DART team confirms orbit of target asteroid
Using some of the world’s most powerful telescopes, the DART research team completed a six-night observing campaign last month to confirm previous calculations of the orbit of Dimorphos—DART’s target asteroid. Dimorphos orbits its larger parent asteroid, Gemini. These observations confirm where the asteroid is expected to be at the time of impact. DART, which is the world’s first attempt to change the speed and path of an asteroid’s motion in space, is testing an asteroid deflection method that could prove useful if such a need arises for planetary defense in the future. “The measurements the team made in early 2021 were critical to ensure that DART arrived at the right place and at the right time for its kinetic effect on Dimorphos,” said Andy Rivkin, co-leader of the DART research team at Johns Hopkins University Applied. Physics Laboratory (APL) in Laurel, Maryland. “Confirming these measurements with new observations shows us that we do not need to change course and that we are already on target.” On the night of July 7, 2022, the Lowell Discovery Telescope near Flagstaff, Arizona captured the asteroid Gemini. Credit: Lowell Observatory/N. Moscovitch Understanding the trajectory dynamics of Dimorphos, however, is important for reasons beyond ensuring the impact of DART. If DART manages to change Dimorph’s path, the moon will move closer to Gemini, reducing the time it takes to orbit it. Although measuring this change is simple, scientists need to confirm that nothing other than the impact affects the trajectory. This includes subtle forces such as the reflection of radiation from the asteroid’s surface heated by the sun, which can gently nudge the asteroid and cause its orbit to change. “The before-and-after nature of this experiment requires an excellent knowledge of the asteroid system before we do anything about it,” said Nick Moskovitz, an astronomer at the Lowell Observatory in Flagstaff, Arizona, and co-leader of the July observing campaign. “We don’t want, at the last minute, to say, ‘Oh, here’s something we hadn’t thought of or phenomena we hadn’t thought of.’ We want to be sure that any change we see is solely due to what DART did.” On the night of July 7, 2022, the Lowell Discovery Telescope near Flagstaff, Arizona captured this sequence in which the asteroid Gemini, located near the center of the screen, moves across the night sky. The sequence here is sped up by about 1,800 times. Scientists used this and other observations from the July campaign to confirm Dimorphos’ orbit and expected location at the time of DART’s impact. Credit: Lowell Observatory/N. Moscovitch In late September to early October, around the time of DART’s impact, Gemini and Dimorphos will make their closest approach to Earth in years. That would put them about 6.7 million miles (10.8 million kilometers) away. As of March 2021, the Gemini system was beyond the range of most ground-based telescopes due to its distance from Earth. However, in early July, the DART research team used powerful telescopes in Arizona and Chile—the Lowell Discovery Telescope at Lowell Observatory, the Magellan Telescope at Las Campanas Observatory, and Southern Astrophysical Research (SOAR)—to observe the asteroid system and to look for changes in its brightness. These changes, called “reciprocal events,” occur when one of the asteroids passes in front of the other due to Dimorphos’ orbit, blocking some of the light they emit. “It was a difficult time of year to get these observations,” Moskovitz said. In the northern hemisphere, the nights are short and it’s monsoon season in Arizona. In the Southern Hemisphere, the threat of winter storms has emerged. In fact, immediately after the observing campaign, a large snowstorm hit Chile, causing evacuations from the mountain where SOAR is located. This resulted in the telescope being out of service for almost ten days. “We asked for six half-nights of observation with some expectation that about half of them would be lost to weather, but we only lost one night. We were really lucky.” In total, the team was able to extract the timing of 11 new mutual events from the data. Analyzing these changes in brightness allowed scientists to determine precisely how long it takes Dimorphos to orbit the larger asteroid. So they are able to predict where Dimorphos will be at certain times, including when the DART makes an impact. The results were consistent with previous calculations. “We really have a lot of confidence now that the asteroid system is well understood and we’re ready to figure out what happens after the impact,” Moskovitz said. This observing campaign not only allowed the team to confirm the orbital period and expected location of Dimorphos at the time of impact, but also allowed team members to refine the process they will use to determine whether DART has successfully changed its orbit. Dimorphos after impact and by how much. In October, after DART crashes into the asteroid, the team will again use ground-based telescopes around the world to search for mutual events and calculate Dimorphos’ new orbit. They expect the time it takes the smaller asteroid to orbit Gemini will be off by several minutes. These observations will also help constrain the theories scientists around the world have put forth about the dynamics of Dimorphos’ orbit and the rotation of both asteroids. Johns Hopkins APL manages the DART mission for NASA’s Planetary Defense Coordination Office as a project of the agency’s Planetary Missions Program Office. DART is the world’s first planetary defense test mission, deliberately performing a kinetic impact on Dimorphos to slightly alter its motion in space. Although neither asteroid poses a threat to Earth, the DART mission will demonstrate that a spacecraft can autonomously navigate a kinetic impact on a relatively small asteroid target and that this is a viable technique to deflect an asteroid on a collision course with Earth. Earth, if ever discovered. DART will reach its target on September 26, 2022.
