Double Asteroid Redirection Test (DART) is a NASAspace mission aimed at testing a method of planetary defense against near-Earth objects (NEOs). Launched from Earth in November 2021, the mission will deliberately crash a space probe into the minor-planet moonDimorphos of the double asteroidDidymos to assess the future potential of a spacecraft impact to deflect an asteroid on a collision course with Earth through a transference of momentum. The asteroid poses no actual threat to Earth; it was merely selected for the test.
DART is a joint project between NASA and the Johns HopkinsApplied Physics Laboratory (APL). The project is 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 are providing technical support. International partners, such as the European Space Agency (ESA), Italian Space Agency (ASI), and Japan Aerospace Exploration Agency (JAXA), are contributing to related or subsequent projects. In August 2018, NASA approved the project to start the final design and assembly phase. The DART spacecraft was successfully launched on 24 November 2021, with collision slated for 26 September 2022 at 23:14 UTC.[3][4][5]
Background
Originally, the European Space Agency (ESA) and NASA had independent plans for missions to test asteroid deflection strategies, and by 2015 they struck a collaboration called AIDA (Asteroid Impact & Deflection Assessment) involving two separate spacecraft launches that work in synergy.[6][7][8] Under the proposal, the European spacecraft, 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 September 26, 2022, during a close approach to Earth.[7] AIM would have studied the asteroid's strength, surface physical properties, and internal structure, as well as measure the effect on the asteroid moon's orbit around the larger asteroid.[citation needed]
This method of asteroid impact avoidance has already been implemented once, for a completely different purpose (analysis of the structure and composition of a comet), by NASA's 370 kg (820 lb) Deep Impact space probe. On impact, it released 19 gigajoules of energy (the equivalent of 4.8 tons of TNT),[10][11][12][13] decreased the perihelion of CometTempel 1 by 10 m (33 ft), changed orbit by 10 cm (3.9 in), and a predicted 0.0001 mm/s (0.014 in/h) velocity change after impacting the comet at the speed of ~10.2 km/s (6.3 mi/s) on July 4, 2005.[9]
The AIM orbiter was cancelled, then replaced by Hera, that would observe the asteroid four years after the DART impact. The effects of the impact by DART would have then been monitored live from ground-based telescopes and radar.[14][8]
In June 2017, NASA approved a move from concept development to the preliminary design phase,[15] and in August 2018 NASA approved the project to start the final design and assembly phase.[16]
On 11 April 2019, NASA announced that a SpaceX Falcon 9 would be used to launch DART.[17]
Description
Spacecraft
The DART spacecraft is an impactor with a mass of 610 kg (1,340 lb),[18] that hosts no scientific payload and has only sensors for navigation such as a Sun sensor, a star tracker called SMART Nav software (Small-body Maneuvering Autonomous Real Time Navigation),[19] and a 20 cm (7.9 in) aperture camera called Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO). DRACO is based on the Long Range Reconnaissance Imager (LORRI) onboard New Horizons spacecraft, and will support autonomous navigation to impact the asteroid's moon at its center. The optical part of DRACO is a Ritchey-Chrétien telescope equipped with telephoto lens with a field of view of 0.29° and a focal length of f/12.60. The detector, of CMOS type, has 2,560 × 2,160 pixels. The spatial resolution of the images recorded immediately before the impact is 20 centimeters. The detector records the wavelength range from 0.4 to 1 micron (visible and near infrared). The instrument has a mass of 8.66 kg (19.1 lb).[20] They used a commercial off-the-shelf detector instead of a CCD as on LORRI. A CCD was chosen for LORRI because they needed the extra sensitivity for the dim lighting conditions at 32 AU; DRACO will be imaging at 1 AU so doesn’t need that. All that being said, DRACO’s detector performance actually meets or exceeds that of LORRI because of the improvements in sensor technology in the decade separating the design of LORRI and DRACO.[21] Fed into an onboard computer with software descended from anti-missile technology, the DRACO images will help DART autonomously guide itself to its crash.[22]
DART spacecraft uses the NEXT ion thruster, a type of solar electric propulsion.[14][23] It will be powered by 22 m2 (240 sq ft) solar arrays to generate the ~3.5 kW needed to power the NASA Evolutionary Xenon Thruster–Commercial (NEXT-C) engine.[24]
Using ROSA as the structure, a small portion of the DART solar array is configured to demonstrate Transformational Solar Array technology, which has very-high-efficiency solar cells and reflective concentrators providing three times more power than current solar array technology.[26][27]
The DART spacecraft is the first spacecraft to use a new type of high gain communication antenna, that is, a Spiral Radial Line Slot Array (RLSA). The antenna operates at the X-bandNASA Deep Space Network (NASA DSN) frequencies of 7.2 and 8.4 GHz. The fabricated antenna in a flat and compact shape exceeds the given requirements and has been tested through environments resulting in a TRL-6 design.[28]
