.

Predictions for New Horizons. New Horizons will tell us a lot about KBOs and test open theories about their formation and collisional history.

Mark Showalter (SETI) on “Orbits and Physical Properties of Pluto’s Small Moons Kerberos (P4) and Styx (P5)” began with “Well, they are not your typical orbits.” The orbits of all the small satellites do wobble with a periodicity defined by Charon. Essentially the system acts like a “time-variable center gravity field.” There are nine orbital elements to fit (semi major axis, a; mean longitude at epoch, theta; eccentricity, e; longitude of pericenter at epoch, w; inclination I; longitude of ascending node at epoch, Omega; mean motion, n; pericenter precession rate dw/dt; nodal regression rate dOmega/dt.). He provided updated parameters for the moons based on this work.

Mark Showalter (SETI) next talked about his preliminary work on “Chaotic Rotation of Nix & Hydra.” He started the presentation with a light curves for Hydra & Nix made the 2010-2012 HST data sets. They do not follow the expected “double sinusoidal.” When plotting phase angle vs. time, Hydra and Nix do get brighter with lower phase angle and he used this information to normalize their light curves. He found that Nix & Hydra’s brightnesses do not correlate with their projected longitude on the sky. They are probably not in synchronous rotation. Also, he is not finding any single rotation period compatible with the data series he has.

His premise is that Nix and Hydra are not following your typical rotation, and are very heavily influenced by the Charon-wobble. Best Guess: Hydra and Nix are in a state of “tumbling.” Bodies that not synchronous have no way to get to synchronous lock.

Until now, Hyperion (one of Saturn’s moons) had been the only chaotic rotator. Not any more! It’s got company!

Marina Brozovic (JPL) spoke about “The Orbits and Masses of Pluto’s Satellites.” She used Pluto & Charon data from photographic plates (1980s), ground-based VLT AO data (1990-2006) and HST data (1990-2012); Nix and Hydra data from HST and VLT AO (2002-2012); and Kerberos and Styx data from HST (2010-2012) to derive orbital parameters for these bodies. They have created plu041 and plu042 ephemeris solutions (i.e. where all the satellites are in the system with time), the latter where they provide orbit predictions for the four smaller satellites. And, they have found interesting puzzles as they are working to find solutions for the new satellite masses. She presented orbital uncertainties at the time of the New Horizons encounter (July 14, 2015).

Andrew Youdin (JILA, CU Boulder) “Using (the stability of)  Kerberos to Weigh Nix & Hydra.” He looked at what was done on the HR8799 (Skemer et al 2012) exoplanet system, where orbital stability technique was used, and applied it to the Pluto System. Kerberos/P4 does appear more unstable, but Styx/P5 may be more stable. To derive the necessary masses for orbit stability, when compared with measured brightnesses, means comet-like albedos are ruled out for small Pluto satellites. Instead, they would have high albedo, clean-icy surfaces. No dirty snowballs here.

Andrew Youdin’s paper on using the P4 data to help constrain the masses of Nix and Hydra can be found here:

Andrew Youdin

Andrew Youdin at the beginning of his talk called out a visual comparison between the Pluto System (left) and the exoplanet system HR8799 (right) 129 light years away characterized by a debris disk and four massive planets confirmed by direct imaging. It served for his inspiration to apply fitting. techniques to the Pluto System small moons. “Pluto’s not a planet. It’s better. In miniature, it’s the richest circumbinary multiplanet system.”

Alan Stern (SwRI) on “Constraints on Satellites of Pluto Interior to Charon’s Orbit and Prospects for Detection by New Horizons.” Alan Stern asks, “Could there be moons inside Charon’s Orbit?” Charon is a big vacuum cleaner, and clears out a big swatch called the CIS, the Charon Instability Strip, clear down to 0.45-0.47 Pluto-Charon separation. Atmospheric drag by Pluto’s atmosphere could also add in the clearing-out the region. Charon’s eccentricity also constrains the problem. And when you combine the recent HST data detection limits, you only have a region from 0.2 to 0.45-0.47 Pluto-Charon separation (the outer edge of the CIS) where you could possibly have moons.

What role will New Horizons bring? New Horizons will do a deep satellite search with the LORRI instrument at seven days prior to Pluto closest approach. This search will reach 6x fainter than current limits set by HST for Pluto companions, to detect objects down to ~1.2 km. If New Horizons does find satellites within Charon’s orbit this will provide new insights into satellite system origins.

Charon Instability Strip

Charon has been a major player in the determining where debris in the Pluto system could remain stable. The Charon Instability Strip is a region between Pluto and Charon that is kept relatively free because of Charon’s gravity.

How can you form Pluto and Charon? Let me just count the ways.

On the afternoon July 23, 2013 at the Pluto Science Conference continued, we switched gears from atmospheres to small satellites. This blog entry is about the formation theories for Pluto and Charon.

Hal Levison (SwRI) started the afternoon with a talk entitled “Unraveling the Early Dynamical Evolution of the Outer Solar system.” The “Nice Model” (Gomes, Levison, Morbidelli, Tsiganis) was devised to introduce possible models that could produce the Outer Solar System as we know it and preserve the Inner Solar System as we know it. The authors have been updating it with planets in resonances (Morbidelli et al 2007), put Pluto-objects in the disk (Levinson et al 2011), restricted the models to “save the Earth” by making sure Jupiter does not encounter an ice giant planet (Brasser et al 2009), and added a third ice giant (Nesvorny & Morbidelli 2011).

To learn more information about the Nice Model, check out a good entry at

The Nice model has told us a lot of good things. It predicts the right number and range or orbits for Jupiter and Saturn, predicts the right number and orbits for Trojans (things in 1:1 resonance with primary body) and reproduces the Late Heavy bombardment of Moon. However, it comes short of explaining the Kuiper Belt.

So, what does this all mean for the Kuiper Belt? The Kuiper Belt is a rich structure. Observationally the sum of all the mass in the Kuiper Belt is <= 0.1 Mass_Earth.  In order to get objects the size of Pluto to grow in the timescales of our Solar System, you need a lot more mass. So we need find this missing mass.

Nice Model vs. Actual KBO Population

Above is a Comparison of updated 2008 Nice Model (green ) vs. Kuiper Belt Data (blue dots). It qualitatively shows similarities however it cannot reproduce the “kernel” (described in Brett Gladman’s talk from July 22nd) nor objects with high inclination (large i) nor objects in the “Cold Classical Population.”

Cold Classical Kuiper Belt Objects have orbits much like the planets; nearly circular, with an orbital eccentricity of less than 0.1, and with relatively low inclinations up to about 10° (they lie close to the plane of the Solar System rather than at an angle). They have characteristics similar to an undisturbed protoplanetary disk. Often the term ‘primordial’ is used when describing Cold Classicals. They tend to be in binaries and have “red” colors.

