New Horizons: Getting to Know a KBO

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Today’s post is written by Simon Porter, a New Horizons postdoctoral researcher at the Southwest Research Institute in Boulder, Colorado. Simon’s work focuses on the small satellites of Pluto.

Hi, I’m Simon Porter, a postdoctoral researcher on NASA’s New Horizons mission. In this blog post, I’m going to talk about our observations of the Kuiper Belt object (KBO) called (15810) 1994 JR1, or simply ”JR1,” with the New Horizons spacecraft.

New Horizons flew past Pluto nearly a year ago and has been sailing through the Kuiper Belt ever since. In November 2015 and April 2016, we used the telescopic Long Range Reconnaissance Imager (LORRI) on board New Horizons to take pictures of JR1 as we flew past it. This was our first “distant flyby” of a KBO (about 66 million miles, about as close as Venus is to the sun), and the first-ever distant observation of a KBO from the Kuiper Belt. We were able to get a huge amount of science out of these images, and they may be a preview of things to come as we observe many more KBOs this way, if an extended mission is approved.

We first observed JR1 at the start of November 2015, taking four sets of 10 images, spaced one hour apart. It was even farther away at that time (172 million miles), and because of an error in targeting, it ended up on the side of picture frames instead of in the middle. However, JR1 was visible in all 40 images, dancing slowly across the field of view. In addition, we pointed the Hubble Space Telescope at JR1 in early November, so that it saw JR1 at almost the same times as New Horizons, accounting for the five hours that it took JR1’s light to reach Hubble. This was the longest-baseline parallax observation ever made – another record for New Horizons! –and allowed us to really improve our knowledge of JR1’s orbit.

The four observations of 1994 JR1 that New Horizons made in November 2015. The KBO is the dot in the center, and the stars are moving past in the background. Credits: NASA/JHU APL/SwRI

The four observations of 1994 JR1 that New Horizons made in November 2015. The KBO is the dot in the center, and the stars are moving past in the background. Credits: NASA/JHUAPL/SwRI

With this new orbit in hand, we pointed the spacecraft to image JR1 again this past April 2016. This would be the closest that New Horizons got to JR1, and we commanded the spacecraft to take lots more pictures than we had in November. We started with two “deep” sets of 24 images each, which could be added together to pick out any moons around JR1. We had already looked at JR1 with Hubble and saw no moons, so it was no surprise to find none in the New Horizons images, but it was worth a check. The ghostly circular pupil image and the little dots that are moving around in the image (and that aren’t JR1) are scattered light from a nearby bright star. LORRI isn’t that big of a telescope – just a little bit smaller than an 8-inch Schmidt-Cassegrain an amateur astronomer would use – so it’s easy for scattered light to bounce around inside the telescope and cause artifacts like these.

New Horizons’ “deep” observations of JR1, from April 2016. Credits: NASA/JHUAPL/SwRI

New Horizons’ “deep” observations of JR1, from April 2016. Credits: NASA/JHUAPL/SwRI

The next sets of observations were to see how the brightness of JR1 changed over time. The first was a sequence of nine sets of three images, spaced half an hour apart, while the second was similar, but an hour apart. We got the half-hour sequence down first and were thrilled to see that it looked like a sine wave! If you are looking an elongated object (say, a tennis shoe) on the side and then turn it to look at the front, the apparent size of the object, from your view, will go down. Turn it back to the side and the apparent size goes up again. Now imagine the shoe is a thousand miles away and someone is turning the shoe for you. You wouldn’t see the shape of the shoe change (because it’s just a point of light), but the brightness of the shoe would change because it can reflect more light to you when you see the side than when you see the front. Making measurements like that is called making a lightcurve. When we do that, we see the brightness of asteroids, KBOs and moons change and we can infer what their shape must be, without actually ever having seen them up close.

