Last year, NASA's DART spacecraft successfully completed its mission: To collide with an asteroid called Dimorphos to see if it was possible to change the trajectory of any potentially planet-killing space rock. Scientists from the DART team have been analyzing the data collected from the mission since then, and they've now published five papers in Nature explaining the details of DART's results. They've also decided that, yes, the method can be used to defend Earth if ever an asteroid big enough to kill us all heads our way.
Apparently, one of DART's solar panels hit Dimosphos first before its body fully collided with the rock at 6km per second (3.7 miles per second). The spacecraft smashed into the asteroid around 25 meters (85 feet) from its center, which was a huge factor in the mission's success, since it maximized the force of the impact. According to the studies, the collision had managed to eject 1 million kilograms or 1,100 tons of rock from Dimorphos. That spray of rubble flew outwards away from the asteroid, generating four times the momentum of DART's impact and changing Dimorphos' trajectory even further.
While NASA has only tested the mission on one space rock, scientists have concluded that for asteroids as big as Dimorphos (around 560 feet across), we don't even need to send an advance reconnaissance mission. As long as we get at least few years of warning time, though a few decades would be preferable, then we will be able to intercept future asteroid threads. Franck Marchis at the SETI Institute in Mountain View, California, told Nature: "[W]e can quickly design a mission to deflect an asteroid if there is a threat, and we know that this has a very high chance of being effective."
We're bound to get an even better look at the mission's effect on the asteroid after European Space Agency's Hera spacecraft arrives at Dimorphos in 2026. The mission will study the binary asteroid system Didymos and Dimorphos to further validate DART's kinetic impact method or future use.