Double Asteroid Redirection Test

The Double Asteroid Redirection Test (DART) was a NASA space mission to test a way to protect Earth from space rocks called near-Earth objects. The goal was to see how much a spacecraft could move an asteroid by crashing into it. The mission targeted a small space rock named Dimorphos, which orbits a larger rock called Didymos. Neither of these asteroids was a danger to Earth, but they were perfect for this test.

DART was launched on November 24, 2021. On September 26, 2022, the spacecraft hit Dimorphos at a speed of 14,000 miles per hour. This changed the path of Dimorphos around Didymos by 32 minutes, which was much more than scientists hoped for. The impact threw material into space, creating a push that helped change the asteroid's orbit.

The DART mission was a teamwork effort led by NASA and the Johns Hopkins University Applied Physics Laboratory. Many parts of NASA helped make it happen, and other space agencies around the world also supported the project. This test gives scientists better ideas on how to protect Earth if a space rock ever heads our way.

Mission history

NASA and the European Space Agency worked together on a plan called AIDA to test ways to move asteroids away from Earth. This plan included two spacecraft. One, called DART, would hit an asteroid, while another would study the asteroid from space.

However, the space study part was canceled and replaced by a new mission called Hera, which will look at the asteroid later. Because of this, scientists watched the DART impact from the ground using telescopes and radar.

NASA moved DART from an idea to a real plan in June 2017 and started building it in August 2018. On April 11, 2019, they announced that a SpaceX Falcon 9 rocket would launch DART into space.

Before DART, NASA had sent a spacecraft called Deep Impact to hit a comet for science reasons. When it hit, it released a lot of energy, enough to make a big crater.

Description

Spacecraft

The DART spacecraft was a 610-kilogram impactor without scientific instruments, except for sensors used for navigation. It crashed into the asteroid Dimorphos in 2022 and cost US$330 million.

Camera

DART’s navigation included a Sun sensor, a star tracker named SMART Nav, and a 20-centimetre camera called DRACO. DRACO was based on a camera used on the New Horizons spacecraft and helped guide DART to its target. It could take detailed pictures just before impact.

Solar arrays

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The spacecraft used special solar arrays tested on the International Space Station. These arrays included advanced technology to produce more power.

Antenna

The DART spacecraft used a new type of antenna for communication, operating at certain frequencies to send and receive data from Earth.

Ion thruster

DART tested a new type of ion thruster powered by solar panels. However, it was not used for the mission because conventional thrusters worked well enough.

Secondary spacecraft

Main article: LICIACube

The Italian Space Agency sent a small spacecraft called LICIACube with DART. It separated before the impact to record images of the crash and the material thrown out.

Effect of the impact on Dimorphos and Didymos

When DART hit Dimorphos, it changed the asteroid’s orbit around its partner, Didymos. The change was small but measured by scientists. This was the first time humans changed the orbit of a celestial body. The impact threw material into space, which also helped change the orbit more than the crash itself.

Observations of the impact

Several telescopes, including the Hubble Space Telescope and James Webb Space Telescope, observed the impact. The change in Dimorphos’s orbit was measured by watching when the two asteroids passed in front of each other. The orbit changed by 32 minutes, showing the impact was successful.

Follow-up mission

In a collaborating project, the European Space Agency launched Hera to Didymos in October 2024. Hera will arrive in 2026 to study the asteroids in detail. It carries two small spacecraft named Milani and Juventas.

AIDA mission architecture

Mission profile

Target asteroid

The mission's target was Dimorphos in the 65803 Didymos system, a binary asteroid system where one asteroid is orbited by a smaller one. The main asteroid (Didymos A) is about 780 metres (2,560 ft) wide; the smaller asteroid Dimorphos (Didymos B) is about 160 metres (520 ft) wide and orbits about 1 kilometre (0.62 mi) from the main one. The mass of the Didymos system is estimated at 528 billion kg, with Dimorphos making up 4.8 billion kg of that total. Choosing this pair of asteroids is useful because changes to Dimorphos's speed can be measured by watching when it passes in front of the other asteroid, creating a dip in light that telescopes on Earth can see. Dimorphos was also chosen because of its size; it is similar to the size of asteroids that might need to be moved if they were heading toward Earth. The pair was also close to Earth in 2022, at about 7 million miles away (0.075 astronomical units; 29 lunar distances; 11 million kilometers). The Didymos system is not an Earth-crossing asteroid, and there was no chance that the test could create a danger of impact.

