When you're riding a comet just 2 1/2 miles wide, you can't rely on its puny gravity alone to hold you down. You're going to need a seatbelt.
So the first thing the lander from the European Space Agency's Rosetta spacecraft will do as it touches down on Comet 67P/Churyumov-Gerasimenko in 2014 will be to fire a barbed harpoon into the comet's icy nucleus.
"The harpoon is actually a hook to fix ourselves on the comet, like an anchor," said ESA spokesman Claus Habfast. "Then you roll up the very thin cord ... [and] softly attach yourself on the comet."
The $980 million mission, one of the longest space voyages ever attempted, is set for launch Thursday from Europe's Spaceport in French Guiana, on the north coast of South America. A booster problem delayed a 2003 launch, and planners had to target a new comet.
If all goes well this time, the three-ton spacecraft will spend the next 10 years whipping around the solar system. It will speed past Earth three times (in 2005, 2007 and 2009), and Mars once (in 2007). With each flyby, it will gain more speed and finally veer into the comet's orbit.
Along the way, Rosetta will fly twice through the asteroid belt that lies between the orbits of Mars and Jupiter. It also will soar 500 million miles from the sun -- farther than any spacecraft operating on solar power.
This will be the first probe to attempt an actual landing on a comet, though a spacecraft built by Johns Hopkins' Applied Physics Laboratory landed on a much closer and larger asteroid in 2001. There have been several comet flybys in the past 20 years, the most recent of which was NASA's Stardust, due to return in 2006 with dust samples collected from a comet's coma.
In May 2014, Rosetta will fire its braking thrusters and slip into an orbit barely 15 miles from Churyumov-Gerasimenko's nucleus. At times, it may dip as close as 1.2 miles from the surface.
By November of that year, scientists expect Rosetta will have mapped the comet's surface and found a safe landing site. They will then dispatch the probe's boxy lander, Philae, to a gentle, walking-speed touchdown.
On the comet, the 220-pound lander will weigh less than a piece of paper, ESA spokesman Habfast said. Without its harpoon, the craft would simply bounce back into orbit.
No one has ever tried to harpoon a comet before. But if the first shot fails, controllers can try again with a spare.
If the second harpoon doesn't work, the Rosetta orbiter will continue its studies, though the landing will have failed.
But if it works, the instrument-packed lander can begin to send back high-resolution photographs, and the first direct measurements of the ice and dust in a comet's nucleus.
Scientists see comets as frozen samples of the early solar system. "They are the keys to understanding the way the whole solar system, the Earth and even [how] we came into being," said Paul Weissman of NASA's Jet Propulsion Laboratory, which has supplied one of Rosetta's instruments.
Comet Churyumov-Gerasimenko, named for its Ukrainian discoverers, circles the sun once every 6.6 years.
Rosetta and its lander will accompany the comet for 18 months as it plunges toward the sun. They'll record at close range what happens as the nucleus warms and begins to spew the geysers of gases and dust that form a comet's characteristic coma and "tail."
A device on the lander called Ptolemy will drill into the comet and measure various chemical elements, frozen gases and the organic compounds that scientists believe may have supplied the young Earth with the basic chemistry for life.