Is Webb at Its Final Temperature?

The Mid-Infrared Instrument (MIRI) on NASA’s James Webb Space Telescope is now cooled by a gaseous helium cryocooler to under 7 kelvins. With the cooler in its final state, the Webb team is operating the MIRI instrument this week as part of seventh and final stage of the telescope alignment. When the instrument is operating, the detectors and electronics produce heat, which is balanced by the cryocooler to keep MIRI at a stable, and very cold, operating temperature. The near-infrared instruments also warm up during operations and have to dissipate heat, although for these instruments this is done with passive cooling; the heat from the detectors and electronics is radiated into deep space.

Now that the instruments are at their operating temperatures, the telescope mirrors will also continue cooling down to their final temperatures, but they are not quite there yet. The primary mirror segments and the secondary mirror are made of beryllium (coated with gold). At cryogenic temperatures, beryllium has a long thermal time constant, which means that it takes a long time to cool or to heat up. The primary mirror segments are still cooling, very slowly.

The secondary mirror, hanging out on the end of its support structure a long way from any heat sources, is the coldest mirror, currently at 29.4 kelvins. The 18 primary mirror segments range in temperature from 34.4 kelvins to 54.5 kelvins. An advantage of beryllium mirrors is that they don’t change shape with temperature the way glass mirrors would at these temperatures, so the temperature range does not affect the telescope alignment process.

Currently, four of the 18 mirror segments are above 50 kelvins: at 52.6, 54.2, 54.4, and 54.5. These four mirror segments emit some mid-infrared light that reaches the MIRI detectors. Since all the mirror temperatures are now below 55 kelvins, it is expected that MIRI will be sensitive enough to perform its planned science, but any additional cooling of these mirrors will only enhance its performance. The Webb team hopes to see the mirrors cool by an additional 0.5 to 2 kelvins.

When we point the telescope at an astronomical target, the telescope and sunshield move together. The angle that the sunshield presents to the Sun is called the pointing “attitude.” The tiny amount of residual heat that makes its way through the five-layer sunshield to the primary mirror depends on this attitude, and since the mirror segment temperatures change very slowly, their temperatures depend on the attitude averaged over multiple days.

During commissioning, Webb is currently spending most of its time pointed at the ecliptic poles, which is a comparatively hot attitude. During science operations, starting this summer, the telescope will have a much more even distribution of pointings over the sky. The average thermal input to the warmest mirror segments is expected to go down a bit, and the mirrors will cool a bit more.

Later in commissioning, we plan to test the thermal dependence of the mirrors on the attitude. We will point Webb at a hot attitude for several days, and point Webb at a cold attitude for several days, in a process called the thermal slew. This will inform us how long it takes for the mirrors to cool down or heat up when the observatory is at these positions for any given amount of time.

Is Webb at its final temperature? The answer is: almost!


–Jonathan Gardner, Webb deputy senior project scientist, NASA’s Goddard Space Flight Center

Webb Will Study Formation, Composition, Clouds of Distant Worlds

The journey of commissioning the Webb telescope continues this week with the successful cooling of the Mid-InfraRed Instrument (MIRI), through the critical ‘pinch point,’ down to its final operating temperature of less than 7 kelvins (-447 degrees Fahrenheit, or -266 degrees Celsius). This was a precondition to completing the seventh and final stage of the mirror alignment process.  The next steps include initial check-outs of MIRI and continue on to the final stages of multi-instrument, multi-field alignment with all four science instruments.

Last week we shared the cool science on star and planet formation planned for Webb. Today, we get into details on how Webb will study planets around other stars, which are known as extrasolar planets, or exoplanets. Knicole Colón, Webb’s deputy project scientist for exoplanet science, takes us into the discovery space of exploring new worlds beyond our solar system. Dr. Colón brings a unique perspective as she is also the project scientist for the Transiting Exoplanet Survey Satellite (TESS), a mission that has found many exoplanet targets that Webb will observe.

“Over the last 30 years, astronomers have discovered over 5,000 extrasolar planets. These discoveries have revealed that exoplanets span a vast range of masses, sizes, and temperatures and orbit all types of stars, leading to extraordinarily diverse worlds.

“With its powerful spectroscopic and imaging capabilities across a wide infrared wavelength range, Webb is poised to revolutionize our knowledge of the composition of these worlds and of planet-forming disks. From small, potentially rocky exoplanets up to giant, gaseous ones, Webb will observe these worlds with the transit technique. Direct imaging techniques will be used to study young, giant exoplanets along with the environments in which planets form and evolve around stars, known as protoplanetary disks and debris disks.

