It sounds like the plot of a Hollywood movie: NASA is hurling a spacecraft on a collision course with a distant asteroid to protect Earth. Except this is real life, minus the nukes and Bruce Willis.
At 7:14 p.m. EDT (23:14 GMT) some 6.8 million miles (11 million km) from Earth, NASA's DART spacecraft successfully slammed into the asteroid Dimorphos, which is about the size of a football stadium. The mission, called Double Asteroid Redirection Test (DART), is humanity’s first-ever test for a planetary defense method, although Dimorphos is totally innocent in this instance, in the sense that it is orbit posed no threat of collision with Earth. It just happened to be the perfect target for practice that checked all of NASA's boxes.
The idea is to slam DART into Dimorphos with just enough energy to move it off course. It’s a test that will provide valuable lessons for when we need to avert a collision with an asteroid that may pose an actual threat. The entire interaction was recorded by a small Italian satellite launching from the DART spacecraft. Although it's been only hours since DART completed its mission, NASA officials have already hailed the mission as a success.
"NASA works for the benefit of humanity, so for us it’s the ultimate fulfillment of our mission to do something like this - a technology demonstration that, who knows, some day could save our home," NASA Deputy Administrator Pam Melroy, a retired astronaut, said minutes after the impact.
DART launched in late November 2021 from a SpaceX Falcon 9 rocket that took off from the Vandenberg Space Force Base in northwestern California. Once in orbit, the DART spacecraft detached from the Falcon 9 and cruised through space for about a year until it encountered a pair of small asteroids — a larger one, known as Didymos, and its orbiting ‘moonlet’, called Dimorphos. Both asteroids are relatively tiny compared to the massive Chicxulub asteroid that struck Earth some 66 million years ago and wiped out all non-avian dinosaurs.
For most of its voyage, the spacecraft was guided by mission control here on Earth. During the final hours of the journey, control was relinquished to the autonomous onboard navigation that is able to respond lighting fast to changing conditions, although mission controllers could intervene up to five minutes before impact if the spacecraft steered off course.
DART slammed directly into Dimorphos at 15,000 miles per hour (24,000 kph), transferring enough kinetic energy to the asteroid that NASA hopes to shift its orbit track closer to the parent asteroid. Preliminary data suggest that DART was completely destroyed by the impact and left a small crater on the asteroid's surface.
The spacecraft had no explosive material, relying solely on its kinetic energy to deflect Dimorphos. This is by design because NASA views blowing up stuff in space as too risky. In the event that a killer asteroid is detected on a collision course with Earth, detonating explosives or even nuclear weapons could break apart the asteroid into many smaller pieces that can be impossible to track and could end up impacting the planet in multiple locations.
The goal is to shorten the orbital path of Dimorphos by 10 minutes, but anything more than 73 seconds is considered a success by NASA, meaning it proves such an exercise is a viable strategy of deflecting a dangerous asteroid -- as long as we have years of warning in advance.
Asteroid deflection and planetary defense, in general, are still in their infancy, but since the consequences can be devastating such missions are critically important. DART cost about $300 million, but scientists think that's woefully insufficient to tackle such a challenging task as planetary defense.
"The nature of the threat is different from most of the threats we face. It's a fairly low probability -- it's unlikely that this will happen in our lifetimes, but if it does it could be catastrophic. Risk depends on both probability and consequences, and an asteroid impact is low probability, high consequences, which means the risk is significant. There's a low probability that we could get hit by an asteroid of the size that could destroy a city or even a nation and we can't let that happen," Mark Boslough, a physicist at Los Alamos National Laboratory, research professor at University of New Mexico and expert in planetary impacts, told ZME Science during an interview that you can watch in full on our YouTube channel.
Although the DART's mission has officially ended, for asteroid scientists this is just the beginning. The moment DART crashed into Dimorphos was captured by a camera mounted on a briefcase-sized satellite released from the spacecraft days in advance of the impact, as well as ground-based telescopes and the Hubble and Webb space telescopes.
Images from these observations are expected to be released in the coming days, but researchers will be busy studying the consequences of this impact for years to come. For instance, scientists will keep an eye out for changes in brightness of the Didymos-Dimorphos system -- by analyzing specific colors of light it is possible to determine the composition of the two asteroids. And if you thought that this was the last time a spacecraft will visit Dimorphos, think again. Years from now, Hera, a spacecraft currently being built by the European Space Agency will visit the two asteroids and take pictures of the crater left by DART, as well as make precise measurements of the asteroid's mass.
“Planetary Defense is a globally unifying effort that affects everyone living on Earth,” said Thomas Zurbuchen, associate administrator for the Science Mission Directorate at NASA Headquarters in Washington. “Now we know we can aim a spacecraft with the precision needed to impact even a small body in space. Just a small change in its speed is all we need to make a significant difference in the path an asteroid travels.”
“DART’s success provides a significant addition to the essential toolbox we must have to protect Earth from a devastating impact by an asteroid,” said Lindley Johnson, NASA’s Planetary Defense Officer. “This demonstrates we are no longer powerless to prevent this type of natural disaster. Coupled with enhanced capabilities to accelerate finding the remaining hazardous asteroid population by our next Planetary Defense mission, the Near-Earth Object (NEO) Surveyor, a DART successor could provide what we need to save the day.”