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If astronomers ever spot an asteroid coming our way, the world will have to decide pretty quickly what to do.

One idea is to knock it off course. But although that sounds pretty straightforward, it hides a multitude of challenges. The most obvious of these is to determine how big an impact would be necessary and how this could be delivered.

In 2002, the European Space Agency began a program called Don Quijote to find out how best to perform such a deflection.

Don Quijote involves sending two spacecraft to a near Earth asteroid; one to smash into it and the other to watch while in orbit above the impact crater. The goal is to change the asteroid’s semimajor axis by more than 100 metres and to measure the change with an accuracy greater than 1 per cent.

But the question is how best to monitor what’s going on in a way that is relevant to other asteroids. After all, the ultimate plan is to use the information from this mission to move some other asteroid with our name on it.

Now, Stephen Wolters at the Open University in the UK and a few friends have published a new analysis of the mission saying that measuring the change in orbit is not enough. Instead, the spacecraft needs to characterise the impact in detail, determining the density of the material near the asteroid’s surface, the size of the surface grains as well as the mass and speed distribution of the impact ejecta.

Only with this information will it be possible to work out exactly how the momentum from the impactor was transferred to the asteroid.

That significantly changes the mission. In addition to an on-board radio transmitter that will allow space scientists back on Earth to work out its distance exactly, the spacecraft will need a sophisticated imaging suite capable of photographing the damage and carrying out infrared spectroscopy to determine the asteroid’s mineral content.

The thermal cameras will also pick up any changes in temperature. That’s important because of the small but potentially significant force from the way the asteroid emits heat.

If the asteroid emits thermal photons equally in all directions, their force cancels out. But if it emits more in some directions than others, then this force will slowly push an asteroid. That can happen if the asteroid tends to cool down at a rate that matches its speed of rotation–so that all the thermal photons emitted after the surface has been in the Sun, travel in the same direction.

This force, called the Yarkovsky effect, could make the difference between whether an impact with Earth occurs or not. So knowing its value is crucial, particularly if the impactor changes the asteroid’s rate of rotation.

So that should make Don Quijote a better mission but it also makes it a more expensive one. And that raises a question mark over whether the European Space Agency will want to pay for it on its own.

But then again, why should it? There’s a good argument that the international community as a whole should shoulder the burden and the cost of planetary protection.

The only question then is how badly we need Don Quijote to keep our planet safe.

Ref: arxiv.org/abs/1107.4229: Measurement Requirements For A Near-Earth Asteroid Impact Mitigation Demonstration Mission

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