“One specific exoplanet observation that will be done with Webb involves collecting observations over the course of a planet’s orbit to enable measurements of the atmospheric composition and dynamics. I am involved in a program to observe the gas giant HD 80606 b as part of Webb’s first year of observations. Because the orbit of HD 80606 b is extremely eccentric (non-circular) and long (111 days), the amount of energy received by the planet from its star ranges from approximately 1 to 950 times what Earth receives from the Sun! This results in extreme temperature variations, which are predicted to cause clouds to rapidly form and dissipate in the planet’s atmosphere on very short timescales. Our science team will probe these predicted cloud dynamics in real-time over the course of a continuous ~18 hour observation of HD 80606 b as it passes behind its star, using the NIRSpec instrument on Webb to measure thermal light from the planet’s atmosphere.

Caption: The orbital configuration of HD 80606 b is shown along with expected temperature variations as viewed from Earth and Webb at several orbital phases. The planned “start” and “end” of the ~18 hour stretch of Webb observations are indicated (credit: adapted from de Wit et al. 2016; courtesy of James Sikora).

“Beyond gas giants, a number of Webb’s exoplanet targets in its first year of observations are small and orbit stars that are smaller and cooler than the Sun, known as M dwarfs. While exoplanet discovery began around 30 years ago, many of these small exoplanets around M dwarfs were just discovered in the last few years by surveys like TESS. Webb observations will start to reveal the diversity of atmospheres that exist on these small planets by searching for evidence of molecules like water, carbon dioxide, and methane in their atmospheres. Because M dwarfs are typically much more active than the Sun and have energetic stellar flares that could potentially strip the atmospheres off of these planets, Webb observations may even reveal that some of these small planets have no atmosphere at all.

Caption: This artist’s illustration shows three small planets discovered by TESS around an M dwarf star called L 98-59. Planets c and d are just 1.4 and 1.6 times larger than Earth and will be observed in Webb’s first year of science (credit: NASA’s Goddard Space Flight Center).

“With TESS and other surveys continuing to discover additional planets in our galaxy at a regular pace and Webb preparing to study the atmospheres of many of these newly discovered worlds, our exoplanet adventures are in many ways just beginning.”

Knicole Colón, Webb’s deputy project scientist for exoplanet science, NASA’s Goddard Space Flight Center


Stefanie Milam, Webb deputy project scientist for planetary science, NASA Goddard

Jonathan Gardner, Webb deputy senior project scientist, NASA Goddard

 

Webb’s Cool View on How Stars, Planets Form

The ongoing success of the multi-instrument optics alignment for NASA’s Webb telescope’s near-infrared instruments has moved the attention of the commissioning team to chill as we carefully monitor the cooling of the Mid-InfraRed Instrument (MIRI) down to its final operating temperature of less than 7 kelvins (-447 degrees Fahrenheit, or -266 degrees Celsius). We are continuing other activities during this slow cooldown which include monitoring the near-infrared instruments. As MIRI cools, other major components of the observatory, such as the backplane and mirrors, also continue to cool and are approaching their operational temperatures.

Last week, the Webb team did a station-keeping thruster burn to maintain Webb’s position in orbit around the second Lagrange point. This was the second burn since Webb’s arrival at its final orbit in January; these burns will continue periodically throughout the lifetime of the mission.

In the last few weeks, we have been sharing some of Webb’s anticipated science, beginning with the study of the first stars and galaxies in the early universe. Today, we will see how Webb will peer within our own Milky Way galaxy at places where stars and planets form. Klaus Pontoppidan, the Space Telescope Science Institute project scientist for Webb, shares the cool science planned for star and planet formation with Webb:

“In the first year of science operations, we expect Webb to write entirely new chapters in the history of our origins – the formation of stars and planets. It is the study of star and planet formation with Webb that allows us to connect observations of mature exoplanets to their birth environments, and our solar system to its own origins. Webb’s infrared capabilities are ideal for revealing how stars and planets form for three reasons: Infrared light is great at seeing through obscuring dust, it picks up the heat signatures of young stars and planets, and it reveals the presence of important chemical compounds, such as water and organic chemistry.