Preflight preparations

Preparations for DART's launch began on 20 October 2021, when the spacecraft started getting fuel at Vandenberg Space Force Base (VSFB) in California. The spacecraft arrived at Vandenberg in early October 2021 after a drive across the country. The DART team got the spacecraft ready for flight, testing its parts and electrical system, wrapping final parts in special blankets, and practicing the launch steps from both the launch site and the mission control center at APL. DART went to the SpaceX Payload Processing Facility on VSFB on 26 October 2021. Two days later, the team got approval to fill DART's fuel tank with about 50 kilograms (110 lb) of hydrazine propellant for moving and controlling the spacecraft. DART also carried about 60 kilograms (130 lb) of xenon for the NEXT-C ion engine. Engineers loaded the xenon before the spacecraft left APL in early October 2021.

Starting on 10 November 2021, engineers attached the spacecraft to the part that sits on top of the SpaceX Falcon 9 rocket. The Falcon 9 rocket without its payload cover moved for a test fire and later returned to the processing area where SpaceX technicians put the two halves of the cover around the spacecraft over two days, on 16 and 17 November, inside the SpaceX Payload Processing Facility at Vandenberg Space Force Base. The ground teams then completed a successful Flight Readiness Review later that week with the cover now attached to the rocket.

One day before launch, the rocket rolled out of the hangar and onto the launch pad at Vandenberg Space Launch Complex 4 (SLC-4E); from there, it lifted off to start DART's trip to the Didymos system and pushed the spacecraft into space.

Launch

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Liftoff of Falcon 9 with DART.

DART separation from second stage

The DART spacecraft launched on 24 November 2021, at 06:21:02 UTC.

Early plans suggested that DART would be sent into a high-altitude, high-eccentricity Earth orbit to avoid the Moon. In that case, DART would use its low-thrust, high-efficiency NEXT ion engine to slowly leave its high Earth orbit for a slightly tilted near-Earth solar orbit, from which it would move onto a path to hit its target. But because DART was launched as a special Falcon 9 mission, the payload and Falcon 9's second stage were placed directly on an Earth escape path and into heliocentric orbit when the second stage restarted its engine for a second burn or escape ignition. So, although DART carries a first-of-its-kind electric thruster and plenty of xenon fuel, Falcon 9 did most of the work, leaving the spacecraft to do only a few trajectory-correction burns with simple chemical thrusters as it headed toward Didymos's moon Dimorphos.

Transit

The trip before impact lasted about 9 months. During its interplanetary travel, the DART spacecraft made a far flyby of the 578-metre (1,896-foot) diameter near-Earth asteroid (138971) 2001 CB21 in March 2022. DART passed 0.117 astronomical units (46 lunar distances; 17.5 million kilometres; 10.9 million miles) from 2001 CB₂₁ on its closest approach on 2 March 2022.

DART's DRACO camera opened its door and took its first light image of some stars on 7 December 2021, when it was 2 million miles (0.022 astronomical units; 8.4 lunar distances; 3.2 million kilometres) from Earth. The stars in DRACO's first light image were used to adjust the camera's direction before it could be used to image other targets. On 10 December 2021, DRACO imaged the open cluster Messier 38 for more testing of the camera's optics and photometric abilities.

On 27 May 2022, DART observed the bright star Vega with DRACO to test the camera's optics using scattered light. On 1 July and 2 August 2022, DART's DRACO camera watched Jupiter and its moon Europa as they came out from behind the planet, as a test for the SMART Nav tracking system to get ready for hitting Dimorphos.

