|Photo of Chelyabinsk meteor and fireball, taken February 15, 2013, from over 100 miles away.|
Image Credit: cyberborean via flickr | Rights Information
An impact from a comet or asteroid about 10 kilometers wide likely caused the most recent and familiar mass extinction that ended the Age of Dinosaurs about 65 million years ago. Such giant impacts are rare, but smaller strikes can still wreak major havoc.
"In the last roughly 100 years, we've had two significant impacts that we know of," said experimental cosmologist Philip Lubin at the University of California, Santa Barbara. "One was the 1908 event in Tunguska, Russia, with an estimated yield equivalent to 10 to 15 megatons of TNT, essentially as large as the largest nuclear weapon the U.S. has ever detonated. The second was the Chelyabinsk event, also in Russia, in 2014, which had an estimated yield of about a half-megaton, equivalent to a medium-sized thermonuclear weapon in the U.S. and Russian arsenals."
Scientists have proposed a variety of ideas to deflect threatening near-Earth objects. For instance, kinetic impactors, perhaps armed with nuclear bombs, could slam into targets to shove them off course. Alternatively, spacecraft with sufficient mass could act like "gravity tractors," using their gravity to pull targets into new orbits. Some have proposed that thrusters mounted on targets can steadily nudge them away — maybe ion engines, or robots that mine rocks off targets and hurl them into space. Others have said that covering one side of a target with paint or mirrors can alter how much it gets warmed by the sun, which in turn can gradually shifts its trajectory.
Now, Lubin and his colleagues have proposed using solar-powered lasers to blast targets. The resulting plumes of vaporized or ablated rock can then push targets away from collision courses.
Lubin's team has run computer simulations to see how well lasers of different strengths might work against incoming asteroids of various sizes. One proposal is DE-STAR, or Directed Energy System for Targeting of Asteroids and exploRation, which consists of a large satellite in Earth orbit that blasts asteroids from afar. (Lubin is coy about whether DE-STAR is a reference to Star Wars' Death Star.)
"The fact that a system could deflect an incoming asteroid from Earth orbit is unique — all the other systems require a spacecraft to go out to the asteroid," said astronomer Paul Chodas, manager of NASA's Center for Near-Earth Object Studies at the Jet Propulsion Laboratory in Pasadena, California, who did not take part in this research.
Lubin's other concept is DE-STARLITE, a much smaller robot spacecraft that flies to an asteroid to deflect it, holding position roughly 10 kilometers from its target. The researchers suggest that DE-STARLITE is the more practical option, because the smaller system could be built more rapidly and inexpensively.
For DE-STARLITE, the researchers modeled a spacecraft powered by a solar array. They simulated asteroids of varying sizes, from 20-meter Chelyabinsk-class and 80-meter Tunguska-class objects to Apophis, a 325-meter-wide asteroid in a potentially hazardous orbit, and larger.
For example, a 20-kilowatt version of DE-STARLITE operating for 15 years could deflect Apophis a distance equal to Earth's diameter. "The military is currently typically focused on 100-kilowatt-range lasers, so 20 kilowatts should eminently be doable," Lubin said.
A more powerful 1-megawatt (1,000 kilowatts) DE-STARLITE that could fit into the first of NASA's upcoming Space Launch System rockets could over the course of five years deflect threats up to 500 meters wide, and deflect Tunguska- or Chelyabinsk-class asteroids in under a year upon arriving at those rocks, the researchers said. "A megawatt sounds like a lot, but there's not any reason we couldn't scale up to a megawatt if we wanted to," Lubin said.
One key advantage of this laser-based strategy "is that it uses mass from the asteroid itself to push the asteroid, instead of bringing a lot of fuel or mass to the asteroid to move it," Lubin said. He added that a laser-based system would weigh less than other options while having as much or more effect on asteroids, and also offer a level of control as fine as the best alternatives.
However, DE-STARLITE does require time to work -- months for it to reach an incoming asteroid, and then years for it to deflect the asteroid a safe amount. Astronomers may not detect a dangerous asteroid in time for DE-STARLITE to intercept it. DE-STAR may then serve as a last line of defense on short notice.
DE-STAR is most effective when targets are relatively close to its lasers. To deflect targets to a safe trajectory, DE-STAR needs a very large, powerful array of lasers, one where the lasers are all in phase, or lockstep.
The researchers calculated that if DE-STAR had a 1-kilometer-wide phased laser array and an equally large solar power array, it could deflect an 80-meter Tunguska-class asteroid by about 1,900 kilometers over the course of four weeks — probably not enough to prevent an impact, but enough to potentially steer an asteroid to hit someplace uninhabited. A 2-kilometer array could deflect an asteroid more than 127,000 kilometers, or about 10 times Earth's diameter.
One set of targets DE-STAR could attack that DE-STARLITE probably could not are long-period comets, ones that take more than 200 years to complete an orbit. The nature of their orbits make it difficult for spacecraft launched from Earth to rendezvous with them and match their speeds and trajectories. "DE-STAR may be one of the few options to defend against long-period comets," Lubin said.
However, building DE-STAR would prove challenging. The International Space Station is currently the largest manmade object in space, and it is only about 110 meters wide. "The engineering involved with building such a large spacecraft is formidable to say the least," Chodas said.
Some people might fear that DE-STAR's powerful lasers could potentially be used as a weapon.
But Chodas noted there could be other peaceful applications for laser satellites besides asteroid defense, such as interstellar exploration.
"Lasers can be used to propel small probes to relativistic speeds, in what may be the only feasible way to get to the nearest stars," Lubin said.
Lubin and his colleagues detailed their findings in a paper accepted by the Publications of the Astronomical Society of the Pacific.
—Charles Q. Choi, Inside Science News