*Update, 1 November, 1:55 p.m.: After several missed attempts to communicate with Dawn over the past 2 days, NASA declared that the spacecraft had run out of hydrazine and reached the end of its mission. Here is our original coverage from 17 October.
After an 11-year journey to Vesta and Ceres, the asteroid belt's two largest members, NASA's Dawn spacecraft is expected to run out of thruster fuel in the next few weeks, ending its mission. The robotic explorer, which gave a close-up view of how the presence or absence of water can shape asteroids, will remain tumbling in orbit around Ceres for decades before ultimately crashing into it.
Launched in 2007, Dawn is the only NASA mission to orbit two planetary bodies, a feat made possible by its efficient ion thrusters. In 2011, it arrived at the egg-shaped, 600-kilometer-long Vesta, orbiting for a year before departing for Ceres, where it arrived in 2015.
The two asteroids, which together account for 45% of the belt's mass, turned out to be a tale of contrasts. Parched Vesta has a composition like the terrestrial planets, with an iron core and a dry, rocky surface carved up into canyons, craters, and mountains, remnants of past impacts and volcanism. Dawn was able to verify that a class of meteorites found on Earth are chips off of Vesta, making it a sort of "reverse sample return mission," says Carol Raymond, the mission's principal investigator and a planetary scientist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California.
Telescopes had already found water-rich minerals on Ceres, a 900-kilometer-wide body classified as a dwarf planet because of its large size and spherical shape. Dawn revealed the remnants of a frozen ocean topped by a heavily cratered crust of clays and salts. "We could not have imagined it would have looked like this," Raymond says.
The young sun could have boiled away much of the water if Ceres had formed in its present location. So, some scientists suspect it was born out past Jupiter and Saturn, only to be drawn in by the gas giants' turbulent early gyrations. Dawn found a marker of this distant birthplace: ammonia, a volatile molecule that could have only formed in the outer solar system.
Dawn also spotted a sign that Ceres remains geologically active: what looked like a lone volcano, 4 kilometers tall, that oozes a "lava" of water, salt, and other materials. Later observations spotted traces of 21 more volcanoes. And more than 100 bright spots rich in salt also suggested the subsurface ice finds its way to the surface here and there, as a slushy brine. One such spot, at Ernutet crater, showed signs of organic molecules—common in the outer solar system and not a signifier of life. But the discoveries have made Ceres a prime candidate for further exploration; a NASA study on a possible lander is expected to conclude next year.
In its final months, Dawn settled into a tight orbit, just 35 kilometers from Ceres's surface. The defunct craft could remain in orbit for a half-century or more, says Marc Rayman, Dawn's mission director at JPL, "an inert celestial monument around the dwarf planet it helped unveil."