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NASA's Dawn spacecraft ran out of fuel on Wednesday and stopped transmitting to Earth, ending an eleven-year mission exploring the two largest objects in the asteroid belt and setting several records in the annals of the history of space.
Dawn failed to contact the controllers at NASA's Jet Propulsion Laboratory in California on a night of communication late Wednesday night to Thursday, and officials said the mission was over after evidence had indicated that space shuttle had lacked hydrazine.
Fuel depletion was long overdue and engineers expected Dawn to be short of hydrazine in September or October. Apparently, Dawn emptied its hydrazine tank on Wednesday, which made the spacecraft orbit around the dwarf planet Ceres unable to keep its antenna pointed at Earth, nor its solar panels sunk to produce electricity.
"Everyone rightly acknowledges that it's bittersweet, but I find it a lot sweeter than bitter," said Marc Rayman, Dawn's chief engineer at JPL, at the time. an interview with Spaceflight Now. "It's the successful conclusion of a successful mission. For me, this is the best way to complete a mission because it is productive to the end and we have pulled as much as possible, even in principle, the probe, so that I can not be happier. "
Spaceflight Now, members can read a transcript of our full interview with Marc Rayman. Become a member today and support our coverage.
"Today, we are celebrating the end of our Dawn mission – its incredible technical achievements, the vital science it has provided, and the team that made the spacecraft make these discoveries." said Thomas Zurbuchen, deputy administrator of NASA's Science Mission Directorate in Washington, in a statement Thursday. "The amazing images and data Dawn has collected from Vesta and Ceres are essential to understanding the history and evolution of our solar system."
Launched from Cape Canaveral aboard a United Launch Alliance Delta 2 rocket on September 27, 2007, the Dawn spacecraft traveled 6.9 billion kilometers (6.9 billion miles) across the interior solar system during of the last 11 years, flying over Mars for a gravity badistance maneuver. in 2009 before reaching the asteroid Vesta, the second largest object of the asteroid belt, in 2011.
The spacecraft was built by Northrop Grumman Innovation Systems, formerly Orbital ATK, and carried three instruments – a framing camera, a visible and infrared spectrometer and a gamma and neutron detector – to study the geology, mineral composition and the water. contents of Vesta and Ceres.
Dawn gravitated around Vesta for over a year, using his ion engines to wrap around the giant asteroid, then retreat and escape Vesta's gravity field to reach Ceres.
Dawn's time at Vesta has resulted in several big surprises, mainly with the discovery of evidence that liquid water may have flowed into the asteroid, Raymond said.
Scientists already have samples of Vesta in laboratories on Earth.
Prior to Dawn's mission, the researchers suspected a special clbad of rock samples called Howardite – Eucrite – Diogenite, or HED, meteorites that fell to Earth from space. They were mbadacred by an old interplanetary collision.
Dawn confirmed this hypothesis and found that Vesta probably had global tectonic activity, something scientists did not expect in such a small world. Vesta is approximately 578 kilometers in diameter along its longest axis.
The camera suite built in Germany by Dawn Shuttle discovered holes in the bottom of several relatively cool craters on Vesta, suggesting volumes of gas – perhaps water vapor – released by violent impacts with other asteroids.
Dawn's journey between Vesta and Ceres took nearly three years, relying on the probe's plasma propulsion system to change its trajectory across the asteroid belt to intercept its next target.
The maneuvers put Dawn on track to be captured by the Ceres gravity field in March 2015.
Before Dawn's arrival, the best images of Ceres from the Hubble Space Telescope gave scientists a glimpse of how the mysterious mini-planet appeared. Scientists knew its size and shape, and they thought that Ceres might contain a subglacial ocean.
Ceres surprised the Dawn team almost as soon as the spacecraft came within sight.
"The big surprise at the beginning of the approach phase was that there was a high reflectivity zone near the Occator (Crater)," said Andreas Nathues, senior investigator of the approach. Max Planck Institute's framing camera team for solar system research in Göttingen, Germany. at a press conference last year. "The first images were so bright that we saturated all the chips (in the camera) because we did not expect such brightness on a dark surface."
