When to Nuke an Asteroid That's Heading Toward Earth (and When Not to)

The best way to protect Earth against asteroids is to be prepared. That's why scientists at the fourth International Academy of Astronautics (IAA) Planetary Defe​nse Conference this week are taking part in a simulated asteroid attack to discuss the best methods to take on incoming asteroids.

Methods up for discussion include the gravity tra​ctor and the ion ​beam shepherd, which would both deflect an asteroid out of Earth's way, as well as the more contentious and high-impact method of nuclear explosives.

"The gravity tractor and the ion beam shepherd are the subtle, slow-moving methods," co-chairman of the conference Richard Tremayne-Smith told me over the phone. He explained that these were the best methods if you wanted to make adjustments to an asteroid's flight path, and had a period of around ten to 20 years to do that in.

The gravity tractor is like a spacecraft, which flies next to the asteroid it wants to nudge off course for several months or years. It gradually pulls the asteroid out of the orbit it was in by a slight gravitational pull. The ion beam shepherd generates a beam of quasi neutral plasma from its engine, and uses the momentum of ​impin​ging propellant ions rather than gravity to deflect the asteroid away from Earth.

A more radical approach to asteroid deterrence is the nuclear option. Unlike the gravity tractor and ion beam shepherd, which apply a small force over a longer period of time, the nuclear explosives method is high-powered. "If you have little time, then you need a bigger force, and the biggest power source we have is nuclear power," explained Tremayne-Smith.

"When you're using a nuclear explosive, you're talking about vaporising or pulverising the object, and totally disintegrating it," he said. This would entail an anti-a​steroid spacecraft, equipped with a nuclear warhead, being sent up to face and blow up the incoming asteroid. While the method is effective, it's not totally foolproof, as if missions are miscalculated, there's still a risk of now-irradiated asteroid debris hitting Earth. But Tremayne-Smith said the whole idea would be to design a mission in a controlled way to avoid any radioactive asteroid smithereens contaminating Earth.

While most scientists lean toward using the less high impact gravity tractor and ion beam shepherd methods, he noted that people would consider the nuclear option "when a force was required for a relatively large object in a short space of time."

The nuclear explosives method is not new. The idea existed even before the planetary defense notion was born, explained Tremayne-Smith.

"This is the oldest method and dates back to when the nuclear bomb was used. People thought, 'What else can we do with nuclear?' Using a nuclear device against an asteroid was proposed before planetary defense was mentioned," he said.

But he said this method would cause the largest divide among people. "There's a feeling among a significant number of people that if that was the only way to save humanity then we should use it. But it's not one that should be publicised," he said, noting the ill-feeling and stigma associated with the word "nuclear." Tremayne-Smith noted that perceptions might change if it was dubbed something like the "High Energy Impactor to Save the World."

"The consensus of most people would be to go more towards using kinetic impactors to deflect an object without changing the momentum of the asteroid, rather than trying to fracture or break it into many parts," he said.

In recent years, much research and many missions have revolved around safeguarding Earth from asteroids. From NASA's asteroid "redirect ​missions" to "sma​shing them with a satellite," the methods are plentiful.​

So where are we at in terms of asteroid defense? Although there have been significant improvements over the years, Tremayne-Smith said that ultimately, we needed "more missions" and the right kit.

"We need a tool box, where you've got all the right spanners and all the right tools, so that if suddenly with relatively short notice we need to go out on a mission, we'll have all the ideas, we've done studies, we've got concepts and we've got the basic communications and mechanisms in place."

He added that one idea would be to make devices needed for asteroid defense cheaper, and bring the weight down on current equipment used out in missions. Aside from the need for more missions, and the data which would allow for better modelling and predictions, he emphasised the importance of international collaboration.

"We need to be looking at who has launch capability if an asteroid happens to be going over the north of India, who doesn't, and how new alliances can be formed, for example," said Tremayne-Smith. "We hear how astronauts looking down on Earth don't see the boundaries. An asteroid knows no national boundaries, it can go from one country to one culture to one religious belief to another one, the whole reaction of that country can be completely different, and we should be prepared to deal with that."