Can the Asteroid Threat Be Averted by a Spacecraft Collision?
A 110-meter meteor is expected to fly past Earth tomorrow, and this past weekend, a 340-meter asteroid skimmed the Earth at a distance of 6.7 million kilometers. In 2013, a 20-meter meteor, undetected by radars, exploded over the Chelyabinsk region in Russia, injuring more than 1,000 people and damaging 7,000 buildings. Both the European Space Agency (ESA) and NASA are taking notice, developing defense systems to avoid asteroid collision with our home planet.
The ESA’s Hera mission plans to achieve a ground-breaking visit in space to explore a binary asteroid system: the Didymos asteroid and a second worldlet. The second body, the size of Egypt’s Great Pyramid of Giza, will become the smallest asteroid ever visited.
Didymos itself, at 780 meters across, is smaller than any asteroid visited by a probe, with the exception of the Itokawa, 350 meters in diameter, visited by Japan’s Hayabusa spacecraft, and Bennu, 500 meters in diameter, around which NASA’s Osiris-Rex mission is currently orbiting. Didymos’s orbiting moonlet, dubbed Didymoon, measuring just 160 meters across, will become the smallest of asteroid ever studied.
“Didymoon’s miniscule size really becomes clear when you look at other asteroids,” comments Hera’s lead scientist Dr. Patrick Michel, CNRS Director of Research of France’s Côte d’Azur Observatory. “Hayabusa2 images show a large boulder near the north pole of Ryugu — and that single boulder is about the same size as Didymoon in its entirety,” he added. The Ryugu asteroid is about 1 kilometer in diameter.
Didymoon was chosen for the planetary defense experiment because of its tiny size. In 2022, NASA’s Double Asteroid Redirection Test (DART) mission will attempt to change Didymoon’s orbit in order to test the possibility of asteroid deflection.
Didymoon is the riskiest class of near-Earth asteroids. Larger bodies can be easily tracked, and smaller bodies are likely to either burn up or cause limited damage. An asteroid the size of Didymoon, however, could easily devastate an entire region upon impact.
“This isn’t the first spacecraft impact into a planetary body,” adds Michel, “NASA’s Deep Impact crashed into comet Tempel 1 in 2005, but not to try and deflect it, instead it was to expose subsurface material — the 6-km diameter body was much too large. But Didymoon is small enough, and in a tight enough 12-hour orbit around its parent, that its orbital period can indeed be shifted in a measurable way.”
Post-impact, Hera will study the Didymos pair in 2026 to assess Didymoon’s mass, surface properties and the shape of the DART impact point.
“This will give us a good estimate of the impact’s momentum transfer, and hence its efficiency as a deflection technique,” said Michael Küppers, ESA’s Hera project scientist.”These are fundamental parameters to enable the validation of numerical impact models necessary to design future deflection missions. We will better understand whether this technique can be used even for larger asteroids, giving us certainty, we could protect our home planet if needed.”
“Didymoon’s small size means we know little about it, but we assume it would be spin-locked around its parent like Earth’s moon, implying a slower spin equal to its orbital period,” said Michel. “The plan is to land at least one CubeSat there, although it will require precise navigation to achieve this. The asteroid will have something like one millionth of Earth’s gravity, with an estimated escape velocity of just 6 cm per second, so one danger might be bouncing back out to space.”
Approximately 15% of known asteroids are binary systems, just like the Didymos duo. The Hera mission will provide more information about the formation of the binaries. In addition, Didymoon-type objects may be optimal for asteroid mining.
Hera is the ESA’s contribution to the Asteroid Impact and deflection Assessment (AIDA) mission, a joint operation between the ESA and NASA. The purpose of the mission is to test whether a spacecraft can successfully deflect an asteroid on a collision course with Earth by crashing an impactor spacecraft into the asteroid moon, thus altering the asteroid’s trajectory. NASA’s DART mission is planned to launch in 2021. Hera is currently pending the approval of the ESA’s Space 19+ Council meeting, and is planned for launch in 2023.
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