Course of the impact

Two months before impact, on 27 July 2022, the DRACO camera saw the Didymos system from about 32 million kilometres (0.21 astronomical units; 83 lunar distances; 20 million miles) away and started improving its path. The LICIACube nanosatellite was released on 11 September 2022, 15 days before impact. Four hours before impact, about 90,000 kilometres (0.23 LD; 56,000 mi) away, DART began to work completely on its own under control of its SMART Nav guidance system. Three hours before impact, DART checked for objects near the target. Ninety minutes before the crash, when DART was 38,000 kilometres (0.099 LD; 24,000 mi) from Dimorphos, the final path was set. When DART was 24,000 kilometres (0.062 LD; 15,000 mi) away Dimorphos could be seen (1.4 pixels) through the DRACO camera, which then kept taking pictures of the asteroid's surface and sending them in real-time.

DRACO was the only instrument that could give a close-up view of Dimorphos's surface. The use of DART's thrusters caused shaking throughout the spacecraft and solar panels, making the images blurry. To make sure the images were sharp, the last path correction was done 4 minutes before impact and the thrusters were turned off after that.

The last full picture, sent two seconds before impact, had a detail of about 3 centimeters per pixel. The impact happened on 26 September 2022, at 23:14 UTC.

The head-on impact of the 500 kilograms (1,100 lb) DART spacecraft at 6.6 kilometres per second (4.1 mi/s) or 22,530 kilometres per hour (14,000 mph) likely released energy of about 11 gigajoules, the same as about three tonnes of TNT, and was expected to reduce the orbital velocity of Dimorphos between 1.75 cm/s and 2.54 cm/s, depending on many factors such as material porosity.^{[failed verification]} The drop in Dimorphos's orbital speed moves it closer to Didymos, causing the moon to feel more gravity and thus have a shorter orbit time. The drop in orbit time from the head-on impact helps make it easier for telescopes on Earth to watch Dimorphos. Hitting the asteroid's back side would instead lengthen its orbit time toward 12 hours and match Earth's day and night cycle, which would make it harder for any single ground telescope to watch all of Dimorphos's orbit every night.

The measured momentum boost factor (called beta) of DART's hit on Dimorphos was 3.6, meaning the impact moved the asteroid about 3.6 times more than if the asteroid had simply taken in the spacecraft and made no pieces fly off – showing that the pieces flying off helped move the asteroid more than the spacecraft did. This means one could use either a smaller spacecraft or less time to move an asteroid than previously thought. The value of beta depends on different factors, like composition, density, and porosity, etc. The goal is to use these results and models to guess what beta could be for another asteroid by watching its surface and possibly measuring its overall density. Scientists think DART's impact sent out over 1,000,000 kilograms (2,200,000 lb) of dusty pieces into space – enough to fill six or seven rail cars. The trail of pieces from Dimorphos made by the DART impact is at least 30,000 kilometres (0.078 LD; 19,000 mi) long with a weight of at least 1,000 tonnes (980 long tons; 1,100 short tons), and possibly up to 10 times that much.

The DART impact on the center of Dimorphos shortened the orbit time, previously 11 hours and 52 minutes, by 33±1 minutes. This big change shows that the push from material dug out from the asteroid and sent into space by the impact (known as pieces) added a lot of extra push to the asteroid, beyond what the DART spacecraft itself gave. Researchers found the impact caused an instant drop in Dimorphos's speed along its orbit of about 2.7 millimeters per second — again showing that the push from pieces played a big role in increasing the push directly given to the asteroid by the spacecraft. That push was increased by a factor of 2.2 to 4.9 (depending on the mass of Dimorphos), showing the push added because of pieces sent out was much more than the push from the DART spacecraft alone. While the change in orbit was small, the change is in the speed and over years will add up to a big change in position. For a made-up Earth-threatening object, even such a tiny change could be enough to stop or avoid a hit, if done early enough. Since the diameter of Earth is about 13,000 kilometers, a made-up asteroid hit could be avoided with as little of a shift as half of that (6,500 kilometers). A 2 cm/s speed change adds up to that distance in about 10 years.

By hitting the asteroid, DART made Dimorphos an active asteroid. Scientists had suggested that some active asteroids are the result of impact events, but no one had ever watched an asteroid become active. The DART mission made Dimorphos active under known and watched impact conditions, allowing the detailed study of the making of an active asteroid for the first time. Observations show that Dimorphos lost about 1 million kilograms of mass because of the crash.

Sequence of operations for impact