Rosetta space probe poised for comet landing

It has been described as "ridiculously difficult" and has taken more than 20 years of preparation, but on Wednesday the scientists behind an ambitious $1.5bn European space mission hope to make the first-ever landing of a spacecraft on a comet. 

Launched in 2004, the European Space Agency’s Rosetta spacecraft has travelled 4 billion miles in pursuit of the duck-shaped comet 67P Churyumov/Gerasimenko.

The comet is travelling through space at more than 40,000 miles an hour, about 40 times the speed of a bullet. To catch up with it, Rosetta used the gravity of Earth and Mars to effectively slingshot itself into the comet’s orbit.

Comets — celestial bodies of dust, ice and complex molecules — are remnants of the birth of our 4.6 billion-year-old solar system. The probe's mission is to gather samples from the comet to analyze the development of Earth and other planets.

"It's basically a time machine containing clues to the composition of the solar system," Fred Jansen, Rosetta mission manager, said in a briefing ahead of the landing attempt.

Matt Taylor, a European Space Agency (ESA) scientist called the project "a big deal, enough to be the sexiest mission ever," in a CNN panel discussion of the mission.

The ESA announced early Wednesday that the active descent system, which uses thrust to prevent the craft from bouncing off the surface, could not be activated. Instead, the agency is relying on ice screws and a harpoon system to secure the lander.

"The cold gas thruster on top of the lander does not appear to be working so we will have to rely fully on the harpoons at touchdown," said Stephan Ulamec, Philae lander manager at the DLR German Aerospace Center. "We'll need some luck not to land on a boulder or a steep slope."

Soon after, the unmanned probe successfully released the lander, putting it on a seven-hour journey to rendevous with the comet. Mission controllers clapped and embraced as the lander's separation was confirmed.

"Philae has gone — it's on its path down to the comet," Rosetta flight director Andrea Accomazzo said. "We are all glad that it worked flawlessly in the past minutes."

It started its close approach to the 2.5 mile-wide comet in August. Since then it has been taking pictures and sampling the dust and gases emitted from the comet. This information has allowed scientists to decide where to attempt to land the smaller, three-legged lander called Philae.

The descent towards the surface is scheduled to begin early Wednesday morning and is projected to take seven hours. Throughout the descent, Philae will send back information on the dust and gases it encounters as it nears the comet.

The timing, angle and speed of the release are crucial because, once on its way, there is nothing the scientists on the ground can do to change the trajectory of the box-shaped 220.5-pound lander.

The ESA team chose a flat, boulder-free area, with good sunlight, which is needed to power Philae’s solar panels. The location was also visible from Rosetta, which the team plans to keep in orbit around the comet and which operates as a relay station for communications with the lander.

But 67P has very little gravity. This means landing Philae from 13.6 miles above, with little control, and keeping the probe on its surface requires effort.

But smooth is relative. "We have to be a bit lucky," Accomazzo said. "If the lander lands in proximity of a boulder, there is nothing we can do. This is the part that worries me the most because we have no control over it."

"There is a risk that the Philae lander will bounce back," said Francisco Diego, Senior Research Fellow at the Department of Physics and Astronomy University College London. "So the three legs have shock absorbers, and instantly, as it touches down, [it] deploys a harpoon with the tether, that make sure the lander remains attached to the surface of the comet."

Philae is carrying a drill, which can take samples as deep as 14 miles below the surface, spectrometers to analyze chemical composition of its findings and other instruments to examine the strength, density, texture, porosity and thermal properties of the comet.

"We know that the surface of the comet is covered by interplanetary dust," said Diego. "But inside we have the pristine material from which the solar system was formed."

The rocks that form 67P are believed to pre-date the formation of our solar system, which makes them catnip to scientists. Scientists are also eager to examine the comet to see what whether the water it carries is similar to that found on Earth. They have suggested much of the water on Earth came from comets and with it came complex organic molecules, including amino acids, the building blocks of proteins.

"When comets come past the Earth we are always interested in taking a look at the gases that are coming out of them and the chemistry of them — a very primitive primordial chemistry," said Chris Riley, of the University of Lincoln. "In this case we are going to get really close and even sample a comet for the first time and do the measurements in situ. So it’s a trip back in time, back four and a half billion years to the birth of the solar system."

Al Jazeera and wire services