Comet 67P/C-G on 30 October 2014 – OSIRIS
(Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA.)
On 12th November 2014, ESA received confirmation that Rosetta’s Philae lander had successfully landed on comet 67P/ Churyumov-Gerasimenko (often referred to simply as 67P, CG or Chury). The beginning of Philae’s up-close-and-personal science projects on the comet is the latest important milestone in Rosetta’s 12 year, deep space journey.
Launched in March 2004, Rosetta has travelled around the Sun five times, picking up gravitational boosts from the Earth and Mars along the way. It has had the chance for fly-bys of two asteroids, taking images of Steins in 2008 and Lutetia in July 2010. But as Rosetta flew further from the Sun its solar panels produced less and less energy, and the spacecraft was put into hibernation.
At 10 am GMT on 20th January 2014, ESA’s version of sleeping beauty was roused from its slumber - not by a handsome prince, but by an internal alarm clock. Rosetta had been hibernating since July 2011, as it travelled close to the orbit of Jupiter, 5.35 AU (800 million kilometres) from the Sun. Now on its return journey into the solar system, Rosetta awoke to continue its mission to rendezvous with comet 67-P.
Once Rosetta woke up, its first job was to warm up the star trackers and find a recognisable star field so that it could point its solar panels towards the Sun and its high gain antenna towards the Earth. The spacecraft then sent a signal home, which had to travel for 45 minutes before it got to Earth. Since then, Rosetta has been taking images of Chury. As the spacecraft got ever closer to the comet, the images became more detailed, and we watched as Chury changed from a dot, to a blob, to a double comet with a deep neck. On 6th August 2014, ESA received confirmation that the Rosetta spacecraft had caught up with its target.
The mission is named after the Rosetta stone, a historical artefact that allowed us to decipher Egyptian hieroglyphics. It’s hoped that through Rosetta we will learn more about the birth of the solar system, as comets were formed at the same time and have been locked away like frozen time capsules ever since. We might find a definitive answer to the question of whether Earth’s water was originally delivered by comets. We might even discover the origins of life itself.
The Rosetta mission is particularly exciting because it is the first to attempt to land on a comet. On 12th November, whilst in orbit around Chury, Rosetta deployed a lander, Philae, equipped with screws and harpoons with which to attach itself to the surface of the comet., Chury doesn’t have enough gravity to keep the lander in position, and as comet 67P gets closer to the Sun, its activity will increase – giving Philae a rough ride as jets of material are ejected from the nucleus to form the comet’s tail (although even at its brightest, the comet won’t be visible to the naked eye). The same lack of gravity means that Rosetta is going to do some tricky manoeuvres to stay in a close orbit around the comet, with the ever-present risk of being damaged by debris.
Unlike comet ISON, 67P is a short-period comet, with an orbital period of less than 200 years. These comets are often called Jupiter Family comets, as their orbits are controlled by Jupiter’s gravity, and they are believed to originate in the Kuiper Belt – a large reservoir of small, icy bodies just beyond Neptune.
Before 1840, the closest 67P/Churyumov-Gerasimenko came to the Sun was 4 AU, too far away for the Sun to heat the nucleus and for a tail to develop, and so the comet was not visible from Earth. In 1840 a close encounter with Jupiter changed the orbit of the comet so that its perihelion distance was 3 AU, gradually decreasing to 2.77 AU over the next hundred years. In 1959 another encounter with Jupiter reduced the comet’s perihelion distance again – to 1.29 AU – and it currently orbits the Sun every 6.45 years.
Philae’s instruments include a thermometer designed to be hammered into the comet, and a gas analyser (the Ptolemy instrument designed and built by RAL Space and the Open University) with an on-board oven for heating comet samples. Philae is able to do high-res microscopy, took images during its descent to the comet surface, and a stereo panorama of the surface. Imperial College provided the Plasma Interface Unit (PIU) that controls five instruments taking measurements of the plasma environment around the comet. Rosetta acts as a data relay for Philae, and the two spacecraft working in tandem are able to do tomography.
Ian Wright, Principal Investigator of the Ptolemy instrument said:
“Many years ago a forerunner of STFC (PPARC, the Particle Physics and Astronomy Research Council) took the bold step of investing in an instrument that would be included on the ‘lander element’ of the Rosetta space mission. Whilst the scientific goals stood on their own merits it was nonetheless impossible to comprehend the magnitude of the challenge since many aspects of the venture were simply unknowable. It was a courageous decision. Well, here we are today having placed a small piece of UK ingenuity onto the surface of a comet. And as we begin our scientific investigations we can reflect on some innovative technological developments that were necessary, along with the highly skilled human capital that the project produced. These now contribute to the UK economy in ways that in some cases were completely unexpected; long may research councils continue to back ‘blue skies’ science.”
After successfully fulfilling its primary mission to investigate the surface of Chury, Philae’s battery has run down. The lander is currently in shadow and is not receiving enough solar power to conduct any more science experiments or communicate with Rosetta. However, the mission scientists were able to rotate the lander into a slightly better position, and there is some hope that it will receive more light and be able to do more work as the comet gets closer to the Sun. For the moment, Philae is dormant, but Rosetta should be able to follow 67P/Churyumov-Gerasimenko until the end of 2015, and possibly beyond. One of the best ways to follow the mission’s progress is via Twitter. Rosetta and Philae each has its own feed.