The Double Asteroid Redirection Test (DART) was a NASA space mission to test a method of planetary defense against near-Earth objects (NEOs). It was designed to assess how much a spacecraft impact can deflect an asteroid by hitting it head-on. The target asteroid, Dimorphos, is a wide moonlet of the asteroid Didymos; neither asteroid poses an impact threat to Earth, but their joint characteristics made them an ideal target.
Launched on 24 November 2021, the DART spacecraft collided with Dimorphos at on 26 September 2022 about from Earth. The collision shortened Dimorphos's orbit around Didymos by 32 minutes, greatly in excess of the pre-defined success threshold of 73 seconds. Deflection resulted from the ejection of debris which caused a recoil that was substantially larger than the impact itself. The collision also very slightly increased the asteroid pair's orbital speed around the Sun—an effect directly relevant to the goal of planetary protection by changing an object's path through space.
DART was a joint project between NASA and the Johns Hopkins University Applied Physics Laboratory. The project was funded through NASA's Planetary Defense Coordination Office, managed by NASA's Planetary Missions Program Office at the Marshall Space Flight Center, and several NASA laboratories and offices provided technical support. The Italian Space Agency contributed LICIACube, a CubeSat which photographed the impact event, and other international partners, such as the European Space Agency (ESA), and Japan Aerospace Exploration Agency (JAXA), are contributing to related or subsequent projects.; ; ;
Mission history NASA and the European Space Agency (ESA) started with individual plans for missions to test asteroid deflection strategies, but by 2015, they struck a collaboration called AIDA (Asteroid Impact and Deflection Assessment) involving two separate spacecraft launches that would work in synergy.AIDA DART Home page at APLDART at Applied Physics Laboratory Johns Hopkins University Under that proposal, the European Asteroid Impact Mission (AIM), would have launched in December 2020, and DART in July 2021. AIM would have orbited the larger asteroid to study its composition and that of its moon. DART would then kinetically impact the asteroid's moon on 26 September 2022, during a close approach to Earth.
The AIM orbiter was however canceled, then replaced by Hera which plans to start observing the asteroid four years after the DART impact. Live monitoring of the DART impact thus had to be obtained from ground-based telescopes and radar.
In June 2017, NASA approved a move from concept development to the preliminary design phase, and in August 2018 the start of the final design and assembly phase of the mission.Asteroid-deflection mission passes key development milestone 7 September 2018 On 11 April 2019, NASA announced that a SpaceX Falcon 9 would be used to launch DART.
Satellite impact on a small Solar System body had already been implemented once, by NASA's Deep Impact space probe's impactor spacecraft and for a completely different purpose (analysis of the structure and composition of a comet). On impact, Deep Impact released 19 gigajoules of energy (the equivalent of 4.8 tons of TNT), and excavated a crater up to wide.
Description
Spacecraft
The DART spacecraft was an impactor with a mass of that hosted no scientific payload and had sensors only for navigation. The spacecraft cost US$330 million by the time it collided with Dimorphos in 2022.
Camera
DART's navigation sensors included a Sun sensor, a star tracker called SMART Nav software (Small-body Maneuvering Autonomous Real Time Navigation), and a aperture camera called Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO). DRACO was based on the Long Range Reconnaissance Imager (LORRI) onboard New Horizons spacecraft, and supported autonomous navigation to impact the asteroid's moon at its center. The optical part of DRACO was a Ritchey-Chrétien telescope with a field of view of 0.29° and a focal length of 2.6208 m (f/12.60). The spatial resolution of the images taken immediately before the impact was around 20 centimeters per pixel. The instrument had a mass of .
The detector used in the camera was a CMOS image sensor measuring 2,560 × 2,160 pixels. The detector records the wavelength range from 0.4 to 1 micron (visible and near infrared). A commercial off-the-shelf CMOS detector was used instead of a custom charge-coupled device in LORRI. DRACO's detector performance actually met or exceeded that of LORRI because of the improvements in sensor technology in the decade separating the design of LORRI and DRACO. Fed into an onboard computer with software descended from anti-missile technology, the DRACO images helped DART autonomously guide itself to its crash.
Solar arrays
Using ROSA as the structure, a small portion of the DART solar array was configured to demonstrate Transformational Solar Array technology, which has very-high-efficiency SolAero Inverted Metamorphic (IMM) solar cells and reflective concentrators providing three times more power than other available solar array technology.;
Antenna The DART spacecraft was the first spacecraft to use a new type of high-gain communication antenna, a Spiral Radial Line Slot Array (RLSA). The circularly-polarized antenna operated at the (microwave) X-band NASA Deep Space Network (NASA DSN) frequencies of 7.2 and 8.4 GHz, and had a gain of 29.8 dBi on downlink and 23.6 dBi on uplink. The fabricated antenna in a flat and compact shape exceeded the given requirements and was tested through environments resulting in a TRL-6 design.
