Double Asteroid Redirection Test facts for kids

Mission profile

Target asteroid

Pre-impact shape model of Didymos and its satellite Dimorphos, based on photometric light curve and radar data

The mission's target was Dimorphos in 65803 Didymos system, a binary asteroid system in which one asteroid is orbited by a smaller one. The primary asteroid (Didymos A) is about 780 metres (2,560 ft) in diameter; the asteroid moon Dimorphos (Didymos B) is about 160 metres (520 ft) in diameter in an orbit about 1 kilometre (0.62 mi) from the primary. The mass of the Didymos system is estimated at 528 billion kg, with Dimorphos comprising 4.8 billion kg of that total. Choosing a binary asteroid system is advantageous because changes to Dimorphos's velocity can be measured by observing when Dimorphos subsequently passes in front of its companion, causing a dip in light that can be seen by Earth telescopes. Dimorphos was also chosen due to its appropriate size; it is in the size range of asteroids that one would want to deflect, were they on a collision course with Earth. In addition, the binary system was relatively close to the Earth in 2022, at about 7 million miles (0.075 astronomical units; 29 lunar distances; 11 million kilometers). The Didymos system is not an Earth-crossing asteroid, and there is no possibility that the deflection experiment could create an impact hazard. On 4 October 2022, Didymos made an Earth approach of 10.6 astronomical units (4,100 lunar distances; 1.59 billion kilometres; 990 million miles).

Preflight preparations

DART being encapsulated in the Falcon 9 payload fairing on 16 November 2021

Launch preparations for DART began on 20 October 2021, as the spacecraft began fueling at Vandenberg Space Force Base (VSFB) in California. The spacecraft arrived at Vandenberg in early October 2021 after a cross-country drive. DART team members prepared the spacecraft for flight, testing the spacecraft's mechanisms and electrical system, wrapping the final parts in multilayer insulation blankets and practicing the launch sequence from both the launch site and the mission operations center at APL. DART headed to the SpaceX Payload Processing Facility on VSFB on 26 October 2021. Two days later, the team received the green light to fill DART's fuel tank with roughly 50 kilograms (110 lb) of hydrazine propellant for spacecraft maneuvers and attitude control. 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 mated the spacecraft to the adapter that stacks on top of the SpaceX Falcon 9 launch vehicle. The Falcon 9 rocket without the payload fairing rolled for a static fire and later came back to the processing facility again where technicians with SpaceX installed the two halves of the fairing around the spacecraft over the course of two days, 16 and 17 November, inside the SpaceX Payload Processing Facility at Vandenberg Space Force Base and the ground teams completed a successful Flight Readiness Review later that week with the fairing then attached to the rocket.

A day before launch, the launch vehicle rolled out of the hangar and onto the launch pad at Vandenberg Space Launch Complex 4 (SLC-4E); from there, it lifted off to begin DART's journey to the Didymos system and it propelled the spacecraft into space.

Launch

Liftoff of Falcon 9 with DART.

DART separation from second stage

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

Early planning suggested that DART was to be deployed into a high-altitude, high-eccentricity Earth orbit designed to avoid the Moon. In such a scenario, DART would use its low-thrust, high-efficiency NEXT ion engine to slowly escape from its high Earth orbit to a slightly inclined near-Earth solar orbit, from which it would maneuver onto a collision trajectory with its target. But because DART was launched as a dedicated Falcon 9 mission, the payload along with Falcon 9's second stage was placed directly on an Earth escape trajectory and into heliocentric orbit when the second stage reignited for a second engine startup or escape burn. Thus, although DART carries a first-of-its-kind electric thruster and plenty of xenon fuel, Falcon 9 did almost all of the work, leaving the spacecraft to perform only a few trajectory-correction burns with simple chemical thrusters as it homed in on Didymos's moon Dimorphos.

Transit

Animation of DART's trajectory
      DART ·       65803 Didymos ·       Earth ·       Sun ·       2001 CB₂₁ ·       3361 Orpheus

The transit phase before impact lasted about 9 months. During its interplanetary travel, the DART spacecraft made a distant 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 in its closest approach on 2 March 2022.

