Landing on Comets

Chris - Now - Comets, with their iconic long tails, are one of the most interesting things we can see out in space from here on earth with the naked eye.  We already know a bit about comets from observing them with ground and space telescopes, and through the Giotto mission, which flew past Halley's comet back in 1986.  

The Rosetta mission sounds like something straight out of Science Fiction, but seeks to do something no spacecraft has ever done before - land on a comet.  This week it has done its final fly-by past Earth, and now it's heading off for its remarkable rendezvous.

Ben Valsler spoke to Professor Ian Wright, from the Open University, to find out more about what we hope to learn from this exciting mission...

Ian -   The Rosetta Mission is a dual spacecraft mission which aims to send both an orbiter spacecraft and a lander spacecraft to rendezvous with, and ultimately land on a cometry nucleus.  Because of the distances involved, and the speeds involved, it's not possible to just launch from Earth and hope to get there directly.  One has to travel around the solar system a few times, building up speed by having gravitational encounters with planets like the Earth and Mars, and then getting flung out into sort of, deeper space, and ultimately getting fast enough so that one can catch the comet up.

Ben -   It sounds like there's a lot that can go wrong.  You're relying on this "slingshot effect" around lots of different planets, and then, you're catching something relatively small, a comet, millions and millions of miles away.

Ian -   It is quite daunting in a way.  I think having been involved with a mission to Mars where, you know, you have something the size of a planet and it's something you can actually, you know, see quite easily from your back garden when the conditions are appropriate.  You know, landing on a planet seems relatively straightforward.  The comet is probably something like a kilometre or two across, and you've got a multi-billion kilometre journey to get there, and then you've got to find it, and ultimately land on it.  It is amazing, and of course, in that sense, I'm in the hands of the spacecraft flight engineers who are experts in this field.

Ben -   And when did it actually launch?

Ian -   It launched in 2004.  It's already been travelling a few years already, of course, building up speed.  It's now going off into a hibernation phase where it'll be effectively in deep space ultimately hoping for a rendezvous and land towards the end of 2014.

Ben -   And once it hits its target, once it lands on this comet and sets the orbiter around it, what are we hoping to learn?

Ian -   The first thing that's quite interesting is, of course, one talks about an orbiter but actually you cannot really orbit a comet.  It's so small, and it has no gravity.  So, one of the first challenges is really to understand how to fly with it.  So the whole concept of orbiting is quite strange.  In answer to the question, you can make of this as, you know, as a human being if you're on Earth and you were going to visit a new country that you've never been to before.  You'd have all kinds of questions that you'd want to know about the place.  What's the weather going to be like?  What's the temperature?  What's the geography?  What's the terrain?  And in a human context, you know, what are the people like?  What's the food like, and all that kind of stuff.  Actually, you can pose very similar questions in relation to the comet, although we don't expect any humans on the comet obviously.  But just simply any question you can think of about somewhere you've never been before, you know.  What's it look like?  What color is it?  How cold is it?  How active is it?  Does it rotate?  What's it made of?  What's the surface like?  Any of those questions, simple or fundamental questions, you can post about the comet.  The Rosetta mission has been designed to try and get answers to as many of those questions as possible.

Ben -   Most space missions are actually very specific.  They're out there to look at certain areas of electromagnetic radiation, or to look at very key things.  It sounds like Rosetta is actually trying to be a bit of a Jack of all trades.

Ian -   Rosetta isn't any, particularly any different in a planetary exploration context.  I think what a lot of planetary missions have is the widest diversity of instruments that they can possibly take.  So for instance, you know, you want a camera.  You want spectrometers.  You want things that can detect magnetic fields.  And once you get down to the surface, you want to know what's the constitution like, physically what's it like.  Is it hard?  Is it soft?  It's not simply a mission that's going to go to a comet and take a picture of it.  It's a mission that goes to ask many different questions, you know.  The order of magnitude for the number of instruments is something like 20.  I mean, there's, you know, there's like 10 on the orbiter, and 10 on the lander.  All different instruments designed for different purposes.  It's quite possible that they won't all work.  I mean, there's a certain amount of redundancy in this.  So, you know, one hopes the mission will be successful whatever happens.

Ben -   It's a very long way to travel if it turns out not to work.  Is there anything that we can use if for on the way?  Can we pick up bits of information about the environment it's travelling through?

Ian -   Well, yes, it is interesting, and although, I said previously it was about to go into hibernation actually, it's got one more stop.  It's going to fly past an asteroid.  It will be the second asteroid it's gone past during this phase.  And yes, we use the opportunity at that point to turn some of the instruments on, again take photographs, look at magnetic fields, and so on.  Again, the mission was designed so that it would actually do this along the way, so that, yes, it spreads out some of the scientific interest along the mission itself, because as you say, I mean, 10 years for a space journey, that is a long time.  And it's a long time to get somewhere, and find things don't work.  So the mission has already done some science, and they say, will continue to do some next year.  Then hopefully, next stop the comet.