NASA and Johns Hopkins teams have successfully tested the world’s first dedicated planetary defense mission: the Double Asteroid Redirection Test (DART). DART impacted Dimorphos, a moonlet of near-Earth asteroid 65803 Didymos, at 7:14 PM EDT (23:14 UTC) on Monday, September 26, while traveling at an approximate speed of 22,000 kilometers per hour.
DART’s collision with Dimorphos produced an impact that is equivalent to that of approximately three tonnes of TNT, and — as one would expect — completely destroyed the DART spacecraft. LICIACube, a small CubeSat released by DART on Sept. 11, imaged DART’s impact with Dimorphos, the subsequent ejecta, and possibly even the impact crater created by DART on Dimorphos’ surface. LICIACube will communicates directly with Earth during the impact and has sent images of the impact back to Earth in the hours and days after the event.
What is DART?
DART is a joint mission between NASA and the Johns Hopkins University Applied Physics Laboratory (APL) in Howard County, Maryland. APL manufactured the DART spacecraft, with LICIACube being built and contributed by the Italian Space Agency (ASI).
DART was a relatively small spacecraft, massing only 610 kilograms. Since DART was a spacecraft designed to be destroyed, the spacecraft body itself was fairly bare and featured no scientific instruments, with only essential spacecraft systems (i.e. power, computers, propulsion, etc.) and a camera onboard.
The first of these spacecraft systems was the Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO). DRACO was a narrow-angle telescope with a 208-millimeter aperture and 0.29-degree field of view. During DART’s descent toward Dimorphos, DRACO imaged the surface of the moonlet and the anticipated impact location. What’s more, DRACO assisted DART’s autonomous SMART Nav (Small-body Maneuvering Autonomous Real-Time Navigation) algorithm by reliably and efficiently guiding DART toward its impact location.
Another important system aboard DART was its propulsion system. DART used NASA’s NEXT-C (NASA’s Evolutionary Xenon Thruster–Commercial) ion propulsion system, which was a solar-powered electric propulsion system that used a gridded ion engine. The ion engine produced thrust by the electrostatic acceleration of ions formed from xenon propellant inside the spacecraft.
DART’s power source came in the form of two Roll-Out Solar Array (ROSA) wings. Each ROSA wing extended 8.5 meters in length and was lighter and more compact than traditional solar arrays used on other spacecraft. The ROSAs were developed by Redwire’s Deployable Space Systems in California and were first tested aboard the International Space Station (ISS) in 2017. Since then, a permanent set of ROSAs have been installed on the ISS, and DART became the first planetary spacecraft to carry the technology — paving the way for their use in future planetary exploration missions.
Installed onto one of the ROSAs was the Transformational Solar Array, a technology demonstration that had extremely efficient solar cells and reflective concentrators that allowed it to produce three times more power than traditional solar technology. Testing and proving this technology will allow solar arrays to be built in smaller sizes, giving future deep-space spacecraft the option of solar-powered systems rather than nuclear-powered systems — decreasing the cost of those missions.
Lastly, the Radial Line Slot Array (RLSA) was a low-cost, high-gain antenna that allowed DART to communicate with Earth with high-efficiency, compact, and planar form communications.
Additionally, DART carried a companion with it on its journey to Dimorphos and Didymos — LICIACube. The Light Italian CubeSat for Imaging of Asteroids (LICIACube) was designed on a 6U platform developed by aerospace company Argotec and features two instruments: LEIA and LUKE. The LICIACube Explorer Imaging for Asteroid, or LEIA, is a narrow-field panchromatic camera that will acquire images of the impact from a long distance with high spatial resolution. LICIACube Unit Key Observer, or LUKE, is an RGB wide field camera that will allow scientists to analyze the impact and the asteroidal environment in multicolor.
DART’s impact and Dimorphos
DART’s target Dimorphos is a small asteroid moonlet located around asteroid 65803 Didymos. Dimorphos was discovered in 2003, with Didymos discovered a few years earlier in 1996. The two asteroids are a part of the Apollo group of asteroids and orbit anywhere between ~2.28 AU and ~1.01 AU around the Sun, completing an orbit once every two years. Dimorphos and Didymos were approximately 9.6 million kilometers away from Earth at the time of impact and posed no threat to Earth.