Effect of the impact on the orbit of Dimorphos and Didymos
The spacecraft must hit Dimorphos in the opposite direction to the asteroid's motion. The actual velocity change and orbital shift are unknown until after it happens because it depends on the topology of the surface. Following the impact, the orbital speed of Dimorphos drops slightly, which has the effect of reducing the radius of its orbit around Didymos. The trajectory of Didymos is also modified but in reduced proportions because the mass of Dimorphos is much lower than that of Didymos. There is a poorly predictable "momentum enhancement" effect due to the contribution of recoil momentum from impact ejecta.[29] The final momentum transferred to the largest remaining fragment of the asteroid could be up to 3-5 times the incident momentum, and obtaining accurate measurements of the effects, which will help refine models of such impacts, is one of the mission's main goals.[30]
The DART impact will excavate 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). The ejecta may eventually hit Didymos's surface. If the kinetic energy delivered to the surface is high enough, reshaping may also occur in Didymos, given its near-critical spin rate. Reshaping on either body will modify the 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 lead to an erroneous interpretation of the effect of the kinetic deflection technique.[31]
Initial estimates of the change in binary orbit period should be known within a week and with the data released by LICIACube.[32] DART's mission science depends on careful Earth-based monitoring of the orbit of Dimorphos over the subsequent days and months. Dimorphos will be too small 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, so any observer that can detect the Didymos system will see the Didymos 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 lots of telescopes to make observations from lots of locations. The asteroid will be near opposition and hanging around high in the night sky through the new year.[33] Detection of the change in Dimorphos' orbit around Didymos will be done by optical telescopes watching mutual eclipses of the two bodies or photometry on the Dimirphos-Didymos pair. A detailed reconnaissance and assessment will be performed a few years later by a spacecraft called Hera, approved by ESA in November 2019.[34][35]
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 15 days before impact on 11 September 2022. It will try to acquire images of the impact and ejecta as it drifts past the asteroid.[34][36] LICIACube will communicate directly with Earth, sending back images of the ejecta after the Dimorphos flyby.[37] LICIACube is equipped with two optical cameras, dubbed LUKE and LEIA.[38]
Follow-up mission
In a collaborating project, the European Space Agency is developing Hera, a spacecraft that will be launched to Didymos in 2024[39] and arrive in 2027[40] (5 years after DART's impact), to do a detailed reconnaissance and assessment.[39]Hera would carry two CubeSats, Milani and Juventas.[39]
The mission's target is 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 m (2,560 ft) in diameter; the asteroid moon Dimorphos (Didymos B) is about 160 m (520 ft) in diameter in an orbit about 1 km (0.62 mi) from the primary.[14] The mass of the Didymos system is estimated at 528 billion kg, with Dimorphos composing 4.8 billion kg of that total.[18] On its approach DART will initially target the binary system, then differentiate the larger and smaller members of the binary pair, and finally steer the spacecraft to strike the smaller Dimorphos.[49] The Didymos system is not an Earth-crossing asteroid, and there is no possibility that the deflection experiment could create an impact hazard.[50]
Choosing a binary asteroid system is advantageous because changes to Dimorphos' 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 it on a collision course with Earth. In addition, the binary system was relatively close (11 million kilometers) to the Earth in 2022.[51]
Preflight preparations
Falcon 9 rocket's payload fairing being attached to NASA's Double Asteroid Redirection Test (DART) spacecraft on 16 November 2021Falcon 9 and DART vertical at SLC-4E
Launch preparations for DART began on 20 October 2021, as the spacecraft began fueling at Vandenberg Space Force Base in California.[52] The spacecraft arrived at Vandenberg Space Force Base (VSFB) near Lompoc, in early October 2021 after a cross-country drive. DART team members have since been preparing 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 kg (110 lb) of hydrazine propellant for spacecraft maneuvers and attitude control. DART also carries about 60 kg (130 lb) of xenon for the NEXT-C ion engine. Engineers loaded the xenon before the spacecraft left APL in early October 2021.[53]
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, November 16 and 17, inside the SpaceX Payload Processing Facility at Vandenberg Space Force Base in California and the ground teams completed a successful Flight Readiness Review later that week with the fairing then attached to the rocket.[54]
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.[53]
