Drama in space: But Philae keeps trying to harpoon comet

The ESA's comet-chaser spacecraft Rosetta will send its lander Philae towards Comet 67P on Wednesday – after some scrambling by boffins here in Germany over the past few hours to get the probe ready for its historic landing attempt.

The comet is hurtling through space at 135,000km per hour towards the Sun, and the ESA hopes Philae will be able to separate from Rosetta, get close enough to harpoon the space rock and draw itself in to land, an effort that will take about seven hours to complete. Once on the rock, the lander will drill into the surface.

Rosetta and Philae are programmed with a schedule of commands to execute to prep for and then perform the separation and landing. Before each prep stage, scientists overseeing the mission at the European Space Operations Centre (ESOC) had to check trajectories and other readings to make sure everything was on course – if it wasn't, an abort signal would have been sent, postponing the landing for at least two weeks.

Which was why it was so nerve-wracking when the third go-ahead was delayed last night. The project manager for Philae, Stephan Ulamec, said the decision to let the systems go ahead stalled due to a problem with the lander’s cold gas system. That means Philae's thruster is out of action, leaving it up to its harpoons and ice-screw legs to ensure it stays on the comet as it lands.

The thruster was designed to counteract the recoil from firing the harpoon, but it'll have to make do without, it seems.

Technicians earlier confirmed Rosetta was on the right trajectory and at the right velocity for Philae to separate successfully towards the comet.

“There were various problems in the preparation activities for the lander. Eventually the decision was slightly later, but the decision was made to go anyway,” Paolo Ferri, ESA head of mission operations, said this morning in Darmstadt.

After the third go-ahead, Rosetta needed to alter its trajectory to prep for separation, which went off without a hitch. Now the lander and Rosetta are definitely on for their attempt today.

ESA hopes to get confirmation the Philae lander separated from Rosetta later this morning, around 9am GMT. Seven hours later, the agency will know whether or not Philae has managed to set down on the rocky Comet 67P. ®