Dimorphos measures only 170 meters in diameter, which is around four times smaller than the much larger Didymos, which has a diameter of approximately 780 meters across — making Dimorphos one of the smallest astronomical objects to have a permanent name. Dimorphos orbits Didymos at a distance of around one kilometer in a near-equatorial and near-circular orbit, with an orbital period of approximately 11.9 hours. Furthermore, Dimorphos’s orbital period is synchronous to its rotation, meaning that the same side of Dimorphos always faces Didymos.
DART launched on November 24, 2021, atop a SpaceX Falcon 9 rocket from Space Launch Complex 4 East (SLC-4E) at Vandenberg Space Force Base in California. Immediately following launch, DART began the transfer phase of the mission, wherein it traveled through space for nine months, preparing for impact.
On July 27, two months before impact, DART’s DRACO camera detected Didymos and Dimorphos from a distance of approximately 32,186,880 kilometers, allowing teams to refine DART’s trajectory to the asteroid pair. DART continued to coast toward the Didymos system for the next two months, until it released LICIACube on Sept. 11, 15 days before impact.
At around four hours before impact, DART became fully autonomous and locked its DRACO camera on Didymos, allowing DART’s SMART Nav guidance system to take over. When Dimorphos was around 176,000 kilometers away from DART (~three hours until impact) and in an effort to locate the moon around Didymos, DART began to perform an inventory of objects near Dimorphos.
Around 90 minutes before impact, DART located and locked DRACO on Didymos, allowing DART’s guidance systems to update the final impact trajectory of the spacecraft. At this point, DART was located approximately 38,000 kilometers from Dimorphos.
When DART was 50 minutes from impact, the spacecraft shifted DRACO and SMART Nav’s targeting from Didymos to Dimorphos. Both asteroids were still in the view of the spacecraft, but DART focused only on Dimorphos.
At 20 minutes before impact, DART and SMART Nav entered “precision lock,” wherein DRACO completely ignored Didymos and solely focused on Dimorphos. Furthermore, DART was producing a high amount of thrust with its NEXT-C engine at this point.
Just two and a half minutes before impact, DART shut down its NEXT-C ion engine and began final preparations for impact. At this point, DRACO and SMART Nav were locked onto Dimorphos and the impact site, and DART was set to impact the moonlet at approximately 22,000 kilometers per hour. DART shut off its engine in these final minutes to prevent trajectory corrections from causing blurry images, as DRACO was imaging DART’s entire descent toward Dimorphos.
Finally, at 23:14 UTC, DART slammed into Dimorphos, generating a blast that was equivalent to that of three tonnes of TNT and spewing surface material away from Dimorphos. As was expected, all data and live views from DART ceased at the moment of impact. The final image from DRACO was transmitted around two seconds before impact.
During the time of impact, LICIACube was imaging Dimorphos and the ejecta that followed the impact with its two camera instruments. LICIACube flew past Dimorphos approximately three minutes after impact.
DART teams gave the spacecraft a 91% to 99% chance that it would successfully impact Dimorphos’ surface if DRACO could see Dimorphos leading up to and during the time of impact. At 8:00 PM EDT on September 26 (00:00 UTC on Sept. 27), NASA held a press conference to discuss the impact and provide additional details.
Timelapse video of NASA's DART spacecraft impacting an asteroid for science. pic.twitter.com/6h88wcGjYc
— Michael Baylor (@MichaelBaylor_) September 26, 2022
DART’s head-on impact with Dimorphos is expected to cause an estimated speed change of around 0.4 millimeters per second — shifting the orbit of Dimorphos around Didymos and leading to a larger orbital shift of both asteroids over time. In fact, DART’s collision with Dimorphos is expected to shorten its orbital period around Didymos, currently 11.92 hours, by approximately 10 minutes.
DART, at its core, is a planetary defense mission, designed to test a system that could be utilized in the event of an asteroid impact threat to Earth. But, what exactly is planetary defense, and why is it important?
Planetary defense, essentially, is all the capabilities needed to detect, warn, and prevent/mitigate the threat of asteroid and comet impacts to Earth and their effects. NASA and planetary defense organizations across the world have a list of near-Earth objects (NEOs) that have heliocentric orbits that bring them inside a zone that is approximately 195 million kilometers from the Sun. If NEOs enter this zone, they can pass within around 50 million kilometers of Earth and its orbit.
