A small body known as 2014 MU69, found by the Hubble Space Telescope barely a year ago, will be the next destination for NASA's New Horizons spacecraft.
NASA's New Horizons team members are still basking in the afterglow of July's historic flyby of Pluto — and still awaiting most of the observations made there. But they're already anticipating the spacecraft's second and likely final encounter in the distant Kuiper Belt.
Last week the space agency announced the spacecraft's target. It's 2014 MU69, an object situated 43.3 astronomical units (6.49 billion km) from the Sun. Astronomers know little about 2014 MU69, other than it's an incredibly dim 25.6-magnitude blip. It's not big — assuming a surface that's 20% reflective (just a guess), astronomers estimate its diameter to be about 45 km (30 miles) across. That's roughly 10 times the size of a typical comet.
But New Horizons isn't zeroing in on this object based on its size or any other physical characteristic. Instead, it's all about location. The spacecraft should be able to reach 2014 MU69 in a reasonable amount of time and still have comfortable fuel reserves for its maneuvering engines once it gets there. Plans call for a series of four trajectory corrections in late October and early November to set course for a rendezvous on January 1, 2019.
It all seems so matter of fact. But finding a suitable post-Pluto target became an urgent problem in early 2014, after 3 years of intensive searching with ground-based telescopes failed to find one. Fortunately, last-ditch searches with the Hubble Space Telescope from July through September turned up three potential targets (PTs). Follow-up observations showed that 2014 MU69 — PT1 in Hubble's short list — was the best all-around choice. Runner-up PT3, designated 2014 PN70, is a slightly larger body, but it would have required more fuel to reach.
Despite its small size, New Horizons scientists are excited about what 2014 MU69 might teach them about the formation and evolution of the distant Kuiper Belt in which it lies. New Frontiers in the Solar System, a "decadal" blueprint for future planetary exploration released in 2003 by the National Academy of Sciences, strongly recommended that small Kuiper Belt objects (KBOs) be included in any mission to Pluto.
This object's orbit is nearly circular (eccentricity = 0.05) and inclined just 2½°. So it has not been strongly perturned or altered since the solar system's formation 4½ billion years ago. "2014 MU69 is a great choice because it is just the kind of ancient KBO, formed where it orbits now, that the Decadal Survey desired us to fly by,” notes Alan Stern, the mission's principal investigator, in a press release announcing the selection.
But just because New Horizons has a suitable target doesn't guarantee that it will be operating once it gets there. Technically the spacecraft is in tip-top shape and fully able to accomplish another close flyby. But first the team needs to make a compelling scientific case for doing so. That proposal, due next year, will be evaluated against other missions competing for the space agency's funds. NASA managers will then decide if it's worth extending the mission's funding for at least three more years.
Meanwhile, astronomers will continue studying this object to refine its orbit and perhaps to get a better handle on its diameter and shape. But don't expect a permanent number or name for it anytime soon. According to Gareth Williams (Minor Planet Center) it will be mid-2016 at the very earliest, and more likely mid-2017, before the orbit of 2014 MU69 is known well enough to justify a better moniker.
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