He then ended his talk by sharing an recent update with his work on Outer Solar System modeling, hoping to explain high inclination Kuiper Belt Object formation, by looking at the formation of Jupiter and Saturn with and without a gas disk present. Jupiter and Saturn, when they are forming, are scattering objects outward. Then if there is a gas disk present, these objects get into what is known as Kozai resonances, where bodies exchange eccentricity for inclination. As the gas disperses, a population at high inclinations in the Kuiper Belt region (30-50 AU) are caught. In the models, if you vary the outside extent of the disk, you spread out the populations. However, this is not in agreement with our solar system (we don’t see those types of objects). Their conclusion is that you needed to have the gas disk truncated and this modification of the Nice Model can explain high inclination KBOs.

Hal Levinson stated strongly “We definitely need New Horizons to visit a cold classical object!”

Anders Johansen (Lund University, Sweden) in “Accretion of Kuiper Belt Objects” stepped us through the two models of major planet formation: Planetesimal (coagulation) vs. Pebble (steaming instability).

Planet Formation Theories

He asked, could Pluto be formed by planetesimal accretion? This will require a cold disk of km-size planetesimals (Kenyon & Bromley 2012) where a key prediction of the planetesimal accretion model gives a differential size distribution that is in agreement with observations (i.e. lots of smaller objects). But, there is a problem this this approach since to make kilometer size objects beyond 20 AU as it would taken 100 Myr which is much longer than the life-time of the gas disk (Lambrechts & Johnansen, in prep). Thus, to make a Pluto-size (few 1000s km size) object would taker longer than the age of the Solar System. (Pluto orbit is 29 AU at closest to Sun to 49 AU, furthest from Sun). However adding streaming instability can speed up the planetesimal growth timeframe.

Could Pluto have been formed by pebble accretion? Pebbles are accreted very efficiently by planetesimals (Lambrechts & Johnansen,  2012; Ormel & Jlahr 2010). This shapes the distribution (makes it steeper) and brings it more into agreement with asteroid and KBO populations.

What new data will New Horizons shed? If data from New Horizons reveals the presence of Aluminum 26, this will imply a formation age for Pluto. Formation time data can be fed back into planet-forming models, be they planetesimal or pebble accretion, and those models can be used to help explain other systems, such as observed proto-planetary disks or exoplanet systems around other stars.

Robin Canup (SwRI) talked about the “Origin of Pluto’s Satellites.” Massive Charon and four very tiny outer moons make up Pluto’s satellites. All of these satellites are co-planar (they are moving in the same plane) and prograde with respect to Pluto’s rotation (they revolve about Pluto in the same direction as Pluto’s rotation). However, Pluto’s rotation is retrograde (in the motion opposite) to its orbit.

It is thought that Charon was formed by a giant impact that could have preserved a lot of angular momentum in the system. Her models (Canup 2011) predict a grazing impact was needed to match the system angular momentum and produce a Charon-mass object. Achieving a Charon-mass object requires an extreme case, as most of them like to create a companion that is 6-8% of mass of the primary object.

She also modeled cases where there is an undifferentiated impactor, and those systems can form “intact-moons.” In many scenarios, Charon-mass objects are created. And “the Charon that was created” forms entirely from impactor material. She postulates that this is the more probable explanation for Charon’s formation.

What about the origin of the tiny moons? The models do create a disk, which has enough mass to form tiny moons. The challenge is that the disk that is made too compact compared to the current existence of Nix & Hydra. Could these moons have been transported out? But no model has been able to drive them out via “resonant transport.”

The alternative theory for the formation of the smaller moons is by capture, but it’s rather very low probability. Plus that could imply far more irregular satellites and Pluto’s smaller moons are more regular. So this opens up the path for other theories. Collisional spreading? Collisional dampening? Preferential re-accretion?

This is an active area of study.

How New Horizons can help. By providing better constraints on masses and densities of Pluto & Charon, compositions of the tiny moons, any information about the differentiation shape of Pluto & Charon, and presence of distance satellites can better constrain these origin model.

More predictions about Pluto’s changing atmosphere. And Charon may have a few surprises of its own.

Blog series continues. These are summaries of talks presented on July 23, 2013 at the Pluto Science Conference. The New Horizons mission will fly by the Pluto System on July 14, 2015, a place that has never been explored before by any other spacecraft. Many questions about the Pluto System remain unanswered. For more information about NASA’s intrepid explorer to the Solar Systems’ Third Zone go to and

Richard French (Wellesley College) presented a talk on “A Comparison of Models of Tides in Pluto’s Atmosphere and Stellar Occultation Observations.”

We have come to understand that Pluto’s atmosphere is cold & tenuous, has a long radiative time constant, shows weak diurnal variations, indicates seasonal transport of volatiles with long term variations of atmospheric mass, and seems to be convectively stable. Current Pluto general circulation models (GCMs) predict smooth T(P) profiles reveal mean circulation and thermal structure. But there are problems. GCMs predictions (with these smooth T(P) profiles) are inconsistent with stellar occultation data, which imply much more complex T(P) profile. The other challenge to this mystery is that stellar occultations are spatially constrained (i.e., map across a particular lat/long swath of Pluto surface at the time of event).

Are there waves in Pluto’s atmosphere? This is one proposition to explain the structures (spikes) seen in the Pluto occultation data. Tidal models they have built make predictions for large scale and small-scale structures. Also they can predict temperature profiles with altitude. Next steps are to apply this model to other occultation geometries. Richard French showed a comparison of a tidal model (Toigo et al 2010) against occultation data from an event on Aug 21, 2002 and they showed qualitative agreement.

Richard French’s predictions for New Horizons fly-by: When New Horizons provides a true frost pattern, they can input this into their models and generate large-scale and small-scale structures for comparison with actual New Horizons atmosphere measurements. Their tidal models do generate regionally variable, latitude dependent thermal changes. If this is what New Horizons observers, their model can help constrain parameters.

Bruno Sicardy (Observatoire de Paris, France) next took us on a rich tour of “Pluto’s Atmospheric Pressure From Stellar Occultations: 2002-2012.”

Pluto Charon Dual Occultation

There was a dual Pluto & Charon occultation event on 4 June 2011. Pluto and Charon each pass in front of the star (at different times). Look at curve shapes. Charon’s curve sharply drops, indicative of no atmosphere, unlike Pluto’s curve, which has not-as-steep ingress/egress that indicates the presence of an atmosphere.

Using the light curve data, Sicardy and his team use a temperature vs. altitude model to fit the light curve depth, width and ingress/egress slope. Then with the temperature, they can derive a pressure. He presented results from the most recent Pluto occultation that was observed May 4, 2013. Good data and good fit. Next were shown the derived pressure (at 1215km) for occultation events observed from 1988 to 2013.

Pluto's Atmosphere 1988 to 2013

Occultation results show the Pluto atmosphere is increasing over the past few years. There is some notable evolution and implies a regular expansion. But a question from the audience stressed caution as we could be seeing just the northern pole facing the Sun with that contributing to the expansion but it could be a localized phenomena.

