Out of this world

Out of this world

Philae phones home – but the mission is about to get riskier

Philae is awake again but a cometary tail threatens its future. ESA

I had planned to spend the day with my mother – nothing special, just a quiet Sunday with family. The plan lasted until midday, when the announcement that the comet lander Philae was finally awake made the twittersphere explode with excitement. Wonderful news – but it spelled the end of my day of rest. As I write this it’s midnight here in Milton Keynes and I’m putting the events into perspective.

On November 12 last year, the Philae spacecraft was released from its Rosetta mothership, travelling for seven hours to the surface of comet 67P/Churyumov-Gerasiemko to carry out a series of observations and analyses. The landing was not straightforward, and failure of the harpoons designed to anchor Philae did not work as planned.

The spacecraft bounced back into space, eventually settling several tens (or possibly hundreds) of metres away from its designated landing site. Unfortunately, the final landing site was in shadow and Philae could not use its solar panels to charge its second set of batteries. After around 60 hours of experiments, the primary battery was exhausted and Philae and its instruments went into hibernation.

Philae as it separated from Rosetta and headed for the comet. ESA

But the scientists and engineers who had worked so hard to build and operate Rosetta and Philae did not despair – Plan B was put into action. As the comet approached the sun, they anticipated that the sunlight falling on the solar panels would be sufficient to charge up the batteries. So Rosetta started a listening campaign, waiting to hear from its erstwhile flight companion. And just before midnight on June 13 – more than 7 months since Philae separated from Rosetta – ESA received a signal. Philae had phoned home!

Troublesome tail

Keeping the connection alive, however, might not be entirely straightforward. Philae has been in hibernation, in the cold, at temperatures well below the -45°C limit for the electronics to work. The first priority has been to warm Philae up – and news from ESA suggests that this has been successful.

Onboard data imply that Philae might have been awake for a couple of days, but unable to communicate with Rosetta until systems were warmer. The solar panels now seem to be generating power but ESA has to redesign Rosetta’s orbit so that Philae and Rosetta can be in better contact.

However, this is more complicated than it was last November, because the comet is much more active. As it moves closer to the Sun, jets and plumes of dust and gas are escaping from the surface – producing a cometary tail. Unfortunately this confuses Rosetta’s pointing device. If care is not taken, Rosetta might lose contact with mission control, which would almost certainly be the end of the mission. A regular Rosetta-Philae communications connection must be re-established, which will allow reliable transmission of data when Rosetta is above Philae’s horizon.

The Philae instrument teams have not been idle over the past seven months. They have had data to reduce and interpret, papers to write and lectures to give. They have also – as optimists with faith in the hardiness of Philae – been planning experiments for when Philae wakes up. Now that day has arrived, and they must return to the fray, ready for a much longer run of observations and analyses.

Over the next two months, Rosetta will fly even nearer to the sun, reaching its closest approach in mid-August. As the nucleus of the comet becomes more active, Philae’s observations and analyses will become more exciting, as fresh parts of the comet are revealed. But the mission also becomes more perilous, increasing surface activity could damage Philae. A worst-case scenario is that a jet blows the spacecraft off the surface and back into space.

Philae has woken up, and has work to do. I wish the instrument teams good luck and will keep my fingers crossed that Philae achieves its experimental goals. I also hope that next weekend is much quieter than this one!