One of the major threats to intelligent life, Stephen Hawking once warned, is the high probability of an asteroid colliding with inhabited planets. Bracing for that eventuality on Earth, scientists are looking at two possible ways to ward off the threat, if and when it comes. Either nudge the asteroid off its Earthbound course, or blow it up before it strikes. The question is which is the better option.
Two new studies have sought to assess the practicability of the two approaches. American scientists have designed a conceptual spacecraft that can either deflect an asteroid by nudging it, or carry a nuclear device to the object. They would prefer deflection with a battering ram, they have stressed, but their evaluation of the spacecraft’s capabilities has indicated that only the nuclear option — detonation to deflect the object — would be viable against a large asteroid within a limited response time.
Russian scientists, meanwhile, have made toy asteroids, blasted them with a laser pulse, and then estimated the size of the nuclear explosion that would be required to blow up an actual asteroid. A big one calls for a big one: to eliminate a rocky asteroid 200 m wide, the bomb needs to deliver the energy equivalent of 3 megatonnes of TNT. For comparison, that is 200 times the TNT equivalent of Little Boy (15 kilotonnes), the atomic bomb that exploded in Hiroshima in 1945.
“We’re also looking into the possibility of deflecting an asteroid without destroying it and hope for international engagement,” study co-author Vladimir Yufa of the Moscow Institute of Physics and Technology (MIPT) said in a statement.
MIPT, which provided The Indian Express with the materials of the study, collaborated with the Russian Federal Nuclear Centre and the State Atomic Energy Corporation. The other study too involved government institutions, including NASA and Lawrence Livermore National Laboratory (LLNL). The paper by the Russians appears in the Journal of Experimental and Theoretical Physics; the one by the Americans in Acta Astronautica.
Short for Hypervelocity Asteroid Mitigation Mission for Emergency Response vehicle, HAMMER is a 9-m-tall, 8.8-tonne spacecraft designed to serve as either a kinetic impactor — essentially a battering ram — or as a transport vehicle for a nuclear device. Nudging an asteroid is the preferred option because blasting it entails the risk of fragments crashing into Earth.
The US team evaluated how effective HAMMER would be in nudging away the asteroid Bennu, 500 m wide and weighing 79 billion kg, which has a 1-in-2,700 chance of striking Earth on September 25, 2135. If it does, the energy released would be equivalent to 1,200 megatonnes, or 80,000 times the energy of the Hiroshima bomb.
If launched from the Delta IV Heavy rocket (the world’s second highest-capacity launch vehicle) 10 years before impact, it would take between 34 and 53 launches of the rocket, each carrying a single HAMMER impactor, to make a Bennu-class asteroid miss Earth. If launched 25 years in advance, it would still need seven to 11 launches. The paper concluded that using a single HAMMER spacecraft as a battering ram would prove inadequate for deflecting an object like Bennu.
“The nuclear option is the only viable option for launches 10-25 years before impact,” LLNL astrophysicist Kirsten Howley, one of the study authors, told The Indian Express by email. “If the object were smaller (say 100 m) or the time to impact were greater (say 100 years), a kinetic impactor may provide a better result.”
In the 1998 film Armageddon, a team led by Bruce Willis drills a hole into an Earthbound asteroid and buries a bomb in it. In a statement, LLNL ruled out that approach. In the nuclear option it proposes, the explosive is detonated some distance from the asteroid. This will flood one side of the asteroid with X-rays, vaporising a layer of the surface. As the vaporised material is ejected, it would propel the asteroid away, like a rocket.
For the Russian team, on the other hand, the nuclear option is to blow the asteroid into pieces, most of which will miss the Earth or burn up in the atmosphere.
The paper takes a leaf straight out of Armageddon’s book. Describing the destruction of miniature “asteroids” with lasers, the MIPT statement says: “In some of the experiments, the laser was targeted at a cavity made in the miniature asteroids ahead of time. By exploiting the cavity, the researchers spent less energy. Similarly, the effect of a buried nuclear bomb is expected to be more pronounced.”
In manufacturing artificial asteroids, the team adjusted their physical properties and chemical composition, which corresponded to that of stony meteorites. Imitation asteroids of various shapes were made, among them spherical, ellipsoidal, and cubical.