After a ten-year journey, the final nerve-wracking countdown is only hours away as a probe prepares for a daring landing on a high-speed comet.
The Rosetta spacecraft has been orbiting the comet 67P/Churyumov-Gerasimenko since August, and will make history when it drops its washing-machine-size lander, Philae, onto the comet at 10:35 a.m. EST (15:35 GMT) on November 12.
Other spacecraft have made contact with comets before, but only as impacts, essentially crash landings such as the Deep Impact probe that smashed into a comet in 2005. This will be the first time a mission tries to soft-land on a comet, allowing it to deploy delicate scientific instruments, and will mark only the eighth body in the solar system to be landed on. (See the National Geographic Channel's "Comet Catcher: The Rosetta Landing.")
The spacecraft has spent the past few months scouting out a suitable landing site as it built up a stunning portrait of the 2.5-mile-wide (4 kilometers) icy visitor to the inner solar system. Rosetta's photographs of the comet have revealed impressive steep canyons, towering cliffs, and jagged boulders of all sizes strewn across its barren surface.
After weeks of poring over potential landing sites, mission scientists chose one with a balance of the least hazardous terrain and greatest scientific potential. (Related: "Incredible Photos From Spacecraft's 4-Billion-Mile Journey.")
Known as Agilkia, the landing site is named after an island in the river Nile in southern Egypt, and is located on the "head" of the duck-shaped comet.
Astronomers are excited for Philae to reach the surface and drill into the comet, which is 311 million miles (500 million kilometers) from Earth and is thought to hold material left over from the origin of the solar system, some 4.5 billion years ago.
European Space Agency officials have described the descent and landing of Philae as their "seven hours of terror," a clear reference to NASA's own risky seven minutes of terror when the Curiosity rover landed on Mars back in August 2012. (See video of "Mars Rover's 'Seven Minutes of Terror.'")
Here is what we expect will unfold in those final anxious hours.
T Minus 7 Hours
The Philae lander detaches from the Rosetta orbiter at about 18.6 miles (30 kilometers) above the comet's surface. It will begin its descent Wednesday at 3:35 a.m. EST (8:35 GMT). We on Earth won't know if the lander was released successfully for another 28 minutes and 20 seconds, when the signal reaches Earth.
About 40 minutes after releasing its lander, the Rosetta mother ship will conduct thruster burns initiated by mission control in Darmstadt, Germany, so that it can pull back into a higher orbit to watch the show as the lander descends.
T Minus 5 Hours
Throughout the final hours of descent, the lander's instruments will be busy snapping images of the comet below and measuring activity on the comet's surface.
By 5:53 a.m. EST (10:53 GMT), Philae will be between 17 and 6.2 miles (about 10 kilometers) above the comet's surface, descending at a leisurely rate of 2.2 miles per hour (3.6 kilometers per hour). It will attempt to open up radio contact with its mother ship and beam back its first data as it approaches, including images and data describing the comet's surface.
The final maneuvers bring the lander down to the surface at its target site, Agilkia. The big unknown: Will there be small boulders that could tip Philae over as it lands? Also, if the gravity is too low (because of its odd dumbbell shape, the comet's already weak gravity varies on its surface), the lander may have a tough time securing itself to the ground with its anchors and harpoons.
We on Earth should find out if Philae touched down safely by 11:02 a.m. EST (16:02 GMT).
If Philae makes it safely to the comet's surface, it will first take a panoramic survey of its new home. A suite of onboard instruments, including radar, magnetometer, and temperature sensors, will probe the comet. The ESA team will also attempt to drill down about 8 inches (20 centimeters) into the comet to retrieve samples and analyze them with Philae's onboard chemistry lab.
For the mission to be a complete success, though, the solar panels will have to work well enough to provide power for the drill and the instruments. But the lander might encounter a very dusty environment when it lands, putting this electrical supply in jeopardy.
If the solar panels don't get enough sunlight, onboard batteries are expected to last for about three days. Hopes are that this will be enough to complete the main scientific mission. But if the panels can collect enough solar power, Philae could keep beaming data well into 2015, until March at least, when the comet's surface will get too hot as it approaches the sun.