In November, the European Space Agency’s hugely ambitious Rosetta spacecraft will set its lander down on the head of Comet 67P/Churyumov Gerasimenko, it was announced in ESA’s conference on 15th September. Five possible landing sites had been earlier chosen from close up images taken by the spacecraft, currently orbiting the 4km wide comet 450 million km away between the orbits of Mars and Jupiter. A fine balance between the most exciting science finding possibilities and safety of the spacecraft was needed in deciding a site from the shortlist of 5 possible landing sites (A, B, C, I & J).
And ‘J’ was the winner.
This is a region on the smallest of the two lobes that make up the rubber duck shaped comet which offers the safest descent and terrain for its risky landing. Cameras and thermal sensors have been mapping the comet since it got to within 100km of the surface (or nucleus) on 6th August to determine the most scientifically valuable sites and the thermal properties of the entire surface. The thermal properties are so crucial to ensuring the survival of the Philae lander, as the 100kg probe, packed with science experiments, needs a reasonably level landing site with enough sunlight for its solar panels to provide power – but not so much continual sunlight that the probe overheats as it makes its way to nearly as close to the sun as Earth’s orbit.
The ESA project team have referred to site J as the least worst of all the sites because of this balance that needs to be struck. The project engineers want the safest landing site to enure the survival of their baby, whereas the project scientists want the most scientifically exciting landing site to gather the most interesting data and findings. The compromise is the 'least worst site' from both team's perspective, site J. But don't let this label fool you into thinking that the science findings from the Philae lander is somehow therefore going to be compromised or diminished. Honestly, it isn't!
Images from the Rosetta orbiter 30km above the surface, show this region is quite active. Even beyond the orbit of Mars, the comet is already heating up and venting trapped gasses into space – but nowhere near as much as it is expected to vent and sublimate (the process of ice turning straight into gas when it’s heated up in space) as it gets nearer to the sun in the coming months. While site ‘C’ has been selected as the back up landing site in the event of any issues that might scupper landing in site ‘J’ – such as too much outgassing between now and November - site ‘J’ has a relatively smoother surface than much of the comet, making it much safer to land upon without losing its scientific interest. “We will make the first ever in situ analysis of a comet at this site, giving us an unparalleled insight into the composition, structure and evolution of a comet”, said ESA’s lead lander scientist, Jean-Pierre Bibring, “Site J in particular offers us the chance to analyse pristine material, characterise the properties of the nucleus, and study the processes that drive its activity.”
Rosetta began at launch in 2004 and was already 10 years into that mission before it even reached its target in August this year. Waking the spacecraft up from hibernation in January and arriving in a close orbit around the comet in August were both anxious moments for the Rosetta team but all systems are currently working as planned, with incredible images being uploaded daily to the ESA website.
At 1.3 billion Euros (£1bn / $1.7bn), the cost of this mission makes it one of ESA’s most expensive and ambitious. ESA launched a spacecraft to fly through the tail of Comet Halley in 1986 to image it close up and analyse the material that was streaming away. It was able to accurately measure its size at 15km long and its composition of water ice covered in thick dust and carbon, while containing carbon dioxide, methane and ammonia. Giotto also showed that Comet Halley was made of the oldest material from the very birth of the solar system.
Rosetta hopes to build upon this knowledge by analysing Comet Churyumov Gerasimenko (pronounced: Churry-you-mov Gerr-ass-ee-men-ko) from orbit with a suite of eleven science instruments and a further ten on the Philae lander for in situ experiments and measurements as the comet begins to heat up and boil away its outer layers to create a large thin atmosphere (coma) and tail behind it in the glare of the sun. This will allow us to see the first ever timeline of a comet going from a relatively stable ‘dirty snowball’ to an active body being heated as it gets nearer to the sun. As the body of the comet (or nucleus) heats up, the ice on the surface turns into a gas (in the vacuum of space, ice cannot exist in a liquid state) and it loses 1/1000th of its mass each encounter – or at least Comet Halley does each orbit. But every comet is different.
For instance, about 1 in 6 of all characterised asteroids are actually two asteroids bound together by gravity – ‘contact binaries’, as they are called. As Rosetta approached Comet 67P/Churyumov Gerasimenko, it looked like this comet was also a contact binary. The rubber duck moniker was used to describe this object consisting of a large and a smaller lobe in contact with one another. However, while the mission scientists aren’t yet able to say with absolute certainty, Rosetta Project Scientist, Matt Taylor, told me that it may not actually be two comets in an elegant cosmic dance. The lobes were more likely carved out of the same chuck of ice and dust by eruptions of gas caused by heating on subsequent approaches to the sun. So perhaps this comet is not so much a rubber duck as a bath bomb. This bath bomb analogy is strengthed by images taken on 26th September (see below) that show the comet venting its gas & dust more violently in the region between the two lobes. This gives more evidence for the comet having once been a larger chunk of ice & dust whose middle has eroded away in the sun's glare, leaving this distorted hourglass shape that we see now.
