The dinosaurs were wiped out by an asteroid. As asteroids swing around our sun, sometimes coming dangerously close to our planet, the sight of saurian fossils in most any science museum is a potent reminder. Scientists have found a way to save the planet. The Double Asteroid Redirection Test crashed into a small asteroid in the past hour.

The impact was not an accident. It's meant to shift the space rock's trajectory by a tiny but noticeable amount, which observers will carefully confirm and track from afar with a plethora of ground and space-based telescopes. If a dangerous asteroid is found on a collision course with Earth, we may be able to use this same technique to get it off course. The DART investigation team is not blowing up the Death Star, according to Andy Rivkin. The asteroid is being changed through the use of the spacecraft.

DART launched in November of 2016 on a collision course with Dimorphos, a 160 meter asteroid that is five times larger than Didymos. As it approached the asteroids, the vending machine-sized, circa 600-kilogram spacecraft took sharper images. At 715 P.M., engineers at APL stopped receiving signals from the craft, which had slammed into Dimorphos.

The director of NASA's planetary science division said during the space agency's livestream of the event that "we are embarking on a new era for humankind." An era in which we have the ability to protect ourselves.

The asteroid was hit with three metric tons of energy and exploded in a shower of metal and asteroid debris. The impact images that will be released in the coming days were taken by a small Italian satellite called LICIA. The true mission is just beginning. Scientists will watch Dimorphos with everything from ground-based telescopes to deep-space observatories to see how much of a difference DART made. Harrison Agrusa of the University of Maryland is a member of the DART team.

Two decades ago, scientists in the U.S. and Europe began to discuss a mission that could practice a technique for asteroids. The mission was originally called AIDA and involved NASA's DART and Europe's AIM. The cancellation of AIM in 2016 was due to a lack of funding. The mission was reborn as the Hera spaceship in the year 2019. Delayed launch means Hera won't lift off until 2024 and won't arrive at Didymos until 2026, making it too late to see DART's impact but still in time to study its effects.

The scientists wanted DART's target to be a pair of asteroids, so that they could measure the impact of a collision. PatrickMichel is the principal investigator of Hera and was the lead scientist of AIM. Scientists chose the Didymos system as the target. The discovery of a small companion in 2003 led to the creation of Dimorphos, or to have two forms.

Graphic shows how the DART mission will work to slow the moonlet Dimorphos’s orbit around the larger asteroid Didymos.
Credit: Matthew Twombly; Source: NASA, Johns Hopkins APL (DART reference)

Didymos is complete by dimorphos every 11.94 hours. The asteroids don't pose a threat to us because they never come closer than a few million kilometers. Dimorphos are able to be measured because they are in front of Didymos. NASA's Lucy probe, which is currently on its way to visit asteroids near Jupiter, is one of the telescopes that will track this eclipse.

DART hit the asteroid almost head on. Mission scientists didn't know whether Dimorphos was a rigid and solid object or a loose pile of rocks and boulders. DART beamed back images of Dimorphos's rubble-strewn surface, which indicated the asteroid was not rock-solid. DART would have transferred only a small amount of inertia to the asteroid if it had been. Rivkin says that the mission needs at least 73 seconds of change. The shabby appearance of Dimorphos suggests the force of outward-spewing material could cause a larger shift in the asteroid's trajectory. Such an event could cause Dimorphos to fall over. The larger the crater, the weaker the asteroid is. We want there to be a lot of flight and ejection.

Graphic shows seven possible ways to deflect an asteroid, including the method employed by the DART mission.
Credit: Matthew Twombly

The DART team is expected to announce preliminary results from the mission at a meeting of the American Geophysical Union in Chicago in December. The mission will not be known for certain until Hera arrives. It will be possible to measure the mass of Dimorphos and get a better sense of how much has changed around Didymos thanks to the observations of that spaceship. An understanding of what Dimorphos is made of will be gained by the DART team.

If DART is ever used to save Earth, that could be important information. Koschny says that this is one of the most important things being done at the moment. We need to show that we can stop an asteroid for a long time. Smaller asteroids like Dimorphos are less likely to be on an impact course with our planet, with only an estimated few percent of their population known. Koschny doesn't feel safe yet. An impact by a Dimorphos-sized space rock could cause widespread damage to a country, meaning there is good reason to look out for such asteroids.

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Upcoming telescopes such as the Vera C. Rubin Observatory will help track these asteroids. If we ever find one on a collision course with Earth, the outcome of the DART mission will affect what we do. Rivkin says that the tool could be used. To divert a hazardous asteroid, perhaps a larger version of DART could be used or a series of DART-sized spaceships to slam into the offending space rock. Rivkin says it depends on how long we have warning time. A perilous event is not likely to happen soon. It is possible that our distant descendants will have this small craft to thank. Agrusa says that if we can divert Dimorphos, we can most likely divert other asteroids.