When NASA’s Double Asteroid Redirection Test (DART) hit Dimorphos, it was guaranteed to change the asteroid's course because of the fundamental physical laws of conservation of energy and momentum. If its orbit around Didymos does not change at all after the impact, then Newton’s laws of motion will have been disproved.

Did it work?

Source: Adapted from NASA/Johns Hopkins University APL; originally appeared in Nature https://doi.org/10.1038/d41586-021-03471-w (2021)

Analysis of data obtained over the past two weeks by the DART investigation team shows the spacecraft's kinetic impact with its target asteroid, Dimorphos, successfully altered the asteroid’s orbit. This marks humanity’s first time purposely changing the motion of a celestial object and the first full-scale demonstration of asteroid deflection technology.

NASA successfully nudged Dimorphos into a different orbit, but was off by a factor of 3 in predicting the change in period, apparently due to the debris ejected. Will we also need to know the composition and structure of a threatening asteroid, to reliably deflect it away from an Earth strike?

The last bits of data that came from the DART spacecraft right before impact showed that it was on course. The fact that the images stopped transmitting after the target point was reached was the first sign of success.

Fifteen days before the impact, DART released a small satellite with a camera that was designed to document the entire impact. The small satellite has been sending photos of the impact back to Earth during early October 2022. A number of Earth-based telescopes as well as some satellites in orbit, including Hubble and James Webb, were watching Didymos at the time of the impact as well.

Using data from these telescopes taken at the time of impact as well as over the following weeks, the DART team at NASA has been able to calculate just how much the impact deflected the orbit of Dimorphos. Before DART, it took 11 hours and 55 minutes for the smaller moonlet to orbit the larger asteroid Didymos. The energy from the impact shortened Dimorphos’s orbit by 32 minutes – showing the impact to be more than 25 times more effective than NASA’s conservative goal of 72 seconds.

Before its encounter, NASA had defined a minimum successful orbit period change of Dimorphos of 73 seconds or more. This early data show DART surpassed this minimum benchmark by more than 25 times. 

The Scientific Impact of Future Impacts

A Hubble Space Telescope image of the asteroid Dimorphos shows a tail of debris 10,000 kilometers long created by the impact of the DART spacecraft, which changed the orbit of Dimorphos around the larger asteroid Didymos by 32 minutes. Credit: NASA/ESA/STScI/Hubble

NASA's Dart strike on Dimorphos modified its orbit by 32 minutes, instead of the 10 minutes NASA anticipated. Uncertainty is expected, and a bigger-than-predicted effect would seem like a good thing, but this is a big difference because of the amount of debris "hurled out into space, creating a comet-like trail of dust and rubble stretching several thousand miles." Does this discrepancy really mean that knowing its mass and trajectory isn't enough to predict what sort of strike will generate the necessary change in the trajectory of an asteroid? Will we also have to be able to predict the extent and nature of fragmentation? Does this become a structural problem, too?

In short, the answer is yes. To a degree.
Knowing the materials of the asteroid will have a large importance in successfully changing the course of an asteroid on its way towards earth. But will this consideration be enough to limit us from redirecting an asteroid away from the earth? Most likely no, provided we impact the asteroid early enough to cause a significant change in its course.

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