On 12th November, the world held its breath as scientists and engineers tried something that hadn’t been done before – a controlled ‘touch down’ on a comet. The Rosetta mission had already seen a lot of ‘firsts’; Rosetta is the first spacecraft to orbit a comet’s nucleus, and it’s the first spacecraft to fly alongside a comet as it heads towards the inner solar system. Rosetta is the first mission able to closely examine how a comet is transformed by the heat of the Sun.
But landing on a comet? That was a tricky endeavour. There were a few nail-biting hours on the day, before we knew that the lander, Philae, had touched down on the surface.
Philae's descent and science on the surface
It wasn’t until Rosetta rendezvoused with comet 67-P (or ‘Chury’) that scientists could get a good look at any potential landing sites. The shape of the comet was unexpected – it has two lobes and is shaped a little bit like a rubber duck (although it’s dark and sooty and too big to fit in the bath tub!). As Rosetta orbited the comet, moving slowly closer, the race was on to narrow down the most suitable landing site before Chury got too close to the Sun. As the comet approaches the Sun, it is warming up and becoming more active, making Philae’s mission very tough indeed.
Examination of the data led the mission scientists to choose landing site J as the best possible option. Site J was renamed ‘Agilkia’ by public vote, after an island in the River Nile. Agilkia is located on the smaller of Chury’s two ‘lobes’, and was considered to pose the minimum risk to the lander, whilst still offering the opportunity to conduct some unique science experiments. The area also gets enough light to recharge Philae’s batteries via the solar panels, extending the life of the lander and allowing more results to be collected.
Despite the detailed planning, there were risks to the mission. Jubilation as the comet touched down in the chosen spot (what an achievement!) turned to concern as the mechanisms designed to anchor Philae to the comet didn’t work, and it bounced back up. And down again. And up again. And down again.
Philae eventually settled into a new and unknown spot, hidden in the shadows on the comet. With a primary battery life of 64 hours, it successfully started its science experiments and uplinked with Rosetta. But then there just wasn’t enough sunlight arriving at the solar panel to keep the batteries charged up, and Philae went to sleep.
We all hoped that, as Chury got closer to the Sun, Philae’s landing spot would be better lit and the little spacecraft would wake up to continue its experiments. And it has! On 14th June 2015 ESA announced that Philae has woken up after 7 months of hibernation, is doing well and has been communicating via Rosetta – what a testament to the quality of the engineering. It will take some time to find out whether Philae is getting enough sunlight to power all of the experiments, but it already has new data to send.
Philae has 10 science instrument packages that will help us learn more about comets:
APXS, the Alpha Proton X-ray Spectrometer, will be examining the chemical composition of the landing site, and (hopefully) how that changes as Chury gets closer to the Sun.
CIVA will use six identical micro-cameras to take panoramic pictures of the comet’s surface, and has a spectrometer to study the composition, texture and reflectivity (albedo) of collected samples.
CONSERT is the COmet Nucleus Sounding Experiment by Radio-wave Transmission, a complex experiment that uses instruments on both Philae and Rosetta to set up a radio link passing through the comet’s nucleus. The aim is to reveal the internal structure of a comet for the first time.
COSAC, the COmetary SAmpling and Composition experiment, is an evolved gas analyser specialised for the detection and identification of complex organic molecules.
RAL Space led the engineering team that built the Evolved Gas Analyser on PTOLEMY – an instrument that will help us to understand the geochemistry of light elements, such as hydrogen, carbon, nitrogen and oxygen, by determining their nature, distribution and stable isotopic compositions. It will work with samples passed to it by the SD2 sample and distribution device, which can obtain samples of the comet down to a depth of 250 mm.
MUPUS, the MUlti-PUrpose Sensor for Surface and Subsurface Science, will be investigating the properties of the surface layers of the comet.
The ROLIS camera will be taking close-ups of Agilkia during Philae’s descent, and after the landing will be taking high-resolution images of the structure (morphology) and mineralogy of the comet surface.
ROMAP is the Rosetta lander Magnetometer and Plasma Monitor, a multi-sensor experiment measuring the comet’s magnetic field, ions and electrons, and the local pressure.
The results from SESAME (the Surface Electric Sounding and Acoustic Monitoring Experiment) will add to our understanding of how comets formed and how the solar system (including Earth) was born.
So we’re in for an exciting summer. Philae’s awake and ready for new experiments, and the Rosetta mission continues, with the orbiter spacecraft accompanying comet Chury on its journey towards the Sun, and then out of the inner solar system once more. The people of Earth will be watching every step of the way, as this historic mission takes us on a journey to discover the origins of life itself. Watch for yourself via the ESA Rosetta website, or keep track of Philae’s progress via its own Twitter feed.