Astronomy Without A Telescope – Impact Mitigation


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 Earth 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 orbit 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 asteroid’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.

Further reading: Wolters et al Measurement requirements for a near-Earth asteroid impact mitigation demonstration mission.

25 Replies to “Astronomy Without A Telescope – Impact Mitigation”

  1. I think a much more effective way of adjusting the orbit of an asteroid would be to focus light onto it. A large Fresnel lens, a lens which formes the base of those overhead projectors so commonly used up until 15 years ago, could focus light onto the asteroid and heat the material there. This heating would then cause an ablation of the dust which by Newton’s third law would impart small delta momentum to the asteroid per unit time.


    1. I think yours is the best plan, if we have enough advance warning. With less warning an impactor , or several, could be used to steer the orbit clear of earth. With even less warning, perhaps guidance by nuclear explosion will work, and as the last resort, direct overwhelming nuclear destruction, resulting in many smaller meteors spread out and a good number going off into space.

      1. The gentle approach I think is preferrable if there is sufficient warning time. The data on NEOs is such that I think this will more or less be the case. Comets are a possible exception, where these can come out of the dark outer reaches of the solar system with little warning. The idea of impacting an asteroid works in principle, but it requires that we impart the “delta vee,” which has its limits. Nuclear bombs are the solution of last resort. Maybe if we find some comet is coming in from around the orbit of Saturn and we have only a few years warning we might then elect to blast it.


      2. Just a thought, but could we, with enough time, attach a VASIMR or several on the surface and then steer it into a lunar parking orbit for future mining? If we just steer it blindly away from Earth, then a future generation may be forced to contend with the same problem. Mining it could pay for the cost of recovery and future similar captures.

      3. Well, we wouldn’t be steering it “blindly” away from Earth – we would try to put it in a new orbit that will see it never encouter Earth (or at least not do so for millions of years). Or perhaps we might destroy it my steering it into another planet or the Sun.

        As for the latter idea, the costs of mining from anywhere in space will probably outweigh the revenue. Metals in the Earth’s crust are far more easily accessible.

      4. I am not sure about things like the space elevator, but if it is built I think the best approach would be to build from up to down. So an asteroid tugged into geosynchronous orbit might then be the material basis for building the elevator. This brings some truth to Jonathan Swift’s floating Isle of Laputia in Gulliver’s Travels where some are trying to build a building from the roof down.


    2. Agree it’s a great concept, but involves a lot of untested technology. You need a solution for maintaining alignment of the lens (or lenses) on a moving target. While it sounds feasible, I can’t think of a precedent for this apart from space telescopes.

      The advantage of DQ is that it’s just one step on from Deep Impact (the mission, not the movie) so you start with a high level of confidence that it’s all going to work.

    3. A large Fresnel lens would be some sort of large panel that is very thin. So there would have to be ways of aiming this at the asteroid and of keeping its shape in line so it does not flap around or wad up. It would require some sort of ion drive to station keep the lens and to point it at the asteroid consistently. Solar wind and other perturbations are an obvious problem.

      This is a technology which probably needs to be explored for other reasons. I suspect we are going to “blow it” with respect to global warming. When the panic button is hit in a few decades we may elect to put large panels at the L1 point. These may consist of bucky tubes woven into a microthin fabric. The material interacts with light according to Mie scattering and diverts a significant percentage of it off on an antenna lobe. This might serve to reduce solar radiation on the Earth by .1 to maybe .5%, which will serve to offset some of the increased heating of the climate. A reduction by .5% would be about a complete offset, which might require other geo-engineering techniques.


      1. I agree we are probably going to ‘blow it’. I hope people will find your message when the time comes.

      2. I actually hope the changing climate kicks us in the face sooner than later. We need a serious kick in the pants to get out of the current trend of delusional thinking and manic-sociopath directions we are unfortunately going down. The Iowa straw poll and recent events illustrates a lot of interest in Bachman and Perry. If you dig a bit these two people are big into ideas about installing a Rushdoony style theonomy over the nation and world in order to bring Jesus back. Check out Rushdoony on the web (Wiki-P etc) to see what sort of insanity he proposed, and how this is gaining traction — scary stuff. Over the last few decades it seems we are seeing a ramp up towards a collective insanity and that we are getting leadership that is increasingly diverging from any sort of reality worldview. A good kick in ass might be what is needed to keep us from having some future President in fervent prayer about the return of Jesus as he or she fondles the nuclear launch device ready to dial in the code. It could happen.


      3. A good kick in ass might be what is needed to keep us from having some future President in fervent prayer about the return of Jesus as he or she fondles the nuclear launch device ready to dial in the code. It could happen.

        You mean like this?

