NASA’s New Horizons is about to encounter the farthest object ever explored by a spacecraft — Ultima Thule. The frigid hunk of ice and rock is more than 4 billion miles (6.4 billion km) away, so far away that it takes the paltry sunlight reflecting from the asteroid to more than 6 hours to reach Earth. Located in the direction of Sagittarius the Archer among the rich star fields of the Milky Way, Ultima is dim mote of light, so faint we still haven’t firmed up its diameter, estimated at 12 to 22 miles (20-35 km) across.
New Horizons first flew by Pluto at a distance of 7,800 miles (12,500 km) in July 2015. This next target lies beyond the dwarf planet in the Kuiper Belt, a repository of asteroids and comets orbiting the sun past Neptune in near absolute zero cold. The New Horizons hazards team studied images taken of Ultima Thule (ul-teh-muh THOO-lee) and its environs looking for moons and rings around the object; having found none, Alan Stern, the principal investigator for the mission, chose a close flyby distance of just 2,200 miles (3,500 km) — more than three times closer than Pluto — to maximize the amount of science that could be done. That also means exquisite photos of Ultima’s surface.
No one’s ever seen the asteroid as anything more than a featureless pinpoint, so we have no idea what to expect. Because of its orbit, numbingly cold distance and small size (too small for geological changes), most scientists believe it formed right where it is and has changed little since. If true, Ultima would be one of the most primitive objects in the solar system we’ve ever visited.
All this goes down starting at 12:33 a.m. New Year’s Day (Eastern Time), when the probe will whoosh by the asteroid at 32,000 mph and run through a preprogrammed set of instructions to map its surface features and composition, determine its rotation rate, look for an atmosphere and search for any moons or rings. Busy, busy, busy. Space nuts like myself can’t wait to see the first photos, but we’ll need to be patient. Ultima is small and will look like a point of light until just two days before closest encounter. A glance at the simulation below will give you an idea of what to expect when.
Because it takes so long for a radio signal traveling at the speed of light to arrive at New Horizons, thousands of “carefully choreographed” commands have been uploaded in advance to be performed in sequence autonomously (without human intervention) during the flyby. The first image from the flyby, a lower resolution one, will arrive the evening of Jan. 1 and shared with public the following day. Much higher resolution images along with all the other juicy data will arrive during the first week of January.
Already a mystery has emerged about the asteroid’s brightness. Most small asteroids and comets have irregular shapes. We’ve seen bowling pins, dumbbells, potatoes and shoes, you name it. As the object rotates it shows variations in brightness as it presents an end, then a side, then an end and so forth. Light and dark patches on its surface also cause repeating variations in light as the object spins. Astronomers take this information to create a curve on a graph plotting the object’s cyclical brightness changes. The puzzle is that Ultima Thule shows no such changes — it has no light curve!
We can only guess what’s up. It’s possible we’re viewing the object pole-on with only one face aimed at us as it spins. Ultima may also be shrouded in dust, something like a comet, cloaking its surface. The most bizarre idea hypothesizes a host of tiny moons around the asteroid, with their combined light curves blurring any light variations. As Alan Stern likes to point out, this mission is pure exploration, so who knows what we’ll learn at Ultima Thule. Exciting stuff. For mission updates, click here, otherwise I’ll post fresh news as soon as it arrives.
Here we go!