In this November 10, 2014 handout photo illustration provided by the European Space Agency (ESA) the Rosetta probe (L) and Philae lander are pictured above the 67P/Churyumov-Gerasimenko comet.
Somewhere on the dark and dusty surface of a distant comet, a little robot should now be taking a long silent look at its novel surroundings.
Meanwhile, back on Earth, euphoria has erupted.
Amid cheers, tears and back slaps, mission controllers at the European Space Operations Centre in Darmstadt, Germany, announced Wednesday that the Philae lander is alive and is talking to them after its one-way trip to Comet Churyumov-Gerasimenko.
The signal confirming touchdown arrived at 11:03 a.m. Eastern Time, about seven hours after Philae detached from its mothership, the Rosetta spacecraft. Both are operated by the European Space Agency (ESA).
"Philae has done its job. We are on the comet!" Stephan Ulamec, manager of the lander portion of the mission, said shortly after the telltale radio bleep was received.
(Read more from Ivan Semeniuk on the dread, hope and excitement of the historic Rosetta mission)
THE TOUCHDOWN: A FIRST IN THE HISTORY OF SPACE FLIGHT
The feat is a first in the history of space flight and it was, in some key respects, significantly more risky than landing a probe on the moon or Mars.
This is because the comet is so small and porous it has barely enough gravity to hold itself together. An ever-present worry during the descent was that Philae would simply bounce off the comet and end up heading back into space.
To prevent this, the lander was supposed to anchor itself by firing two harpoons into the comet, but initial telemetry suggests that the harpoons did not fire.
Because of a glitch that emerged Tuesday night, Philae also did not have the use of a thruster that was supposed to prevent it from recoiling during the final moments of its decent.
Although scientists have long understood that comets are made of a mixture of icy and rocky material, little is known about their detailed physical characteristics.
The data thus far suggest the lander is resting lightly on a surface that has a consistency of soft sand. Dr. Ulamec speculated that the lander may have even lifted up before it settled back down again in a different location.
"Maybe today we didn't just land once, we landed twice," he said.
Philae's radio signal became intermittent shortly after touchdown. Although the signal was always restored, it meant that plans to send back a panoramic image of the surface were disrupted and had to be postponed till Thursday.
THE MISSION: A DECADE-LONG JOURNEY
The successful landing has provided both the emotional highlight and the biggest wild card in the $2-billion Rosetta mission, which embarked on its journey to Churyumov-Gerasimenko more than a decade ago, in March 2004.
On Aug. 6, Rosetta reached its goal and became the first spacecraft to orbit a comet and it soon began scouting out possible landing sites for Philae. The final selection of a site was made in October.
Previous images from orbiter reveal that the landing site lies on a sunlit plain that is mostly smooth and flat. However, it includes enough boulders, steep slopes and other hazards to have kept mission controllers, and everyone else following the event via webcast and social media, on their edges of their seats right to the last minute.
"We're moving in for kiss," project scientist Matt Taylor said shortly before the lander reached the comet.
WHAT HAPPENS NOW THAT PHILAE HAS LANDED?
If the lander proves to be in good condition, it is preprogrammed to undertake a diverse set of science experiments over the next 60 hours that includes drilling into and sampling the comet and hunting for complex organic molecules that could be the ancient precursors to life.
"We could be comet stuff ourselves," said Mark McCaughrean, a senior science adviser with the ESA.
Effectively frozen in time, comets are thought to have formed directly from the cloud of debris that surrounded the newborn sun billions of years ago and eventually gave rise to the planets. The measurements that Philae makes are expected to yield insights into Earth's origins and, more generally, into the process of planet formation through the galaxy.
One of the lander's 10 instruments, known as the Alpha Particle X-Ray Spectometer, was co-built and programmed 15 years ago by Ralf Gellert, now a professor of physics at the University of Guelph.
If all goes according to plan, a robotic arm will place the device near some of the comet's surface material so that it can expose the material to a radioactive source. The way the material behaves will reveal its elemental composition.
"I think this mission will open up a lot of theories about what comets are made of," Prof. Gellert said.
Ivan Semeniuk