Impact mitigation

Aug 15, 2011 By Steve Nerlich, Universe Today
The Don Quijote mission, which may launch by 2015. The Sancho spacecraft will orbit an asteroid, collecting data as another spacecraft Hidalgo collides with that asteroid, in an attempt to alter its trajectory. Credit: ESA.

The save-the-Earth rehearsal mission Don Quijote, commissioned by the European Space Agency, is planned to test the potential of a real life-or-death mission to deflect a mass-extinction-inducing asteroid from a collision course with Earth.

Currently at ‘concept’ stage, the Don Quijote Near Asteroid Impact Mitigation Mission – has been modelled on a proposed flight to either 2002 AT4 or 1989 ML, both being near-Earth asteroids, though neither represent an obvious collision risk. However, subsequent studies have proposed that Amor 2003 SM84 or even 99942 Apophis may be more suitable targets. After all, 99942 Apophis does carry a marginal (1 in 250,000) risk of an Earth impact in 2036.

Whatever the target, a dual launch of two spacecraft is proposed – an Impactor called Hidalgo (a title Cervantes gave to the original Don Quixote) and an Orbiter called Sancho (who was the Don’s faithful companion).

While the Impactor’s role is self-explanatory, the Orbiter plays a key role in interpreting the impact – the idea being to collect impact momentum and trajectory change data that would then inform future missions, in which the fate of the Earth may really be at stake.

The extent of transfer of momentum from Impactor to asteroid depends on the Impactor’s mass (just over 500 kilograms) and its velocity (about 10 kilometres a second), as well as the composition and density of the asteroid. The greatest momentum change will be achieved if the impact throws up ejecta that achieve escape velocity. If instead the Impactor just buries itself within the asteroid, not that much will be achieved, since its mass will be substantially less than any mass-extinction-inducing asteroid. For example, the object that created the Chicxulub crater and wiped out the dinosaurs (yes, alright – except for the birds) is thought to have been in the order of 10 kilometres in diameter.

So before the impact, to assist future targeting and required impact velocity calculations, the Orbiter will make a detailed analysis of the target asteroid’s overall mass and its near-surface density and granularity. Then, after the impact, the Orbiter will assess the speed and distribution of the collision ejecta via its Impact Camera.

However, accurately measuring the degree of deflection achieved by the impact represents a substantial challenge for the mission. We will need much better data about the target asteroid’s mass and velocity than we can establish from Earth. So, the Orbiter will do a series of fly-bys and then go into around the asteroid to assess how much the asteroid is affected by the spacecraft’s proximity.

A precise determination of the Orbiter’s distance from the asteroid will be achieved by its Laser Altimeter, while a Radio Science Experiment will precisely determine the Orbiter’s position (and hence the asteroid’s position) relative to the Earth.

Having then established the Orbiter as a reference point, the effect of the collision of the Impactor will be assessed. However, a significant confounding factor is the Yarkovsky effect – the effect of solar heating of the asteroid, which induces the emission of thermal photons and hence generates a tiny amount of thrust. The Yarkovsky effect naturally pushes an ’s orbit outwards if it has a prograde spin (in the direction of its orbit) – or inwards if it has retrograde spin. Hence, the Orbiter will also need a Thermal Infrared Spectrometer to separate the Yarkovsky effect from the effect of the impact.

To estimate the effect of Hidalgo's collision, the Yarkovsky effect must be acounted for. Heating of an asteroid's surface by the Sun causes thermal radiation. The nett cumulative momentum of that radiation is from surfaces that have just turned out of the Sun's light (i.e. 'dusk'). In asteroids with prograde spin, this will push the asteroid into a higher orbit - i.e. further away from the Sun. But, for asteroids with retrograde rotation, the orbit decays - i.e. towards the Sun.

And of course, given the importance of the Orbiter as a reference point, the effect of solar radiation on it must also be measured. Indeed, we will also need to factor in that this effect will change as the shiny new spacecraft’s highly-reflective surfaces lose their sheen. Highly reflective surfaces will emit radiation, almost immediately, at energy levels (i.e. high momentum) almost equivalent to the incident radiation. However, low albedo surfaces may only release lower energy (i.e. lower momentum) thermal radiation – and will do so more slowly.

To put it another way, a mirror surface makes a much better solar sail than a black surface.

So in a nutshell, the Don Quijote impact mitigation mission will require an Impactor with a Targeting Camera – and an Orbiter with an Impact Observation Camera, a Laser Altimeter, a Radio Science Experiment and a Thermal Infrared Spectrometer – and you should remember to measure the effect of solar radiation pressure on the spacecraft early in the mission, when it’s shiny – and later on, when it’s not.

Explore further: NASA team lays plans to observe new worlds

More information: Wolters et al., Measurement requirements for a near-Earth asteroid impact mitigation demonstration mission.

