DS1 is a testbed for new space technologies
A partially damaged spacecraft, Deep Space 1, is set for a comet fly-by at the weekend. Very few probes have got as close to a comet - and taken detailed measurements - as DS1 hopes to get to Comet Borrelly on Saturday.
"We wanted to fly by a comet," said mission director Marc Rayman. "It's something few spacecraft have tried before.
"Our goal is to capture a black and white photo of the sunlit side of its nucleus," he added.
Close encounter
DS1 was launched in October 1998, carrying a range of untried technologies into space.
Among them: a long-lasting ion propulsion engine, a solar array that focused sunlight for extra power, and software that endowed the craft with crude artificial intelligence to conduct its own affairs.
"Deep Space 1 tested these advanced technologies so that other missions wouldn't have to bear the costs of being first," said Rayman, of the American space agency's Jet Propulsion Laboratory in California, US.
Rayman added: "When we were firing the ion engine during tests in 1998 and 1999, we always steered to keep our options open for a comet encounter."
Only one spacecraft has gone closer to a comet than DS1 expects to get at the weekend and obtained close-up pictures and measurements. That was the European Space Agency's Giotto probe, which flew a mere 596 kilometres (370 miles) from the core of Comet Halley in 1986.
Its images showed Halley's nucleus to be one of the blackest objects in the Solar System.
Fuzzy blob
In visiting Borrelly, the DS1 team hope its probe will get as close as 2,000 km (1,243 miles) - may be nearer.
Astronomers can easily spot Comet Borrelly through a medium-sized telescope. It is glowing in the constellation of Gemini, looking like a fuzzy blob with a short, faint tail.
"Unlike Giotto," said Rayman, "DS1 was not built to encounter a comet. It has no shielding to protect it from flying debris. A single particle of comet dust the width of a human hair can deliver as much energy as a bowling ball does when it crashes into the pins."
Because of this, DS1 could be hit hard by such particles dozens of times, as it travels to the heart of the comet at 16.5 km/s (37,000 mph).
"This journey has been filled with adventure - much of it different from what we had planned," said Rayman. "But that's OK. You're not going to win the big prizes if you don't take the big risks."