A first step in planetary defense

Shortly after 7 p.m. Monday evening, the probe DART of NASA will crash into an asteroid in order to divert its trajectory. Rest assured: the asteroid Dimorphos is harmless. But if it works, the tactic could be reused against a real threat.

Mathieu Perreault
The Press

DART and Licia

Launched last November, the probe DART – the acronym stands for “Twin Asteroid Redirection Test” – NASA is targeting Dimorphos, a moon of the asteroid Didymos. Together they form a binary asteroid. The goal is to measure how much Dimorphos’ orbit around Didymos will have changed after the collision. “In theory, it’s a viable tactic to ward off an asteroid heading for Earth, but whether it works in practice has to be seen,” says Sabina Raducan, an astrophysicist from the University of Bern, Switzerland. She does not participate in the mission, but studies this type of strategy.

The collision will be filmed by a small satellite, the lightweight Italian CubeSat for asteroid imaging (LICIACube), which was ejected from DART September 12. Dimorphos was chosen because its movement around Didymos is easy to observe. It will only take 38 seconds for the photos of LICIACube arrive on Earth. In 2026, the European probe Hera will arrive near Didymos to accurately confirm the change in orbit of Dimorphos. Didymos was discovered in 1996 and Dimorphos in 2003.

Hayabusa and Osiris-Rex

The big unknown in this maneuver is the composition of Dimorphos. “Before, we thought that asteroids were relatively compact, especially small ones,” says Ms.^me Raducan. But the Japanese Hayabusa mission in 2005, and Osiris-Rex in 2020, showed that asteroids that were thought to be full are in fact agglomerations of rocks. If Dimorphos is a single rock, its orbit will only change by a few seconds, but if it’s a cluster of rocks, the difference could be up to half an hour. The orbit of Dimorphos lasts 12 hours. M^me Raducan studies the effects of impacts such as that of DART digitally – and also physically, with a pile of sand where there are rocks. Hayabusa brought back asteroid samples in 2010 and Osiris-Rex will bring back its own next September.

Autonomous navigation

DART is also the first space probe to approach an asteroid using autonomous navigation software. Route corrections (so that the impact takes place in the center of Dimorphos) cannot be made effectively from Earth in the four hours preceding the collision, due to the communication delay. “Before, we sent the orders in advance and we hoped that it would work as planned, explains Nathalie Ouellet, astrophysicist at the University of Montreal. We used autonomous navigation for the first time with the landing of the Mars probe Perseverance in 2020.” The autonomous navigation software was tested last week with the sighting of Jupiter’s moon Europa.

25,000 threats

About 500 near-Earth asteroids (NEA, near-Earth asteroids), with a diameter greater than 150 meters, are identified each year. But only 10,000 of the estimated 25,000 NEAs are known. This includes 97% of NEAs longer than 1 km. For comparison, the asteroid that signed the death warrant for the dinosaurs 65 million years ago was 10 km in diameter, and the one that injured nearly 1,500 people in Siberia in 2013, 20 m in diameter. . The space telescope NEO Surveyor (NEO for Near-Earth Object), which NASA plans to launch in 2026, should complete the identification of NEAs larger than 150 meters by the mid-2030s.

Patrol

It will be difficult to determine the solid or clumped composition of an Earth-threatening asteroid from a distance, according to Ms.^me Raducan. “It will be essential to know what power of impact to have to deflect it. So she thinks that probes like NEA Scouta CubeSat (very small satellite) to be launched with the Artemis I lunar mission, could play a crucial role in planetary defense.

NEA Scout will be propelled by a solar sail allowing it to acquire high speeds, with very little fuel. The designers of NEA Scout have argued that hundreds of probes like NEA Scout could patrol the vicinity of our planet and head towards a threatening asteroid to better characterize it. Would a clustered asteroid be less dangerous if it hit Earth? “Not necessarily, because an asteroid could hit the ground without exploding at altitude,” says M.^me Raducan.

Tractor

For asteroids too large or dense to be deflected by an impact, another tactic is considered. It would be for the probe to cling to the asteroid and deflect its trajectory with an ion engine or a solar sail, or else to deflect it simply by remaining at its side, by gravity.

Asteroids in movies

Don’t Look Up (2021): Leonardo DiCaprio plays a scientist unable to convince politicians and the media to worry about an asteroid threatening Earth.

Armageddon (1998): Bruce Willis saves the Earth from an asteroid by splitting it in two with a nuclear bomb.

Deep Impact (1998): The President of the United States (Morgan Freeman) secretly plans an asteroid destruction mission which fails partially, causing a planetary catastrophe.

Meteor (1979): Sean Connery convinces a Soviet scientist to participate in a joint mission to destroy asteroids threatening the Earth, with nuclear missiles in orbit.