Science Interviews


Thu, 7th Feb 2013

Asteroids and Near Earth Objects

Dr Simon Green, Open University

Listen Now    Download as mp3 from the show Analysing Asteroids

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.  Thatís 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 -   Weíre 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?Timelapse of Asteroid 2004 FH

Simon -   How long is a piece of string?  It depends on the size.  We know of about 10,000 objects at the moment, but thatís 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 -   Thatís right.  The larger ones attract Ė 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 youíve got a near-Earth object that might come into collision with the Earth, how do you go about knowing how itís 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 doesnít 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 itís 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 donít 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.  Itís around 45 meters if you do the calculation, and itís possible if itís very fragile that it would explode in the upper atmosphere, much like an object in 1908 that damaged a large area of Siberia Ė fortunately, there were no people involved.  If itís 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 weíll probably know that thatís 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 itís 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 itís reassuring to know that those options are there.  Thank you very much, Simon.  Thatís Simon Green from the Open University.


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The typical assumption is to detonate a nuke next to it such that the pressure of the radiation will exert so much force as to change its velocity a bit. Then if we've done this in enough advance time then we can change its orbital path enough so that it will miss the Earth. Pmb, Fri, 15th Feb 2013

The best thing to do is not see them - if someone sees one, it forces a collapse of the wavefunction and makes the path of the asteroid become certain, at which point it may hit us. If we avoid seeing it, it will remain uncertain as to where it is so it can miss us and hit us at the same time, and then we'll continue to exist in a universe where it missed. David Cooper, Fri, 15th Feb 2013

A lot depends on the size and pre-warning.

Perhaps yesterday's meteorite, estimated at about 10 tons would be considered the maximum "safe" size.  Had there been advanced warning, the people choosing to seek shelter would have been quite safe.  Of course, many people would have been standing at the windows, or outside to see it.

2012_DA14 which missed Earth shortly afterwards was estimated at 190,000 metric tons.  And, while still small by celestial standards, striking Earth would cause much more severe damage. 

If one Nuked the middle of the asteroid, one would create a cloud of debris.  However, each fragment would be put on a significantly different trajectory than the original object.  Depending on when it was blasted, perhaps only a couple of fragments would collide with Earth at close to the original expected time. 

The question would then be how small the fragments would be.  Could one blast it so that no fragment remained greater than 10 tons?

Hitting Earth with the "shotgun effect" would be brutal, but with advanced warning, most people could survive.

CliffordK, Fri, 15th Feb 2013

I like it in theory, but in practice when I've tried to collapse the waveform of a substantial payrise I always wind up in the wrong universe. The same thing could happen here, so I vote for installing solar sails. Ophiolite, Fri, 15th Feb 2013

Sorry, but if you're not just joking around then you must not have a very good understanding of quantum mechanics.. You're missing some important facts. For macroscopic ovbjects the uncertainties become certainties. Where there was an absense of trajectories now has the ani                                                                                                                                                                                                                                                                Pmb, Sat, 16th Feb 2013

The solar sail would have to be HUGE to make much difference. 

However, a solar powered plasma engine would be more efficient than the solar sails, and it would be an optimal scenario for the solar plasma engine, with essentially unlimited mass to expel, if you could just get the right energy density. CliffordK, Sat, 16th Feb 2013

A number of those techniques like the nuclear explosion or solar sail rely on evaporating volatile material, forming a diffuse gas jet which would deflect the object. Presumably this would work better on objects with lots of volatiles (like a comet) than one with lots of iron or stone.

Another suggestion is to use something like a shotgun blast, which would create a distributed pressure on the whole asteroid, keeping it largely intact; any bits blown off would be small in size.

I like the idea of preparing a satellite with compartments of black powder (eg graphite) and white powder (eg Titanium dioxide). You could aim it at an asteroid, and while playing orbital catch-up, analyse its shape and rotation. With judicious placement of black and white powder, solar pressure can divert it over several orbits.

The problem with all of these schemes is that they assume you can spot the satellite several orbits before it strikes the Earth. If you only see it on its final approach, you won't be able to rendezvous with it, and so some high-speed flyby with a nuclear weapon may be the only option, and that will not deflect an object greatly if it only has a fraction of an orbit for the deflection to take effect.

Wiki article on the topic: evan_au, Sat, 16th Feb 2013

We can't. yor_on, Sat, 16th Feb 2013

Sorry, but if you're not just joking around then you must not have a very good understanding of quantum mechanics.. You're missing some important facts. For macroscopic ovbjects the uncertainties become certainties. Where there was an absense of trajectories now has the ani

The end of your post appears to be missing. But no - it is not clear that macroscopic objects can't be in highly uncertain states, and it's possible that they are forced to take up more certain states when observed by something complex like a mind which cannot sustain the same uncertain state in the course of processing the data. The very act of processing the data may force a simplification of the data (meaning that its state must become more certain) and this may force the real object to simplify too to conform to the certainty of the data. David Cooper, Sat, 16th Feb 2013

How far could one maintain a high power laser in space?

Could one nudge an asteroid off course with a powerful laser?

What about forcing the ice on a comet to melt, at least reducing the initial kinetic impact of an intact comet. CliffordK, Mon, 18th Feb 2013

We have to be sure that no one with an IQ over 10 observes it. Atomic-S, Tue, 19th Feb 2013

In principle, irradiating one side the asteroid could  vaporize that side, causing jet thrust to redirect its course. Atomic-S, Tue, 19th Feb 2013

We could try catching them with a net an move them off course three space shuttle connected then detached with a net wrapping around the meteor using side rockets to push the three shuttles apart grabbing the meteor.....( Meteor Exstraction Service ) shane, Wed, 20th Feb 2013

Try catching the asteroids in a net and move them else where with a powerful rockets of some kind shane, Wed, 20th Feb 2013

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