“Let us look at each reason in more detail. We often hear that infrared light passes through obscuring dust, revealing newborn stars and planets that are still embedded in their parental clouds. In fact, mid-infrared light, as seen by MIRI, can pass through 20 times thicker clouds than visible light. Because young stars are formed quickly (by cosmic standards, anyway) – in as little as a few 100,000 years – their natal clouds have not had time to disperse, hiding what is going on in this critical stage from visible view. Webb’s infrared sensitivity allows us to understand what happens at these very first stages, as gas and dust are actively collapsing to form new stars.

Hubble Space Telescope images in the optical (top) and near-infrared (bottom) of the Eagle Nebula’s Pillars of Creation. These images show how infrared light can peer through obscuring dust and gas and reveal star and planet formation within these giant galactic stellar nurseries. Credit: NASA, ESA/Hubble and the Hubble Heritage Team.

“The second reason has to do with the young stars and giant planets themselves. Both begin their lives as large, puffy structures that contract over time. While young stars tend to get hotter as they mature, and giant planets cool, both typically emit more light in the infrared than at visible wavelengths. That means that Webb is great at detecting new young stars and planets and can help us understand the physics of their earliest evolution. Previous infrared observatories, like the Spitzer Space Telescope, used similar techniques for the nearest star-forming clusters, but Webb will discover new young stars across the galaxy, the Magellanic Clouds, and beyond.

“Finally, the infrared range (sometimes called the “molecular fingerprint region”) is ideal for identifying the presence of a range of chemicals, in particular water and various organics. All four of Webb’s science instruments can detect various important molecules using their spectroscopic modes. They are particularly sensitive to molecular ices present in cold molecular clouds before stars are formed, and NIRCam and NIRSpec will, for the first time, comprehensively map the spatial distribution of ices to help us understand their chemistry. MIRI will also observe warm molecular gas near many young stars where rocky, potentially habitable planets may be forming. These observations will be sensitive to most bulk molecules and will allow us to develop a chemical census at the earliest stages of planet formation. It is no surprise that a significant number of Webb’s early scientific investigations aim to measure how planetary systems build the molecules that may be important for the emergence of life as we know it.

“We will be keeping a close eye on MIRI as it cools down. As the only mid-infrared instrument on Webb, MIRI will be particularly important for understanding the origins of stars and planets.”

Simulated MIRI spectrum of a protoplanetary disk, as it might appear in a number of Cycle 1 science programs. The spectrum shows many features that demonstrate the presence of water, methane, and many other chemicals. Credit: NASA, STScI.

–Klaus Pontoppidan, Webb project scientist, Space Telescope Science Institute


By Jonathan Gardner, Webb deputy senior project scientist, NASA’s Goddard Space Flight Center

And Stefanie Milam, Webb deputy project scientist for planetary science, NASA Goddard

Webb’s Mid-Infrared Instrument Cooldown Continues

“The Mid-Infrared Instrument (MIRI) and other Webb instruments have been cooling by radiating their thermal energy into the dark of space for the bulk of the last three months. The near-infrared instruments will operate at about 34 to 39 kelvins, cooling passively. But MIRI’s detectors will need to get a lot colder still, to be able to detect longer wavelength photons. This is where the MIRI cryocooler comes in.

By necessity, MIRI’s detectors are built using a special formulation of Arsenic-doped Silicon (Si:As), which need to be at a temperature of less than 7 kelvins to operate properly. This temperature is not possible by passive means alone, so Webb carries a “cryocooler” that is dedicated to cooling MIRI’s detectors. Credit: NASA/JPL-Caltech.

“Over the last couple weeks, the cryocooler has been circulating cold helium gas past the MIRI optical bench, which will help cool it to about 15 kelvins. Soon, the cryocooler is about to experience the most challenging days of its mission. By operating cryogenic valves, the cryocooler will redirect the circulating helium gas and force it through a flow restriction. As the gas expands when exiting the restriction, it becomes colder, and can then bring the MIRI detectors to their cool operating temperature of below 7 kelvins. But first, the cryocooler must make it through the ‘pinch point’ – the transition through a range of temperatures near 15 kelvins, when the cryocooler’s ability to remove heat is at its lowest. Several time-critical valve and compressor operations will be performed in rapid succession, adjusted as indicated by MIRI cryocooler temperature and flow rate measurements. What is particularly challenging is that after the flow redirection, the cooling ability gets better as the temperature gets lower. On the flip side, if the cooling is not immediately achieved due to, for example, larger than modeled heat loads, MIRI will start warming.