Launch
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 eccentricityEarth orbit designed to avoid the Moon. In such a scenario, DART would use its low thrust, high efficiencyNEXT 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 homes in on Didymos's moon Dimorphos.[55]
The transit phase before impact lasts about 9 months. During its interplanetary travel, the DART spacecraft made a distant flyby of the 578-meter-diameter near-Earth asteroid (138971) 2001 CB21 on 6 March 2022.[56][57]
DART's DRACO camera opened its aperture door and took its first light image of some stars on 7 December 2021, when it was 3 million km (2 million mi) away from Earth.[58] 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.[58] On 10 December 2021, DRACO imaged the open clusterMessier 38 for further optical and photometric calibration.[58]
On 27 May 2022, DART observed the bright star Vega with DRACO to test the camera's optics with scattered light.[59] 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.[60]
Jupiter and its four Galilean moons imaged by DRACO in a SMART Nav test
Course of the impact
Two months before the impact, on 27 July 2022 the DRACO camera detected the Didymos system from approximately 32 million km (20 million mi) and refined its trajectory. The LICIACubenanosatellite was released on 11 September 2022, 15 days before the impact.[61] Four hours before impact, DART becomes completely autonomous and implements its SMART Nav guidance system. Three hours before impact, when the target is 176,000 km (109,000 mi) away, DART performs an inventory of objects near the target. The final trajectory is fixed 90 minutes before the collision when the asteroid is 38,000 km (24,000 mi) away from Dimorphos. When it is 24,000 km (15,000 mi) away, Dimorphos begins to be observable (1.4 pixels). Until impact, DRACO takes continuous images of the asteroid's surface which are transmitted in real-time. In the last minutes, trajectory corrections are no longer allowed so that the images taken by DRACO (the only ones to provide a detailed view of the surface of Dimorphos) remain sharp. Indeed, due to the length of the solar panels, each use of the propulsion causes vibrations that make the images blurry. The last image, transmitted two seconds before impact, should have a spatial resolution of fewer than 20 centimeters. The impact will take place on 26 September 2022 at 23:14 UTC.[2]
The head-on impact of the 500 kg (1,100 lb)[62] DART spacecraft at 6.6 km/s (4.1 mi/s)[63][64] will produce an energy equivalent of about three tonnes of TNT[65] and an estimated velocity reduction of Dimorphos on the order of 0.4 mm/s.[66] 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. Although the change in Dimorphos's orbit is small, the offset in its orbital position will accumulate and become more noticeable over time.[7][50][67] The orbital period reduction from the head-on impact serves to facilitate ground-based observations of Dimorphos; if Dimorphos were impacted on its trailing side such that its orbital period would increase up to 12 hours, its period would coincide with Earth's day and night cycle, which would limit ground-based observers from observing all orbital phases of Dimorphos nightly.[68] Over a span of years, the cumulative trajectory change from such a small change in velocity could mitigate the risk of a hypothetical Earth-bound asteroid hitting Earth.[69] The impact will target the center of figure of Dimorphos and should decrease the orbital period, currently 11.92 hours, by roughly 10 minutes.[18]
DART impact course illustrated from different angles
DART's Collision with Dimorphos (side view) An animation taking a wide-angle view from the side as NASA's first planetary defense test mission, the Double Asteroid Redirection Test (DART)
Telescopic Observations An animation illustrating how observations by telescopes are able to measure the orbital period of Dimorphos about Didymos by measuring the changing brightness of the system over time
DART's Collision with Dimorphos (back view) An animation looking from behind as NASA's first planetary defense test mission, the Double Asteroid Redirection Test (DART), collides with the asteroid moonlet Dimorphos
DART's Collision from Dimorphos' Point of View
DART's Collision from DRACO and SMART Nav's Point of View
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^Bray, Matthew (2020). "A Spiral Radial Line Slot Array Antenna for NASA's Double Asteroid Redirection Test (DART)". 2020 IEEE International Symposium on Antennas and Propagation and North American Radio Science Meeting. pp. 379–380. doi:10.1109/IEEECONF35879.2020.9330400. ISBN978-1-7281-6670-4. S2CID231975847.
^ abcTalbert, Tricia (22 December 2021). [With Its Single “Eye,” NASA’s DART Returns First Images from Space "NASA's DART Captures One of Night Sky's Brightest Stars"]. NASA. Retrieved 25 September 2022. ((cite web)): Check |url= value (help)
Launches are separated by dots ( • ), payloads by commas ( , ), multiple names for the same satellite by slashes ( / ). Crewed flights are underlined. Launch failures are marked with the † sign. Payloads deployed from other spacecraft are (enclosed in parentheses).
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