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DART: Everything you need to know about NASA's mission to deflect an asteroid – BBC Science Focus Magazine

On 27 September the space agency deliberately crashed a spacecraft into an asteroid as part of its planetary defence programme.
NASA's not just about sending robots to Mars and building space telescopes. They are also testing plans to keep our planet safe in the event that we discover an asteroid or comet on a collision course with the Earth. A large part of these plans, of course, is studying the sky for any potentially hazardous objects. But that's not all: the planetary defence programme also includes a mission called DART.
First things first: there isn't an asteroid on a collision course with Earth. We're not going the way of the dinosaurs just yet. The aim of the DART mission is to prepare ourselves. What would we do if we did find a dangerous space rock heading our way? And can we be confident our plan would work?
NASA isn't planning to blow up a dangerous asteroid like Bruce Willis in Armageddon. Instead, the idea behind the DART mission was to crash a spacecraft into it, knocking it into a safer orbit.
And, at 00.14 BST on 27 September 2022, NASA did just that. The DART spacecraft successfully crashed into its target asteroid, Dimorphos.
Here's everything you need to know about the first-of-its-kind mission.
The Double Asteroid Redirection Test (DART) mission is a trial of NASA's planetary defence plans. It's part of the Asteroid Impact and Deflection Assessment (AIDA) mission, a joint collaboration between ESA, NASA, the German Aerospace Center (DLR), Observatoire de la Côte d´Azur (OCA) and the John Hopkins University Applied Physics Laboratory (JHU/APL).
An asteroid or comet with a diameter above 140 metres that approaches Earth at a distance of less than 5 per cent of the distance from the Earth to the Sun is called a 'potentially hazardous object' (PHO). Most of these will pose no danger to us – in fact, NASA says that none of the known PHOs have a significant chance of hitting Earth in the next 100 years. However, it estimates that only 40 per cent of these objects are known.
So, just in case we should discover a PHO that is heading straight for Earth, NASA has a plan: to use a spacecraft to deflect the oncoming asteroid. DART is the first attempt to do exactly that, using a near-Earth binary asteroid called Didymos. Orbiting Didymos is a moonlet called Dimorphos, which the spacecraft successfully knocked into a different orbit in September 2022.
DART is the first-ever mission to attempt to redirect the path of an asteroid by crashing a spacecraft into it.
The mission is part of NASA’s planetary defence strategy and aims to build on our ability to model, predict and prepare for an asteroid that might provide a threat to Earth, should one be discovered.
It was launched on a SpaceX Falcon 9 rocket from Vandenberg Space Force Base in California on 23 November 2021 and made impact on 27 September 2022.
“It's essentially a big science experiment to see if crashing a spacecraft into an asteroid is a good way to change its orbit around the Sun and potentially deflect an Earth-crossing asteroid away in the future should that happen, or rather when that happens,” said cosmochemist and author of Meteorite Dr Tim Gregory.
“It sounds impossible that something as lightweight as a spacecraft, even a spacecraft like DART, which weighs more than half a tonne, could possibly nudge something like an asteroid, which weighs millions of tonnes. But you don't need to nudge an asteroid by very much for it to miss the Earth entirely. Just fractions of a degree, and you'll miss the Earth by millions of miles.”
DART’s target was a binary asteroid system made up of a larger asteroid named Didymos, which is Greek for ‘twin’, and a smaller companion asteroid named Dimorphos, which is Greek for ‘two forms', that orbits it roughly every 12 hours. Didymos is around 780m across and Dimorphos is around 160m across.
“It's important to stress that this particular asteroid system doesn't pose a threat to the Earth. It was merely just chosen as a target for this science experiment, and it was chosen from a few different candidates based on its orbit around the Sun,” said Gregory.
The spacecraft collided with Dimorphos when it is around 11 million km from Earth. At the moment of impact, it was estimated to be travelling at around 6.6km/s.
Yes, the DART mission is safe. Didymos, the target asteroid, is no threat to Earth. When DART reached the asteroid in 2022, it was roughly 11 million kilometres away, and the aim was only to move the moonlet Dimorphos into a different orbit around the asteroid.
DART was designed and built by teams at Johns Hopkins Applied Physics Laboratory (APL) in Maryland, USA. The main structure is a cube roughly 1m across fitted with large, flexible solar arrays on opposite sides that stretch out around 8m each. It has a mass of around 610kg.
It is powered by NEXT–C, NASA's Evolutionary Xenon Thruster–Commercial, a solar-powered ion propulsion system that produces thrust using xenon as fuel.
On board is a high-resolution camera DRACO, or Didymos Reconnaissance and Asteroid Camera for Optical navigation. As well as being used to support navigation, the camera was also used to measure the size and shape of the asteroid target to investigate the geology of the impact site. The images acquired by DRACO before the kinetic impact was streamed back to Earth in real-time.
In the final four hours before impact the SMART Nav, or Small-body Maneuvering Autonomous Real-Time Navigation, worked alongside DRACO to autonomously manoeuvre the spacecraft into position for impact.
The spacecraft was also carrying a companion CubeSat named LICIACube (Light Italian CubeSat for Imaging of Asteroids) that was designed by the Agenzia Spaziale Italiana (ASI). LICIACube was deployed on 11 September during the spacecraft's approach to Dimorphos and captured images of the impact.
NASA used the 'kinetic impactor' technique – that is, the spacecraft crashed into the asteroid to alter its course.
DART manoeuvred using its onboard camera, DRACO, and autonomous navigation software. These directed the spacecraft to collide with Dimorphos at a speed of about 6.6km/s (14,700mph), altering the moonlet's speed by less than 1 per cent.
Following the impact, the DART investigation team are comparing the results of the spacecraft's collision with Dimorphos via observations with ground-based telescopes, with sophisticated computer simulations that they have already run. This way they will be able to assess the efficacy of the kinetic impact and figure out the most effective way to employ it should any future planetary defence scenarios arise.
“The nature of doing any sort of science is sometimes you just don't know what's going to happen," said Gregory.
"And with this mission being the first of its kind, I think the margin for success is very wide. And I guess, to paraphrase the Apollo astronauts, hopefully, it will be a success, but it might be a very successful failure.”
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Jason is the commissioning editor for BBC Science Focus. He holds an MSc in physics and was named Section Editor of the Year by the British Society of Magazine Editors in 2019. He has been reporting on science and technology for more than a decade. During this time, he's walked the tunnels of the Large Hadron Collider, watched Stephen Hawking deliver his Reith Lecture on Black Holes and reported on everything from simulation universes to dancing cockatoos. He looks after the magazine’s and website’s news sections and makes regular appearances on the Instant Genius Podcast.
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