The team behind ESA’s groundbreaking Rosetta mission to a comet has selected five possible spots where the Philae lander could touch down in November. Rosetta has now been flying alongside comet 67P/Churyumov-Gerasimenko since 6th August, and has spent that time scanning and mapping the strange dusty surface of this alien world in the search for a good place to land.
Philae can’t just drop down anywhere it likes. The photos of the comet taken by Rosetta show many craters, rocks and ravines on the 4km-wide comet that could prove lethal to the little lander, a galactic tiddler about a metre across and with a mass of 100 kilogrammes. What’s more, the site has to be somewhere the lander won’t get too hot, won’t get too cold, with enough sunlight to power it and not too much to overheat it.
The landing site also has to be considered scientifically interesting – although arguably just about anywhere on comet 67P is scientifically interesting, given that this is the first time anyone has ever dared to try to get down and touch a real-life, space-cruising, water-carrying, secrets-of-the-universe-bearing bundle of cometary space rubble.
So, the five sites vying for Rosetta’s pre-Christmas No.1 hit are:
Site A is an interesting region located on the larger lobe, or body of the rubber duck-shaped comet, benefiting from what estate agents would call ‘outstanding views’ of the smaller lobe. The terrain between the two lobes is likely the source of outgassing, which means it could be really interesting to watch what comes flying off this area of the comet as it heats up towards the sun later this year and into 2015. The downside of Site A is that there could be small depressions and slopes that would pose a problem, and the amount of light it receives needs to be better studied.
Site B, down in the crater-like area on the smaller lobe, is one of those spots that just about every Earth-bound comet landing pundit would pick as a fairly safe bet. It has flat terrain, and is thus considered relatively safe for landing, but it could end up being too well lit, meaning poor Philae may get too sunburnt to last long on the surface. There are also visible boulders down there, which could prove a hazard.
Site C is a lovely little spot on the larger lobe with a nice mix of features like cliffs, hills and smooth plains, and a good, but not overly good supply of sunlight. However, it really does need to be studied in more detail if it’s to emerge as a favourite, because there may well be hidden risks that we haven’t yet spotted.
Site I is a bit of a mixed bag, as it looks a bit rough in some places, but there are also other patches on this part of the smaller lobe that are smooth and rather more welcoming looking. This part of the comet could contain material from within that has recently come up to the surface, so that would make it very interesting to scientists.
Site J is quite a lot like site I – a bit hit and miss but worthy of further inspection. It would make a good spot for communication with Rosetta, but there are still areas of terracing and boulders that could catch out the unwary space explorer. Again, plenty of light and what the estate agent would call a ‘nice aspect’.
The Rosetta team are working at top speed to tie down where Philae should land, because they’re worried that if they don’t manage to get the probe onto the surface by November then it may be too late. At the moment the comet is just over 500 million kilometres from the Sun, and by mid-November it should be 450 million kilometres from our star, a distance at which this mass of dust and ice should still be relatively inactive.
If Philae waits any later then it could risk landing on a comet that has begun to spit like a sausage on a barbecue – not only does that make it more dangerous to land on, it also crucially robs the science team of the chance to analyse the cometary material before it is modified by solar radiation. As one of the key objectives of Rosetta is to better understand what comets are made of, and in so doing grasp what our early solar system was like, this is a very crucial stage, and nobody wants to miss it.
The current timetable sees the list of sites expected to be whittled down from five to two in the next few weeks, and then, in October, the No. 1 choice will be announced, with the Philae lander aiming to plop onto 67P at walking pace on 11th November. The team at ESA’s space flight centre in Darmstadt, Germany, will take Rosetta down to an altitude of 20 kilometres during that decision-making period in a bid to spot and map as many boulders, ravines and hazardous zones as they can.
However, working out precisely where Philae will lodge itself with its ice screw-equipped feet isn’t like tracing out a landing pad for a helicopter. The challenges of orbiting Rosetta around a comet about 30 kilometres from the surface and then releasing the probe mean that each landing zone is defined in terms of an ellipse of around one square kilometre. So Philae won’t have an X on the comet map to aim for, it will have a general landing field defined for it and a green light from the engineers at ESA to do whatever it takes to reach the surface, and cling on for dear life for as long as possible.
As communication with Earth will take a good half-hour, Europe’s little probe will have to do the landing job on its own, without instruction from ground control. Once in place, Philae has a nominal battery life of 64-hours, and it’s hoped its solar panels won’t get covered with primordial dust and grime and it will be able to recharge and carry on for many more days.
If, and it’s a big ‘if’, everything goes to plan, then Philae will become one of the most famous space probes in history, a true first in man’s quest to understand the solar system and the universe around it. If it doesn’t work, then nobody should accuse Europe’s space engineers and scientists of lacking ambition and panache. Whatever happens, the Rosetta mission is already considered a huge step forward in robotic space exploration, both in science and the art of spaceflight.