      4. The following articles about Perry of Texas and his “Response” prayer meeting are interesting,

        where a lot of this is becoming a tad more than just fiction. We are becoming a nation which denies lots of science, with creationism and climate denialism, a nation which lives and consumes far beyond its actual productive means, and which seems to be plunging deeper into various collective delusions. It is a worrisome trend.


      5. I noticed on the YouTube link there was a link to this part from that 1980’s movie “The Day After.”

        It astounds me to see these ever more extreme people rise to the political mainstream. The problem is that these guys are not just wrong, they are insane. This is delusional sociopathic power rising to consume everything. With every decade the level of demagoguery, the yelling and bullying, the extremism the theonomic nonsense rises to ever higher decibels. Our chances of survival are minimal if this is not reversed — permanently.


    4. Much as I like the idea of a giant Archimedes’ burning glass in space, I don’t think this would work. If this is in earth’s orbit, then the sun will subtend half a degree. You are wanting to point this light at an object a kilometer across which is starting out beyond the orbit of Jupiter. We might be able to hit an object like this with a giant laser pumped with sunlight, but passive optics can only improve the finesse of the sun’s light by collimating it. There was a suggestion of a project to put lots of CD-sized lenses at L1, but the object was to make light miss the earth, which is a lot easier.

      If LC is actually trying to sneak funding for his 10 km space telescope, then we won’t tell 😀

      1. The idea is to have the craft rendezvous with the asteroid. It would illuminate the surface of the asteroid in an orbit or trajectory that is in some ways parallel to the orbit of the asteroid.


      2. Ah. If you are going out to meet the asteroid, then the focus problem is solved. I would be tempted to use a large solar sail anchored to a pole of the asteroid, that would also act as a mirror. The thrust on the sail would be used to keep it in shape, but the thrust on the asteroid would be provided by ablation.

      3. That might work, where the mirror is also a sort of solar kite. That might make the station keeping a bit easier.


      4. In returning to this it occurred to me that if you are to tether something to the asteroid it would be best to use this as a solar sail. A mirror meant to irradiate the surface might be a push-pull effect. The solar sail would then provide the long term force that moves the asteroid. Tethering and control would be an obvious set of problems to work through.


      5. Why not put the solar sail on a very heavy spacecraft, and use it as a gravity-tug to move the asteroid along? No tether needed! 🙂

    5. An H-bomb is a very efficient way of packaging energy, but it will have no more momentum than an indenter. The classic 60’s airburst weapon would do much of the destruction with the shockwave from the fireball and its reflection from the ground. In effect, it knocked over buildings by flinging the atmosphere at them. In space, things will be very different. There is no atmosphere to throw, so we must bury the bomb to get a decent push. We cannot impact an H-bomb, because nukes are delicate things. Landing and drilling a hole may not be practical, particularly if we are meeting the target at speed, but there are known techniques for digging in explosives to get at underground targets. We might use a conventional impactor, or impactors to smack a narrow hole, followed within a tenth of a second or so while things are still moving outwards by the main bomb.

      If something is heading for Earth and you have nothing to push against, then you have to divide the asteroid into two halves that will pass either side. In this case, a lot of the asteroid on the side of the explosion would be blown all over the place, and so should not present an impact threat. If you knew your trajectory accurately, the pushing process should work.

  2. An impactor is a nice, simple way of delivering a large amount of energy to a point. However, the figures for how effective it might be vary by 30:1. The worst case scenario in the paper is that the impactor just buries itself into the asteroid, while the best cases have it throwing out a lot of dust back from the impact, which gives a much larger push to the asteroid then the original impact itself.

    Unfortunately, there is a worse scenario where the impact generates a shock wave in the asteroid, and the shock wave carries the momentum to the other side. When the shock wave arrives at the surface, then the surface just scabs off into space. People who design bulletproof armor know all about this sort of thing. This could give an impactor efficiency of less than 1.0. It is in fact possible for the efficiency to be less than zero if the bulk of the asteroid remains in one place, and the ejecta all fly off the opposite side. I am pretty sure this won’t happen, because a small asteroid would have an uneven composition, and so it will tend to break up a shock wave rather than transmit it coherently. However, I am not sure enough to bet the survival of the Earth on it.

    We could have a lander that could collect loose material and fling it away with just more than the escape velocity in the right direction. This would be slower than in impactor, so it would not be a solution for something that is headed straight for Earth, but it is much more energy efficient, so it could go on working for years. This would be good for something that would make several passes by the Sun before threatening Earth.

  3. The amount of momentum change might not even be measurable. I wish them luck. Attaching a plasma engine (ion drive or VASIMR) might have a larger long term effect, but until we map the inner solar system and most things that impinge on it, I don’t think we’ve have enough warning to do this properly before an earthly impact. Might be fun in a few decades to give the moon some of it’s own satellites, or observe a lunar impact from orbit or the lunar surface.

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