Averting Armageddon, with ROI
by Mike Combs
Copyright © 2008
Tunguska of that era was a sparsely-inhabited region. For that reason, human fatalities and property damages were minimal. In the intervening century, the growth in human population has resulted in far fewer regions of the Earth similarly standing near-empty. The next such impact will surely result in greater loss of life and property. If such a strike takes place in an ocean (which chance would favor), the losses might paradoxically be even greater as global-scale tsunamis engulf shorelines.
Then we look nervously at the paleontological records of Extinction Level Events (ELEs) where some disaster overtakes the entire planet resulting in more species than not becoming extinct. And in at least some cases, an asteroid or comet is implicated.
So what can be done about it? If there is an asteroid out there with our name on it, then there are two considerations:
Shouldn't we be working to develop the techniques and technologies needed to move small solar system bodies around? This is a position being vigorously advocated by a number of groups, chief among them Rusty Schweickart's B612 Foundation.
For quite some years, the received wisdom on this issue was that we would use nuclear explosives to change the course of a small body on a path intersecting the Earth. Nuclear devices are certainly the biggest stick in humanity's technological arsenal.
But according to a 1998 Nature paper, while stand-off nuclear detonations might have some utility, nuclear devices might not be as useful as one might think. Many asteroids seem to be multi-lobed. A nuclear detonation might see a significant fraction of its energy going into the shifting of one lobe relative to the other, with correspondingly less energy going into a course deflection of the overall body.
Even if we had the good luck to be threatened by a non-lobed body, many asteroids may be less like a single rock and more like a loose conglomeration of bodies embedded in dust. Subjected to a nuclear explosion, such a "flying gravel pile" might separate. So we would find we had only succeeded in converting one single disaster into many separate ones. Sometimes Hollywood dramas speak of pulverizing an asteroid such that all of its pieces would "burn harmlessly in the atmosphere". But an entire sky alit with the blazes of entering debris would touch off surface fires of incomprehensible scale.
So are there any better methods for changing the course of an asteroid; something which can apply low but steady thrust for weeks or months rather than a sudden shock?
In the 1970's Princeton physicist Gerard O'Neill was working out a method for establishing settlements beyond the Earth which were not dependent on the conditions we find on the surfaces of other planets or moons. In both scientific papers and "The High Frontier" he advocated the construction of enormous, orbiting structures which rotated for artificial gravity and would contain within them Earthlike environments in closed ecologies. The unceasing power of the sun would be used to drive those ecologies and for power generation (indeed, such settlements might help pay for themselves by constructing solar power stations in orbit). This approach would seem to have many advantages over trying to make a go of it on other planets.
But one can immediately discard the notion of manufacturing the parts for these space settlements here on Earth and then launching them into orbit on a rocket. With habitat masses ranging from 4 to 10 million tons, this was clearly unfeasible for any plausible rocket system, regardless how improved over those currently in use.
This meant such space settlements (and solar power stations, if desired on a scale large enough to make a global difference) had to be constructed from materials already in space. O'Neill first turned to the moon, designing and even building working models of a device called a mass driver to function as a catapult capable of launching lunar ores to the L-2 point. This was a position in space from which the material could be collected for delivery to an orbital ore refinery.
The mass driver was a long, solar-powered structure with a series of electromagnetic coils running down its length. Recirculating buckets equipped with magnetic coils of their own could be rapidly accelerated down the length of the mass driver and then sharply decelerated, ejecting their contents out the end of the structure.
It was quickly realized that such a device built in space rather than on the lunar surface could function as a reaction engine. And moreover, a reaction engine which could use literally anything as reaction mass. This neatly solved the problem of getting the kit for the lunar mass driver along with the needed mining and other support equipment from Earth orbit to lunar orbit. A shuttling mass driver reaction tug could haul the loads from one orbit to the other.
Shortly after, Brian O'Leary wrote papers advocating the use of mass driver reaction tugs to retrieve asteroidal resources to high orbits around the Earth using a portion of the material for reaction mass. It turns out asteroidal resources might have advantages over lunar ones. For one thing, a wider variety of materials are available, including volatiles rare or nonexistent on the moon. For another, a number of the Near Earth Asteroids then known (and even more are known now) have round trip delta-V's which compare favorably with that for the moon.
O'Leary had investigated possible trajectories back from certain NEAs which capitalized on gravity assists from Venus and the moon, and concluded that the requirements fell inside the technical capabilities of the mass driver reaction tug.
This is good news. If the goal is bringing many tons of asteroidal material back into near-Earth space, it's tempting to propose aerobraking through the Earth's atmosphere to slow down. But such maneuvers might never be permitted by Earth dwellers fearing the consequences of a load of ore going off course. But calculations seemed to indicate that capture of ET materials into cislunar space need not depend on anything so dramatic as a screaming dive through our atmosphere.
