NASA’s DART Mission Blasts Off
Last Wednesday, Nov. 24th, saw NASA launch the Double Asteroid Redirection Test (DART) Mission, the world’s first full-scale planetary defence test. The DART spacecraft will travel for 10 months covering 6.8 million miles before reaching its final destination with a bang: a 15,000 mph high speed kinetic impact with the asteroid moonlet Dimorphos. If successful, the mission will be the first demonstration that impacting an asteroid with a spacecraft can alter its trajectory.
NASA’s target is the near-Earth, binary asteroid system Didymos, which is made up of a larger asteroid Didymos (780 metres in diameter), and a smaller, orbiting moonlet Dimorphos (160 metres in diameter). Together, they form an eclipsing binary system. From the perspective of Earth, Dimorphos regularly passes in front of and behind Didymos as it orbits. This is important because when one asteroid is eclipsed by the other, the total amount of light the two reflect decreases. Telescopes on Earth can monitor the light intensity that is reflected from the binary system, and from the periodic changes in light intensity, scientists can calculate the time Dimorphos takes to orbit around Didymos.
When DART rams into Dimorphos, scientists expect that the time Dimorphos takes to orbit Didymos will change by several minutes. Though this change is only a tiny fraction of its current orbital period (11 hours and 55 minutes), it is enough to be observable on earth.
Colin Stuart, Space Author and Writer, puts everything into perspective for us. “The mass of [Dimorphos] is five billion kilograms, and the mass of DART is about 500 kilograms. So you’re throwing a 500 kilogram object at a 5 billion kilogram target. So, we’re actually only going to change the speed of its orbit by a few millimetres per second.”
Alongside the earth-based observations, DART will livestream its final moments back to the earth using DRACO, a high-resolution onboard telescope. DRACO (Didymos Reconnaissance and Asteroid Camera for Optical navigation) also guides the autonomous navigation system in the final hours before impact, making sure DART is aiming for Dimorphos, and not Didymos, before it coasts into its high speed impact. Shortly before the impact, DART will deploy a small CubeSat satellite, which will sit a comfortable 3 minutes behind the spacecraft and be spared DART’s explosive fate. NASA are hoping that the CubeSat will be able to witness the kinetic impact event, and relay images of the plume of asteroid debris as it is flung into space from the collision, and even the impact crater and back-side of the asteroids during its fly-by.
The Hera Mission, a European Space Agency follow up project, is planned to launch in 2024. Hera will rendezvous with the Didymos system in 2026, approximately 4 years after impact, and conduct detailed post-impact investigations that will provide scientists with data that will improve current impact models. These models are invaluable in the event that a future asteroid looks like it’s headed our way.
Whilst Didymos and Dimorphos pose no real threat, asteroids of a similar size, 140 metres or larger, have the potential to cause regional devastation if they were to hit Earth. Colin calls these asteroids “city killers” because typically, an asteroid will leave a crater 10 to 20 times its size on impact. For a 140 metre asteroid, that can mean destruction over kilometres.
Since asteroids don’t emit their own light, but only reflect sunlight, they can be difficult to detect. The smaller an asteroid is, the more difficult it is to find. According to Colin, it’s only a matter of time before we find an asteroid on a path dangerously close to Earth, “It’s a question of when and not if. It’s hard to put an exact timescale on it. As we start to discover more and more of these smaller ones, the chances naturally go up.” Scientists estimate that there are roughly 25,000 near-Earth asteroids that are 140 metres or larger in size, and only 39% of those have been discovered. Colin believes one game-changer will be the Vera C. Rubin Observatory, due to come online in 2023. “It’s going to discover a lot more of these smaller asteroids than we’ve been able to discover so far”.