A Robust, Nonnuclear Defense Against Asteroids
An SSI Paper from the Asteroid Mitigation Workshop
Advantages of mass drivers:
Rugged design that uses off-the-shelf parts
Variable specific impulse
Performance degrades gracefully if individual units are faulty
Can use any mass as fuel (that is, as reaction mass)
Very high efficiency in conversion of electrical energy to kinetic energy
Mass drivers have benefited from the advances in solid-state switching and in high-performance ultracapacitors
No design surprises are expected, a mass driver of adequate performance should be able to be constructed with straightforward engineering. Such a machine could be a bit better or a bit worse depending on the talent of the engineer, but we expect no fundamental show stoppers to the construction of such a machine adequate to deflect even large asteroids given a reasonable lead time.
Mass drivers do not require a nuclear power source, a solar power source similar to that used on the international space station will be sufficient to power the mass driver during its operational lifetime.
Because of its inherent rugged construction, mass drivers should be able to be relied on for the years-long thrusting necessary to divert global killer asteroids.
Critical mass driver subsystems have been developed to technology readiness level 6 by SSI at Princeton University under contract NASA and private funding.
Historical note on mass drivers
Mass drivers were initially conceived to launch aluminum and silicon and glass from the moon to support the construction of satellite solar power stations in high Earth orbit. Remarkable progress was made in reducing the size and weight of the mass driver over the course of a nine-year development program. Brian O’Leary at the Space Studies Institute pointed out the utility of mass drivers in returning small asteroids to high Earth orbit to use in a program of solar power satellite construction in an article in science in 1979. Following the publication of that article, mass drivers decreased in size and complexity and the threat of near Earth objects began to be appreciated, making his conclusions all the more relevant.
For the rare objects that could cause global damage, warning-times are likely to be longer and the mass-driver can be used effectively to avoid impacts. To give an idea of the power required to operate the mass-driver, consider a NEO with mass one billion tons, a mass-driver with a throw-velocity of 100 meters per second and an energy-efficiency of 50 percent, and a warning-time of ten years. Then the power required to move the object by 5 earth-diameters and safely avoid the impact will be 140 kilowatts. This is a modest amount of power that could easily be supplied by solar collectors or by a small nuclear reactor.
A concept that has received relatively little attention in the literature is the employment of a mass driver to apply a steady acceleration to the asteroid which, given sufficient time, will develop lateral movements that can convert a strike on earth into a miss. The major advantage of this approach is that all the energy comes from the sun and all the reaction mass is obtained from the mass of the asteroid itself. The only mass that needs to be transported to the asteroid is that of the solar collectors, power supplies and acceleration coils which convert electrical energy to kinetic energy.
It is recommended that this little-studied third approach receive research funding. One key issue that deserves examination is the design of mass drivers which are optimized towards low velocity ejecta since these designs convert the highest percentage of the collected solar energy to momentum transfer to the asteroid.