DART's DRACO camera opened its aperture 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) away from Earth. The stars in DRACO's first light image were used as calibration for the camera's pointing before it could be used to image other targets. On 10 December 2021, DRACO imaged the open cluster Messier 38 for further optical and photometric calibration.

On 27 May 2022, DART observed the bright star Vega with DRACO to test the camera's optics with scattered light. On 1 July and 2 August 2022, DART's DRACO imager observed Jupiter and its moon Europa emerging from behind the planet, as a performance test for the SMART Nav tracking system to prepare for the Dimorphos impact.

Course of the impact

Two months before the impact, on 27 July 2022, the DRACO camera detected the Didymos system from approximately 32 million kilometres (0.21 astronomical units; 83 lunar distances; 20 million miles) away and started refining its trajectory. The LICIACube nanosatellite was released on 11 September 2022, 15 days before the impact. Four hours before impact, some 90,000 kilometres (0.23 LD; 56,000 mi) away, DART began to operate in complete autonomy under control of its SMART Nav guidance system. Three hours before impact, DART performed an inventory of objects near the target. Ninety minutes before the collision, when DART was 38,000 kilometres (0.099 LD; 24,000 mi) away from Dimorphos, the final trajectory was established. When DART was 24,000 kilometres (0.062 LD; 15,000 mi) away Dimorphos became discernible (1.4 pixels) through the DRACO camera which then continued to capture images of the asteroid's surface and transmit them in real-time.

DRACO was the only instrument able to provide a detailed view of Dimorphos' surface. The use of DART's thrusters caused vibrations throughout the spacecraft and solar panels, resulting in blurred images. To ensure sharp images, the last trajectory correction was executed 4 minutes before impact and the thrusters were deactivated afterwards.

The last full image, transmitted two seconds before impact, has a spatial resolution of about 3 centimeters per pixel. The impact took place 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) likely imparted an energy of about 11 gigajoules, the equivalent of 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 numerous factors such as material porosity. The reduction in Dimorphos's orbital velocity brings it closer to Didymos, resulting in the moon experiencing greater gravitational acceleration and thus a shorter orbital period. The orbital period reduction from the head-on impact serves to facilitate ground-based observations of Dimorphos. An impact to the asteroid's trailing side would instead increase its orbital period towards 12 hours and make it coincide with Earth's day and night cycle, which would limit any single ground-based telescope from observing all orbital phases of Dimorphos nightly.

DART impact and its corresponding plume as seen by using the Mookodi instrument on the SAAO's 1-m Lesedi telescope

The measured momentum enhancement factor (called beta) of DART's impact of Dimorphos was 3.6, which means that the impact transferred roughly 3.6 times greater momentum than if the asteroid had simply absorbed the spacecraft and produced no ejecta at all – indicating the ejecta contributed more to moving the asteroid than the spacecraft did. This means one could use either a smaller impactor or shorter lead times to produce a certain deflection in an asteroid than previously expected. The value of beta depends on various factors, composition, density, porosity, etc. The goal is to use these results and modeling to infer what beta could be for another asteroid by observing its surface and possibly measuring its bulk density. Scientists estimate that DART’s impact displaced over 1,000,000 kilograms (2,200,000 lb) of dusty ejecta into space – enough to fill six or seven rail cars. The tail of ejecta from Dimorphos created by the DART impact is at least 30,000 kilometres (0.078 LD; 19,000 mi) long with a mass of at least 1,000 tonnes (980 long tons; 1,100 short tons), and possibly up to 10 times that much.