Bruno Sicardy’s predictions for New Horizons fly-by: Atmosphere will be present for the fly-by.

Michael Person (MIT) next described “Trends in Pluto’s Atmosphere From Stellar Occultations.” He started his talk with the advantages of occultation measurements:  you get spatial resolution (~1 km at Pluto) with direct measurements of atmospheres (temperature, pressure, number profiles). MIT has collected data sets from 1988 through 2013. Their group tends to separate the upper vs. lower atmosphere when they fit their data. He next showed a light curve comparison over. Are we seeing a gradually decrease lower atmosphere slope? Is there a gradual lowering of the separation boundary?

“Haze or No Haze? That is the question.” Best evidence of haze is from the occultation event of 2002, where there is a distinct change in brightness as a function of wavelength (Elliot et al 2003).  Attempt to look for haze in the 2011 occultation event with SOFIA in three bands was not successful. The main question is why does the haze come and go, and what is causing it?

What Mike Person is looking forward to: New Horizons will finally provide the size of Pluto! Knowing where the Pluto surface really is, or equivalently, the size of Pluto, is a key data point, as all these interpretations of occultation light curves and interpretations to atmosphere assumes a Pluto size.

Alex Dias de Oliveira (Observatoire de Paris, France).“Pluto’s Atmosphere from Jul 18, 2012 stellar occultation.” This is his PhD work and he provided an updated status of the steps taken from prediction of the event, the observation data collected, various calibration items, and first attempts to invert the light curve to get a temperature profile. He observed this Pluto occultation event with the ESO VLT (8m telescope in Chile) with the NACO instrument in the H band (1.65 microns). Comparison with the June 12, 2006 AAT event showed that spikes seen in the light curves were repeated in the July 18, 2012 event he observed wit the VLT in Chile.

Cathy Olkin (SwRI) presented results from “The May 4th, 2013 Stellar Occultation by Pluto and Implications for Pluto Atmosphere in 2015.” This was an event where Pluto passed in front of a R=14.4 mag star with a slow shadow velocity of 10.6 km/s. The event was observed from the southern hemisphere, from Cerro Tololo in Chile.

Erika Barth (SwRI). “Is Methane Supersaturation Consistent with the Presence of Haze Particles in Pluto’s Atmosphere?” She asked the question: If you put haze particles into Pluto’s atmosphere how do they interact with the methane in Pluto’s atmosphere?” She developed a model to ingest haze particles into a supersaturated environment and this predicts the growth of clouds and condensation of methane. Then when methane condenses out, that reduces the amount of observable methane. Her model requires the existence of a troposphere (which we learned earlier in Emmanuel Lellouch’s talk today that there is no evidence for this, but its existence could explain some phenomena, some observed to date, other predicted) and also predicts a thick troposphere as well. She created a Pluto version of CARMA, the Community Aerosol and Radiation Model for Atmospheres.

Jason Cook (SwRI) next spoke about his “Analysis of High Resolution Spectra of Pluto: A Search for Cold Gaseous Methane Layer, and Spatial Variation in Methane Column Abundance.” Occultations have told us that Pluto’s upper atmosphere (above 1195 km) is pretty warm (100 K). But 2.15 micron N2 ice measurements of Pluto’s surface tells us the surface is ~40 K. So this implies there needs to be a cold-layer in the atmosphere. To investigate a search for this “cold layer of air” they took NIR (near infrared) spectra with NIRSPEC on Keck with R=35,000 in 2011. They need to move to a two-temperature model to help constrain the observed data (i.e. measured methane line depths from the high-res NIR spectra), but the hot/cold ratio of the two temperatures is an unresolved topic (pun intended).

They also took spectra of Pluto over several nights to probe the different longitudes of Pluto (Pluto has a rotational period of 6.4 days) and they got a fairly consistent number except near 180 deg longitude where gaseous CH4 was not easily detected. They would like more data to probe this temporal measurement.

Methane Spectra on Pluto

Selection of high-resolution NIR spectra of Pluto obtained over several days. This series probes a range of Pluto rotations and show how methane lines (Q-branch) vary.

Eliot Young (SwRI) spoke about “Evidence for Recent Change in Pluto’s Haze Abundance.” Hazes have been observed on Titan (photolysis products from higher up in the atmosphere) and Triton (condensates from surface). The August 21, 2002 occultation showed evidence for haze (change in brightness with color, Elliot et al 2003), but 2007 (0.51 & 0.76 micron) and June & July 2011 occultation events in different bands (I & K bands) showed no change in color.

Occultations can only probe down to a certain depth, so they are limited. We don’t really how close you got to the Pluto surface. If you have a special case where you can have a central flash or sets of flash spikes, you can derive more information. By applying a new technique on the 2007 Mt John light curves, he proposed they can determine amount of haze by evaluating the attenuation in those parts of the light curve.

Central Flash Description:  A central-flash occultation is visible when the observer is located near the center of the shadow path of the object. It is here where the atmosphere near the edges of the occulting body (for Pluto occultations, this is Pluto) refracts extra star light (from the background star) directly opposite from the star, forming a “brightening” in the middle of the deep light curve.

Mark Gurwell (Harvard CfA) provided a talk entitled “Atmosphere CO on Pluto: Limits from Millimeter-Wave Spectroscopy.” Carbon monoxide (CO) is expected based upon the presence as an ice on its surface. The first direct detection of CO was done in the NIR with the VLT (Lellouch et al 2011). Then JCMT (Greaves et al 2011) revealed a CO(2-1) line in the submillimeter, but this line had not been there a few years back, leaving a mystery. There is still mystery in fitting the CO abundance based on the measured submillimeter width and strength of this line. He did show that Pluto had been in the fore-ground of a galactic emission during the JCMT observations. He supposes that they had contamination. They did their own observations using the SMA sub-millimeter telescope multiple times and did not detect the CO(2-1) line in the spectra (they have upper limits). So he is excited about using the ALMA array that has 30-50x SMA sensitivity to really address the CO, nitriles and isotopes.

And the final talk of the morning Atmosphere session just could not leave Charon out of it.

Alan Stern (SwRI) “Cometary Impact Produce Transient Atmospheres on Charon.” Most scientists have come to accept that Charon does not have an atmosphere (see earlier posting in Bruno Sicardy’s talk showing the dual occultation event for Pluto and Charon in Jun 2011.) But he postulates what about impacts? Coming from the Kuiper Belt, impactors (assuming cometary-level amounts of volatiles) could provide volatiles to the surface to Charon and therefore creating a “nanobar” atmosphere on Charon. Similar events could lead to atmospheres on Kuiper Belt objects. Their modeling (Trafton & Stern 2008) predicts presence of N2, CO, Ar, CH4, with CO dominating after impacts, and N2 being the dominate species (in terms of amount).

