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The NASA Dawn spacecraft, orbiting the asteroid Ceres, died quietly, the space agency said Thursday.
Dawn missed her scheduled recording on Wednesday. Mission officials concluded that the booster propeller was dry and Dawn could no longer control its orientation. His antenna is far from Earth and his radio signal has been lost forever.
It was an expected end of the mission, although the spacecraft lasted two years longer than originally planned.
Launched in 2007, Dawn sends home a close-up view of Ceres and Vesta, the largest asteroids in the belt between Mars and Jupiter, as well as clues to the constituent elements of the planets of the solar system.
"These are time capsules from the very beginning of the solar system," said Carol Raymond, the mission's lead investigator, at a glimpse of Dawn's disappearance last month by NASA.
Here are some of the biggest discoveries of Dawn.
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The reflections of Ceres
Ceres is the most important object of the asteroid belt, although it is smaller than most of the larger moons of the solar system. Giuseppe Piazzi, an Italian priest and astronomer, discovered in 1801, and it was first declared to be a planet. But then, other astronomers continued to find more rocks in this area, and they were eventually all classified as asteroids. In the last reworking of the planets, Ceres received a promotion. It is now classified as a dwarf planet because it is large enough to be round.
Among the discoveries of Dawn, the most unexpected were brilliant spots on Ceres – about 300 of them. The discovery sparked waves of scientific curiosity.
Was it frozen water? Other ice cream? How did they get there? What was happening under the surface? "What we saw was completely mind-blowing," said Dr. Raymond.
The white matter turned out not to be snow or ice, but sodium carbonate, a type of salt. On Earth, sodium carbonate is often called washing soda or sodium carbonate. 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 appears in two fascinating places, she said: drying up of lake beds on Earth and in the feathers coming out of Enceladus, a moon of Saturn known to have an ocean under its outer ice shell.
On Ceres, it seems that salty brine tanks – the remains of a submarine ocean – sometimes stay on the surface to create bright spots. This means that Ceres, even though it is only 588 miles in diameter, is still geologically active and spitting ice instead of the so – called cryovolcanism.
The bright spots are almost all in craters or nearby. This suggests that the meteor impacts created the spots, either by lifting materials beneath the surface or by cracking the outer crust, thus allowing the underground brines to flow upward. On the surface, water escaped into space leaving deposits of sodium carbonate and ammonium chloride, another type of salt.
The biggest, the brightest of all
The most striking feature of Ceres lies in the illuminated regions of a crater 40 km wide called Occator.
In the crater, a central dome called Cerealia Facula is thought to have been formed by ice lava spewing through fractures, possibly pushed by gases in the brine. Nearby is another luminous region named Vinalia Faculae, which is more diffuse in shape and texture, and it seems to have been formed by a somewhat different process. The scientists assume that the gases dissolved in the liquid made it melt on the surface, like the champagne flowing out of the bottle that has just been opened.
A mountain
Another interesting feature is the 13,000-foot mountain near the Equator 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: a thick melted material is compressed like a toothpaste, without an explosive eruption, to create a dome shape.
The volcano is not active today. Dr. Raymond said that over time, Ahuna Mons, probably a few hundred million years old, would spread, flatten and eventually disappear, and that there were probably other mountains. volcanic in the past.
A (very) thin atmosphere
Ceres is too small and its gravity too weak to maintain a significant atmosphere. Yet in 2014, the European Space Agency's Herschel Space Telescope detected water vapor around the asteroid, which Dawn later confirmed.
This transient atmosphere is generated by high energy particles from the sun that turn into water molecules on or near the surface of Ceres. the The same phenomenon occurs at Mercury and on the Moon of the Earth.
A tale of two asteroids
Before Dawn turned around Ceres, he had visited Vesta, another asteroid, from 2011 to 2013. Exploring this 330-kilometer-wide rock – which looks like a cratered potato – and its contrasts with Ceres, rounder and wider, offered astronomers an additional insight into how the objects formed the solar system.
The differences go beyond the size. Vesta is dry and strongly cratered, resembling the moon, while Ceres is full of water.
"It's almost day and night," said James L. Green, NASA's chief scientist.
Why are they so different? Planetary scientists now think that Ceres was formed much further in the solar system and then pushed inward by the jostling of giant planets like Jupiter. Vesta, on the other hand, is probably formed near where it is today, a region in which the ice would have been heated early in the history of the solar system.
Even though Vesta is a diminutive, he still has planetary qualities. As he trained, he heated enough to melt, the heavier elements sinking into the nucleus.
The Dawn discoveries also confirmed that some meteorites found on Earth came from Vesta.
Transfer to the belt
Dawn's journey from Vesta to Ceres made it the first spaceship to orbit a world, and then leave another world in orbit. This is possible thanks to the three ion engines of the spacecraft.
Unlike most space-based propulsion systems, which produce thrust by chemical reactions, the electric fields of ion engines accelerate the xenon atoms. The amount of thrust is tiny. Each engine generates up to 91 Newtons, which corresponds to the force needed to hold a sheet of paper on the Earth. But ion engines are much more efficient and can work for long periods of time instead of the shards used in chemical propulsion.
Return to Ceres?
Spacecraft carry microbial hitchhikers from the Earth that can contaminate the worlds where they land. NASA is trying to minimize this risk at the end of a mission. Engineers piloted the Cassini probe in the atmosphere of Saturn last year.
For the last part of his mission, Dawn was sent into an elliptical orbit that plunged about 30 kilometers from the surface, orbiting every 27 hours. This provided the sharpest images of features such as the Occator Crater.
Although the spacecraft is no longer powered, it will continue in this orbit for at least 20 years, or even more decades, at which point it could crash into Ceres.
It's not long enough for all the terrestrial microbes on Dawn to die, but NASA officials hope that 20 years would be enough for the space agency to go there to determine if Ceres had ever had conditions. Satisfactory life before Dawn crushes it.
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