Ion thruster
DART demonstrated the NEXT gridded ion thruster, a type of solar electric propulsion. It was powered by solar arrays to generate the approximately 3.5 kW needed to power the NASA Evolutionary Xenon Thruster–Commercial (NEXT-C) engine. Early tests of the ion thruster revealed a reset mode that induced higher current (100 A) in the spacecraft structure than expected (25 A). It was decided not to use the ion thruster further as the mission could be accomplished without it, using conventional thrusters fueled by the of hydrazine onboard. However, the ion thrusters remained available if needed to deal with contingencies, and had DART missed its target, the ion system could have returned DART to Dimorphos two years later.NASA's DART Mission Post-Asteroid-Impact News Briefing, 26 September 2022, 8pm EDT, at 27 minutes
Secondary spacecraft
The Italian Space Agency (ASI) contributed a secondary spacecraft called LICIACube (Light Italian CubeSat for Imaging of Asteroids), a small CubeSat that piggybacked with DART and separated on 11 September 2022, 15 days before impact. It acquired images of the impact and ejecta as it drifted past the asteroid.Asteroids have been hitting the Earth for billions of years. In 2022, we hit back. Andy Rivkin, The Johns Hopkins University Applied Physics Laboratory, 27 September 2018 LICIACube communicated directly with Earth, sending back images of the ejecta after the Dimorphos flyby. LICIACube is equipped with two optical cameras, dubbed LUKE and LEIA.
Effect of the impact on Dimorphos and Didymos
The spacecraft hit Dimorphos in the direction opposite to the asteroid's motion. Following the impact, the instantaneous orbital speed of Dimorphos therefore dropped slightly, which reduced the radius of its orbit around Didymos. The trajectory of Didymos was also modified, but in inverse proportion to the ratio of its mass to the much lower mass of Dimorphos and therefore not much. The actual velocity change and orbital shift depended on the topography and composition of the surface, among other things. The contribution of the recoil momentum from the impact ejecta produces a poorly predictable "momentum enhancement" effect. Before the impact, the momentum transferred by DART to the largest remaining fragment of the asteroid was estimated as up to 3–5 times the incident momentum, depending on how much and how fast material would be ejected from the impact crater. Obtaining accurate measurements of that effect was one of the mission's main goals and will help refine models of future impacts on asteroids.
Scientists later determined that the impact also slightly changed the orbit of the asteroid pair around the Sun--the first time human action measurably had such an effect on a celestial body. The orbital speed increased by about two inches an hour from the pre-impact speed of . Amateur astronomers in several countries contributed data by precisely measuring the pair's occultation of background stars along their orbit after impact. Scientists also used radio telescope data about the pair obtained before and after impact. According to a study author, even such a tiny orbital change might prevent a catastrophic asteroid collision with Earth.
The DART impact excavated surface/subsurface materials of Dimorphos, leading to the formation of a crater and/or some magnitude of reshaping (i.e., shape change without significant mass loss). Some of the ejecta may eventually hit Didymos's surface. If the kinetic energy delivered to its surface was high enough, reshaping may have also occurred in Didymos, given its near-rotational-breakup spin rate. Reshaping on either body would have modified their mutual gravitational field, leading to a reshaping-induced orbital period change, in addition to the impact-induced orbital period change. If left unaccounted for, this could later have led to an erroneous interpretation of the effect of the kinetic deflection technique.
Observations of the impact
DART's companion LICIACube,LICIACube Twitter feed the Hubble Space Telescope, James Webb Space Telescope, and the Earth-based ATLAS observatory all detected the ejecta plume from the DART impact.ATLAS twitter feed On September 26, SOAR observed the visible impact trail to be over long. Initial estimates of the change in binary orbit period were expected within a week and with the data released by LICIACube. DART's mission science depends on careful Earth-based monitoring of the orbit of Dimorphos over the subsequent days and months. Dimorphos was too small and too close to Didymos for almost any observer to see directly, but its orbital geometry is such that it transits Didymos once each orbit and then passes behind it half an orbit later. Any observer that can detect the Didymos system therefore sees the system dim and brighten again as the two bodies cross.
The impact was planned for a moment when the distance between Didymos and Earth is at a minimum, permitting many telescopes to make observations from many locations. The asteroid was near opposition and visible high in the night sky well into 2023. The change in Dimorphos's orbit around Didymos was detected by optical telescopes watching mutual eclipses of the two bodies through photometry on the Dimorphos-Didymos pair. In addition to radar observations, they confirmed that the impact shortened Dimorphos's orbital period by 32 minutes. Based on the shortened binary orbital period, the instantaneous reduction in Dimorphos's velocity component along its orbital track was determined, which indicated that substantially more momentum was transferred to Dimorphos from the escaping impact ejecta than from the impact itself. In this way, the DART kinetic impact was highly effective in deflecting Dimorphos.
Follow-up mission