On September 26, 2022, NASA deliberately crashed a spacecraft into Dimorphos, a small moon orbiting the asteroid Didymos. The collision changed how the moon moved around its asteroid.
The mission was DART, the Double Asteroid Redirection Test. Neither Didymos nor Dimorphos threatens Earth; they offered a place to test whether a spacecraft could push an asteroid off course. At impact, they were about 11.3 million kilometers from Earth, nearly 30 times the distance to our Moon.
Didymos and Dimorphos at the September 2022 impact. The lower ruler shows their distance from Earth using average Earth–Moon distances. Planet symbols are enlarged; the paths around the Sun are not DART’s flight. Source: JPL Horizons; Luniter calculations.
Didymos is about 780 meters wide. Its moon, Dimorphos, is about 160 meters wide, and DART’s target was that smaller rock. Before impact, Dimorphos orbited with its center about 1.2 kilometers from Didymos’s center. Astronomers could time those orbits before and after the collision to see what changed.
Above, Didymos and Dimorphos at their preimpact separation. Below, the asteroid models and Lower Manhattan share one physical scale. Violet marks Didymos; blue marks Dimorphos, DART’s target. Placement and colors are illustrative; neither asteroid threatens Earth. Sources: NASA PDS; NYC Open Data; Luniter rendering.
The impact
Johns Hopkins Applied Physics Laboratory built and operated DART for NASA. The spacecraft had a mass of about 579 kilograms and struck at 6.1 kilometers per second. Imagine a 10,000-metric-ton freight train moving at 105 miles per hour: it would carry roughly the same energy of motion. DART packed that energy into a spacecraft weighing less than a small car.
The collision also blasted material off Dimorphos. As debris flew away, the remaining moon recoiled in the opposite direction, adding to the push from DART itself. How much material escaped, and where it went, helped determine the moon’s response.
Escaping debris adds recoil to Dimorphos. The archived asteroid shape and impact site anchor this illustration; the approach, particles and arrows explain the mechanism rather than reproduce measured debris paths. Sources: NASA DART results; NASA PDS.
Dimorphos’s changed orbit
The push acted against Dimorphos’s direction of travel, slowing it slightly. With less forward speed, the moon could no longer follow its old path around Didymos. Gravity pulled it onto a path that passed closer to the asteroid.
As Dimorphos moved inward, gravity increased its speed. The moon followed a smaller orbit and completed each circuit sooner. The backward push changed the path it would take; it did not move the moon sideways into a new orbit at the instant of impact.
Before and after the push. The new path begins at the same location and passes inside the former orbit. This and the next animation share a simplified model; inward changes are enlarged fourfold for visibility. Sources: NASA orbital mechanics and measured periods; Luniter illustration.
In the measurements, Dimorphos’s average separation from Didymos decreased by about 37 meters. An orbit that had taken about 11 hours and 55 minutes now took about 11 hours and 22 minutes, a 33-minute reduction. Its actual path became slightly elliptical and continued changing as more material escaped.
Astronomers measured the new timing with telescopes on Earth. Whenever Dimorphos and Didymos eclipsed or passed in front of one another, their combined brightness dipped. Timing those repeated dips revealed how long Dimorphos took to return to the same position in its orbit.
Two views of the same moon, running on one clock. The after-impact model initially moves more slowly, passes closer to Didymos and completes its lap first. Periods are measured and rounded; paths are illustrative, with inward changes enlarged fourfold. Source: NASA’s orbital analysis; Luniter calculation.
Hera’s return
Four years later, the European Space Agency’s (ESA) spacecraft Hera is approaching Didymos and Dimorphos to investigate the moon’s altered orbit, mass and structure. To stay nearby, Hera must use its thrusters to slow its approach and closely match the asteroids’ motion. Otherwise, it would fly past.
ESA’s published approach plan called for a 93-minute braking burn on October 15 and another lasting about 31 minutes on October 22. Smaller adjustments would follow, with close-up observations beginning in November, initially from about 25 to 30 kilometers away.
Follow the labeled Hera spacecraft along the planned route. At October 22, the view zooms about 24 times closer, revealing a bend that is hard to see at the wider scale. Red marks scheduled burn days. This uses a prediction retained October 13, not confirmed flight results. Sources: ESA; JPL Horizons.
Hera will refine our estimate of Dimorphos’s mass, which is needed to work backward from the changed orbit to the strength of the push. A lighter moon would respond more to the same transfer of momentum. Cameras and laser ranging will map the surface, while radio tracking measures how gravity affects the spacecraft’s motion. A small spacecraft traveling with Hera, Juventas, carries radar designed to probe inside Dimorphos.
The hardware behind the missions
Redwire (RDW), a spacecraft hardware supplier, contributed to both DART and Hera. Its roll-out solar arrays and Sun sensors helped power and orient DART. For Hera, Redwire supplied the onboard computer through OHB, the main spacecraft contractor. That computer controls spacecraft systems and handles mission data, helping the instruments’ measurements reach researchers on Earth.
Hera’s planned measurements connect surface shape, gravity and interior structure to Dimorphos’s response. Redwire supplied the onboard computer. Sizes and spacing are illustrative; the asteroid uses a preimpact model and no measured interior is depicted. Sources: ESA; NASA PDS; Redwire.
A hazardous asteroid would have its own mass, structure and orbit. Redirecting it would require a well-placed push, early enough to change where it would be when its path approached Earth’s. With years or preferably decades of warning, a small change in motion could accumulate into a large enough miss.
DART showed that a spacecraft could reach a small asteroid moon and change its motion. Hera will help explain how that moon responded, so we can better estimate what a collision would do to another asteroid. If we ever have to make that attempt for real, we will need more than the ability to hit the target. We will need to know how hard to push.
Anthony
The Luniter Observatory
Disclosure: I own shares of Redwire (RDW).
Research and educational analysis only — not financial advice. Every investment carries risk.
