Based on NASA’s observation data, an asteroid called “Bennu” has a 1 in 2,700-chance of striking Earth on Sept. 25, 2135. So, for now, don’t plan anything on that date.

The good news: We have some time to figure how to nudge the 500-meter rock off course.

Lawrence Livermore National Laboratory (LLNL) scientists have an idea to deflect the Earth-bound asteroid: a 9-meter-tall, 8.8-ton spacecraft dubbed the HAMMER.

The conceptual “Hypervelocity Asteroid Mitigation Mission for Emergency Response” vehicle would serve as a kind of a battering ram – delivering an impact speed of 22, 000 miles per hour.

So, how many HAMMERs would it take to bring down an asteroid?

Research from LLNL concluded that a blow from a single HAMMER spacecraft would not provide enough force to deflect an object like Bennu, which weighs in at 79 billion kilograms.

The study instead suggests deploying a whole fleet of HAMMERs. And the earlier, the better, according to Megan Bruck Syal, LLNL physicist and coauthor on the paper.

“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,” said Syal in a March 2018 press release.

The researchers’ study examined a number of deflection scenarios, ranging from launching 10 years before impact to 25 years in advance.

The 8.8-ton conceptual HAMMER spacecraft (right) is designed to fit within the Delta IV Heavy launch vehicle from United Launch Alliance. (Image Credit: LLNL)

With 10 years of lead time between launch and anticipated Earth impact, however, a single HAMMER impactor could deflect a 90-meter-diameter object by only 1.4 Earth radii – an insufficient amount to stop an asteroid hit, according to Syal.

“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,” said the LLNL physicist.

To defend against Earth-bound asteroids, the team proposes to send the “battering rams” via Delta IV heavy launch vehicles, developed by the Centennial, CO-based manufacturer United Launch Alliance.

The sooner the spacecraft reaches the asteroid, the less force the HAMMER will require to disrupt the asteroid’s path.

According to the analysis, 10-year scenarios would call for between 34 and 53 launches of the Delta IV Heavy rocket, each carrying a single HAMMER impactor. A 25-year lead time would reduce the number to 7 to 11 launches.

“The push you need to give it is very small if you deflect the asteroid 50 years out,” said Kirsten Howley, LLNL physicist and fellow co-author of the study.

The asteroid research is part of a planetary-defense collaboration between NASA and the National Nuclear Security Administration (NNSA), which includes LLNL and the Los Alamos National Laboratory.

While the LLNL team provides the threat-mitigation and emergency-response tasks, NASA is in charge of finding the asteroids.

In September of 2016, NASA launched a probe called OSIRIS-Rex. The “Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer” will spend two years in space, chasing down Bennu.

Using a 10'-long “pogo stick,” OSIRIS-Rex aims to collect samples from the asteroid and return them to Earth in 2023.

Read our interview with OSIRIS-Rex project scientist Jason Dworkin, who explains how the probe’s samples and trajectory calculations will help us better defend against asteroids — and better understand the clues to the formation of the solar system.