DART’s purpose within planetary defense is to address and test the “mitigation” portion of the initiative. DART’s impact demonstrated a way to deflect an asteroid/comet away from its predicted impact trajectory with Earth, should an asteroid or comet ever threaten Earth.
However, DART’s impact will also allow scientists to study asteroids, their surfaces, and how their orbits are affected by impacts from micrometeoroids or other cosmic material.
One of the primary goals of DART’s mission is to measure how much of Dimorphos’ motion is disrupted and changed by DART’s impact. DART didn’t just bump into Dimorphos; it slammed into the moonlet at around 22,000 kilometers per hour, or around six kilometers per second — enough energy to alter the orbit of Dimorphos around Didymos.
DART’s impact into Dimorphos not only pushed the asteroid out of its normal orbit but is expected to have ejected somewhere between 10,000 and 100,000 kilograms of surface material from the asteroid, with the recoil “kick” from the excavation of this material rivaling, or even exceeding, the energetic push given by DART during impact.
Though high-speed impacts on planetary bodies are extremely common throughout the solar system, scientists have not yet had the capabilities to study their effects or understand how they work. Up until DART, scientists have only been able to use computer simulations to simulate high-velocity impacts into planetary bodies, but those simulations have their limits — especially when modeling an impact event that is of similar magnitude to DART’s impact into Dimorphos.
Furthermore, in order to accurately model DART’s impact using computer simulations, scientists would need to know the physical properties of Dimorphos and how those properties affect the ejecta from the impact event. However, this information is largely unknown, as no spacecraft prior to DART has studied Dimorphos and Didymos.
Though using computer simulations to model the impact may be difficult, scientists could still make predictions as to what would happen during the impact by using previously-collected data and information on Dimorphos from telescopes and other spacecraft.
DART teams predict that Dimorphos’ strength, or ability to withstand bending or breaking from the impact, and porosity could possibly double the orbital enhancement experienced from DART’s impact. Images and data collected during the impact event, as well as future missions to Dimorphos and Didymos, will help scientists confirm their predictions and models, and will help refine simulations for future high-velocity impact missions like DART.
LICIACube wasn’t the only spacecraft observing DART’s impact, though. Following the impact event, telescopes measured the brightness of the Didymos system over time to plot a light curve. When selecting an asteroid for DART’s mission, scientists selected Didymos and Dimorphos because the asteroids are binaries, and cause dips in the brightness of the system as Dimorphos crosses in front of and behind Didymos during its orbit.
Scientists will measure this change in brightness to plot a light curve, which can give them information on Dimorphos’ orbital period and location after impact, similar to how scientists use light curves to analyze exoplanets. Scientists will compare the light curves from before and after the impact to investigate and analyze how DART’s impact changed the orbit of Dimorphos around Didymos.
Roughly four years after DART’s impact, a separate mission developed by the European Space Agency (ESA) will visit Dimorphos and Didymos and investigate the effects of DART’s impact on the asteroids. Named Hera, the mission is scheduled to launch in 2024 and rendezvous with the Didymos system in 2026.
Hera and its two CubeSat companions will survey both Didymos and Dimorphos in extreme detail, with a specific focus on observing the crater left by DART on Dimorphos and measuring the mass of Dimorphos after the impact. Furthermore, the detailed post-impact observations from Hera will enhance our knowledge of planetary defense substantially.
DART, LICIACube, and Hera teams are a part of the Asteroid Impact and Deflection Assessment (AIDA), an international collaboration that aims to produce some of the most accurate data and knowledge possible from DART’s asteroid deflection technology demonstration.
What’s more, AIDA is also comprised of researchers from around the world who will work together to understand and analyze planetary defense tactics and solar system science from the DART and Hera missions. AIDA’s international collaboration demonstrates the understanding and importance of planetary defense worldwide, and that scientists, engineers, and agencies are willing to seek and solve the problems posed by near-Earth objects throughout the solar system.
(Lead image: Artist’s illustration of DART at Dimorphos. Credit: NASA/Johns Hopkins APL)