The DART investigative team finished a six-night observation campaign last month using some of the most potent telescopes in the world to verify earlier estimates of the orbit of Dimorphos—target DART’s asteroid. Dimorphos is orbiting Didymos, its bigger parent asteroid. These data validate the projected location of the asteroid at the time of impact. DART tests an asteroid deflection technique that might be helpful if a need for planetary defence emerges in the future. It is the first attempt in history to alter the speed and course of an asteroid’s motion in space.
According to Andy Rivkin, a co-lead on the DART investigation team at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, “the team’s measurements in early 2021 were crucial for ensuring that DART arrived at the right place and time for its kinetic impact into Dimorphos.” “Confirming those measurements with new observations shows us that we don’t need any course changes and we’re already right on target.”
However, there are additional benefits to comprehending Dimorphos’ orbit dynamics in addition to ensuring DART’s effectiveness. If DART is successful in changing Dimorphos’ trajectory, the moonlet will approach Didymos and require less time to orbit it. It is simple to measure that change, but researchers must be certain that the orbit is only being affected by the hit. This includes more subdued forces like radiation recoil from the Sun-warmed surface of the asteroid, which can gently press on the asteroid and alter its orbit.
“The before-and-after nature of this experiment requires exquisite knowledge of the asteroid system before we do anything to it,” said Nick Moskovitz, an astronomer with Lowell Observatory in Flagstaff, Arizona, and co-lead of the July observation campaign. “We don’t want to, at the last minute, say, ‘Oh, here’s something we hadn’t thought about or phenomena we hadn’t considered.’ We want to be sure that any change we see is entirely due to what DART did.”
Didymos and Dimorphos will come as close to Earth as they have in recent years in late September or early October, around the time of DART’s collision. They will be about 6.7 million miles (10.8 million kilometres) distant after this. Due of its distance from Earth, the Didymos system had been out of reach for the majority of ground-based telescopes since March 2021. But at the beginning of July, the DART Investigation Team used powerful telescopes in Arizona and Chile to observe the asteroid system and search for variations in its brightness.
These telescopes included the Southern Astrophysical Research (SOAR) Telescope, the Lowell Discovery Telescope, and the Magellan Telescope at Las Campanas Observatory. Due to Dimorphos’ orbit, these alterations, known as “mutual occurrences,” take place when one of the asteroids passes in front of the other, obstructing some of the light they emit.
“It was a tricky time of year to get these observations,” said Moskovitz. In the Northern Hemisphere, the nights are short, and it is monsoon season in Arizona. In the Southern Hemisphere, the threat of winter storms loomed. In fact, just after the observation campaign, a major snowstorm hit Chile, prompting evacuations from the mountain where SOAR is located. This resulted in the telescope being shut down for close to ten days. “We asked for six half-nights of observation with some expectation that about half of those would be lost to weather, but we only lost one night. We got really lucky.”
Overall, the team was able to determine 11 new mutual events’ time from the data. Scientists were able to properly calculate how long it takes Dimorphos to orbit the larger asteroid by analyzing those brightness variations. By doing so, they are able to forecast Dimorphos’ whereabouts at particular points in time, particularly when DART will have an impact. The outcomes agreed with earlier calculations.
“We really have high confidence now that the asteroid system is well understood and we are set up to understand what happens after impact,” Moskovitz asked.
The team was able to validate Dimorphos’ orbital period and predicted location at the moment of impact thanks to this observation effort, and it also improved the procedure they will use to evaluate whether DART effectively altered Dimorphos’s orbit post-hit and by how much.
After DART collides with the asteroid in October, the team will once more use ground-based telescopes all across the globe to search for mutual events and determine Dimorphos’ new orbit. They anticipate a change of several minutes in the smaller asteroid’s orbital period around Didymos. The mechanics of Dimorphos’ orbit and the spin of both asteroids are two more topics that these studies will help put constraints on.
As a project of the Planetary Missions Program Office of NASA, Johns Hopkins APL oversees the DART programe on behalf of the agency’s Planetary Defense Coordination Office. DART, the first planetary defence test mission ever, deliberately impacts Dimorphos in order to gently alter its path through space. Although neither asteroid poses a threat to Earth, the DART mission will show that a spacecraft is capable of autonomously navigating to a kinetic impact on a relatively small target asteroid and that this is a workable method to divert an asteroid if one is ever detected. The DART project will succeed on September 26, 2022.
