Rosetta: ‘At the limit of what humankind can do’

The perils and triumphs of the audacious mission to land a probe on a comet.

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Part of Rosetta’s lander Philae is visible in this image it took on the surface of Comet 67P/Churyumov-Gerasimenko.

The European Space Agency claimed a new benchmark of human achievement last week with the historic landing of the probe Philae on the surface of Comet 67P/Churyumov–Gerasimenko, currently some 512 million kilometres from Earth and speeding through space at 135,000 kilometres per hour.

It was the culmination of two decades’ preparation – the mission cost $1.3 billion and saw Philae’s mothership Rosetta travel six billion kilometres over 10 years to chase down the comet.

As we reported in our preview of the mission earlier this year, Rosetta is the latest in a series of space missions that have transformed our understanding of comets.

Philae has now completed its main science mission and, with power drained, has gone into hibernation. But ESA scientists are overjoyed about the data they have received and the insights they are likely to get about the comet’s make-up and behaviour.

Philae’s planned landing site, covering about one square kilometre, is located close to the top of this image taken with Rosetta’s navigation camera on 6 November. – ESA/Rosetta/NavCam
A close-up of the comet on Philae's first approach to the surface. A failure of its booster meant it bounced more than a kilometre off the surface when it first came down on the comet. – ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

From past studies and fly-bys we’ve had glimpses of what comets are made of. We know they carry amino acids and other organic chemicals. Now scientists are keen to test the hypothesis that they may have seeded our world with the essential ingredients for life, including Earth’s abundant water supply. This is the question the Rosetta mission was designed to answer.

While Rosetta has already collected much useful data, the lander mission nearly came undone when hitches occurred during Philae’s touchdown procedure.

Philae’s parting shot of its mothership Rosetta as it began its descent. – ESA/Rosetta/Philae/CIVA
Philae as seen from Rosetta after separation. – ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Two malfunctions – one with the thruster on top of the probe, designed to push Philae onto the low-gravity comet, and one with the harpoons designed to anchor the craft to the surface – caused the craft to bounce off the surface twice.

The first bounce catapulted Philae more than a kilometre above the surface – it took two hours for it to float back down. It then bounced a second time before coming to rest a few minutes later.

It landed for the final time on its side with two legs on the surface and one pointing upwards, lodged against a cliff face. The harpoons for that leg did not fire.

One of the first images from Philae, with an illustration of the landing probe superimposed to indicate the angle at which it is lodged against a cliff. – ESA/Rosetta/Philae/CIVA

This created some difficult decisions for the scientists. The cliff face blocked much of the sunlight from Philae, which needs the energy to charge its batteries if it is to carry out an extended scientific mission. The original landing site ensured Philae received six or seven hours of sunlight each 24 hours, now reduced to just 1.5 hours every 24.

Last Friday, a successful last-ditch attempt to nudge it into better sunlight apparently came too late to charge its batteries and keep systems up and running.

Philae’s primary mission was always designed to last around 60 hours on its initial battery charge (in fact it lasted 57). The solar panels covering the Philae were designed to charge a set of secondary batteries and extend the mission for months. While Rosetta will continue to listen for messages from Philae the ESA says the low recharge current coming from the solar panels means it is unlikely that contact will be re-established with the lander in the near future.

Nevertheless, Stephan Ulamec, Philae lander manager, estimates that 80-90% of the science ESA was hoping for has been achieved. And the challenges have failed to dampen the enthusiasm of the ESA’s lead lander scientist Jean-Pierre Bibring who told reporters Philae was “in a gorgeous place”.

Nor has it meant we are short of data already relayed back to Earth from Philae – British scientists using the lander’s “Ptolemy” gas analysis instrument say they are particularly pleased with results already.

“It’s amazing where we are,” Bibring said. “We are at the limit of what humankind can do.”

The planned manoeuvres of Rosetta after Philae's descent. Philae must relay data to Rosetta for transmission to Earth. It can only do so when Rosetta is within range. – ESA

The following gallery shows images of the comet surface taken by Rosetta as the craft manoeuvred into position for Philae’s descent. They were taken from 10 kilometres above the surface of the comet.


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Bill Condie is head of publishing at The Royal Institution of Australia and former publisher of Cosmos.
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