NASA's dawn legacy, near the end of its mission

Dawn's mission has been extended several times, exceeding scientists' expectations in his exploration of two planet-like bodies, Ceres and Vesta, which account for 45% of the mass of the main asteroid belt. Now, the spacecraft is about to miss an essential fuel, hydrazine. When that happens, probably between mid-September and mid-October, Dawn will no longer be able to communicate with Earth. It will remain in silent orbit around Ceres for decades.

"Although it will be sad to see Dawn leave our missionary family, we are extremely proud of her many accomplishments," said Lori Glaze, acting director of the Planetary Science Division at NASA's headquarters in Washington. "Not only did this spacecraft unlock scientific secrets about these two small but significant worlds, but it was also the first to visit and orbit bodies at two extraterrestrial sites during its mission.

When Dawn left Cape Canaveral Air Force Base in Florida in September 2007, attached to a Delta II-Heavy rocket, scientists and engineers had an idea of ​​what Ceres and Vesta looked like. Through terrestrial and space telescopes, including NASA's Hubble Space Telescope, the bodies of the asteroid belt between Mars and Jupiter were visible, but even the best images were blurred.

From 2011 to 2012, Dawn invaded Vesta, capturing images that went beyond the imagination of everyone: craters, canyons and even mountains. On Ceres in 2015, Dawn showed us a cryovolcano and mysterious luminous spots, which scientists later discovered were salt deposits produced by the exposure of a brackish liquid from within Ceres. Through Dawn's eyes, these brilliant dots were particularly astonishing, shining like diamonds scattered on the surface of the dwarf planet.

"Dawn's legacy is that he has explored two of the last unexplored worlds of the internal solar system," said Marc Rayman of NASA's Jet Propulsion Laboratory in Pasadena, California, mission director and chief engineer. from Dawn. "Dawn showed us extraterrestrial worlds that for two centuries were just points of light in the middle of the stars, and he produced these richly detailed intimate portraits and revealed exotic and mysterious landscapes different from all that we have never seen. "

Engineering practices

Dawn is the only spacecraft to orbit a body in the asteroid belt. And it's the only spacecraft to orbit two extraterrestrial destinations. These achievements were made possible by ionic propulsion, an extremely efficient propulsion system known to science fiction fans and space enthusiasts. Dawn has pushed the limits of the capabilities and endurance of the system, showing how useful it is for other missions aimed at visiting multiple destinations.

Driven by ionic propulsion, Dawn reached Vesta in 2011 and examined it from the surface to the heart for 14 months in orbit. In 2012, engineers maneuvered Dawn out of its orbit and piloted it into the asteroid belt for more than two years before inserting it into orbit around the dwarf planet Ceres, where it collection of data since 2015.

Window on the past

Meanwhile, scientists were able to get acquainted with the early stages of our solar system, thus meeting Dawn's goal. The mission was named for its purpose: to learn more about the dawn of the solar system. He targeted Ceres and Vesta as they function as time capsules, intact survivors of the first part of our story. And the duo gave the scientists an overview of the basic elements of the solar system.

"Vesta and Ceres each told how and where they were formed and how they evolved – a fiery magmatic story that led to rocky Vesta and a colder, water-rich history that resulted in the ancient world Oceanic Ceres ". Raymond de JPL, principal investigator of the Dawn mission. "These information treasures will continue to help us understand other bodies of the solar system in the future."

Ceres spectacular

There were so many things that scientists did not know about Ceres before Dawn arrived. Raymond was wondering if they could find Ceres covered with a smooth, young surface – a huge ball with a frozen crust. Instead, they found the dwarf planet carrying the chemistry of its former ocean. "What we found was absolutely breathtaking – the story of Ceres is just divided over its entire surface," she said.

Some of the bright, shiny dots turned out to be bright salty deposits, mostly composed of sodium carbonate that made its way to the surface in a melting brine inside or below the crust.

The results reinforce the idea that dwarf planets, not just icy moons like Enceladus and Europa, could have hosted oceans in their history – and could still do so. Analysis from the Dawn data suggests that there may still be some liquid surface below the surface of Ceres and that some areas were relatively recently geologically active, feeding in a deep reservoir.

One of Dawn's biggest revelations about Ceres was in the Ernutet crater area. Organic molecules have been found in abundance. Organic matter is one of the building blocks of life, although Dawn's data do not allow us to determine whether organic processes formed the organic compounds of Ceres.

"It is becoming increasingly evident that organic matter from Ernutet originated in the interior of Ceres, in which case it would have existed for some time in the inland ocean," said Julie Castillo-Rogez, Dawn Project Scientist and JPL Assistant Researcher.

Vibrant Vesta

At Vesta, Dawn's investigation of the first solar system found a 4.5 billion-year-old witness. Dawn mapped craters of the planet-like world and revealed that the northern hemisphere had suffered larger impacts than expected, suggesting that there were more large objects in the asteroid belt than scientists thought.

Vesta also had other surprises. Although it is technically classified as an asteroid, this label contradicts the rich and varied terrain that Dawn has revealed and the planet-like processes that Vesta has experienced. Hubble had relayed images of a mountain in the center of a huge pool now called Rheasilvia. Dawn's mapping showed that she was twice the height of the mount. Everest, and he revealed canyons that rival the size of the Grand Canyon. Dawn also confirmed that Vesta was the source of a widespread family of meteorites.

Now, towards the end of Dawn's second mission to Ceres, the spacecraft has continued to collect high resolution images, gamma and neutron spectra, infrared spectra and gravimetric data. Almost once a day, it will fly over Ceres about 35 kilometers from its surface, about three times the altitude of a jet plane, collecting valuable data until the last hydrazine that powers the thrusters controlling its orientation is exhausted. . Dawn's reaction wheels failed earlier in the mission, leaving her heavily dependent on this vital fuel. When Dawn runs out in the next two months, the spacecraft will lose its ability to communicate with Earth, but it must not hit Ceres.

As Ceres presents interesting conditions for scientists studying the chemistry that leads to the development of life, NASA follows strict planetary protection protocols for the removal of the Dawn spacecraft. Unlike Cassini, who deliberately plunged into the atmosphere of Saturn to protect the system from contamination, Dawn will remain in orbit around Ceres, which has no atmosphere.

Engineers have designed Dawn's final orbit to ensure that it will not fail for at least 20 years, and probably decades more.

Rayman, who led the team that flew Dawn throughout the mission and reached its final orbit, likes to think of the end of Dawn as "a celestial monument inert to human creativity and ingenuity."

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