Footprint of DART spacecraft over the spot where it impacted asteroid Dimorphos

The DART impact on the center of Dimorphos decreased the orbital period, previously 11 hours and 52 minutes, by 33±1 minutes. This large change indicates the recoil from material excavated from the asteroid and ejected into space by the impact (known as ejecta) contributed significant momentum change to the asteroid, beyond that of the DART spacecraft itself. Researchers found the impact caused an instantaneous slowing in Dimorphos' speed along its orbit of about 2.7 millimeters per second — again indicating the recoil from ejecta played a major role in amplifying the momentum change directly imparted to the asteroid by the spacecraft. That momentum change was amplified by a factor of 2.2 to 4.9 (depending on the mass of Dimorphos), indicating the momentum change transferred because of ejecta production significantly exceeded the momentum change from the DART spacecraft alone. While the orbital change was small, the change is in the velocity and over the course of years will accumulate to a large change in position. For a hypothetical Earth-threatening body, even such a tiny change could be sufficient to mitigate or prevent an impact, if applied early enough. As the diameter of Earth is around 13,000 kilometers, a hypothetical asteroid impact could be avoided with as little of a shift as half of that (6,500 kilometers). A 2 cm/s velocity change accumulates to that distance in approximately 10 years.

By smashing into the asteroid DART made Dimorphos an active asteroid. Scientists had proposed that some active asteroids are the result of impact events, but no one had ever observed the activation of an asteroid. The DART mission activated Dimorphos under precisely known and carefully observed impact conditions, enabling the detailed study of the formation of an active asteroid for the first time. Observations show that Dimorphos lost approximately 1 million kilograms of mass as a result of the collision.

Sequence of operations for impact

Date (before impact)	Distance from Dimorphos	Raw image	Events
27 July 2022 (T-60 days)	38 million kilometers (0.25 astronomical units; 99 lunar distances; 24 million miles)		The DRACO camera detects the Didymos system.
11 September 2022 23:14 UTC (T-15 days)	8 million kilometers (0.053 astronomical units; 21 lunar distances; 5.0 million miles)		Ejection of LICIACube, which maneuvers to avoid crashing into the asteroid.
26 September 2022 19:14 UTC (T-4 hours)	89,000 kilometers (0.23 lunar distances; 55,000 miles)		Terminal phase—start of autonomous navigation with SMART Nav. DRACO locks onto Didymos since Dimorphos is not visible yet.
22:14 UTC (T-60 minutes)	22,000 kilometers (0.057 lunar distances; 14,000 miles)		The DRACO camera detects Dimorphos.
22:54 UTC (T-20 minutes)	7,500 kilometers (4,700 miles)		SMART Nav enters precision lock onto Dimorphos and DART begins thrusting toward Dimorphos.
23:10 UTC (T-4 minutes)	1,500 kilometers (930 miles)		Start of final course correction
23:11 UTC (T-2 minutes 30 seconds)	920 kilometers (570 miles)		Last image with both Didymos (lower-left) and Dimorphos entirely in frame is taken
23:12 UTC (T-2 minutes)	740 kilometers (460 miles)		End of final course correction
23:14 UTC (T-20 seconds)	130 kilometers (81 miles)		The photos taken reach the expected spatial resolution.
23:14 UTC (T-11 seconds)	68 kilometers (42 miles)		Last image showing all of Dimorphos by DART
23:14 UTC (T-3 seconds)	18 kilometers (11 miles)
23:14 UTC (T-2 seconds)	12 kilometers (7.5 miles)		Final complete image of Dimorphos transmitted. Resolution roughly 3 cm per pixel (~ 30m across).
23:14 UTC (T-1 second)	6 kilometers (3.7 miles)		Last partial image taken by DART before impact, transmission of this image was terminated by the destruction of the transmitter. Resolution per pixel to be determined at a later date by analysis of image and timing.
23:14 UTC (T-0)	0 kilometers (0 miles)		Impact Dimorphos (estimated impact velocity 6 kilometers/second)
23:17 UTC (T+2 min 45 s)	56.7 kilometers (35.2 miles)		Closest approach to Dimorphos by LICIACube.

Gallery

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  Size comparison of DART and the two Didymos asteroids

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  This chart offers insight into data the DART team used to determine the orbit of Dimorphos after impact.

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  Images of the impact captured by the Hubble (left) and Webb space telescopes (right).

See also

In Spanish: Double Asteroid Redirection Test para niños

• Asteroid impact avoidance
• B612 Foundation
• Deep Impact (spacecraft)
• Don Quijote (spacecraft)
• NEO Surveyor
• The Spaceguard Foundation