Heyimorphos is easily one of the least interesting objects in the solar system. It’s a rock—a moon, really—just 160 m (525 ft) in diameter, orbiting the asteroid Gemini, which is only 780 meters long. At a distance of 11 million kilometers (6.8 million miles) from Earth, the Gemini-Dimorpho system is only a tiny part of the river of debris that circles the sun in the asteroid belt between Mars and Jupiter.
But on Monday, September 26 at 7:14 p.m. ET, the attention of much of the astronomical community will turn to Dimorphos. This is when NASA’s DART (short for Double Asteroid Redirection Test) spacecraft will hit the moon In the nose — deliberately collided with it at about 28,200 k/h (17,500 mph). The results of this cosmic break-in could go a long way in determining how NASA and the world’s other space agencies can protect the planet from incoming asteroids: by destroying or deflecting them before they have the kind of cataclysm of damage that occurred when a 10 by 15 kilometer (6.2 to 9.3 mile) space rock crashed into the Yucatan Peninsula 65 million years ago, triggering the global extinction event that marked the end of the dinosaurs.
The danger to modern Earth is real. NASA’s Center for Near-Earth Object Studies (CNEOS) maintains a running count of near-Earth asteroids (NEAs), defined as space debris that is not locked in the asteroid belt but circles the sun in an orbit that brings them within 45 million kilometers (28 million miles) of Earth. That would seem like a pretty safe fail-safe distance, but there’s always the chance that some other free-flying piece of space junk could collide with an NEA, altering its course and sending it our way. According to the CNEOS inventory, there are 855 known NEAs with an area of at least 1 km (0.62 mi) and more than 10,000 that are at least 140 m (460 ft) in diameter. In total, there are 29,801 known NEAs of all sizes in the CNEOS database.
Interception and deflection are our best defenses against NEAs, and as a first test of the as-yet-unproven technique, NASA built DART and launched it toward the Gemini-Dimorphos pair on November 23, 2021. The spacecraft is actually two spacecraft. The DART’s main body is 2.6 meters (8.5 ft) in diameter and weighs 600 kg (1,320 lb). It carries a small toaster-sized spacecraft built by the Italian Space Agency (ISA), called the Light Italian CubeSat for Imaging Asteroids (LICIACube). It is DART itself that will collide with Dimorphos. The job of LICIACube, which separated from DART on September 11, is to fly close and take images of the moon before and after the impact.
“We are working with ASI to get LICACube within 25 to 50 miles [40 to 80 km] of Dimorphos just two to three minutes after DART impact—close enough to get good images of the impact and ejecta plume, but not so close LICIACube could be hit by ejecta,” said NASA’s LICIACube navigation director, Dan Lubey, in a statement from the space agency.
LICACube’s work will be important when it comes to gathering evidence about the kind of physical damage a spacecraft impact can do to an asteroid. But the real indicator of the mission’s success will come in measurements of how dramatically DART changes Dimorph’s orbit around Gemini. This will be determined by an array of Earth-based telescopes, including NASA’s Deep Space Radio Telescope Network in Barstow, California. Madrid, Spain; and Canberra, Australia.
For now, NASA’s best guess is that DART will speed up the moon’s 11.9-hour orbit around Gemini by several minutes. This seemingly small difference is actually very large, as even a small change in an asteroid’s speed or trajectory when it is millions of miles from Earth could send it flying far away from us when it finally reaches our planetary neighborhood.
Space has always been a dangerous place. The DART mission could help make it safer. How much safer will be known next week.
This story originally appeared in TIME Space, our weekly newsletter covering all things space. You can register here.
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