If mass driver reaction tugs can economically deliver asteroidal material to high Earth orbits, then there's no reason to suppose that orbital habitats need be any more difficult to build than similar settlements on the moon or Mars. The needed raw material would be close at hand.
But for this concept are we limited to studies conducted back in the era when disco was king and bell-bottom pants the fashion? Perhaps not. In 2004, SpaceWorks Engineering, Inc. did a study involving a large number of robotic space craft which could be dispatched to an asteroid, latch on, and use telescoping mass drivers to fire off bits of the asteroid sequentially, thus altering its trajectory.
Undaunted by words with negative connotations, they cheerfully gave their system the acronym MADMEN: Modular Asteroid Deflection Mission Ejector Node. But far from madness, developing the technology to change the courses of small bodies in our solar system might be the sanest thing we could do.
Any system which can use the raw material of the asteroid as reaction mass has a considerable advantage over any system which has to haul the needed fuel to the asteroid. Additionally, if we're interested in retrieval of useful resources, a system which can use any raw material as reaction mass is even more advantageous. The problem with some proposals such as solar or nuclear steam rockets is that you're throwing away the most valuable part of the asteroid. It matters little to the mass driver what material is coming out the exhaust. If we're engaging in ore refining on the trip back to cislunar space, we might well use liquid oxygen as reaction mass, which means we're throwing away the least valuable part. (Any space mining operation can expect to have a surplus of oxygen available.)
High Frontier enthusiast Steve Whitting has drawn a comparison between mass driver reaction tugs and the steam-powered paddleboats which cruised the Mississippi back in frontier days. When their fuel of coal neared depletion, the crew could always pull up to a bank, gather deadwood, and use that to continue firing their boilers. Like steamboats, mass driver spacecraft offer the advantage of being able to "refuel" (technically, gather reaction mass) from available material, including the regolith of an asteroid.
One of the newest proposals for changing the course of asteroids involves a "gravity tractor" which gets us around the technical challenges of halting the rotation of an asteroid before we can begin operations. But there seems no reason to doubt that a mass driver reaction tug could be used in such a configuration. One of the conceptual designs for an ore retriever featured three parallel mass driver engines in a triangular array with struts connecting them to each other and to an enormous sack of asteroidal soil in the middle. If the three mass drivers were angled slightly apart such that their exhaust missed an asteroid being pulled by the vehicle's gravity, then we have a mass driver powered gravity tractor.
We've talked about using asteroidal resources to create solar power stations and human settlements in high orbit. Might there be any materials in asteroids worth bringing back to the surface of Earth? Some have tried to make a case for platinum group metals. Detractors have pointed out that dumping megatons of precious metals on the market will serve to make them less precious. But consider this. A world in which we are manufacturing several Solar Power Satellites a year might be a world where electric cars come into their own. Their fuel cells will require platinum as a catalyst. An asteroidal platinum retrieval operation might largely piggy-back off of an existing asteroidal-ore-for-SPS operation. The expansion of the electric car industry enabled by a new, renewable source of abundant electricity might make the platinum market considerably more elastic than otherwise (demand might expand nearly as dramatically as the supply).
Any technology developed to haul asteroids (or even portions thereof) around the solar system for profitable use can certainly be applied to preventing the next big asteroid strike. The problem with convincing Joe Six-pack that many tens of billions of dollars of taxpayer's money should be spent developing the techniques to shift the courses of asteroids is that, if honest, we must tell him that the next big impact could happen next Tuesday, or a thousand years from now. It's hard to argue for this generation spending the money as opposed to a future one.
What we're essentially trying to sell is a planetary insurance policy. But what if it could be a purchase instead?
Entrepreneurs who have an interest in building large solar power stations or settlements in high Earth orbit might well spend their own money to develop the technologies needed to change the orbits of asteroids. And if we do spot that rock which has the Earth in its cross-hairs, wouldn't it be better to go out and face it with technologies and techniques which have already been in routine economic use for years than to try and develop them in a crash-course program? When the stakes are this high, that's not the time to realize you haven't quite shook all of the bugs out of the system yet.
A large program of using asteroidal resources for economic gain would make our planet safer in the long term. Such a program would seek out low delta-V targets, which means NEAs crossing the Earth's orbit would get used up first. But even a long time before then, a sizable asteroid striking the Earth would simply never happen in such a High Frontier universe. This would be a universe of space telescopes the size of large buildings, and changing the course of an asteroid or comet would be all in a day's work. There would be people living out there between us and an approaching boulder. They would be in a far better position to do something about it than we here at the bottom of our gravitational hole.
Some have said we should establish independent space settlements in case the Earth suffers an ELE impact. I say we should advance to the kind of society capable of building those settlements as such a society would never allow a significant Earth impact in the first place.
Those who are concerned about the dangers from asteroids should certainly advocate near-term strategies and technologies. But I think they should also stand as advocates of a High Frontier future, as this future would see Earth at its safest from such hideous threats.