NASA’s Dawn mission to the asteroid belt says goodnight
NASA’s Dawn spacecraft, orbiting the asteroid Ceres, has died quietly, the space agency said Thursday.
Dawn missed her scheduled recording on Wednesday. Mission officials concluded that the thruster on her thrusters was dry and Dawn could no longer control her orientation. Its antenna turned away from Earth and its radio signal was lost forever.
It was an expected end of the mission, although the spacecraft lasted two years longer than originally planned.
Launched in 2007, Dawn sent 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 planets in the solar system.
“These are time capsules from the very beginning of the solar system,” Carol Raymond, the mission’s principal investigator, said during a NASA snapshot last month on Dawn’s disappearance.
Here are some of Dawn’s greatest discoveries.
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The reflections of Ceres
Ceres is the largest object in the asteroid belt, although it is smaller than most of the largest moons in the solar system. Giuseppe Piazzi, Italian priest and astronomer, discovered it in 1801, and it was first declared a planet. But then other astronomers kept finding more rocks in this area, and eventually they were all classified as asteroids. In the most recent planetary reshuffle, Ceres received a promotion, and she is now classified as a dwarf planet because she is large enough to be round.
Among Dawn’s findings, the most unexpected were bright spots on Ceres – about 300 of them. The discovery sparked waves of scientific wonder.
Was it frozen water? Other ice cream? How did they get there? What was going on below the surface? “What we saw was completely breathtaking,” said Dr Raymond.
The white stuff turned out not to be snow or ice, but sodium carbonate, a type of salt. On Earth, sodium carbonate is often referred to 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,” said Dr Raymond.
But it does show up in a few intriguing places, she said: dry lake beds on Earth and in the gushing plumes of Enceladus, a moon of Saturn known to have an ocean under its icy outer shell.
On Ceres, what seems to be happening is that reservoirs of salt brine – remnants of an underground ocean – sometimes rise to the surface to create the points of light. This means that Ceres, even though it is only 588 miles in diameter, is still geologically active and spits ice instead of lava – this is called cryovolcanism.
Light spots are almost all found in or near craters. This suggests that meteor impacts created the spots, either lifting material below the surface or cracking the outer crust, allowing underground brines to flow upward. On the surface, water escaped into space, leaving deposits of sodium carbonate as well as ammonium chloride, another type of salt.
The biggest, the brightest of all
The most striking feature on Ceres are the bright regions in a 57 mile wide and 2.5 mile deep crater called Occator.
In the crater, a central dome called Cerealia Facula it is believed to have been formed by icy lava that gushed out through fractures, possibly pushed by gases into 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 the gases dissolved in the liquid caused it to squirt through cracks on the surface, like champagne spilling out of a bottle that has just been opened.
Another eye-catching feature is a 13,000-foot-high mountain near the equator of Ceres. Named Ahuna Mons, it is indeed the only mountain of Ceres. Scientists have described it as the result of an unusual type of volcanism involving salt water and mud: thick molten material is squeezed like toothpaste, without an explosive eruption, to create a dome shape.
The volcano is not active today. Dr Raymond said that over time Ahuna Mons, perhaps a few hundred million years old, would likely expand, flatten out and eventually disappear, and that there were likely more volcanic mountains in the past.
A (very) thin atmosphere
Ceres is too small and its gravity too weak to retain a meaningful 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 that crash into water molecules at or near the surface of Ceres and lift them up. The same phenomenon occurs in Mercury and on Earth’s Moon.
The story of two asteroids
Before Dawn orbiting Ceres, she visited Vesta, another asteroid, from 2011 to 2013. Exploring this 330-mile-wide boulder – which looks like a cratered potato – and its contrasts with the rounder and wider Ceres offered astronomers additional information about 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.
“It’s almost like night and day,” said James L. Green, NASA chief scientist.
Why are they so different? Planetologists now believe that Ceres formed much further into the solar system and then was pushed inward by the stampede of giant planets like Jupiter. Vesta, on the other hand, likely formed near where it stands today, an area in which ice would have been heated early in the history of the solar system.
Even though Vesta is diminutive, he still exhibits planetary qualities. As it formed, it heated up enough to melt, with the heaviest elements sinking into the heart.
Dawn’s findings also confirmed that some meteorites found on Earth originated from Vesta.
Dawn’s journey from Vesta to Ceres made her the first spacecraft to orbit a world, and then to orbit a second world. This is possible thanks to the three ion engines of the spacecraft.
Unlike most propulsion systems for ships in space, which produce thrust through chemical reactions, the electric fields of ion motors accelerate the xenon atoms. The amount of thrust is tiny. Each motor generates up to 91 millinewtons, which is roughly the amount of force you need to hold a sheet of paper on Earth. But ion engines are much more efficient and can run for long periods of time, instead of the short bursts used in chemical propulsion.
BepiColombo, the recently launched European Space Agency mission to Mercury, also uses ion propulsion, and NASA intends to develop a more powerful version to carry large amounts of cargo to Mars.
Go back to Ceres?
Spaceships carry microbial hitchhikers from Earth that can infect the worlds they land on. NASA tries to minimize this risk when a mission ends; engineers flew the Cassini probe, for example, into Saturn’s atmosphere last year.
For the last part of her mission, Dawn was sent into an elliptical orbit that plunged to within 22 miles of the surface, orbiting every 27 hours. This provided the sharpest images to date of features like Occator Crater.
Although no longer powered, the spacecraft will continue in that orbit for at least 20 years, if not decades longer, at which time it could crash into Ceres.
It’s not long enough for all of the Earth microbes on Dawn to die, but NASA officials hope 20 years would be long enough for the space agency to make another visit there to investigate whether Ceres had ever had conditions. conducive to life before Dawn crashes and infects him.