This morning at 4AM Eastern time, the European Space Agency's Rosetta orbiter jettisoned a washing-machine-sized probe, which began its fall towards the comet Rosetta is monitoring. The 250-pound probe, called Philae, will fall for seven hours; if everything goes to plan, it will land on a patch of the 2.5-mile-long comet.

"the most critical part was and still is the landing itself" "The most critical point was and still is the landing itself, the touchdown on the surface," says Stephan Ulamec, the head Rosetta lander. "This is where we will need some portion of luck."

Philae could, for instance, overshoot and miss the comet, dubbed 67P/Churyumov–Gerasimenko; it could bounce, or land at a bad angle. And the landing site, though chosen because it seemed clear of debris, is near plumes of gas that erupt from the surface — which could reduce the lander to shrapnel. Working in Philae's favor though, is the comet's small size; its relatively gentle gravitational pull means that the touchdown will occur at "walking speed," or less than 3 feet a second, according to ESA.

In order to help itself land, Philae will shoot harpoons into the comet as an anchor. The probe is also equipped with thrusters, meant to fire at landing to keep the probe upright. Unfortunately, there's a problem with the cold gas system, which means the thrusters may not work, says Ulamec. That means Philae could bounce off 67P before its harpoons and screws are inserted into the icy surface.

Once it's anchored, Philae will begin sending back images, as well as information on the comet's crust. The lander also has a twitter account, @Philae2014, because of course it does.

The harrowing landing has been a decade in the making The harrowing landing has been a decade in the making. The Rosetta orbiter launched in 2004, and orbited earth three times to pick up the speed required to let it catch 67P. At its furthest point from the sun, Rosetta got only 3 percent of the sunlight we receive on earth — a problem for its solar panels, which is how it generates its electricity. So it was designed to go into a kind of hibernation, with only the most vital functions running. In January, Rosetta woke up and in August, it caught 67P/Churyumov–Gerasimenko.

Comets are essentially icy chunks of rock left over from the planets' formation more than 4.6 billion years ago. "Comets have the beauty of having preserved the ingredients with which the solar system formed," says Jean-Pierre Bibring, Philae's lead scientist. Some of the complex molecules thought to be the first building blocks for life may be preserved in 67P's ice, he says. After Philae lands, it will have about 60 hours at full power to run experiments using its 10 on-board instruments before its batteries drain. If the lander isn't stuck in a shadow, Philae's solar panels will recharge the craft.

No spacecraft has ever had a controlled landing on a comet, though NASA's Deep Impact slammed a probe into one in 2005 to find out what composed the comet's core.

Whether or not Philae succeeds, Rosetta has already gathered an unprecedented amount of information about 67P; no comet has been tailed for nearly as long. It has discovered the comet's personal scent, composed mainly of rotten eggs with notes of horse urine and formaldehyde. Rosetta has also been tracking 67P as the comet's surface temperature increases, and it sprays off more water.

The craft, which cost 1.4 billion Euros (about $1.75 billion), will accompany 67P around the sun, and may supply more years of data. Its mission ends in December 2015, after flying with the comet to its closest pass with the sun in August, according to ESA.