By sending the Philae lander to the surface (to see how this will happen, watch this ESA video), we may be able to answer this question as we learn more about the composition and density, the mass-loss as it heats up, the effects of the stream of particles leaving the sun and much much more. But most exciting for the public, Rosetta and Philae should tell us more about one of the biggest questions in planetary science. Were water and life brough to Earth by comets that collided with our planet billions of years go?
Philae should be able to tell us whether the ratio of hydrogen to deuterium in the water ice on the comet matches Earth water’s hydrogen/deuterium ratio (deuterium is a hydrogen atom with a neutron. Common hydrogen has no neutrons). It should also help answer the question about organics (the carbon molecules that are needed for the RNA and DNA that make up you and me). It will analyse the organic molecules that have already been detected in abundance on the surface of the comet from orbit to see if they have a similar signature to the organic molecules that make up life on Earth.
Each orbit around the sun strips another layer off the surface of comets to constantly reveal the fresh contents beneath, giving us a look at the most primitive constituents in the solar system. These water and carbon dioxide ices, dust and organic molecules survived – or in some cases were created by - the sun exploding into life 4.6 billion years ago, and provide us with a pristine time capsule; a historical window into the earliest days of the solar system.
Whichever site the lander does finally touch down upon, after its anticipated landing date of 12th November, using the following instruments on the surface of the comet, Philae will surf the comet around the sun taking constant readings and measurements to give us a richer picture of the birth of our solar system:
ROLIS (Rosetta Lander Imaging System) will take images during descent to the surface, and close up images while on the surface.
ÇIVA - Six identical micro-cameras to take panoramic pictures of the surface. A spectrometer studies the composition, texture and albedo (light reflectivity) of samples collected from the surface.
SD2 (Sample and Distribution Device) drills more than 20 cm into the surface, collects samples and delivers them to different ovens or for microscope inspection.
COSAC (Cometary Sampling and Composition experiment) a gas analyser that detects and identifies complex organic molecules from their elemental and molecular composition.
PTOLEMY, another gas analyser, which obtains accurate measurements of isotopic ratios of light elements including the amino acid building blocks of life.
MUPUS (Multi-Purpose Sensors for Surface and Subsurface Science) uses sensors on the Lander's anchor, probe and exterior to measure the density, thermal and mechanical properties of the comet’s surface.
APXS (Alpha X-ray Spectrometer) will be lowered to within 4 cm of the ground, it detects alpha particles and X-rays, which provide information on the elemental composition of the comet's surface.
SESAME (Surface Electrical Sounding and Acoustic Monitoring Experiments) is three instruments to measure the properties of the comet's outer layers. 1) The Cometary Acoustic Sounding Surface Experiment (CASSE) measures the way in which sound travels through the surface. 2) The Permittivity Probe (PP) investigates its electrical characteristics, and 3) the Dust Impact Monitor (DIM) measures dust falling back to the surface
ROMAP (Rosetta Lander Magnetometer and Plasma Monitor) is a magnetometer and plasma monitor to study the comet’s magnetic field and how the comet interacts with the solar-wind.
CONSERT (Comet Nucleus Sounding Experiment by Radiowave Transmission) will probe the internal structure of the nucleus. Radio waves from the CONSERT experiment on the orbiter travel through the nucleus and are returned by a transponder on the lander to tell us what the interior and core are made of – similar to how we reveal the internal structure of the crust, mantle and core of the Earth without being able to drill far enough into our planet.
So we wait with fingers crossed that the instruments continue to work as well as they have, while we enjoy the daily image updates and news briefings from ESA on this mission into the very birth of the solar system. This time next year we may have answers to many of the questions we’ve asked here. But while we wait for Philae to land with baited breath, we can let the excitable enthusiasm of Rosetta Project Scientist, Matt Taylor, ring in our ears, “Stay tuned because it’s happening now! We’re trying our best to get out there and let everyone know what’s happening and we’re on the edge of being able to do that in terms of some solid results. So, really, stay tuned. We really appreciate people’s excitement and their engagement in this. Stay with us this ride’s gonna be great!”