DART, which is the world’s first attempt to change the speed and trajectory of an asteroid in space, is trying a method to deflect asteroids that could be useful if such a need arises in the future to protect Earth. The reason an asteroid’s moon was chosen for this is because its orbit around the asteroid will change slightly. The asteroid itself will not be deflected so as not to accidentally hit Earth
The DART team verified the trajectory of the target asteroid
Using some of the world’s most powerful telescopes, the DART probe team completed a set of observations over six nights in July to verify previous calculations of the orbit of Dimorphos—DART’s target asteroid. Dimorphos orbits its larger parent asteroid Didymus. These observations verify where the asteroid is likely to be at the time of impact. DART, which is the world’s first attempt to change the speed and trajectory of an asteroid in space, is trying a method to deflect asteroids that could be useful if such a need arises in the future to protect Earth.
“The measurements the team made in early 2021 were critical to ensuring that DART arrived at the right place at the right time for its kinetic impact into Dimorphos,” said Andy Rivkin, co-director of the DART investigation team at Johns Hopkins University. “Verifying these measurements with new observations will show us that we don’t need to change course and are already on target.”
Understanding the dynamics of the dimorphos pathway, however, is important for reasons other than ensuring the DART hit. If DART is successful in changing Dimorphus’ orbit, the moon will move closer to Didymus, shortening the time it takes to orbit it. Although this change can be measured directly, the scientists need to make sure that nothing other than the impact affects the trajectory. This includes weak forces such as radiation recoil from the sun-heated asteroid, which can nudge it gently and cause its orbit to change.
“This experiment is kind of ‘before and after’ and therefore requires excellent knowledge of the asteroid system before anything is done to it,” said Nick Moskowitz, an astronomer at the Lowell Observatory in Flagstaff, Arizona, and co-director of the July Observing System. “We don’t want to say at the last minute, “Oh, here’s something we didn’t think about or a phenomenon we didn’t take into account”. We want to be sure that any changes we see will be solely because of what DART did.”
In late September to early October, around the time of DART’s impact, Didymus and Dimorphos will be closest to Earth in years, at a distance of about 10.8 million km. As of March 2021, the Didymus system was out of range of most ground-based telescopes because of its distance from Earth. But in early July This year the DART investigation team used powerful telescopes in Arizona and Chile—the Lowell Discovery Telescope at Lowell Observatory, the Magellan Telescope at Las Campans Observatory, and the Southern Astrophysical Research Telescope (SOAR)—to observe the asteroid system and look for changes in its brightness. These changes, called “reciprocal events,” Occur when one of the asteroids passes in front of the other, blocking some of the light they emit.
“This time of year is problematic for obtaining these observations,” Moskowitz said. In the northern hemisphere the nights are short and this is the monsoon period in Arizona. In the Southern Hemisphere, the threat of winter storms was at its peak. In fact, right after the observatory system, a big snowstorm hit Chile, and the mountain where SOAR was located had to be evacuated. As a result, the telescope was closed for almost ten days. “We asked for six half-nights of observation with the expectation that we would lose about half of them due to the weather, but we only lost one night. We were very lucky.”
In total, the team was able to extract from the data the timing of 11 new mutual events. Analyzing these brightness changes allowed scientists to determine precisely how long it takes for the dimorph to orbit the larger asteroid. This way they can predict where Dimorph will be at specific points in time, including when DART hits. The results matched previous calculations.
“We are very confident now that the asteroid system is well understood and are prepared to understand what will happen after the impact,” Moskowitz said.
In addition to verifying Dimorphus’ orbit duration and expected position at the time of impact, these observations also allowed team members to refine the process they will use to determine whether, and by how much, DART was able to alter Dimorphus’ trajectory after impact.
In October, after DART collides with the asteroid, the team will again use ground-based telescopes around the world to look for mutual events and calculate Dimorphos’ new orbit. They predict that the time it takes the small asteroid to orbit Didymus will vary by a few minutes. These observations will also help constrain theories put forward by scientists around the world about the dynamics of Dimorphos’ orbit and the rotation of the two asteroids around their axes.
DART is the world’s first experimental planetary defense mission, the deliberate execution of a kinetic strike on a dimorph to slightly alter its motion in space. Neither asteroid poses a threat to Earth, but the DART mission will demonstrate that a spacecraft can autonomously navigate to kinetically strike a relatively small target asteroid, and this technique is applicable to deflecting an asteroid on a collision course with Earth if such an asteroid is ever discovered. DART will reach its target on September 26, 2022.
Operation DART on the NASA website
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