Predictions for New Horizons. Duty cycle would be short lived so it will be rare if New Horizons catches this event. However, with smaller impactor sizes, there could be a possibility that those events could have occurred within the “photoionization time” (before the molecule breaks down or escapes) or resulting implanted molecules by the time New Horizons gets there. Alan Stern coyly stated, “could be as much as a 25% chance” to see an nano-bar atmosphere on Charon.

A good question regarding surface volatiles that are revealed by impacts got the crowd excited. After all, when you describe an atmosphere you can categorize things as sources and sinks. Sources bring material to the system and here they could be not only the KBO impactor but also the materials that are revealed from the impacted-body after the impact.

On July 14, 2015 New Horizons will be doing a very sensitive experiment via the observations of the Charon occultation (Charon passing in front of our Sun as viewed from New Horizons).

Charon may indeed hold a few surprises of her own!

Pluto’s uppermost atmosphere. How big is it?

This is a blog series about talks presented that Pluto Science Conference, held July 22-26, 2013 in Laurel, MD.

Darrell Strobel (JHU) next took us through a study about  “Pluto’s Atmosphere: Escape and the Relationship to Density and Thermal Structure.”

But first, what hydrodynamic escape discussion could be complete without a few equations….

Atmosphere Escape Equations

Yes, this is what an atmospheric modeler solves for. He/she solves numerous energy-balance, energy-transport, etc. equations to derive properties for making an atmosphere.

The Hydrodynamic escape rate is a key output from these numerical models for Pluto, with predictions in the range of 1.5-6.7 x 1027 particles/s. The basic problem with computing hydrodynamic escape is due to the presence of a gravity well that these molecules need to escape from. Essentially, you need an additional energy input (such as thermal) to drive the escape process.

Some other key terminology: “The exosphere is a thin, atmosphere-like volume surrounding a planetary body where molecules are gravitationally bound to that body, but where the density is too low for them to behave as a gas by colliding with each other.” (Wikipedia) It is the uppermost layer where the atmosphere thins out and merges with interplanetary space. The exobase is the lower boundary of the exosphere, defined as the altitude at which the atmosphere becomes collisionless.  Atmospheres can lose atoms from stratosphere, especially low-mass ones, because they exceed the escape velocity (Ve= (2GM/ R)½). This is known as (Jeans escape or Thermal Escape). The Jeans parameter (lambda) is a measure of how efficient the loss mechanism is. Larger lambda values implies less loss (smaller escape rates).

Models by different groups predict Pluto’s exobase between 5 and 10 Pluto radii. Assuming Pluto radius of 1200 km, Pluto’s exobase is in the 6000-12,000 km range. New Horizons’ nominal trajectory will bring the spacecraft to within ~10,000 km of Pluto’s surface and ignoring the slight fact that there are uncertainties in deriving Pluto’s size in the 20-100 km range and ignoring whether you determine a planet size by including or excluding the atmosphere, there is a possibility New Horizons could be flying through Pluto’s exosphere. Such an extended atmosphere could be affected by Charon and could affect Pluto’s interaction with the solar wind at the New Horizon encounter, as measured by New Horizons instruments PEPSSI and SWAP. (For more talk summaries about solar wind, see later blog entry).

A plot of temperature in Kelvin (x axis) vs. altitude in km (y axis) is a typical output of this type of model. Below is a particular plot shows the effect of adiabatic cooling, which Darrel Strobel stressed, is a key component that cannot be ignored. Another key output from these models is the computation of number density (N2 molecules/cm2/s) as a function of altitude.

Temperature Profile with Altitude

Temperature profile with altitude for models with (blue) and without (red) adiabatic cooling. The surface is at 40 K (which is from observation evidence) and upper atmosphere temperatures are in the 100s of Kelvin (supported by NIR spectral observations of methane). The two models predict wildly different temperatures at high altitudes depending on whether cooling is occurring.

Darrel Strobel’s predictions for New Horizons fly-by: Escape rate 3.5×1027 N2/s, exobase at 8 Rpluto ~9600 km, Jeans Parameter Lambda ~ 5.


Weather on Pluto. Fair, haze patches at first. Moderate calm with the occasional chop.

The July 23, 2013 morning session of the Pluto Science Conference started with a collection of talks addressing what we know and what we don’t know about Pluto’s atmosphere.

Emmanuel Lellouch (Paris Observatory, France) spoke about “Pluto’s Atmosphere: Current Knowledge and Open Questions.”

What do we know about Pluto’s Atmosphere? We know that it is a nitrogen (N2) dominated atmosphere with methane (CH4) (tens of %) and probably carbon monoxide (CO). It’s about 10-microbar (pressure) class showing evidence for changes in pressure on year/decade timescales. There is also evidence for waves (dynamic changes), and the atmosphere does have a thermal structure, despite the details being hotly debated in the community (pun intended). People do agree that the surface is cold (40-50 K) and then the atmosphere is around 100K at micro-bar pressure levels. The details of the cold/warm layers in between are the stuff that thermal models are made of!

Pluto was discovered in 1930, but it was only in 1985 that the first observation detecting an atmosphere around Pluto was made. It was discovered through a measurement called a stellar occultation, when Pluto crossed between a star and an observer on Earth, on August 19, 1985 (Brosch, MNRAS, 1995). A higher signal-to-noise light curve was obtained on the June 9, 1988 (Elliot, et al 1989) occultation events whose light curves indicated existence of waves.

Occultation light curve for Pluto passing in front of a star on Jun 9, 1988

Occultation light curve for Pluto passing in front of a star on June 9, 1988 (signal vs. time) Features in this dataset indicate the upper atmosphere (above the ‘kink’) and lower atmosphere (below the ‘kink’). The ‘kink’ presence is theorized to be due to heating by methane (Hubbard et al 1990). Waves are indicated by the “spikes” in the light curve. When the scientists create this light curve from the occultation event, they then “invert” it to fit a temperature model and derive pressures for different scale heights.

The first molecular detection of anything in the Pluto’s atmosphere was methane (L. Young et al 1997) using the IRTF (3.5 m telescope) in May 1992. This was confirmed and re-measured in 2008 with higher resolutions and sensitivity (Lellouch et al 2009) using the VLT (8 meter telescope) with more recent observations in 2012. Emmanuel Lellouch showed that with those two latter datasets there was no evidence of change in the last four years. With this higher resolution data they can use it to provide a fit to the temperature using the line widths.

Carbon monoxide (CO) was detected in the submillimeter at 240 GHz with JCMT (Greaves et al 2011), but this detection and the inferred amount has lead to questions that the current models cannot produce this molecule with the temperature and amount inferred from the observations. This particular topic was addressed by Mark Gurwell’s talk later in the morning.

There is also evidence for diluted methane (CH4) and pure CH4 ice on Pluto’s surface. The atmosphere CH4 is much greater than what is expected from an ideal mixture, so this implies there must be a mechanism to enrich the CH4 component in Pluto’s atmosphere. Recent thermo-dynamic models and “GCMs” (general circulation models) predict a consistent mixture for CH4.

