This is Stardust, NASA's comet hunter about 312,000,000 kilometers from Earth. Yesterday, they ordered her main engines to burn at full throttle until they consumed all the available fuel, and then turned off her radio. But why?
Launched in February 7, 1999, the brave 661-pound spaceship was the first spacecraft in history to return to Earth samples from a comet (Wild 2) and the first to record the sound of the dust of a comet (Temple 1) hitting her instruments.
During her more than 12 years lifetime it has performed 40 major flight maneuvers, firing her rockets a combined 2 million times for a total of 5.67 billion kilometers (3.52 billion miles) burning most of her 80 kilograms of fuel load. In all these years, Stardust's systems have completed her main and extended missions. In fact, the spacecraft was still healthy, humming across the solar system with Temple 1 way behind her stern now.
Stardust-NExT project manager Tim Larson of NASA's Jet Propulsion Laboratory in Pasadena, California, explains how the burning process itself works:
We call it a burn to depletion, and that is pretty much what we're doing—firing our rockets until there is nothing left in the tank. It's a unique way for an interplanetary spacecraft to go out. Essentially, Stardust will be providing us useful information to the very end. [...] What we think will happen is that when the fuel reaches a critically low level, gaseous helium will enter the thruster chambers. The resulting thrust will be less than 10 percent of what was expected. While Stardust will continue to command its rocket engines to fire until the pre-planned firing time of 45 minutes has elapsed, the burn is essentially over.
And Allan Cheuvront, Stardust program manager at Lockheed Martin Space Systems, has the explanation:
We'll take those data and compare them to what our estimates told us was left. That will give us a better idea how valid our fuel consumption models are and make our predictions even more accurate for future missions.
The results were sent on its last transmission—yesterday, at 7:33pm ET: The engines fired up for exactly 146 seconds. They will now compare the data to their projections and see how accurate NASA's fuel consumption models are.
But why turn off Stardust's computer systems and deplete her batteries turning its solar panels away from the Sun? Perhaps it could find something new, as it travels around the solar system. The answer is simple: To avoid future interferences. Since the radio spectrum for space transmission is limited, NASA doesn't want to have spacecrafts transmitting data in frequencies that may be used by other ships in the future.
While some ships—like the Voyager missions—are kept alive after their main missions end, most of NASA's ships go through the same process. Others, like the Galileo mission in Jupiter, are destroyed by crashing them into planets. Larson has a good description of the final burn of Stardust:
This kind of feels like the end of one of those old western movies where you watch the hero ride his horse towards the distant setting sun-and then the credits begin to roll. Only there's no setting sun in space.
Godspeed, comet hunter. You served us well.