We owe our very existence to asteroids. But if one of these cosmic boulders, let’s say the size of a mountain, should come hurling towards Earth at any day now, it would be a nemesis. The odds of asteroids hitting us remains astronomically low, but if it should happen, how do we stay prepared?
Scientists at Massachusetts Institute of Technology (MIT) have concocted a framework of decision to stipulate the highly effective mission to deflect an asteroid that’s on collision course with our planet. The framework factors in things like – an asteroid’s mass and momentum, amount of warning time of an impending collision estimated, and how long until it crosses the Earth’s gravitational keyhole.
The researchers also key in degrees of uncertainty associated with the aforementioned determinants to single out the most successful mission for a given asteroid.
Following careful assessment, the framework helps the researchers settle on three choices, and they are:
- Pushing the asteroid off course with a projectile,
- Sending an investigatory scout to measure the asteroid so as to calibrate the projectile that would be dispatched thereafter, or
- Sending two scouts, one for measuring the asteroid and the other for moving the asteroid slightly off the rail prior to launching a larger projectile.
“People have mostly considered strategies of last-minute deflection, when the asteroid has already passed through a keyhole and is heading toward a collision with Earth,” explains Sung Wook Paek, lead author of the study. “I’m interested in preventing keyhole passage well before Earth impact. It’s like a preemptive strike, with less mess.”
For mission to yield positive outcome, variables such as the asteroid’s mass, momentum, trajectory, and surface composition must be known precisely. So this means that the mission literally is chock full of uncertainty because some of these variables are almost impossible to know at all that precisely.
In order to pare down the level of uncertainty involved, Peak and his team developed a simulation code to identify mission with the highest possibility of success. They fed the simulation all the variables involved and the degrees of uncertainty associated with each of those. They also took into account the distance between the asteroid and the Earth’s gravitational keyhole, and the amount of time before it zooms past the keyhole.
“Does it matter if the probability of success of a mission is 99.9 percent or only 90 percent? When it comes to deflecting a potential planet-killer, you bet it does,” says Oliver de Weck, a professor of aeronautics and astronautics and engineering systems. “Therefore we have to be smarter when we design missions as a function of the level of uncertainty. No one has looked at the problem this way before.”
The team ran their simulation on the asteroids Apophis and Bennu, which are two of the closest with known gravitational keyholes positions with respect to Earth, and among those with the highest probability of hitting Earth one day. They simulated the proximity between the asteroids and their respective gravitational keyholes, and defined a “safe harbor” region where an asteroid would have to be deflected so as to avoid both an impact with Earth and transit to any other nearby keyhole.
In 2068, the asteroid Apophis will make its closest approach at 19,400 miles (31,200 kilometres) above Earth’s surface, and the chance of it hitting us is 1 in 150,000. Bennu, on the other hand, will pass Earth at 460,000 miles (750,000 kilometers), and the odds of making an impact are 1 in 2,700 between 2175 and 2199.
So this means that scientists have considerable amount of time to weigh which of the three missions would have highest probability of success in pushing them into a safe harbor region.
If Apophis, for example, is five or more years away from hitting the Earth’s gravitational keyhole, instead of resorting to projectile, sending two scouts should do just the job, that is – of measuring the asteroid’s dimensions and sending it off the track. If it crosses a keyhole within two to five years, sending a scout to measure it and work out the parameters of a larger projectile before sending the impactor up might still get the job done. But once the keyhole passage occurs in less than a year, then it might already be too late to stop it from hitting Earth; even the projectile might not be able to reach it within this span.
The results were same in the case of Bennu. Although less is known about its composition, launching a projectile might be the best approach to deflect it.
“Instead of changing the size of a projectile, we may be able to change the number of launches and send up multiple smaller spacecraft to collide with an asteroid, one by one. Or we could launch projectiles from the moon or use defunct satellites as kinetic impactors,” Paek says. “We’ve created a decision map which can help in prototyping a mission.”
With Jupiter actively engaged in pelting dangerous objects into the inner solar system, it is never too early to come up with strategies for deflecting them if one were to ever sneak up on us. The team’s new simulation tool may serve just the purpose.
The study has been published in the journal Acta Astronautica.