The combination of both the infrared spectral results and the visible (and in some case near-infrared) occultation light profiles helps resolve temperature profile (i.e., how temperature varies with altitude) inconsistences.

Speaking of temperature profiles, one of the hotly debated topics for Pluto atmosphere specialists is whether their models contain a tropopause. Per Emmanuel Lellouch’s overview talk, he stated, “There is no proof there is a troposphere. And deep troposphere are not predicted by the GCMs.” However, many Pluto atmosphere specialists often invoke a troposphere in their calculations to help predict other things that have been inferred to occur on Pluto.

Pluto’s atmosphere seems to be changing. There is observation of pressure evolution. Specifically, the pressure appears to have doubled from 1988 to 2002 (Sicardy et al 2003, Eliot et al 2003). Evidence that the pressure is continuing to increase is based on recent 2013 occultation data. This has led to the development of Volatile Transport Models. These are basically computations that track the dominant species, and for Pluto, it is nitrogen, through multiple temperature and pressure ranges, heat exchanges such as sublimation cooling in summer and condensation heating in winter. A schematic of a Volatile Transport Model from Leslie Young, New Horizons deputy Project Scientist, is shown below.

Schematic of a volatile transport model for Pluto

Schematic of a volatile transport model for Pluto. More details about the model are in a blog on Leslie Young’s volatile transport model talk later in the conference.

Other Oddities in Pluto’s Atmosphere. There appears to be evidence for photochemical haze from a 2002 occultation (Elliott et al 2003) but occultations in 2007 and 2011 did not show evidence of this. Hazes are large particles in the atmosphere (almost cloud-like) and the 2002 occultation had suggested hazes since there had been a distinct change in brightness as a function of wavelength. Why does the haze come and go, and what is causing it?

Pluto has also indicated “reddening” (color-change) that occurred between 2000 and 2002 (Marc Buie using color photometry with HST). That’s a mystery.

Waves (dynamic changes) in atmosphere are indicated by some of the occultation measurements. What could cause waves? There are multiple suggestions what could form these dynamic changes (even evoking the elusive gravity wave mechanism). Could it simply be Pluto’s atmosphere response due to the diurnal variation of sublimation of N2 particles?

What will Pluto’s Atmosphere be like when New Horizons comes to take a close look?

  • Will the atmosphere be there in 2015? Lellouch’s best guess: Yes.
  • Will there be a thermal structure (i.e. see a troposphere)? Lellouch’s best guess: Hopefully (helps modelers out).
  • Will there be other gases present (i.e. C2H2 HCN, etc.)? Lellouch’s best guess: Maybe.
  • Will there be clouds or hazes? Lellouch’s best guess: Maybe.
  • When will the atmosphere collapse (i.e. pressure drops by orders of magnitude)? Lellouch’s best guess: “Your guess is as good as mine.”

Comparative compositions of Pluto and friends, even long-distant friends.

Continuing coverage of the July 22, 2013 first day of the Pluto Science Conference being held this week in Laurel, MD.

Bill McKinnon (Washington University) next provided an engaging talk about implications for composition and structure for Pluto and Charon.

Where did Pluto Accrete (i.e. where was Pluto born)? Pluto is not alone in its location on that a/e plot for Trans-Neptunian Objects (see previous posting).  It’s part of an ensemble of bodies on the 2:3 resonance with Neptune, coined the group “Plutinos.” Was Pluto formed around 33 AU (Malhotra 1993, 1995) and then migrated outward? What does this Nice I Model (Levinson et al 2008) which migrates the giant planets predict for the KBO population? The Nice I Model implies that for Pluto, Pluto could have formed 20-29 AU (i.e. closer in) to allow it to achieve its high inclination. Then a subsequent model, Nice II (Levinson et al 2011), suggests Pluto may have formed in the 15-34 AU range. This is in okay-agreement with accretion models since Pluto, a 1000-km size body, would need 5-10 million years (i.e. within a nebular life) if it were formed in the 20-25 AU range. McKinnon’s best guess: Pluto formed between 15-30 AU.

How long did accretion take and what are the implications (i.e. how long for Pluto to grow up)? If we have an accretion time (10’s of million years), there is time enough to form Aluminum-26, which provides a form of heat through its decay. Heat then can melt ices and create a differentiated body (i.e., rocky core, icy mantle) and also drive water out. McKinnon’s best guess: Pluto formed rapidly and early.

What are Pluto & Charon made of? They are understood to be made of rock+metal, volatile ices, and organics, with rock+metal more than ice, and ice more than organics. The rock will be some combination of hydrated & anhydrous silicates, sulfides, oxides, carbonates, chondrules, CAIs (calcium-aluminum-rich inclusions), CHONPS (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur). We don’t really know what sort of composition these KBO volatile ices: will they be more like Jupiter Family Comets or Oort Comets? And we know even less about organic components: will the Nitrogen to Carbon ratio tell us whether KBO N2 (nitrogen) comes from organics rather than NH3 (ammonia)? Solar models (which lock up CO (carbon-monoxide) into carbon) can influence understanding of what rocks in the outer solar system are made of but their models are not in agreement with the best understanding of Pluto/Charon make-up. McKinnon’s best guess: Rock/Ice nature of Pluto-Charon is 70/30.

What are the implications for Pluto & Charon internal structure? New Horizons will not directly detect the differentiation state of Pluto & Charon because it does not fly close enough.

Alain Doressoundiram (Paris, France) came next. Using MIOSTYS, multi-fiber front-end to a fast EMCCD camera, on a 193 cm telescope in France, they observe outer solar system bodies using stellar occultations. Other science objectives for variable stars, transiting exoplanets. They confirmed two TNO occultation events, one in 2009 and one in 2012 and continue monitoring.

Luke Burkhart (Johns Hopkins University) talked about his work on a “Non-linear satellite search around Haumea.” Haumea is another Trans-Neptunian Object (TNO) that has multiple satellite companion, like Pluto. Using HST (10 orbits) they observed the Haumea system and used a method of stacking & shifting to identify satellites. But this method fails to capture objects which are close in, moving fast, and on highly curved orbits. So they developed a new method using a non-linear shift-rate. Their approach, when applied to the Haumea system, had a null-result. However, this approach could be used on images of the Pluto system and other TNOs.  Specifically, in answer to a question from the audience, Luke would be eager to use his technique on any of those long-range KBO targets the New Horizons project is currently investigating.

Family portraits of the eight largest trans-Neptunian objects (TNOs).

Family portraits of the eight largest trans-Neptunian objects (TNOs) (From Pluto is shown with its 5 companions.

