Philae so far: Following the Rosetta comet probe’s progress

The European Space Agency’s Rosetta mission achieved a world-first this week, soft-landing its Philae probe on comet 67P/Churyumov-Gerasimenko. Here’s its status so far.

Many watched live yesterday, 12 November, as the Philae probe reached its destination on a comet 510m kilometres away; a landmark moment in a pioneering mission from the European Space Agency (ESA).

The €1bn Rosetta mission began more than 10 years ago when the spacecraft was launched from a spaceport at Kourou, French Guiana in March 2004.

Travelling to the 4.5km-wide comet 67P/Churyumov-Gerasimenko located between 800m and 186m kilometres from the sun, Rosetta required gravity assists, which were provided by planetary flybys of Earth and Mars. Thus, it was a long and circuitous trip.

Wake up, Rosetta

To conserve energy, the Rosetta spacecraft spent 31 months of its epic journey in deep-space hibernation. In January this year, the spacecraft woke from its solar slumber, having returned to a distance within 673m kilometres from the sun, and re-established communication with the ESA.

By July, 5,500km from comet 67P, the spacecraft was beaming back images from its narrow-angle OSIRIS camera. This trio of images captured at two-hour intervals revealed surface details of the comet, which is about the size of Mont Blanc and has a smaller ‘head’ connected to a larger ‘body’ by a ‘neck’ – a shape that presented a challenge to the scientists back on Earth.

^{Rotating view of comet 67P/C-G on 14 July 2014. Image via ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA}

^{Comet 67P/C-G on 20 July 2014. Image via ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA}

Agilkia marks the spot for Philae

In August, Rosetta reached a position close enough to begin mapping the surface of the comet in search of a suitable landing spot. By September it was determined that the head of the comet provided the best option, at a site with few hazards and a enough daily light to recharge the solar-powered Philae.

^{Philae’s primary landing site on Site J on comet 67P/Churyumov–Gerasimenko. Image via ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM}

With a site selected – and newly named ‘Agilkia’ courtesy of a public competition – Philae’s launch date was set for 12 November.

In October, the CIVA camera on the Rosetta orbiter’s Philae lander snapped a ‘selfie’ of the side of the Rosetta spacecraft and one of Rosetta’s 14m-long solar wings, with comet 67P/Churyumov–Gerasimenko about 50km away in the background.

^{Rosetta’s selfie. Image via ESA/Rosetta/Philae/CIVA}

What’s next for Philae?

Finally, after journeying for a cumulative distance of 6.4bn kilometres, the day arrived for Philae’s big moment.

At 9.03am (UTC) on 12 November, the ESA team received the signal telling them that the Philae lander had successfully detached from Rosetta, 22.5km from comet 67P. A tense seven-hour wait followed until, at 4.03pm (UTC), Philae’s touchdown was confirmed.

The plan was to use harpoons to anchor the 100kg probe to the low-gravity comet, but there was a problem with the small thruster designed to counteract the recoil of the harpoons and the conditions of landing, along with the exact location of Philae on the comet, are currently being analysed.

^{Rosetta’s OSIRIS narrow-angle camera captures a parting shot of the Philae lander after separation. Image via ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA}

Philae’s primary science mission post-landing is expected to deliver a full panoramic view of the landing site, including three-dimensional high-resolution images of the surface immediately below it. Philae will also conduct on-the-spot analysis of the composition of comet 67P’s surface materials, taking samples from a depth of 23cm.

The lander will also measure the electrical and mechanical characteristics of the surface, and low-frequency radio signals will be beamed between Philae and the Rosetta orbiter through the nucleus to probe the internal structure.

Meanwhile, Rosetta will shift back into an orbit around the comet, at greater distances as needed, and coming in for flybys as close as 8km from the comet centre on occasion.

^{The ROLIS instrument aboard the Philae lander captures comet 67P just 3km from the surface. Image via ESA/Rosetta/Philae/ROLIS/DLR}

What’s the point of it all?

Philae’s mission is to escort comet 67P around the sun up to December 2015, collecting scientific data from the comet nucleus. Critical to this mission is an on-board processor unit built by Space Technology Ireland, which will play a key role in passing streams of commands and data between the Rosetta spacecraft and the instruments on the Philae lander.

^{Rosetta captured by Philae’s CIVA-P imaging system shortly after separation. Image via ESA/Rosetta/Philae/CIVA}

Like all comets, comet 67P is home to a 4.6bn-year record of the solar system and this is the first time a robotic system has been deployed to take direct measurements of these properties. This information – it’s hoped – will provide important insights into the formation of our solar system.

“It’s been an extremely long and hard journey to reach today’s once-in-a-lifetime event, but it was absolutely worthwhile,” said ESA Rosetta mission manager Fred Jansen yesterday.

“We look forward to the continued success of the great scientific endeavour that is the Rosetta mission as it promises to revolutionise our understanding of comets.”