Rosetta: The Comet Chaser

Rosetta spacecraft

Melanie Davies reports on her visit to Airbus Defence & Space on the day that a British-built spacecraft delivered a probe to a very alien world.

There are not many space missions that have touched the hearts and minds of people throughout the world as much as Rosetta. Named after the Rosetta Stone – a slab of volcanic basalt responsible for revealing the mysteries of ancient Egyptian hieroglyphics – this intrepid comet chaser has inspired a generation of science explorers, uncovering mysteries bound up in its icy companion – comet 67P/Churyumov-Gerasimenko (67P for short). 

The International Rosetta Mission was approved as far back as November 1993 by the European Space Agency’s (ESA) Science Programme Committee. The initial target for the mission was comet 46P/Wirtanen, but a ten month launch delay meant that a new rendezvous location was needed: the now famous 67P. 

Roger Bonnet with Rosetta model
ESA Director, Roger Bonnet, inspecting an early model of Rosetta in January 1998

Stevenage-based aerospace company, Airbus Defence and Space (formally Astrium UK), was responsible for the design and build of Rosetta, which began construction in 1997, ‘making the impossible possible’. A recent statement from Airbus proudly exclaimed “When the scientific community needs something done that has never been done before, they come to us.”

Although we think of this mission as being bang up-to-date, the technology within the spacecraft is of another era. Even at the time of building, Rosetta’s onboard kit had to be reassuringly tried and tested, using 1980s computer chips to ensure stability throughout its long journey. Designed with a box-type central structure (measuring 2.8 x 2.1 x 2.0 metres) containing its precious payload of 11 scientific instruments plus the Philae lander, it uses thermal balancing to enable Rosetta to endure temperatures between 140 and -270 degrees Centigrade. Outstretched from its central hub are the craft’s enormous wings: 32 meters from tip to tip. These light-absorbing solar panels have a combined area of 64 square metres – enough to capture and process solar energy from a distance of over 800 million kilometres from the Sun, where levels of sunlight are only 4% those on Earth. Rosetta is the first space mission to journey beyond the main asteroid belt relying solely on solar cells for power generation. And when it costs up to £25,000 per kilogram just to get into space, weight is paramount. So from insulation and ducting to solar panels – the choice of materials for the comet chaser was of prime importance, with the final weigh-in at around 3000 kilograms.

Lift off
Rosetta on the launch pad in French Guiana aboard an Airbus Ariane 5 G+ rocket

Rosetta’s epic journey started on 2nd March 2004 when it launched from ESA’s spaceport at Kourou, near the equator in French Guiana, atop an Ariane 5 G+ rocket, also built by Airbus. “Launching from the equator means that less fuel is needed to escape Earth’s gravity” said Izabala Zajac, Graduate Thermal Engineer at Airbus Defence and Space. Rosetta has now travelled about 6.5 billion kilometres, including four gravity-assist slingshots (three around Earth and one around Mars) and two asteroid flybys.

This ultra-ambitious mission has already met with a host of milestones. After a decade-long cosmic voyage, navigating freely by reading the stars, it has chased and caught up with a comet and started a year-long ride around its nucleus; measured the comet’s surface and sub-surface temperatures; studied violent eruptions of gases; and generated images the likes of which have never been seen before.

Philae separates from Rosetta
Philae separates from the mother ship on 12th November 2014

And if that’s not enough for one small spaceship, on 12th November last year, it also ejected a lander into unknown territory. Rosetta at this time was travelling at 18.3 kilometres per second. Philae was now on its own, free-falling the 10 long kilometres to its landing site below – equivalent to throwing luggage out of an aeroplane and expecting it to land in a passenger’s front garden! On this momentous day, ESA’s down-to-earth Rosetta Project Scientist, Matt Taylor, said “Rosetta has something that no other mission has… a wow factor. I live the mission. There’s something about this alien landscape… it just blows your mind!”

A close-up photo of comet 67P
A close-up photo of comet 67P taken by the CIVA instrument onboard Philae

After successful release and three touchdowns, the audacious probe came to rest in a dark cave and set to work. Philae had its own payload of 10 scientific packages including visual and acoustic recording devices (cameras and microphone), a gas analyser, subsurface science sensor (a drill!), and a ‘Cometary Sampling and Composition experiment’. Within the first 64 hours of deployment, the lander was able to capture descent images, record the sound of the first ever touchdown on the surface of a comet, and create panoramic images at ground level; completing its main mission before bedding down into hibernation mode. Because of its final resting place, Philae’s solar panels are unable to capture enough sunlight to re-charge its batteries so the planned long-term experiments will have to wait. Hopefully, when 67P gets closer to the Sun, sufficient light will reach the panels for Philae to awake from its slumber. That is, of course, if it survives potential outgassing (explosive eruptions) as Rosetta’s comet heats up.

The black surface of Rosetta's comet
With a surface as black as coal, Rosetta’s comet is revealing its dark secrets

It may take many months of data analysis before we get results from the experiments conducted by the lander, but Rosetta’s scientific discoveries have already started to unfold, revealing some of 67P’s dark secrets. Probably the most interesting revelation is that water found on this comet is significantly different to that found here on Earth. Water molecules found at the comet contain considerably more ‘heavy hydrogen’ – hydrogen atoms with an extra neutron, known as deuterium. Rosetta expected to confirm a long-held belief that Earth’s abundant water was delivered in part by the bombardment of comets in the young Solar System, after the Earth had cooled and solidified. Kathrin Altwegg, principal investigator for ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) said “Our finding… adds weight to models that place more emphasis on asteroids as the main delivery mechanism for Earth’s oceans.” Matt Taylor added “We knew that Rosetta’s in situ analysis of this comet was always going to throw up surprises for the bigger picture of Solar System science, and this outstanding observation certainly adds fuel to the debate about the origin of Earth’s water. “As Rosetta continues to follow the comet on its orbit around the Sun throughout next year, we’ll be keeping a close watch on how it evolves and behaves, which will give us unique insight into the mysterious world of comets and their contribution to our understanding of the evolution of the Solar System.”

© Melanie Davies 2015

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