Dimorphos is easily one of the least interesting objects in the solar system. It’s a rock, a small moon, actually, that’s only 160m (525ft) across and orbits the asteroid Didymos, which itself is only 780m (2,560ft). Located 11 million kilometers (6.8 million miles) from Earth, the Didymos-Dimorphos system is just a small part of the river of debris that surrounds the sun in the asteroid belt between Mars and Jupiter.
But on Monday, September 26, at precisely 7:14 pm ET, the attention of much of the astronomical community will turn to Dimorphos. That’s the time when NASA’s DART (short for Double Asteroid Redirection Test) spacecraft will pierce the moon. Yon the nose, deliberately colliding with it at a speed of approximately 28,200 k/h (17,500 mph). The results of that cosmic collapse could go a long way toward determining how NASA and the world’s other space agencies can keep the planet safe from incoming asteroids: by destroying or deflecting them before they can cause the kind of cataclysmic damage that occurred when A 10 to 15 km (6.2 to 9.3 mi) space rock crashed off the Yucatan Peninsula 65 million years ago, triggering the global extinction event that spelled the end of the dinosaurs.
The risk to modern Earth is real. NASA’s Center for Near-Earth Object Studies (CNEOS) maintains a running account of what it calls Near-Earth Asteroids (NEAs), defined as space debris that they are not locked in the asteroid belt, but instead revolve around the sun in an orbit that carries it. within 45 million km (28 million miles) of Earth. That would seem like a pretty safe fault distance, but there’s always the chance that some other piece of free-flying space debris could collide with an NEA, change its course, and send it hurtling toward us. According to the CNEOS census, there are 855 known NEAs that are at least 1 km (0.62 mi), and more than 10,000 that are at least 140 m (460 ft) wide. Overall, there are 29,801 known NEAs of all sizes in the CNEOS database.
Interception and diversion are our best defense against NEAs, and as a first test of the as-yet-unproven technique, NASA built DART and launched it toward the Didymos-Dimorphos pair on November 23, 2021. The spacecraft is actually two spaceships. The main body of the DART is 8.5 ft (2.6 m) wide and weighs 1,320 lb (600 kg). It carries with it a small toaster-sized spacecraft built by the Italian Space Agency (ISA), called the Light Italian CubeSat for Imaging Asteroids (LICIACube). It is the DART itself that will collide with Dimorphos; The job of LICIACube, which was spun off from DART on 9/11, is to fly close by and take images of the tiny moon before and after impact.
“We are working with ASI to take LICIACube to between 25 and 50 miles [40 to 80 km] of Dimorphos just two or three minutes after the DART impact, close enough to get good images of the impact and ejection plume, but not so close that LICIACube could be hit by the ejection,” said Dan Lubey, director of NASA’s LICIACube navigation, in a statement from the space agency.
LICIACube’s work will be important when it comes to collecting evidence on the kind of physical damage an impactor spacecraft can do to an asteroid. But the true indicator of mission success will come in measurements of how dramatically DART changes Dimorphos’s orbit around Didymos. That will be determined by a number of ground-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 accelerate the tiny moon’s 11.9-hour orbit around Didymos by several minutes. That seemingly small difference is actually very big, as even a slight change in an asteroid’s speed or trajectory when it’s millions of miles from Earth could cause it to fly a long way 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 as early as next week.
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