The Stardust NExT probe is preparing for an encounter on St Valentine's Day with comet Tempel 1.
Next Monday will mark the first time two different comets have been surveyed with the same set of scientific instruments. And Tempel 1, explored by NASA's Deep Impact mission in 2005, will become the first comet to be visited by two spacecraft.
Seven years ago, the probe visited Wild 2 and captured a thimbleful of comet dust.
"Had we known at the time of the Wild 2 flyby how comets worked, we would have been even more nervous. There were jets at sonic speeds, and there were clumps of material coming out from the comet and breaking up," said University of Washington astronomer Donald Brownlee.
"That's scary when you know a particle larger than a centimeter across – less than half an inch – could destroy the spacecraft, along with years of planning and work."
Stardust NExT can't gather any samples this time, as its collector returned to Earth following the encounter with Wild 2. But it will take photos of the crater formed on Tempel 1 during the Deep Impact mission to learn more about the interior of comets.
Deep Impact couldn't gather images itself because its camera's vision was obscured by a cloud of debris from the creation of the crater.
It will also measure the size and distribution of particles flowing from Tempel 1 and analyze their composition - the science team plans to make detailed observations of how interaction with the sun has physically changed Tempel 1 in the six years since the Deep Impact encounter.
Stardust started out with about 22 gallons of hydrazine fuel for its thrusters. Now, two missions and about 3.6 billion miles later, there's only about a cupful left, not enough for any meaningful operations.
So, after 12 years of being guided by mission controllers at NASA's Jet Propulsion Laboratory, the office-desk-sized spacecraft will continue to orbit the sun on its own.
It will still have its scientific instruments and its camera, a spare from the Voyager program. It also still carries two microchips bearing the names, in microscopic type, of more than a million people who signed up before the launch.
"When it's chucked from the solar system by Jupiter, the spacecraft and its 'crew' of signatures will keep going for billions of years," said Brownlee. "The chip will probably be readable after the Earth is gone."