LAUREL, MARYLAND—Humanity is getting its first good look at a primordial planetary building block, in images sent back by this afternoon by NASA’s New Horizons spacecraft after its flyby of MU69, a small icy body at the far fringes of the solar system.
For a half-year, the New Horizons team had puzzled over the possible shape of MU69, which was little more than an oblong dot in Hubble Space Telescope images. Is it two icy objects orbiting each other, or a single “peanut”? It turns out to be both. Resembling a 33-kilometer-long interplanetary “snowman,” in the words of Alan Stern, a planetary scientist from the Southwest Research Institute (SwRI) in Boulder, Colorado, and principal investigator for the $800 million mission, MU69 appears to have formed when two spherical objects gently smooshed together billions of years ago. Mutual gravitational attraction keeps them married despite their gentle, 15-hour rotation. “What you’re seeing is the first contact binary ever explored by a spacecraft,” Stern said today at a press conference here at the Johns Hopkins University Applied Physics Laboratory.
The 140-meter-resolution image, taken 28,000 kilometers from MU69 half an hour before the spacecraft’s closest approach, reveals two bumpy, reddish spheres, with one three times the volume of the other. The object has a mottled look, with the bright patches concentrated in mysterious circles while the darker areas seem more linear. The “neck” between the two lobes is particularly bright, perhaps because small, reflective particles tumbled into its crevasse, said Cathy Olkin, a deputy project scientist and planetary scientist at SwRI. Although the photo doesn’t show shadows, MU69’s profile suggests it could have hills as tall as a kilometer.
MU69 appears to have few craters or other signs of violent impact, supporting the idea that the solar system’s building blocks formed when friction and gravity gently drew together clouds of dust and gravel—a theory known as pebble accretion. Closer to the sun, these building blocks would go on to form Earth, Mars, and all the other planets. But in the Kuiper belt, the region of icy bodies beyond Neptune, they formed but did not evolve further, said Jeff Moore, a planetary scientist at NASA’s Ames Research Center in Mountain View, California, and the mission’s geology lead. Kuiper belt objects “are the first planetesimals,” he said. “These are the only remaining basic building blocks.”
Seeing one close up could clear up many debates. For example, comets that visit Earth from the Kuiper belt have had a peanutlike profile similar to MU69, prompting debate about whether they were sculpted by the sun’s heat or looked that way from the start. The latter now seems likely. “This really puts the nail in the coffin now,” Stern said. “We know this is how many objects like this form.” (The discovery also suggests the rubber-duck comet 67P/Churyumov-Gerasimenko was actually the first contact binary to be explored by a spacecraft when the European Space Agency’s Rosetta visited it in 2014.)
The MU69 story is only starting to unfold, Stern added. Less than 1% of the data has returned so far. More will be presented tomorrow, perhaps giving a better indication of its composition. New Horizons’s journey into the solar system’s past has just begun.
*Correction, 3 January, 10:55 a.m.: This story has been updated to note that MU69 may not be the first contact binary visited by a spacecraft.