Late Wednesday night, the Dawn spacecraft traveled 35 km above the hidden surface of Ceres, halfway between Mars and Jupiter. He used his thrusters cautiously, but frequently in flight, adjusting his wing-shaped solar array whenever the gravity of the dwarf planet caused a dip. Then, without warning, the next impulse broke and nothing came out. Unable to stall his panels with the sun, Dawn went into safe mode and darkened. His support team, which coincidentally was on duty, immediately suspected that the 11-year mission was over.
"At that time, we were practically working on fumes," says Marc Rayman, Dawn's chief engineer for NASA's Jet Propulsion Laboratory.
The fact that Dawn's tank dried up just two days after the Kepler telescope, looking for an exoplanet, hit the headlines, was a startling but not shocking coincidence, as both teams were aggressively managing their declining stocks for months. While both spacecraft have worked in different regions of the solar system, the arcs of their lives – and their deaths – are similar. Everyone has had to deal for a decade in the fight against the inevitability of breaking parts, backed by a team of engineers forced to balance the same scarce and handicapped resources by the same design feature imperfect.
Both machines charging their batteries to solar energy, so the limiting factor was their maneuverability. To stay focused on scientific targets and rotate to Earth to transmit this knowledge to the home, the spacecraft used two strategies. Most of the time, they used "reaction wheels", disks that turned in one direction to rotate the machine in the other direction. But occasionally, in order to adjust the wheels still turning without changing direction, the spacecraft would use hydrazine to fire thrusters. Charlie Sobeck, Kepler Systems Engineer at NASA's Ames Research Center, compares the installation to a hybrid car. "Usually you use the electric motor," he says, "but from time to time the battery will completely discharge and the gas engine will have to turn on and power up the battery."
Dawn and Kepler were launched in 2007 and 2009 with four wheels each (one for each axis of rotation and one backup) and dozens of kilos of fuel. Arrazine would have been a limiting factor, but the reaction wheels were mysteriously beginning to give way – the first on Dawn in 2010, then the one on Kepler in 2012 -. The engineers then realized that they were facing an unexpected challenge. They could compensate for the lack of wheels by increased use of thrusters, but each shot has brought a little closer the end of the missions.
Neither Rayman nor Sobeck know exactly what's wrong with these wheels. The possibilities range from poor lubrication to internal static electric shocks leaving tiny craters in their bearings. In the absence of far space mechanics, they had to fly their best with the broken parts. A naïve choice might have been to accept shorter delays and to navigate exclusively with less efficient thrusters, but in both cases the engineers were creative and found alternative options.
When a failure of Kepler's second drive in 2013 ended his main mission, his team found a way to turn the sun into a third virtual wheel to complete the other two. The light exerts the slightest push, and while heading straight for the sun, the spacecraft found the right angle to stay in balance. The new flight mode was slightly more inefficient from the point of view of hydrazine and limited the observations to certain bands of the sky, but there were stars everywhere. The resulting K2 mission then added more than 350 new exoplanets over a five-year period to the discovery of thousands of missions by the main mission. "We were not expecting a two-wheeled solution," says Sobeck, "but this solution has proven to be very, very effective."
Unlike Kepler, the dominant force on Dawn was a destabilizing force. After exploring the second largest object of the asteroid belt, Vesta, from 2011 to 2012, he still had two wheels in rotation. Rayman knew that he would need aging reaction wheels to prevent the unstable gravity attraction of Ceres from pushing Dawn into a fall, so he turned them off and headed for the second destination, mainly on hydrazine . "We admitted we had this important vulnerability," he says. "We had to manage carefully [the wheels’] lifetime. "In December 2015, Dawn relaunched the wheels for low-level intelligent navigation.
The dawn buzzing around Ceres continued until Wednesday – despite the loss of the third wheel last year – consolidating its legacy as an explorer of unexplored worlds inspired of science fiction. In addition to having an "ion engine" popularized by Star wars and Star TrekDawn became the first spacecraft to orbit an extraterrestrial body, then take off and jump to another.
Even with a thorough shepherd, hydrazine aboard both craft could only help the troubled jet wheels. Pressure in Kepler's fuel line dropped in early July. Ames engineers then stopped the machine to preserve the remaining falls. They thought that their final download of the data would take place in August. "Many of us thought we would be able to do it," recalls Sobeck. They were able to retrieve this information and even make a last observation session before the hydrazine tank was completely emptied in mid-October. Kepler is now waiting for a last watch command. Dawn remains completely inaccessible. Unable to stabilize, the spacecraft probably entered a slow and imposing downfall that will continue around Ceres for decades, according to Rayman.
Although NASA was unable to determine the precise reasons why so many Kepler and Dawn wheels failed, they took steps to improve the technology of future spacecraft. The wheels of the two missions came from the same company, UTC Aerospace Systems, which provided a similar model to that of Hayabusa, a Japanese jumper of asteroids – another mission that overcame failures and orientation problems. . So, while Kepler and Dawn dying within two days are a wild coincidence, the fact that they both ran out of gas together may not be totally random. Rising internal temperatures meant that Kepler's days were numbered regardless of fuel resources, but Dawn was otherwise in excellent health and potentially could have continued.
UTC Aerospace Systems referred the inquiries to NASA, but Rayman said the other wheels of the same contractor were working "very well" and Dawn and Kepler's reaction wheel product line was no longer available. .
Although the reaction wheels proved to be the Achilles heel of the duo in this case, Rayman praised the technology in general, citing Cassini as an example of a spaceship whose wheels directed it "beautifully" during almost 20 years old. Sobeck says something must break in the end, and these capricious gyroscopes can often be the first critical element to go through. Most components do not move much, but the reaction wheels must rotate thousands of times per second for years, so any imperfection can quickly produce catastrophic amounts of friction and heat.
Thanks in part to each team's MacGyvering team, the $ 500 million Dawn has exceeded its three-year core mission, and the $ 600 million Kepler has exceeded his expectations of six. The performance does not surprise either Rayman, who suggests the reason why NASA spacecraft so often become super-efficient, which amounts to the economy and good engineering. "If you commit hundreds of millions of taxpayer dollars," he says, "for an 8-year mission, you do not design it so that everything fails at 8.01 years old."
Rayman says he would like both missions to continue, but it's paradoxical to say how much the community is lucky to have it for so long. He compares the situation to receiving $ 1,000 a day for a decade, and then this week, the windfall profits stop abruptly. "You may think, well, I want a thousand dollars today too," he says. "But the other way to look at things is: look at my wealth." Kepler and Dawn both made fabulous riches for a wonderfully long time.