A 15-kilometer-wide asteroid hit the planet 66 million years ago, ending the Cretaceous Period. It wiped out 99 percent of all life and 75 percent of all species of life on Earth.
If we were to discover another such a threat coming from space today, what could we do about it?
NASA’s Double Asteroid Redirection Test launched Nov. 24, 2021 from Vandenberg Space Force Base in California. The DART spacecraft was built and is operated by the Johns Hopkins Applied Physics Laboratory under NASA’s Planetary Defense Coordination Office.
DART’s mission was to slam into Dimorphos, the smaller asteroid of a binary pair, and change its orbital period. The purpose is to test our ability to achieve a kinetic impact on an asteroid and to observe the asteroid’s response. The mission team will measure how much the impact changed the asteroid’s orbit of its counterpart Didymos. DART is using kinetic deflection as a straightforward method of diverting an asteroid. Kinetic impact deflection is just one of several proposed ways to redirect potentially hazardous asteroids and is currently the most technologically mature.
DART’s Target
Didymos is the primary body of the binary system. The name means “twin” in Greek. It orbits the sun once every 770 days. The elliptical orbit passes through the asteroid belt beyond Mars to within 10 million kilometers from Earth. This orbit classifies it as an Apollo asteroid. Dimorphos is the smaller of the pair. Its name means “two forms.” If Dimorphos impacted the Earth it could cause regional devastation with an explosive force of three hundred Hiroshima bombs. Its orbit does not place it within range of being any threat to Earth, but the pair was ideally suited for the redirection test. They are eclipsing binary asteroids as seen from Earth. Dimorphos, at 160 meters diameter, passes in front of larger Didymos, at 838 meters diameter. Dimorphos’ orbit of Didymos, 11 hours 55 minutes, is known exactly. The impact was planned for Sept. 26, 2022, with the pair passing Earth at its closest orbital point.
DART is testing several innovative technologies, including a new autonomous guidance system. The Small-body Maneuvering Autonomous Real Time Navigation system allows the DART spacecraft to guide itself. The DART spacecraft has a single instrument, the Didymos Reconnaissance and Asteroid Camera for Optical navigation. It is a high-resolution imager derived from the New Horizons Pluto mission camera. DRACO performs navigation and targeting to measure the size and shape of the asteroid target. It also defines the impact site and geologic context. DRACO autonomously maneuvers the spacecraft during its last four hours before impact. The DART team fine-tuned the DRACO system early in the mission by sighting on Jupiter’s moon Europa. DART is also testing NASA’s Xenon Thruster-Commercial, or “NEXT-C. It is a solar-powered ion propulsion system with potential application to future deep-space missions.
The DART spacecraft is a box with dimensions of roughly 1.2 by 1.3 by 1.3 meters – slightly larger than a washing machine. Two very large solar arrays were deployed. They are each 8.5 meters long.
The DART craft navigated to crash into Dimorphos at a speed of approximately 3.8 miles per second – about eight times the speed of a rifle bullet. The total mass of the DART spacecraft was approximately 1,345 pounds at launch and roughly 1,260 pounds at impact. This was expected to divert Dimorphos into a lower orbit and shorten its orbital period by at least 10 minutes.
Mission Outcome
The DART impact was spectacularly successful. It occurred on Sept. 26, 2022, 7:14 p.m. EDT. The event was broadcast live on NASA TV. The live feed showed Didymos and Dimorphos getting larger by the second. The last frames showed the target asteroid Dimorphos in detail as a rubble pile with car-sized boulders strewn everywhere. The first proof of impact was that the last image frame sent was truncated with a crimson red “end of signal”-message. The impact was photographed from 55 kilometers’ distance by the Light Italian CubeSat for Imaging, a probe about the size of a microwave oven. It was released by DART days earlier to be the robotic witness. The impact was also photographed by Earth-based and other space-based telescopes, notably the Hubble and James Webb Space Telescopes.
The impact spread dust and debris, masking the impact crater created on Dimorphos’ rubbly surface. Images taken days later showed the dust expanding like a comet’s tail driven by solar wind. Detailed observations are ongoing.
NASA held a news briefing on Oct. 10, two weeks after the impact, announcing the mission success. JPL Goldstone radar observations and southern hemisphere JPL Los Cumbres Observatory telescope independently verified that DART’s impact had indeed changed the orbital period of Dimorphos to 11 hours and 23 minutes. That’s a change of 32 minutes, or 4 percent – much more than the 10 minutes for which planners had hoped.
The DART Mission proved that if an asteroid is discovered to be an imminent impact threat to Earth, kinetic impact deflection could be used to slightly alter the asteroid’s trajectory to avert the impact disaster. The chances of success are greatly improved if the asteroid threat is discovered early, at a great distance from Earth.
Guarding the Earth from Extraterrestrial Threats
A number of surveys are looking for potentially hazardous asteroids that could pose a global threat. These are asteroids greater than 300 meters diameter that cross closer than 8 million kilometers from Earth. The sky surveys are part of the Near-Earth Object Observations program that began in 1998. The Catalina Sky Survey of University of Arizona Lunar and Planetary Laboratory has the largest asteroid discovery rate. Their Spacewatch Survey runs follow-up observations of found asteroids to detail their size and orbital parameters. The University of Hawaii runs the Pan-STARRS Survey, which is currently the largest digital sky survey. It used two 1.8-meter telescopes equipped with 1.4 gigapixel cameras. The survey not only finds NEOs, but has also found numerous comets, Kuiper-belt objects and supernovae. NEOWISE is an orbiting infrared probe that can boast to have discovered more than 34,000 new asteroids, detailed their orbits, sizes, and probable compositions.
The surveys collectively have found 90 percent of the NEOs greater than a kilometer in diameter. Only 40 percent of asteroids greater than 140 meters diameter have been found. None of these have the potential to strike Earth in the next century. This still leaves us to wonder about the unknown 60 percent of that population. If an asteroid were found to be a threat to the Earth, a kinetic deflection mission like DART must be ready to address the threat in time. An impactor could be scaled up and explosives, even a nuclear device, could be added to address a threat. The DART mission was just a first test of kinetic asteroid deflection.
In the DART post-impact briefing, NASA Administrator Bill Nelson said, “All of us have a responsibility to protect our home planet. After all, it’s the only one we have. This mission shows that NASA is trying to be ready for whatever the universe throws at us. NASA has proven we are serious as a defender of the planet. This is a watershed moment for planetary defense and all of humanity, demonstrating commitment from NASA’s exceptional team and partners from around the world.”