On a Sunday morning in September about 117 years from now, a 500-meter (1,640-foot) asteroid will fly past Earth five times closer than the Moon. The asteroid has a one-in-2,700 chance of hitting us on September 25, 2135.
The asteroid is not just another anonymous space rock, but is actually quite well known. Its name is Bennu and NASA currently has a mission, called OSIRIS-REx, that aims to go to the asteroid, collect a sample, and then return it to Earth in 2023. The asteroid was selected because it is a remnant of the early Solar System and it could help us understand the beginning of the Earth in greater detail. So we decided to go to it before it comes to us.
If Bennu was to hit Earth, the impact would have a kinetic energy equivalent to 80,000 Hiroshima nuclear bombs, about 1,200 megatons. For this reason, a group of US researchers used this potential impact as a case study on what to do. The research, published in Acta Astronautica, describes a potential spacecraft that could be used to deviate the asteroid.
“The chance of an impact appears slim now, but the consequences would be dire,” co-author Kirsten Howley, from the Lawrence Livermore National Laboratory (LLNL), said in a statement. “This study aims to help us shorten the response timeline when we do see a clear and present danger so we can have more options to deflect it. The ultimate goal is to be ready to protect life on Earth.”
The mission is called Hypervelocity Asteroid Mitigation Mission for Emergency Response vehicle, or HAMMER, which is a fitting backronym because the spacecraft can be rammed into the asteroid. This impact would nudge the asteroid on a slightly different orbit.
The team looked at what it would actually take to deliver such an impact – 7.4 years from construction to delivery. And just one might not be necessary. They modelled that if we wanted to deflect Bennu 25 years before its closest approach, it would take between seven and 11 HAMMER impacts. If we only had 10 years, that range would increase to between 34 and 53.
“When many launches are required for a successful deflection, the mission success becomes more difficult, due to the failure rate associated with each individual launch,” added Megan Bruck Syal, LLNL physicist and co-author on the paper. “If we only had 10 years from launch, we would need to hit Bennu with hundreds of tons of HAMMER mass just to barely deflect it off of an Earth-impacting path, requiring dozens of successful launches and impact at the asteroid.”
A single HAMMER can deflect an object 90 meters (295 feet) in diameter by about 17,000 kilometers (10,500 miles) with a 10-year lead-up.
However, there is an alternative to multiple HAMMER vehicles. The craft can be also fitted with a nuclear device to deliver that kick. The impact needs to be powerful enough to move Bennu, but not too powerful that it breaks apart. The team hasn’t modelled the effect of a nuclear device on a big NEOs such as Bennu, but it's believed that such an explosion would deliver a more drastic change in trajectory.
While it’s likely that Bennu will not hit us, we should not underestimate the threat of these space rocks. Currently, we have only discovered 1.5 percent out of the 1 million hazardous asteroids larger than 30 meters (100 feet) that could one day hit us.