Scientists "soft-landed" a space probe on a comet for the first time on Wednesday, capping a ten-year, four-billion-mile journey.

The European Space Agency's Rosetta mission sent a washing-machine-size probe named Philae to the surface of comet 67P/Churyumov-Gerasimenko.

Despite a thruster failure on Wednesday, the three-legged, 220-pound (100-kilogram) probe set down successfully at 11:03 a.m. EST Wednesday. Two harpoons designed to attach Philae firmly to the rubber-duck-shaped comet failed at the last minute, but ESA scientists say there's no reason to believe at this point that Philae's mission has been compromised. The probe, they believe, is ready to begin what could be up to a year or more of intensive scrutiny of the comet's composition and structure.

"We have definitely confirmed that Philae is sitting on the surface talking to us," said a mission flight controller after the landing. "We are on the comet."

The thruster failure, detected by ground controllers after Philae separated from the main craft, could have turned the landing into a disaster. Even if the thruster were working perfectly, the cracked, craggy, boulder-strewn surface of comet 67P/Churyumov-Gerasimenko, not to mention the gas-venting cracks near the landing site, would have made for a risky touchdown. (See more on the landing process in our Viewing Guide.)

The Rosetta spacecraft already made history in August by going into orbit around a comet for the first time. Though Wednesday's touchdown is the first soft landing on a comet, NASA soft-landed a spacecraft on an asteroid in 2001.

Coupled with ongoing observations from the orbiter's suite of cameras and scientific experiments, the pair of robotic probes will "provide us with a quantum leap forward in understanding comets," says Rosetta's principal investigator Matt Taylor, a planetary scientist with the ESA.

That's no exaggeration: Until Rosetta, everything scientists knew about comets came from telescopes or from a handful of probes that have whizzed past comets at high speed. (Related: "Incredible Photos From Spacecraft's 4-Billion-Mile Journey.")

Now they have a chance to look at this visitor from the edge of the solar system up close and at leisure, and it's hard to overstate how scientifically important this could prove to be. (See the National Geographic Channel's "Comet Catcher: The Rosetta Landing.")

To start with, what they learn could be crucial to understanding the origin of Earth's oceans, since icy comets may have delivered water to our young planet in the solar system's early days. It could help explain the origin of life as well, given that comets contain tarry deposits of organic chemicals, which theorists believe could have provided some of the raw material biology used to get started.

And it could be crucial to understanding the origin of the solar system itself, because comets are widely understood to be the leftover, unused building blocks from the time the planets took shape nearly 4.6 billion years ago—chunks of ice and rock that congealed from a vast cloud of gas and dust floating between the stars.

Why Comet Chemistry Matters

In the inner solar system, chunks like the comets (but made mostly of rock, not ice) were drawn together by gravity to form Mercury, Venus, Earth, and Mars, plus the massive rocky cores of the gas giants Jupiter, Saturn, Uranus, and Neptune.

Out beyond Neptune, however, the pieces were mostly too widely spread to form planet-size objects. The best known exception is Pluto, discovered in 1930, which was joined in the 2000s by Pluto-size Eris and the somewhat smaller Makemake, Haumea, and Quaoar.

But swarming around these, planetary scientists began to realize about two decades ago, are up to ten billion much smaller icy bodies, ranging from a mile to a few tens of miles across. The swarm is known as the Kuiper Belt, and the bodies themselves are known as Kuiper Belt Objects, or KBOs.

When a gravitational nudge sends these objects plunging from the Kuiper Belt into the inner solar system, their ices begin to warm into vapor, carrying dust along with them to form a temporary atmosphere, and sometimes a glowing tail—the familiar trappings of a comet.

Since KBOs have been in a deep-freeze since the solar system began, planetary scientists think they've largely retained their primordial chemical composition, which the Rosetta orbiter's 14 instruments and the Philae lander's 10 will be able to study in unprecedented detail.

View Images The Philae lander separated from the Rosetta spacecraft and anchored itself on the surface of the comet. Illustration by ESA/ATG Medialab

Team Approach

Rosetta and Philae will be able to make their measurements in relative luxury: Both will ride along with 67P through the comet's closest approach to the sun next summer, when the comet, which is still frozen at this point, will be expelling more than a ton of material into space every hour.

The observations will be largely complementary. The orbiter, for example, currently about 14 miles (22.5 kilometers) away from 67P, will slowly spiral in until it's only a mile or two from the comet's surface, at which point its cameras will be able to pick out details just a few inches across.

"That's really good," says Holger Sierks, of the Max Planck Institute for Solar System Research in Göttingen, Germany, who is responsible for the cameras: The best images before Rosetta, from spacecraft that flew by comets, were more like 30 feet (9 meters) across.

Philae, named after an island in the Nile where an obelisk used to help decipher the celebrated Rosetta stone was located, will see far finer detail, albeit over a much smaller area. In both cases, light reflected from the surface and into spectrographs will tell scientists what 67P's outer crust is made of.

Both craft will also "sniff" the comet's atmosphere with mass spectrometers to reveal the composition of gases released from the interior. Already, the orbiter has detected hydrogen sulfide, ammonia, formaldehyde, and hydrogen cyanide along with water and carbon dioxide. As the atmosphere grows thicker over the next several months, the instruments should detect other gases as well.

Philae also carries a drill, which will bore about eight inches (20 centimeters) below the surface, drawing out samples for yet another round of chemical analysis.

And the lander will join the mother ship in a joint experiment, performed when the two are on opposite sides of the comet, in which they'll send radio waves back and forth through the body of 67P, which is about 2.5 miles (4 kilometers) thick at its thickest point.

"It's conceptually like an x-ray," says Michael Küppers, a mission scientist on the ESA staff, and should help scientists understand the comet's deep structure—whether the comet is relatively solid, for example, or more of a "rubble pile" of smaller chunks held loosely together.

If it's the latter, says Hermann Boehnhardt, of the Max Planck Society for the Advancement of Science, "this is, for me, proof that the comet is truly primordial," a snapshot of the process that could have led to the formation of an icy planet like Pluto, but ultimately didn't.

All of this is little more than informed speculation at this point—the same sort of speculation planetary scientists have been indulging in for decades. By this time next year, the avalanche of data already pouring in from Rosetta and Philae will have turned much of that speculation into understanding.