Andy Rivkin (JHU/APL) ended the afternoon’s lively discussion by addressingDistant Cousins: What the Asteroids Can Teach us About the Pluto System”. He started his talk with a comparison of sizes between Ceres (the largest asteroid in the asteroid belt between Mars and Jupiter) and the Pluto System. He used as his framework Emily Lakdawalla’s chart, which can be found on the Planetary Society blog

Here the relative sizes of objects in the Pluto system are represented by objects from the Saturn system. Saturn’s moon Rhea serves as Pluto, Dione as Charon, Prometheus as Nix, Pandora as Hydra, Helene as Kerberos, and Telesto as Styx. Superimposed is where Ceres (an asteroid in our asteroid belt between Jupiter & Mars) fits on this scale. Andy Rivkin did a comparison of his observations of Ceres to postulate what that might mean for the Pluto system.

Pluto system and Ceres shown to scale, represented by objects from the Saturn system.

Pluto system and Ceres shown to scale, represented by objects from the Saturn system.

Ceres has an icy interior, but much too warm to keep ice on surface. HST images reveals rather smooth surface. IR spectra (from reflected sunlight) are very rich and indicative of ice-type features. Could there some sort of layering? On Pluto, you could have the same thing, but it’s also cold enough for ice to remain on the surface. There is also a mystery that several large C asteroids have similar 3 micron spectra to Ceres like 10 Hygiea and 704 Interamnia.

Implications for Pluto: Large main-belt asteroids could serve as comparisons for KBOs. Geophysical comparisons may be easier than compositional ones.

So the big take-away from the introductory talks on the “Kuiper Belt Context” is that we can learn more by sharing the knowledge: Learning from Other Bodies  (Other TNOs, Comets, Asteroids) will help us learn more about Pluto & Charon, and vice versa.

Finding that distant KBO needle in a deep space haystack.

Next up at the Pluto Science Conference were a series of talks dedicated to recent work in the searches for another Kuiper Belt Object (KBO) for the New Horizons spacecraft to fly by after its Pluto fly-by. Fuel on board the New Horizons spacecraft after the July 2015 Pluto fly-by could enable a fly-by of a distant KBO in the late 2010s through 2020s, pending identification of targets reachable within New Horizon’s remaining fuel budget.

John Spencer (SwRI) has been leading the ground based campaign to search for New Horizons’ next target. With an on board ~0.13 km/s delta-v (measure of propellant), traveling at 14 km/s, this translated to a ~0.5 deg half-angle cone through the Kuiper Belt for accessible targets, a type of “survey beam.” Previous KBO searches had been for R>26.0 over 1-2 degrees. But right now Pluto is in Sagittarius which is in the direction of the Galactic Center and there are a lot of other stars in the field that make searching for a slowly-moving object, this KBO, difficult.

KBO Survey Star Field

Above are what the star fields the team is inspecting look like. They observe the same star field night after night and look for shifts in a object between frames, indicating it’s a KBO and not one of the “fixed stars.”

Outer Solar System“Known objects in the Kuiper belt, derived from data from the Minor Planet Center. The pronounced gap at the bottom is due to difficulties in detection against the background of the plane of the Milky Way.” This is exactly where John Spencer and his team are focusing their efforts because a subsection of that part of the sky is what is reachable by the New Horizons spacecraft after 2015. Image taken from

The ground based search program area is entering a sweet spot, where they can cover a smaller area of sky from the Earth that falls within the expected New Horizons travel zone.

KBO Search Zone

The team has found 31 objects from 2011 data including a TNO. However, as of 2012 season, they have not found an object that could have a fly-by encounter by the New Horizons spacecraft.  But there are three objects (2011 JW31, 2011 JY31, 2011 HZ102) that New Horizons could get to within 0.15-0.2 AU of in 2018-2019. The team is in the middle of the 2013 observing season and based on the current number densities they are predicting to see 1.78 objects down to  R=26.0mag and 4.15 objects in 26.5 mag.

Alex Parker (one more month at Harvard before moving to Berkeley) provided a more in depth view of unique observations New Horizons can still make of these long-distance KBO fly-by, that is, a fly-by in the 0.1-0.2 AU range of the spacecraft. At 0.2 AU range, New Horizons’ LORRI will have 140 km/pixel range compared to our “sharpest eyes” by Hubble at 1200 km /pixel from low-Earth orbit.

His excitement over the unique discovery space New Horizons provides that you cannot get from anywhere else: Proximity. High angular resolution. High phase angles.

He’s been studying trans-Neptunian binaries as binaries provide a direct mechanism to measure their masses. “Wide” Kuiper Belt binaries have been discovered already (e.g.  Gemini observations of wide binaries 2006 BR284 separated by 0.82 arcseconds; 2000 CF105 separated by 0.95 arcseconds).

To visualize a ride on looking over the New Horizons shoulders as it journeys into the Kuiper belt, check out this one of Alex Parker’s visualizations at

Make note to hold onto your seat when the craft enters the Cold Kuiper Belt region in 2018!

Susan Benecchi (Carnegie Institute) rounded out the talks with HST Follow-Up Observations of Long-Range Candidates for New Horizons post Pluto. They observed 2011 JW31, 2011 JY31, and 2011 HZ102 with HST. Those objects had been detected with the KBO ground based search program described by John Spencer and Alex Parker (previous presentations). Her team has not confirmed detection of 2011 JW31. Her team has confirmed the colors of the two other objects being “red” which is consistent with the Cold Classical Population (i.e. primordial). Implications for New Horizons: HST can provide this extra characterization step for new candidates.

Gustavo Beneditti-Rossi (Brazil) described a summary of “Astrometric Analysis of 15 years of Pluto Observations.” Using the Pico dos Dias Observatory (1.6m and 0.6m telescopes), they monitored Pluto-Charon (which is not separated in their data) for 154 nights over 1995 to 2012. They do refraction correction (due to viewing angle from earth) and photo-center correction (due to the fact they cannot separate Pluto from Charon). And show that their tracking of Pluto’s location is in agreement with occultation data.

To end this post, I could not resist showing you Alex Parker’s vision of what New Horizons brings to this field of study. He created this montage of images illustrating the proximity (within artistic license) and equally important the high phase (objects as crescents) and high angular resolution (we can see surface features), all that New Horizons will provide in 2015 that no other observation platform can.

NH Silhouette

2015 will be the “Year of Pluto” and so much more!

Pluto, “King of the Kuiper Belt, Prince of the Plutinos.” Certainly an object that inspires odes, songs, and ballads.

After the New Horizons’ instrument overviews on the first day at the Pluto Science Conference (Jul 22, 2013, we jumped right into Pluto in the Kuiper Belt Context.

Brett Gladman (University of British Columbia, Vancouver, Canada) started the conversation by addressing “How does Pluto fit in our understanding of the Kuiper Belt?”

But before we get into that, discussing the Kuiper belt today can be pretty complex. It was only discovered in 1992, but in the years since, over 130,000 bodies with sizes 100 km and larger have been identified (Petit et al 2011), with Pluto being the largest member.