“Once the cryocooler overcomes the remaining heat loads, it will settle into its lower-power steady science operation state for the rest of the mission. This pinch point event has been extensively practiced in the cryocooler testbed at NASA’s Jet Propulsion Laboratory (JPL), which manages the MIRI cryocooler, as well as during Webb testing at the agency’s Goddard Space Flight Center and Johnson Space Center. Performing it on orbit will be supported by the operations team comprised of personnel from JPL, Goddard, and the Space Telescope Science Institute. The MIRI cryocooler was developed by Northrop Grumman Space Systems. MIRI was developed as a 50/50 partnership between NASA and ESA (European Space Agency), with JPL leading the U.S. efforts and a multi-national consortium of European astronomical institutes contributing for ESA.”

– Konstantin Penanen and Bret Naylor, cryocooler specialists, NASA JPL

“MIRI stands out from Webb’s other instruments because it operates at much longer infrared wavelengths, compared to the other instruments that all begin with an ‘N’ for ‘near-infrared.’ MIRI will support the instrument suite to explore the infrared universe with depth and detail that are far beyond anything that has been available to astronomers to date.

“The imager promises to reveal astronomical targets ranging from nearby nebulae to distant interacting galaxies with a clarity and sensitivity far beyond what we’ve seen before. Our grasp on these glittering scientific treasures relies on MIRI being cooled to a temperature below the rest of the observatory, using its own dedicated refrigerator. Exoplanets at temperatures similar to Earth will shine most brightly in mid-infrared light. MIRI is therefore equipped with four coronagraphs, which have been carefully designed to detect such planets against the bright glare of their parent stars. The detailed colors of exo-giant planets (similar to our own Jupiter) can then be measured by MIRI’s two spectrometers to reveal chemical identities, abundances, and temperatures of the gases of their atmospheres (including water, ozone, methane, ammonia, and many more).

MIRI is inspected in the giant clean room at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, in 2012. Credit: NASA/Chris Gunn.

“Why so cold? MIRI’s state-of-the-art light sensitive detectors that are tuned to work in the mid-infrared are blind unless they are cooled below 7 kelvins (-266 degrees Celsius, or -447 degrees Fahrenheit). For contrast, a standard domestic freezer cools its contents to about 255 kelvins (-18 degrees Celsius, or -0.7 degrees Fahrenheit). At higher temperatures, any signal that may be detected from the sky is lost beneath the signal from its own internally generated ‘dark current.’ Even if the detectors are cooled, Webb images would still be swamped by the glow of thermal infrared light emitted by MIRI’s own mirrors and aluminum structure if they are to get warmer than 15 kelvins (-258 degrees Celsius, or -433 degrees Fahrenheit). The engineering solution was to stand MIRI off from the instrument mounting structure behind Webb’s primary mirror like a high-tech metal spider on six carbon fibre legs. These insulate MIRI from the much hotter telescope (where 45 kelvins, or -228 degrees Celsius/-379 degrees Fahrenheit, qualifies as hotter). The instrument’s body is also swathed in a shiny aluminum-coated thermal blanket, which reflects the radiant heat of its surroundings.

“Getting this instrument cold is one of the last major challenges faced by Webb before the MIRI team can truly relax, and passing through the cooler’s ‘pinch point’ will be the most daunting step in this challenge. At that time, the cooler will have pulled out almost all of the heat left in MIRI’s 100 kilograms (220 pounds) of metal and glass from that tropical launch day morning, three months ago. MIRI will be the last of Webb’s four instruments to open its eyes on the universe.”

– Alistair Glasse, Webb-MIRI Instrument Scientist, UK Astronomy Technology Centre
and Macarena Garcia Marin, MIRI Instrument and Calibration Scientist, ESA

Webb Completes First Multi-Instrument Alignment

The sixth stage of aligning NASA’s James Webb Space Telescope’s mirrors to its scientific instruments so they will create the most accurate and focused images possible has concluded. While the Mid-Infrared Instrument (MIRI) continues its cooldown, optics teams have successfully aligned the rest of the observatory’s onboard instruments to Webb’s mirrors. Previous alignment efforts were so accurate that the team concluded no additional adjustments to the secondary mirror are necessary until the seventh and final stage, which will involve MIRI when it has fully cooled.

“As a general rule, the commissioning process starts with coarse corrections and then moves into fine corrections. The early secondary mirror coarse corrections, however, were so successful that the fine corrections in the first iteration of Phase Six were unnecessary,” said Chanda Walker, Webb wavefront sensing and control scientist, Ball Aerospace. “This accomplishment was due to many years of planning and great teamwork among the wavefront sensing team.”