The second, longer set of images confirmed this variation and allowed us to determine the rotational period of JR1 was 5.47 hours—something that had ever before been measured. That’s pretty fast for a KBO this size, most of which spin at half this speed. Unlike asteroids, the sun is too far from KBOs to spin them up with solar radiation, so KBO spins mostly record the collisions that they have had with other KBOs. Since JR1 is spinning so fast, it probably had a pretty big glancing impact at some time in its distant past.

The lightcurve of JR1. Credits: NASA/JHUAPL/SwRI

The lightcurve of JR1. Credits: NASA/JHUAPL/SwRI

Lightcurves and deep images could be taken with Earth-based telescopes, but what no telescope other than LORRI could do is see a KBO from the side. From the KBO’s perspective, Earth is always a few degrees away from the sun, which means that from Earth we always see KBOs at high noon, with no shadows. From a spacecraft in the Kuiper Belt (like New Horizons), we can look at different times of JR1’s day. The November observations of JR1 were either late morning or early afternoon (we don’t know, because we don’t know if JR1’s pole points up or down). The April observations were at either early morning or in the late evening on JR1. Both of these times should have had shadows on the surface, especially the April observation. Sure enough, when we put all the brightnesses together in a time series, we found that there was enough dimming from shadows that the surface must be at least as rugged as Saturn’s rough-surfaced moon Phoebe. This makes sense, as Phoebe is thought by some to be a captured KBO, and is therefore probably our best guess for what (15810) 1994 JR1 looks like.

The brightness of JR1 from Earth (red and green) and New Horizons (blue). Credits: Porter et al 2015, under review

The brightness of JR1 from Earth (red and green) and New Horizons (blue). Credits: Porter et al 2015, under review

 

Saturn’s moon Phoebe may be similar to JR1. Credits: NASA/JPL-Caltech/SSI

Saturn’s moon Phoebe may be similar to JR1. Credits: NASA/JPL-Caltech/SSI

Finally, our April observations were the closest-ever of a KBO (other than Pluto), and we used that fact to refine the orbit of JR1. From Earth, we can predict the motion of a KBO as seen from Earth very well, but can’t as well predict how far away it is. Because the New Horizons observations were taken at a very different angle to how the Earth sees JR1, we were able to drop the uncertainty of how far JR1 is from the sun (and thus Earth) from around 60,000 miles (100,000 kilometers) to around 600 miles (just under 1,000 kilometers). That’s a huge improvement in JR1’s orbit, and should enable other astronomers to predict when JR1 will go in front of stars, a measurement we call an “occultation” (from the Latin word for “hidden”). Observing an occultation of JR1 would allow a measurement of both its size and shape.

Having this high-precision orbit in hand also allowed us to make a computer simulation of what JR1’s orbit will do in the future, and did in the past. JR1 is a “plutino” (pseudo-Italian for “little Pluto”) because, like Pluto, it goes around the sun three times for every two times that Neptune goes around the sun. In fact, JR1 is only 2.7 astronomical units (AU) away from Pluto – an AU being the average distance between the sun and Earth, about 93 million miles (or 149 million kilometers) – which is pretty close on outer system scales (it’s 35.5 AU from the sun).  However, the orbits of Pluto and JR1 are different enough that this close encounter is cosmically fleeting, only lasting a few hundred thousand years, and not coming together again for another 2.4 million years. Pluto does have a gravitational effect on the orbit of JR1, but it’s mainly to add a bit of chaos into JR1’s orbit, causing it to be unpredictable over timescales longer than about ten million years (again pretty short, cosmically speaking).

The orbit of JR1 compared to Pluto’s orbit with Pluto perturbing JR1 (red) and without (blue). Credits: Porter et al, 2015, under review

The orbit of JR1 compared to Pluto’s orbit with Pluto perturbing JR1 (red) and without (blue). Credits: Porter et al, 2015, under review

The primary mission of New Horizons will end this year, when it is finished downloading all the data from the Pluto system. NASA is currently deciding whether or not to approve an extended mission for New Horizons to do a close (within 6,000 miles or 10,000 kilometers) flyby of a KBO even smaller than JR1. If approved, this would also enable New Horizons to observe dozen more KBOs in a similar way to JR1.