WASHINGTON — A NASA spacecraft that deliberately collided with a near Earth asteroid last month changed its orbital period by more than a half-hour, exceeding expectations for the planetary defense demonstration.
At an Oct. 11 briefing, NASA Administrator Bill Nelson announced that the Double Asteroid Redirection Test (DART) spacecraft, which collided with the asteroid Dimorphos Sept. 26, changed the asteroid’s orbit around a larger asteroid, Didymos, by 32 minutes. Dimorphos, which previously took 11 hours and 55 minutes to orbit Didymos, now completes an orbit in 11 hours and 23 minutes, based on ground-based optical and radar observations.
“This is a watershed moment for planetary defense and a watershed moment for humanity,” said Nelson. “This mission shows that NASA is trying to be ready for whatever the universe throws at us.”
DART collided with Dimorphos as a test of the “kinetic impactor” technique that could be used to deflect an asteroid on a trajectory to impact the Earth. The mission had a requirement to change the orbit by at least 73 seconds, and models from before the impact predicted a change of between a few minutes and several tens of minutes, said Lori Glaze, director of NASA’s planetary science division. The revised orbit, with a precision of plus or minus two minutes, is consistent with those models, “but clearly at the upper end of that range.”
The revised orbit came from observations from four telescopes in Chile and South Africa that monitored the lightcurve, or change in brightness over time, of the combined Didymos-Dimorphos system. Those data, analyzed by two groups in different ways, reached the same measurement of the new orbital period, said Nancy Chabot, DART coordination lead at the Johns Hopkins Applied Physics Laboratory. Radar observations from observatories in California and West Virginia also measured the same orbital period.
“This is a very exciting and promising result for planetary defense,” Chabot said. “It’s definitely indicating that you’re getting an enhanced deflection due to the amount of ejecta, that rocky material being thrown off when DART’s collision happened.”
That ejecta may be linked to Dimorphos being a “rubble pile” asteroid, a collection of smaller rocks rather than a single intact body. “We expect a solid rock to be less responsive than a pile of gravel,” said Tom Statler, DART program scientist at NASA Headquarters.
“When I saw Dimorphos come into view and when I saw there was not a single crater on it, and there were a lot of what appeared to be loose rocks,” he said, recalling some of the last images returned by DART before its impact, “I looked at it and I said, ‘This is not going to be 73 seconds.’”
Scientists are still working to measure the overall effectiveness of the impact, a value known as beta; a higher value means a greater change in momentum of the asteroid. “It seems virtually certain that the ejecta were a significant contributor to the period change, so we know beta is not equal to one, because that would have been no ejecta,” Statler said.
Observations of Didymos and Dimorphos will continue for months, including those from large ground- and space-based telescopes that show a tail of debris extending for 10,000 kilometers from Dimorphos. An Italian cubesat called LICIACube, deployed from DART in the weeks before the impact, flew by the asteroid minutes afterwards and took 720 images that show details of the ejecta plume not visible from Earth.
The test provides optimism for NASA that a similar spacecraft could deflect a hazardous asteroid. “Dimorphos is a size of asteroid that is a priority for planetary defense,” said Chabot. The object is 160 meters across, large enough to cause damage on a regional scale if it hit the Earth. “It’s the first time we’ve been to an object that size and see how it reacts.”
Statler cautioned not to generalize the success of this test for all asteroids. “This is one test done on one asteroid. What we’re learning every time we send a mission to another asteroid is that each asteroid has a different part of the story of our solar system’s past to tell,” he said. “We should not be too eager to say that one test on one asteroid tells us how every other asteroid would behave in a similar situation.”
NASA has no plans to conduct additional impact tests like DART. Glaze said NASA’s priority now for planetary defense is the Near Earth Object (NEO) Surveyor mission, a space telescope to discover more such asteroids. However, NASA’s fiscal year 2023 budget request slashed funding for the mission and pushed out a planned 2026 launch to 2028. NASA is also considering a recommendation by the planetary science decadal survey in April to follow up NEO Surveyor with a “rapid response” mission to study an asteroid on short notice as practice if a threatening asteroid was discovered.
“We are capable of deflecting an asteroid” of the size of Dimorphos, Glaze said. “It’s been an incredibly successful demonstration of the kinetic impactor. Hopefully, we’ve got that tool in our toolkit now.”