This is Stardust, NASA's comet hunter about 312,000,000km from Earth. Later today, they'll order her main engines to burn to depletion, accelerating "anywhere from 2.5 to 35.2 metres per second", and turn off her radio. But why?
Swinging around the solar system
Launched in February 7, 1999, the brave 300kg 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 two million times for a total of 5.67 billion km, burning most of her 80kg of fuel load. In all these years, Stardust's systems have completed her main and extended missions. In fact, the spacecraft is 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.
Killing her softly
But why kill her completely then? 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.
Stardust will just fade out and keep swinging among the stars. Godspeed, comet hunter. You served us well.