The bright spots of the Occator crater immediately sparked speculation that it could be icy patches, or perhaps an erupting volcano spewing water into the space. Scientists initially favored the explanation of ice, but further investigation by Dawn's scientific instruments revealed that they were deposits of sodium carbonate, a type of salt.
Scientists believe that shiny salt deposits have risen to the surface when an old impactor hit Ceres, releasing melted rock and water into a complex or cryovolcanic hydrothermal system. Dawn also discovered Ahuna Mons, a three-kilometer high peak (5 km) which, according to his team, is a dormant volcano that spits an aqueous matter in the sky instead of a rocky magma.
Dawn's exploration of Ceres contributed to the scientists' conclusion that dwarf planets could have harbored oceans and contained the ingredients necessary for life.
Ceres stretches for about 950 kilometers (590 miles) in diameter, about one-thirteenth the size of the Earth. It is larger than the moon of Saturn, Enceladus, which hides a global ocean under its ice shell warmed by the constant gravity of Saturn's gravity inside the moon, a phenomenon known as the warming of tides.
"There is an affinity between some of the Ice Moons and Ceres, and they certainly carry similarities," said Carol Raymond, Dawn's senior investigator at JPL, during an interview with Spaceflight Now. 39, last year. "But since Ceres lives in such a warm environment, it seems very different. His ocean froze. He does not have tidal heat. The ocean is frozen and its surface is baked compared to the frozen moons. The way it has formed, what it is formed in, seems to be similar, but the paths of evolution are very different. "
Dawn's main mission ended in 2016 and NASA approved an extension to continue exploration of the Ceres probe, the largest world between the Mars and Jupiter orbits. Senior agency officials have not approved a proposal to light Dawn's loyal ion engines and escape Ceres to fly over an asteroid. They therefore concluded that there was more scientific progress to be made at Ceres than at another target.
Dawn has almost never reached the launch pad.
Cost overruns and difficulties with Dawn's electric propulsion system led NASA to cancel the mission in March 2006. The space agency reinstated the mission less than a month later following a call from responsible for the JPL.
"There were some dramatic points," said Raymond last year. "The first took place right before the launch, when we learned that we had faulty reaction wheels and that we could not do anything about it. We moved to a mode where we tried to preserve the life of the wheel. "
Three of the spacecraft's four reaction wheels failed during the mission, forcing the engineers to devise a new way to control the pointing of the probe with a combination of inertia wheels and hydrazine-powered thrusters. The rotating gyro wheels are designed to change their rotational speed to rotate the spacecraft.
With the failure of a third reaction wheel last year, Dawn began to consume more hydrazine for pointing control. The probe was launched with approximately 45 kg of hydrazine to fuel its propellants.
The lasting legacy of Dawn
Rayman said Dawn's exploration mission will leave a lasting scientific and technical legacy.
"In science, it's the unveiling of two of the last unexplored worlds of the inner solar system," Rayman said. "Vesta and Ceres are the two largest bodies between Mars and Jupiter and, prior to the Dawn mission, Ceres was the largest object between the sun and Pluto that a spaceship had not yet visited.
"The main asteroid belt really contains millions of objects, but Vesta and Ceres, which Dawn has explored on its own, contain 45% of that total mbad," Rayman said. "I think it's quite impressive and it showed us that Vesta is not just an asteroid like the others. Many people call it a big rock or something like that. Geologically, it is more closely related to terrestrial plants, one of which is just under our feet. It has a dense iron-nickel core surrounded by a mantle, surrounded by a crust and resembles more terrestrial planets than rocks that we consider asteroids. "
Dawn was carrying three ion engines to move the spacecraft around the solar system, setting a record for the longest duration of use of a plasma propulsion system in space.
Using a combination of xenon fuel and electric power to generate low thrust levels, ion engines are not as powerful as conventional thrusters, but they produce more pulses over time, allowing fuel economy to gain in efficiency.
It took Dawn's ionic propulsion system four days to accelerate to 96 km / h (60 mph), but the probe pushed its ion engines for 5.9 years of cumulative operation, changing the gear speed of 41,700 km (25,700 km) on the course. of his mission.
This ability has allowed Dawn to become the first spacecraft to orbit two solar system destinations outside the Earth and Moon.