So when we start looking at large numbers of objects, it’s time to classify. So a typical plot to describe these “populations” is shown below, where semi-major axis (distance of body from the Sun) is plotted (horizontal axis, labeled ‘a’ in units AU, where 1 AU is the distance of the Earth from the Sun, Jupiter is ~5 AU, Saturn ~10AU) versus eccentricity (value between 0 and 1 that describes how circular an orbit it, e=0 is circle, e=1 is parabola, 0<e<1 describes ovals).

And then you have your Classical, your Cold Classical, Hot Classical, Detached, Resonant, and SDO (aka Scattered Disk Objects), etc. Sometimes they group together, others are more uniform across the parameter space.

“Kuiper Belt in a/e space.”

Kuiper Belt in a/e space.” Cold classical (black open triangles). Resonant Kuiper (open red square). Detached (blue triangles). Pluto is indicated with the yellow-box, it’s a Resonant, as it is in 3:2 Resonance with Neptune. This group of objects, all in 3:2 Resonance with Neptune are the “Plutinos.” (that clumping around 40 AU, red triangles, spanning over a range of e). Resonance numbers are shown at the top of the graph.

Plutinos are also a family of TNOs, Trans-Neptunian Objects, characterized by being in 3:2 mean-motion resonance with Neptune (i.e., every time Neptune makes 3 trips about the Sun, the Plutinos make 2 trips). Plutinos are the most dominant of the TNOs. Less numerous are the 1:1 objects, objects known as Trojans.

Definitely KBO soup!

For more information about TNOs and their period relationships with Neptune

After getting down those nomenclature basics, Brett Gladman (who is also lead for a huge ground-based survey of KBOs called the Canada-France Ecliptic Plane Survey/CFEPS) discussed the strengths and pitfalls of the theories put forward to explain the formation and structure of this complex KBO menagerie.

For more information about CFEPS check out

How did Pluto get to where it is today? Two leading theories (1) Resonant sweeping of objects formed in TNO regions and (2) resonant trapping explain many things, but no published models explain those resonant structures of the Kuiper belt. And any of these models have issues with the classical and scattering disk populations as well. Theorists, better sharpen your pencils.

So he left us with questions to ponder. Is there a cold primordial Kuiper Belt with edge at 45 AU? Did Pluto likely form as one of hundreds to thousands of >1000km embryos? Did some of these become implanted into the nearby non-scattering belt? Are there others out beyond 100 AU (considered likely, but to discover them, you need to get down to 23-24th mag which is beyond the current survey capabilities until new telescopes and.or techniques come available)?

No doubt, searches for more TNOs will continue, the classification of the KB will undergo evolution, and theorists will refine their models. And New Horizons will provide a unique data set of an up-close-and-personal visit to Pluto and its companions to help constrain those models.

Putting Centaurs and TNOs in Context

Putting Centaurs and TNOs in Context. This time plotting inclination (the degrees from the ecliptic plane) vs. semi-major axis in AU. Object sizes are reflected in the symbol sizes. Location of Saturn, Uranus and Neptune are shown. Just another way to look at that awesome & diverse Outer Solar System. From:

Next, Cesar Fuentes (Arizona State) phoned in about his work on the “Size Distribution (SD) of the Kuiper Belt.” Size distribution is basically counting the number of objects as a function of size.  Coagulation (of small particles) and gravitation instability (of larger particles) shape the size distribution. Size distribution is expected to change due to collisions. Different distances from the sun also appear to have different size distributions.

He stepped us through recent size distribution models from Schlichting, Fuentes & Trilling (2013) and Kenyon & Bromley (2013) where they even have some including the “collisional factor” influence on the size distribution over time periods.

Snapshot of Size distributions by KBO-classification

All the Size Distributions show a “rollover” around H~9, D=70km. Nesvorny et al. 2013 investigates this further. Is the break due to collisional and therefore separate the primordial from the evolved KBO populations?

Even more questions to ponder:  Can we use size distributions to evaluate primordial from the evolved KBO populations?

And then he left us with a tantalizing experiment with the New Horizons mission: If New Horizons can provide data sets enabling “crater counting,” we will be able to measure the impactors on Pluto. This can aid in understanding KBO populations, addressing specifically, formation time, timescales for surface activity, and origins of bodies like Nix & Hydra. What would a 0.1-100 km impactor size distribution look like?

Pluto, be it Prince of the Plutinos or King of the Kuiper Belt, will always remain a key part to these questions above. And data sets from New Horizons will provide many new angles to answering questions about “Where did Pluto form and why did it wind up where it is now.”


Introducing the New Horizons Instrument Menagerie.

During the first day of the Pluto Science Conference, being held July 22-26, 2013, in Laurel, MD, the conference participants listened to a series of talks describing the rich instrument suite aboard the New Horizons Spacecraft. This entry is just a very brief synopsis of the instruments.

Ralph, Alice, MVIC, LEISA, LORRI, REX, SWAP, PEPSSI, SDC. Those are instrument names and acronyms of the New Horizons science instruments.

New Horizons Instrument SuiteNew Horizons Instrument Suite at a Glance.

LORRI (Long Range Reconnaissance Imager), among many things, “Enables Far-Out Encounter Science. ” That is, at 10 weeks from closest approach, LORRI can observe the Pluto system with spatial resolution better than Hubble. It is a visible camera, equipped with a 1024 x 1024 pixel CCD, with a 0.29 x 0.29 degree field of view (5 microradian pixel iFOV). LORRI also will be used, on approach, for optical navigation. The LORRI Instrument Principal Investigator and Instrument Scientist is Andy Cheng (JHU/APL) and Hal Weaver (JHU/APL), respectively.

Ralph & Alice form New Horizons’  “Remote Science Suite.” Ralph is both a color-imager (MVIC) and an infrared mapping spectrometer (LEISA). Alice is a ultraviolet spectrometer.

Ralph’s MVIC (Multi-Spectral Visible Imaging Camera) consists of seven independent CCD arrays. Four channels are filtered to map blue (400-550 nm), red (540-700 nm), near infrared (780-975 nm) and a narrow methane absorption band (860-910 nm). Six of the MVIC arrays (including all the filter channels) have a 5.7 x 0.037 degree field of view (20 microradian pixel iFOV). LEISA (Linear Etalon Imaging Spectral Array) is a grating spectrometer covering 1.25 to 2.5 microns wavelength range at a resolving power of R~240. A second segment covers 2.1 to 2.25 micron range with a resolving power of R~560. The Ralph Instrument Principal Investigator and Instrument Scientist is Alan Stern (SwRI) and Dennis Reuter (NASA Goddard), respectively.

Alice is an ultraviolet imaging spectrometer. It has two entrance apertures, a large airglow channel and a small SOCC aperture for solar occultation measurements. The entrance slit has two sections, a “box” with a 2.0 x 2.0 degree field of view, and a “stem” with a 0.1 x 4.0 degree field of view. The wavelength coverage spans from 520 to 1870 Angstroms, with a resolution of 3.6 Angstroms.  The Alice Instrument Principal Investigator and Instrument Scientist is Alan Stern (SwRI) and Maarten ver Steeg (SwRI, San Antonio), respectively.

