We just landed a spacecraft on a comet for the first time. Here's why it matters                     

Joseph Stromberg, 11/12/14

At around 10:30 am EST Wednesday morning, a small probe named Philae made history-- by becoming the first spacecraft to ever land on a comet.

The 220-pound, box-shaped lander was carried to the comet, called 67P/C-G, by a larger probe named Rosetta, launched and operated by the European Space Agency. In August, Rosetta became the first spacecraft to ever be put into orbit around a comet, and at 3:35 am EST on Wednesday morning, it released Philae.

Philae successfully landed on comet 67P/C-G

After a seven hour journey down from Rosetta, the lander successfully made contact with the comet, sinking down an estimated four centimeters into its dusty surface and shooting out the ice screws on its legs. However, ESA scientists are saying that the lander's harpoons did not correctly fire, raising the possibility that it might roll over.

It's unclear what might happen next, but the ESA says it's looking into the possibility of trying to re-fire the harpoons.

Whether it's able to use them or not, the lander seems to be stable at the moment, and has already begun transmitting scientific data, including a high-resolution photos of the descent to the comet -- the first ever captured:

It has a total of about 64 hours (the amount of time its battery power will last) to analyze soil samples and collect other sorts of data, telling us more about comets than we've ever known before.

The Rosetta mission

Rosetta was launched from French Guiana in March 2004, and has now traveled more than 3.5 billion miles in space.

The reason it's been traveling so long is that, to reach the comet, Rosetta had to loop around the solar system several times, passing by Earth and Mars so that it could use the planets' gravity as a slingshot. In 2008 and 2010, the spacecraft also flew by a pair of asteroids, allowing engineers to take photos and calibrate Rosetta's cameras before it approached its final destination.

In 2011, after Rosetta had traveled too far from the sun to sustain itself with solar power, it went into hibernation to conserve energy, and was woken back up at the start of 2014. Finally, in August, Rosetta successfully rendezvoused with the comet, a 2.5 mile-wide chunk of rock and ice that approaches the sun in an elliptical orbit, then goes back out once every six and a half years.

Scientists had been expecting a nice, even, roughly spherical object, which would have hopefully provided several level landing areas. Instead, they found a lopsided comet that they've been calling "duck-shaped."

Two previous probes had flown by comets, and in 2005, NASA intentionally crashed a probe into a comet in order to analyze its interior. But putting a craft into orbit around a comet is much more difficult (it is, after all, a spinning, mountain-sized object orbiting the sun at about 84,000 miles per hour, millions of miles from Earth) and Rosetta is the first craft to ever achieve the feat.

Over the past three months, Rosetta has orbited the comet, taking photos of the surface and searching for landing sites. The craft has also analyzed the gases given off by the comet as it nears the sun and warms up.

Rosetta will stay in orbit around the comet for the next year, tracking it as gets continually hotter and begins to vaporize. Data collected during this period will provide new information about the plume of dust and water vapor that forms a comet's tail as it passes by the sun. Then, in December 2015, Rosetta will break away from the comet, which will travel back out toward the outer reaches of the solar system.

Even without the lander, this is by far the most ambitious project ever attempted by the European Space Agency -- it involved 10 years of planning and 10 years of travel just to reach this point, and cost an estimated $1.59 billion.

The Philae lander

Despite the many firsts of Rosetta, the most remarkable part of this mission was landing Philae, a dishwasher-sized probe, on the comet's surface.

Much was rightfully made of the difficulty of landing Curiosity on Mars' surface in 2012. But some experts have said that Philae's landing was even more difficult: partly because we've mapped the comet much less thoroughly than we have Mars, and because the comet's uneven shape made it impossible to find a landing site that was entirely level.

Still, after a descent of about 19 miles over the course of seven hours, Philae successfully landed on the comet at a speed of about one meter per second (similar to that of a person walking). Data indicated it was a relatively soft landing, perhaps due to loose dust and other material covering the area.

However, early reports are that its harpoons did not engage, making it possible that the probe might roll over. Scientists may try to re-fire the harpoons to prevent this from happening, though its leg screws have reportedly dug in to the comet.

In either case, Philae has less than three days' worth of battery life to tell us a ton of new information about both this comet and comets in general. It'll immediately begin using its panoramic camera to provide photos of the comets' surface of unprecedented detail. One concern is that the data link between Philae and Rosetta has been somewhat intermittent, although it's been sufficient for the lander to send back a few photos and some scientific data already.

Philae carries 10 scientific instruments in total, including a system capable of drilling up to nine inches down into the comet's surface and extracting soil samples. Other instruments onboard will analyze the contents of these samples, telling us far more about the composition of comets than we've ever known before.

Another instrument will emit radio waves that will travel through the comet's center, then get picked up by the orbiting Rosetta. Based on the time it takes for these waves to arrive, scientists will be able to map the comet's interior.

Why scientists wanted to put a lander on a comet

All this data will be especially interesting because the comet is believed to have formed 4.6 billion years ago, from material leftover as Earth and the solar system's other planets were coalescing. As a result, understanding the composition of comets could help us better model the formation of the solar system.

Moreover, many scientists believe that in the period afterward, when the solar system was still a chaotic, collision-filled system, comets and asteroids were responsible for bringing water and perhaps even organic molecules to Earth. If water ice is present on this comet, as scientists hope, Philae will calculate the ratio of different sorts of hydrogen isotopes present in it -- information that could provide an important clue as to whether the hypothesis is correct.

In other words, data collected by a tiny robot on this lopsided, spinning comet, millions of miles away, could provide a window into the history of all life on earth.

Update: This article has been edited to reflect ongoing developments.

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