"For me, Dawn was the first interplanetary spacecraft," Rayman said. "The ability to travel to a distant alien world, orbit it, then maneuver into orbit, then out of orbit and travel through the solar system – it's been two and a half years and 900 million miles To go from Vesta to Ceres, go into orbit into another extraterrestrial world and explore it, I think it's really extraordinary. Indeed, it's truly unique in more than 61 years of space exploration, and I think it bodes well for our species as we continue to reach out into the cosmos. "
"In many ways, Dawn's legacy is just beginning," Raymond said in a statement. "Dawn's data will be deepened by scientists working on planetary growth and differentiation, as well as the time and place where life could have formed in our solar system. Ceres and Vesta also play an important role in the study of distant planetary systems, as they provide insight into the conditions that may exist around young stars. "
Rayman said Dawn's ground crew found for the first time on Wednesday that the spacecraft, more than 300 million kilometers from Earth, could be out of fuel. NASA's Deep Space Network, which includes antennas in California, Spain, and Australia, was tracking Dawn's radio signal to measure its Doppler shift, collecting data that allowed scientists to accurately map the Ceres gravity field. information that can help determine variations in the internal structure of the dwarf planet.
"We lost the signal at the end of the track, so we continued to watch during this track and we did not see it, but that was not enough to definitively decide that the mission was over," he said. Rayman.
Dawn did not transmit any telemetry during the Doppler track – it was simply a blank radio signal – so the controllers could not be sure of the status of the spacecraft. They waited for another communication late Wednesday, when the engineers heard only silence.
"We have not seen the spaceship at all," Rayman said. "That was enough to confirm that the mission was over, because we had known for so long that we were about to run out of hydrazine."
Without hydrazine to power its control jets, Dawn was unable to correct its orientation, which requires regular maintenance to counteract the natural forces that fire at the spacecraft, such as gravity and solar pressure from Ceres.
Engineers were expecting these forces to pull out Dawn's solar panels, which cover a depth of 20 meters, the latch against the sun, preventing the satellite from recharging its batteries. Under such circumstances, the on-board software was programmed to automatically turn off the Dawn radio transmitter to conserve power until the batteries could be recharged.
"It's smart enough to turn off this radio, save energy, save battery, until its solar panels are in the sun, but it will never reach that goal. She turned off the radio and did not turn it back on. Said Rayman.
Dawn is the second mission declared NASA ended this week.
NASA announced Wednesday that the Kepler Space Telescope was running out of fuel, ending its search for planets around other stars. The engineers plan to link the latest orders to turn off Kepler's radio as early as next week.
NASA selected the Dawn and Kepler missions on December 21, 2001, following a competition among other mission proposals to secure federal funding through the Space Agency's Discovery program, a relatively small series of robotic space missions. costly science-based.
Kepler was launched in March 2009 and also experienced reaction wheel problems during his mission.
"They were selected the same day," Rayman said. "Of course, they ran away from each other. Kepler was launched in 2009 and Dawn in 2007. So, the same beginning and the same end, but very different lives between the two, but it's an interesting coincidence.
Dawn will remain in its current orbit around Ceres in the near future. The spacecraft evolved into an egg-shaped orbit earlier this year, which takes Dawn around Ceres once every 27 hours, rising to a distance of about 35 km above its surface on each orbit, closer to Ceres than the one Dawn had ever seen before.
"We have a planetary protection requirement that Dawn does not contact Ceres for at least 20 years," Rayman said. "The reason is that Ceres has a lot of water. Most are frozen, but some can still be liquid. It contains organic material detected by Dawn. It also has a rich inventory of other chemicals … so it contains many of the ingredients that are important or interesting for the study of chemistry that leads to the development of life. "
NASA does not want to contaminate Ceres with the debris of Dawn, thus ensuring that the frozen world of the asteroid belt remains blank for future study missions.
"(Our) badysis clearly shows that there is no chance of being impacted in 20 years and even in 50 years, there is less than 1% chance that the spacecraft will hit the ground. So we will stay in orbit for a very long time, "Rayman said.
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Follow Stephen Clark on Twitter: @ StephenClark1.
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