REX, New Horizons’ Radio Science Experiment, is enabled by adding a small amount of signal processing hardware to the existing communication hardware on New Horizons’ main antenna. It will be used, among other observations of Pluto, to showcase a “Different Kind of Radioscience” via 20kW uplink experiments from the DSN during the Pluto and Charon occulations at flyby. The REX Principal Investigator and Instrument Scientist are G.L. Tyler and Ivan Linscott (Stanford University), respectively.

PEPSSI (Pluto Energetic Particle Spectrometer Science Investigation) & SWAP (Solar Wind Around Pluto) are modern particle instruments designed to capture Pluto’s interaction with the solar wind. PEPSSI can measure ions and electrons from 10s of keV to 1 MeV over a 160 x 12 degree fan-shapped beam. SWAP can measure particles with energies 35 eV to 7.5 keV over a 276 x 10 degree field of view. PEPSSI’s Principal Investigator and Instrument Scientist are Ralph Mcnutt (JHU/APL) and Matthew Hill (JHU/APL). SWAP’s Principal Investigaor and Instrument Scientist are David McComas (SwRI, San Antonio) and Heather Elliott (SwRI, San Antonio).

SDC, the Student Dust Counter, designed and built by students at the University of Colorado, Boulder, is “The First Student Experiment on a Deep-Space Probe.” The Principal Investigator is Mihaly Horanyi (University of Colorado). There have been numerous Instrument Scientists, all students at Univ. of Colorado. The current Instrument Scientist is Jamey Szalay. Students continue to be active in supporting data analysis as SDC collects dust rates on its voyage through the solar system. More information about the SDC and the students behind it at

More details about each of the instrument descriptions and performance can be found at
New Horizons Instruments

Locations of the science instruments on the New Horizons Spacecraft

The Architecture of New Horizons’ Pluto Fly-By Sequence.

In her presentation at the Pluto Science Conference, Dr. Leslie Young (SwRI), deputy Project Scientist and chief architect of the Pluto Encounter Sequence, stepped us through the New Horizons’ Science Objectives and the types of observations that will be pre-programmed aboard the craft for the entire year of 2015. Unique science is not just around Pluto Closest Approach on Tues, Jul 14, 2015, but many months prior and post the encounter. Although most of the “Group 1” (see below for description) science objectives for the mission will be met by measurements made in the -2hr to +3hr from closest approach. Closest approach is on July 14, 2015.

Leslie Young Presents Group Science Objectives

Leslie Young (deputy Project Scientist) describes the overview of the science highlights for the year 2015. Also shown in the slide is a mapping of the Science Objectives per each phase.

Science space missions typically have a set of “science requirements,” specific measurements to address specific questions, set forth to be met by the mission design. The main science questions that the New Horizons mission is designed to answer were asked in the proposal call (AO 01-OSS-01) that NASA put out in early 2001, the competition which the New Horizons team won. The proposed series of measurements that New Horizons will do with its instrument suite provide measurements to answer Group 1, Group 2 or Group 3 objectives. Group 1 are measurements that must be done and define baseline science mission success. Group 2 are highly desired measurements and Group 3 are desired measurements. To obtain data that meets Group 1, 2 & 3 measurements is full-science success.

That single slide that Leslie showed (above) is the sum of many, many, many months of work with the New Horizons Science Team, along with support from the project’s Mission Design team, to identify which measurements of which body at which time (or times), as an ensemble meet the Group objectives. She specifically calls out the Group 1 by showing those categories in Bold Italics.

As the New Horizons Science Fly-By mission is a temporal series of measurements, the mission has been constructed to compartmentalize the measurements as a function of day from the closest approach. Hundreds of unique measurements are scheduled in rapid-formation within the day prior and after closest approach, called the NEP or Near Encounter Period.

Some Pluto Encounter Design Temporal Terminology:
AP= Approach Phase, NEP=Near Encounter Period, DP=Departure Phase
AP1: Jan 6-Apr 4, 2015, P-180 to P-100 days to Closest Approach
AP2: Apr 4-Jun 23, 2015, P-100 to P-21 days to Closest Approach
AP3: Jun 23-Jul 13, 2015, P-21 to P-1 days to Closest Approach
NEP: Jul 13-15, 2015, P-1 to P+1 day from Closest Approach
DP1: Jul 15-Aug 4, 2015, P+1 to P+21 days from Closest Approach
DP2: Aug 5-Oct 22, 2015, P+21 to P+100 days from Closest Approach
DP3: Oct 22, 2015-Jan 1, 2016, P+100 to P+180 days from Closest Approach

Leslie Young Present Near Encounter Phase

Leslie Young describes the mission science measurements on a timeline near closest-approach. The instruments are color-coded in this representation of the distance to Pluto (y axis) vs. distance from Earth/Sun (x axis) with respect to the closest approach (nominal July 14, 2015 11:50 UTC). A larger version of that slide is shown below. The x-axis spans 5 hrs of time.

NEP Timeline

Below is a summary of the best spatial resolution measurements anticipated from New Horizons’ Remote Sensing Suite within a few hours of closest approach. Panchromatic (LORRI camera), Color (Ralph MVIC), and Infrared (Ralph LEISA) resolutions are shown against each target body for the closest-distance to those target bodies in the nominal sequence. The science requirement for the equivalent Group 1 objective is shown in italicized text.

NH Best Resolution Specs

With our current best resolution of Pluto spanning 100 km/pixels taken with the Hubble Space Telescope, the New Horizons mission with its up-close-and-person will rewrite the textbooks on this elusive system with more than 2 orders of magnitude resolution improvement, plus spectral, radioscience, and plasma unique measurements.

Our best on Pluto from Hubble can be found form these links for observations taken in 1994 & 2010: and, respectively.

Calling for proposals to observe the Pluto System from Earth and Earth satellite-assets! “As planetary astronomers, we love phases” as Rick Binzel (MIT) describes “Earth-Based Observing Campaign for the New Horizons Encounter.” We’re going to need to make a link to connect decades of earth-based observations of the Pluto system before the fly-by and continue it for decades after the New Horizons fly-by. There is a website set up for information on how to participate and get more information. Specifically observations are needed in 2014, 2015 (encounter year), and 2016.

Earth Campain Obs Pluto

The website will be based at . Check back later since they are actively working the content, but you can always email nhobs “at” for information.

Rick Binzel also introduced the campaign to get a Lego set made of the New Horizons Spacecraft. It needs to vote to get it approved for production. Note: this requires you to register for free-account to log in to vote.

NH Lego

Summing up the first session of an exciting beginning to the Pluto Science Conference,  per Alan Stern, the Principal Investigator for NASA’s New Horizons’ Mission: “The most exciting discoveries will likely be the ones we don’t anticipate” and  “Revolution in Knowledge is in Store.”