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User comments : 14

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StandingBear
1 / 5 (1) Aug 15, 2011
If that artist's depiction of 'Sancho' is accurate, it seems to indicate electric main propulsion. I hope that this is so.
Jeddy_Mctedder
2 / 5 (8) Aug 15, 2011
nuclear rockets, not bombs. obvious solution. greenies can't handle the truth.
Pyle
1 / 5 (1) Aug 15, 2011
How capable are we to position asteroids into near earth orbits, and use them as interceptors? This would give us considerably more mass to throw at a potential threatening object.
SemiNerd
4 / 5 (2) Aug 15, 2011
How capable are we to position asteroids into near earth orbits, and use them as interceptors? This would give us considerably more mass to throw at a potential threatening object.

Directing an asteroid of significant mass so it is CLOSER to earth probably wouldn't be hugely popular. It actually sounds like the plot of a great 'technology goes mad' thriller where the asteroid that was supposed to save us plunges into the Pacific and drowns half the earths population.

Can you say.... 'Ooops?'
micahgtb
3 / 5 (1) Aug 16, 2011
How capable are we to position asteroids into near earth orbits, and use them as interceptors? This would give us considerably more mass to throw at a potential threatening object.


With any degree of confidence of safety? None. That will take sometime before we have the propulsion capabilities to CORRECT a slip up or malfunction, because no doubt those will happen, but until that time, None none none and we'd be dumb to try.
Thrasymachus
5 / 5 (3) Aug 16, 2011
How capable are we to position asteroids into near earth orbits, and use them as interceptors? This would give us considerably more mass to throw at a potential threatening object.
One would think that if we were capable of positioning asteroids into near earth orbits to be used as interceptors, nudging a somewhat larger one just enough so it misses us wouldn't be that challenging.
antialias_physorg
3.7 / 5 (3) Aug 16, 2011
How capable are we to position asteroids into near earth orbits, and use them as interceptors?

We aren't capable of doing that - and probably won't be able to do so for quite some decades (or even centuries).

We have neither the technology nor the resources to move a multi kilo (or mega)tonne object anywhere in space. Our probes are a few kg to half a tonne at best - and a large proportion of that is fuel to get the tiny rest with the scientific experiments anywhere.
Pyle
1 / 5 (1) Aug 16, 2011
Hmmm. Ok, everybody is pretty negative here. I just thought that it would be considerably less effort to use a mass already "free" of the Earth's gravity. Then we could use the 500 kg of impactor probe as fuel instead to move a larger impactor.

We could probably get a couple of these asteroids tethered, or something. Set them spinning and send them off, sling-like to smack an incoming object. If they got dangerous themselves we could detach them at a time that sends them both careening away from us. You could probably set several of these up with the spin properly aligned such that you could intercept any likely trajectories and have the safety release with no danger of impacting the planet.
Pyle
1 / 5 (1) Aug 16, 2011
One would think that if we were capable of positioning asteroids into near earth orbits to be used as interceptors, nudging a somewhat larger one just enough so it misses us wouldn't be that challenging.
I think that would depend on how fast it was coming. If it was fast enough, we probably wouldn't detect it anyway. But there is a huge range that we might detect and still not be able to sidle up next to it in time to give it a nudge. Throwing something at it seems like an option, but it would probably make more sense if the something were a nuke.
Thrasymachus
5 / 5 (2) Aug 16, 2011
Of course detection is the problem, but this article, and the efforts behind these satellites, aren't about the issues surrounding detection, but about the much simpler problem of how to move the asteroids where we want them to go. Sure, everybody wants a comprehensive system to detect and deflect incoming asteroids, but we're still figuring out how to get there. It doesn't make much sense to read an article that describes how we're just now testing how to move a relatively small asteroid, then propose that we should move a bunch more of 'em to act like a shield. We don't even know how to move one small asteroid to where we want it to go yet, let alone do any of the precise maneuvering of multiple asteroids your proposal would require.
LKD
3 / 5 (2) Aug 16, 2011
Throwing something at it seems like an option, but it would probably make more sense if the something were a nuke.


My recommendation would be to attach a solar sail to the asteroid and let the sun alter the trajectory so it's less a threat.
antialias_physorg
4 / 5 (3) Aug 16, 2011
We could probably get a couple of these asteroids tethered, or something. Set them spinning and send them off, sling-like to smack an incoming object.

The idea is to hit it far away, so that something as miniscule as 500kg will have an effect. The closer you get the more you need to affect the incoming rock. But far enough out a few percent of a degree are enough to ensure a clean miss.

So if anything we want to hit asteroids anywhere BUT in Earth orbit.
zadoc_paet
not rated yet Aug 16, 2011
No freaking way should the ESA attempt this. They could make a 1 in 250,000 chance more like a 1 in 10 chance. No one has any idea how to do something like this.

POLL: Should the ESA go through with plans to fire a projectile at the asteroid 99942 Apophis to see if they can change its course?
Vote: wepolls.com/p/1805342
xNico
not rated yet Aug 22, 2011
Throwing something at it seems like an option, but it would probably make more sense if the something were a nuke.

Nuking Asteroids would be pointless and stupid. For one, the nuke might not even be able to do sufficient amounts of damage, and if it does, we have more than one object about to crash into our planet.