New Horizons Team Does Science in the Shadow

2015 MU69 Occults a Star

sofia occultation run
SOFIA cartoon showing flight path of airborne observatory catching the occultation; observers also watched it from the ground. Courtesy NASA

On January 1, 2019, the New Horizons spacecraft is going to pass close to a Kuiper Belt object called 2014 MU69. Not much is known about this distant little world, so it’s important to know how big it is, whether it has companions or a debris field or even a ring around it. Most importantly, the New Horizons team needs to get a very precise fix on the object’s position and orb it so they can plan the flyby to gather maximum science with minimum danger to the spacecraft. This is very similar to work done by the team as their spacecraft approached Pluto in 2015 during the fantastic flyby exploration of the distant planet.

One way to get that information is to watch as MU69 passes in front of (occults) a star. As the star “winks out” and then reappears, the team can get a much better guide on the position and size of the object. So, the team gathered star position data using a star catalog amassed by observations using the European Space Agency’s GAIA telescope, and the positions of the object gathered by the Hubble Space Telescope to plot out a series of occultation observations. The second of those observation runs took place on July 10th aboard the SOFIA aircraft high over the southern hemisphere skies. Groups of ground-based observers also looked for the occultation, as well.

This is an incredibly tough set of measurements to make. MU69 is not very bright, it’s small, and its very far away. The good news is, the SOFIA observers reported they were able to fly through the “shadow” of this tiny world and get occultation measurements. It’ll be a short while (perhaps a few days) before we know the full results, but coupled with the ground-based info, the chances are very good that we’ll know more about this little object. That’ll be of immense help to the New Horizons planners as they chart the close flyby in less than a year and a half.

Occultations and their Meaning for New Horizons

So, how can an object occulting a distant star reveal information about the object?
If you know the distance of the object, you can use simple geometry to figure out at least one of its dimensions using time it takes for the object to traverse in front of the star. The starlight winks out (or dims) at one point and then some minutes (or hours) later, it comes back. Astronomers can use that to plot an angular distance that the object traveled as it passed in front of the star. Imagine watching a car pass in front of a tree. You note the time it begins the pass, and then note when its rear bumper clears the tree. That gives you the time it took to occult the tree. If you know how far away it is, you can the figure out how long the car is. Occultations by asteroids are most commonly studied, as well as occultations of stars by the Moon.

Multiple occultations from different places can also help astronomers understand more about the object’s shape. Different parts of the object will occult the star at different times, and when you put all that data together, you will get a rough estimate of the “roundedness” or “lumpiness” of its shape (what planetary scientists refer to as its “morphology”). If there are multiple occultations before and after the main occultation, that might be a clue to the existence of a ring system or moons or co-orbital objects (maybe it has a clump of objects traveling with it).

For planets and other solar system objects, the more distant stars act as an indirect measuring stick that reveals a lot more information than you’d think. While it’s likely not an issue at MU69 (but never say “never”), an occultation could also reveal whether it has an atmosphere or not. The starlight passing through that atmosphere would absorb certain wavelengths of light, and that would show up in a spectrum of the starlight taken during the occultation.

We Should Hear Soon from the New Horizons Team

I hope we hear soon from the New Horizons team about this set of occultation studies. The first one was somewhat inconclusive, but this one appears to be successful. You can follow the news at the New Horizons mission web site, and on Twitter using the hashtag #mu69occ — presumably, there’ll be an announcement in the next few weeks, once the data analysis is complete.

Other Observations Today

Also, keep an eye out for images from the Juno spacecraft. It’s flying over Jupiter’s Great Red Spot today and the images should be downright amazing! Look for those in a few days, too.

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