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MU69 Hazards, November-December 2018, APL

I've been working with NASA's New Horizons mission since the time of its selection in 2001. It was launched in 2006, and passed by Jupiter a year later. The big event was the Pluto flyby on July 14, 2015, which was an amazing success, and an incredible experience to be part of.

Not content with just revolutionizing our knowledge of Pluto, New Horizons is still cruising, and exploring more of the outer solar system. Its first close flyby after Pluto is with the Kuiper belt object (KBO) named '2014 MU69', aka Ultima Thule. Ultima is far smaller than Pluto: about 30 km across rather than 2400 -- making it perhaps one one-millionth the mass of Pluto. And it is colder, slower, and more primitive, and by far the smallest body in the outer solar system we've ever flown past.

New Horizons' nominal job is to fly a pre-programmed path past the body, and execute a pre-set sequence of observations. We'll get the vast majority of our data from within the central hour of our flyby. The spacecraft is moving at some 15 km/sec (52,000 kph).

At the speed that we're traveling, a millimeter-sized dust grain could end the mission. And, while space itself (the 'interplanetary medium') is pretty empty, the region around the planets is not. For all we know, MU69 could have moons, rings, or a cloud of dust around it. Other planets, asteroids, and comets often do, so it's plausible that MU69 could.

It was hard enough to detect MU69 itself with the Hubble Space Telescope in 2014 when it was discovered, and it's essentially impossible to use Hubble to probe thoroughly for dust near MU69. The target is just too faint, too small, and too far away.

Enter the New Horizons Hazard Team! Our job was to keep the spacecraft safe at encounter, by assessing the impact risk from dust as we were closing in. The result of our work was a binary decision: do we fly the spacecraft on its ideal 'prime' trajectory that goes just 3500 km above the surface, or do we take a backup 'alternate' trajectory that is 10,000 km away?

After 18 months of telecons and planning, we got to work on December 1, 2018, a month before encounter. By this point New Horizons' image of MU69 were becoming good enough that we could start to resolve structure in the system. (The images were far better at this point than what we had gotten from Hubble: HST is much larger, but further away.) We were far from seeing any detail on the body itself, but if there were very large, very bright clouds of dust thousands of km across, then even our initial surveys on December 1 would see them. And as we closed in, our scale got much better, to the point where we could resolve rings or clouds on the 100-km scale (and moons far smaller than that). These images would inform our decision over which trajectory to recommend.

The Hazard Team is comprised of a dozen people:

For 17 days we worked in a single room together at the Johns Hopkins Applied Physics Lab (APL), outside Laurel, MD. The culmination of this work was a decision made with the spacecraft 'stakeholders' (i.e., PI Alan Stern, managers, and engineers), where they chose whether to fly the Prime trajectory, or the Alternate.

In the end we found: absolutely nothing! No moons, no rings, no clouds of dust, down to our limit of detectability (for moons, 1.4 km assuming albedo = 0.09 outward of 1000 km; for rings and dust, I/F < 5e-7). Furthermore, we identified no stable orbits that dust even could theoretically survive for long times at MU69. We were confident that, with the data we had, the MU69 system looked clear, and we recommended the mission fly the Prime trajectory.

Thanks much to Carl Engelbrecht and Alan Stern for arranging to allow photography on the lab. This was a rare privilege and I'm grateful we were able to document the preparations for this historic flyby. Credit also to Mark Showalter who led us in the crow's nest this whole time.

Photo credits: NASA / JHUAPL / SwRI / Henry Throop, with a few by NASA / JHUAPL / SwRI / John Spencer, as noted.