Spacecraft that studied asteroids goes quiet
NASA’s Dawn spacecraft, in orbit around the asteroid Ceres, has died quietly, the space agency announced Thursday.
Dawn missed its appointed check-in Wednesday. Mission managers concluded that the propellant for its thrusters had run dry and Dawn could no longer control its orientation. Its antenna turned away from Earth, and its radio signal was lost forever.
It was an expected end to the mission, although the spacecraft lasted two years longer than originally planned.
Launched in 2007, Dawn has been sending home close-up views of Ceres and Vesta — the largest asteroids in the belt between Mars and Jupiter — as well as clues to the building blocks of the solar system’s planets.
“These are time capsules from the very beginning of the solar system,” Carol Raymond, principal investigator of the mission, said during a NASA preview in October of Dawn’s demise.
Here are some of Dawn’s biggest discoveries.
Ceres is the largest object in the asteroid belt, although it is smaller than most of the solar system’s larger moons.
Giuseppe Piazzi, an Italian priest and astronomer, discovered it in 1801, and it was declared at first to be a planet. But then other astronomers kept finding more rocks in that region, and eventually they were all classified as asteroids. In the most recent reshuffling of planets, Ceres received a promotion, and it is now classified as a dwarf planet because it is large enough to be round.
Among Dawn’s findings, the most unexpected were shiny splotches on Ceres — some 300 of them. The discovery set off waves of scientific wondering.
The white stuff turned out not to be snow or ice, but sodium carbonate, a type of salt. On Earth, sodium carbonate is often known as washing soda or soda ash. It is used in the manufacture of glass, in some detergents and as a water softener.
“Sodium carbonate is not common in the solar system,” Raymond said.
The most striking feature on Ceres are the bright regions within a 57-mile-wide, 2.5-miledeep crater called Occator.
In the crater, a central dome called Cerealia Facula is thought to have been formed by icy lava sputtering up through fractures, possibly pushed by gases in the brine. Nearby is another bright region named Vinalia Faculae, which is more diffuse in shape and texture, and it appears to have been formed by a somewhat different process. Scientists hypothesize that gases dissolved in the liquid caused it to sputter through cracks onto the surface, like Champagne spilling out of a just-opened bottle.
Another eye-catching feature is a 13,000-foot-high mountain near Ceres’ equator. Named Ahuna Mons, it is indeed the only mountain on Ceres. Scientists described its formation as a result of an unusual type of volcanism involving salty water and mud: Thick molten material is squeezed up like toothpaste, without an explosive eruption, to create a dome shape.
The volcano is not active today. Raymond said that over time, Ahuna Mons, perhaps a few hundred million years old, would likely spread, flatten and eventually disappear, and that likely had been other volcanic mountains in the past.
Ceres is too small and its gravity too weak to hold onto a significant atmosphere. Yet in 2014, the European Space Agency’s Herschel space telescope detected water vapor around the asteroid, later confirmed by Dawn.
This transient atmosphere is generated by high-energy particles from the sun slamming into water molecules at or near Ceres’ surface and kicking them up. The same phenomenon happens on Mercury and on Earth’s moon.
Before Dawn orbited Ceres, it visited Vesta, another asteroid, from 2011-13. Exploring this 330-mile-wide rock — which looks like a cratered potato — and its contrasts with the rounder and wider Ceres offered astronomers additional insights into how objects in the solar system formed.
The differences go beyond size. Vesta is dry and heavily cratered, resembling the moon, while Ceres is full of water.
The Dawn findings also confirmed that certain meteorites found on Earth originated from Vesta.
Dawn’s trip from Vesta to Ceres made it the first spacecraft to orbit one world, and then leave to orbit a second world. That was possible because of the spacecraft’s three ion engines.
Though out of power, the spacecraft will continue in that orbit for at least 20 years, possibly decades longer, at which point it could crash into Ceres.