Throughout the majority of the alignment process, Webb’s 18 hexagonal mirrors and secondary mirror were focused into alignment to the Near-Infrared Camera (NIRCam) instrument only. Upon completing this most recent step, the observatory is now aligned to the Fine Guidance Sensor (FGS), the Near-Infrared Slitless Spectrograph (NIRISS), and the Near-Infrared Spectrometer (NIRSpec) as well as NIRCam.

Once MIRI fully cools to its cryogenic operating temperature in the weeks ahead, a second multi-instrument alignment will occur to make final adjustments to the instruments and mirrors if needed. When the telescope is fully aligned and able to deliver focused light to each instrument, a key decision meeting will occur to confirm the end of aligning the James Webb Space Telescope. The team will then transition from alignment efforts to commissioning each instrument for scientific operations, which are expected to begin this summer.

Webb Continues Multi-Instrument Alignment

While telescope alignment continues, Webb’s Mid-Infrared Instrument (MIRI) is still in cooldown mode. MIRI, which will be the coldest of Webb’s four instruments, is the only instrument that will be actively cooled by a cryogenic refrigerator, or cryocooler. This cryocooler uses helium gas to carry heat from MIRI’s optics and detectors out to the warm side of the sunshield. To manage the cooldown process, MIRI also has heaters onboard, to protect its sensitive components from the risk of ice forming. The Webb team has begun progressively adjusting both the cryocooler and these heaters, to ensure a slow, controlled, stable cooldown for the instrument. Soon, the team will turn off MIRI’s heaters entirely, to bring the instrument down to its operating temperature of less than 7 kelvins (-447 degrees Fahrenheit, or -266 degrees Celsius).

In the meantime, after achieving alignment with the Near-Infrared Camera (NIRCam), Webb engineers have begun aligning the telescope to the remaining near-infrared instruments. For more about this six-week process, we hear today from Michael McElwain and Charles Bowers, members of the Webb team at NASA’s Goddard Space Flight Center:

“Webb’s alignment at the NIRCam field showed some spectacular diffraction-limited images, producing a tantalizing glimpse of the capabilities this observatory will carry for its science program. This was a major milestone because it required nearly all of the observatory systems to be functioning as designed. It all worked as well as we dared to hope, and it was certainly a moment to celebrate.

“The next step is to ensure the telescope is well-aligned to the instruments other than NIRCam, including the guider (the Fine Guidance Sensor, or FGS) and the other three science instruments: the Near-Infrared Slitless Spectrograph (NIRISS), Near-Infrared Spectrometer (NIRSpec), and MIRI. All the near-infrared instruments have already been passively cooled, are approaching their operational temperatures, and are participating in this next alignment stage. MIRI requires active cooling by a cryocooler, which is now underway, and it will be ready for alignment in a few weeks.

“This is the sixth stage of our telescope alignment plan, the Telescope Alignment Over Instrument Fields of View. Each of the instruments occupies a part of the telescope focal plane, just slightly offset with respect to each other. NIRCam was intentionally placed at the center of the telescope field where the telescope’s optical performance is best due to its demanding imaging performance requirements. Additionally, NIRCam was equipped with some specialized optical tools used to align the telescope. However, the initial alignment using only NIRCam could lead to an incorrect placement that compensates errors from primary-to-secondary mirror misalignments with the primary mirror itself. Small misalignments of this type will be evident in images in instruments farther from the center of the telescope field of view.

“The first step was to simply look at star fields as seen by NIRCam, NIRISS, FGS, and NIRSpec to see whether they were in focus. The stars looked nearly in-focus, which was a sign that the primary to secondary mirror alignment was already very good. A more accurate optical error measurement has been carried out at five to 10 field positions within each operational science instrument, using data taken with the secondary mirror positioned out of focus. This dataset provided a conclusive determination of the telescope alignment state.

“The Webb optics team analyzed the multi-instrument dataset and determined that only minor focus adjustments are needed on the secondary mirror and science instruments. Since the telescope is still cooling along with the MIRI instrument, we will not apply the corrections at this time and will defer them until the next round.

“When MIRI is available, an additional round of measurements will be conducted by each science instrument to determine the final state of the telescope alignment. We will iterate this process as needed to ensure the telescope performance is optimized for all of the instruments. After the telescope alignment to all instruments is complete, we will transition to the final two months of commissioning, where we will carry out optical stability tests and measure the science instrument performance before embarking on the Cycle 1 science program.”

Michael McElwain, Webb observatory project scientist, NASA Goddard

Charles Bowers, Webb deputy observatory project scientist, NASA Goddard