Asteroids and Near Earth Objects

07 February 2013

Interview with

Dr Simon Green, Open University

On the 14th-15th of February 2013, Earth will have a truly close encounter.  Asteroid 2012 DA14, a lump of rock weighing somewhere around 130,000 tonnes will pass just 24,000 km from Earth.  That's closer than many satellites.  Objects like these are known as Near-Earth Objects and they're of interest to scientists, but also to a group of entrepreneurs who are aiming to mine asteroids for their minerals.

Dominic -   We're joined by Near Earth Object Specialist Dr. Simon Green from the Open University.  First of all Simon, what do we mean when we talk about asteroids?

Simon -   Asteroids are lumps of rock which formed in the inner Solar System, but never developed into a planet.  So, they're essentially the building blocks of terrestrial planets like the Earth.

Dominic -   And some of these are classified as near-Earth objects.  Is there a strict definition of what that means?

Simon -   Yes, anything that comes within 1.3 astronomical units.  So, 1.3 times the distance of the Earth from the Sun is counted as a near-Earth object.  Most likely, an asteroid, but it also could potentially be a comet.

Dominic -   Roughly, how many objects are we talking about?

2004 FH is the centre dot being followed by the sequence; the object that flashes by near the end is an artificial satellite.

Images obtained by Stefano Sposetti, Switzerland on March 18, 2004. Animation made Raoul Behrend, Geneva Observatory, Switzerland. (c) NASA' alt='Timelapse of Asteroid 2004 FH's flyby (NASA/JPL Public Domain)' >

Simon -   How long is a piece of string?  It depends on the size.  We know of about 10,000 objects at the moment, but that's just a tiny fraction of the total population.  The smaller you go, the more there are, and so, there are many millions down to sizes of meters.

Dominic -   I guess the big ones are quite easy to see, but the smaller ones are much harder to pick out.

Simon -   That's right.  The larger ones attract - we probably of 90 to 95% of all objects bigger than about a kilometre in size.  Of things bigger than maybe 100 meters or so, we certainly know less than 10% of them.

Dominic -   So, how do we go about looking for them?

Simon -   With telescopes, we see it the best way using a wide field, CCD cameras and tracking the sky and looking for objects that move.  So essentially, you'll look for objects that produce trails and images, or look like stars, but are changing position from minute to minute.  And from the change in position, you can calculate the orbit.

Dominic -   So, if you've got a near-Earth object that might come into collision with the Earth, how do you go about knowing how it's going to travel through the Solar System in the future?

Simon -   The orbits themselves, you can calculate and predict where an object will go if it doesn't have any other forces and gravity acting on it, but you need enough observations in order to track its orbit as it is now, and then you do calculations based on the perturbations from other planets, other asteroids.  And the predictions are fine until you have a close encounter with a large body, and then it's very, very difficult to predict afterwards.  So, we can predict for objects when they might come close to a planet, but after that, we probably don't know.  Now, most near-Earth asteroids, because they're in the inner Solar System, perhaps close to planets at some point, their orbits are not stable over long time periods.  So, they probably only exist in the inner Solar System between 1 and 10 million years.

Dominic -   Now, the object 2012 DA14 which is coming close to the Earth next week weighs 130,000 tonnes.  How dangerous is an object that large?

Simon -   This is actually the very bottom end of the kinds of objects that can penetrate the atmosphere and reach the ground.  It's around 45 meters if you do the calculation, and it's possible if it's very fragile that it would explode in the upper atmosphere, much like an object in 1908 that damaged a large area of Siberia - fortunately, there were no people involved.  If it's solid iron which might have come from the core of a larger asteroid, then it would certainly reach the ground and make a crater maybe 100 meters or larger in size.  But this is at the very bottom end of the sort of damage scale, if you like, that we need to be worried about.

Dominic -   We heard earlier about the asteroid that might have wiped out the dinosaurs.  If we were to find an object that was quite large coming towards us, what could we do about it?

Simon -   Something that big would be quite tough, but we'll probably know that that's not going to be the case, but something maybe a few hundred meters in size up to a kilometre, we would need to be able to try and deflect it.  And then we need to therefore know its orbit and predict where it's going to go with many years, preferably decades ahead of time.  

We can then use a number of different techniques.  It might be a kinetic impactor firing a spacecraft into the target that produces a tiny change in its momentum and therefore, its track, but magnified over a number of years, it can be enough to miss the Earth.  We could use something like a gravity tractor where you bizarrely use the gravity of the spacecraft itself, take it close to the object and then fire the rockets very, very gently and use that small gravity of the spacecraft to just gradually change its orbit.  Both of those techniques will take a long time and we may not have that level of warning.  In the end probably, the only alternative is to let off nuclear weapons very close to the object, vaporising some of the material in a jet effect, and then move the target.

Dominic -   I guess it's reassuring to know that those options are there.  Thank you very much, Simon.  That's Simon Green from the Open University.

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