Earlier this week, NASA ad that his DART spacecraft had successfully deflected an asteroid. This raises a good question: How on earth did scientists figure this out, given that Dimorphos is almost 11 million kilometers away?
NASA’s DART mission managed to shorten the amount of time it takes for Dimorphos to orbit Didymos, as the spacecraft pushed the asteroid a little closer to its companion. Dimorphos’s orbital period used to be 11 hours and 55 minutes, but now it’s 11 hours and 23 minutes, a change of 32 minutes, give or take. As far as the altered distance is concerned, this represents “tens of meters,” as Nancy Chabot, DART mission coordination lead from Johns Hopkins Applied Physics Laboratory, told reporters.
A ‘decisive moment’
NASA Director Bill Nelson described this successful test as a “defining moment for humanity.” In fact, it marks the first time our species has deliberately changed the motion of a celestial object. It is also the first large-scale demonstration of a strategy to deflect asteroids, showing that it could eventually actually protect us from an asteroid threat.
Dimorphos posed no real danger to Earth, but it was the perfect place to test kinetic impact technology. The 600-kilogram DART spacecraft traveled for 10 months to reach this binary asteroid system, where it crashed into a 160-meter-wide rock while traveling at a speed of 22,500 kilometers per hour. DART hit the asteroid with near-millimeter precision on September 26, but it was not immediately clear whether this impact had any effect.
That the DART mission hit the asteroid was immediately obvious, since observations from both From space What From the earth they showed that a comet-like plume of material had been projected out during the hours and days after impact. However, it took astronomers about two weeks to confirm the new orbital dynamics of the Didymos-Dimorphos system. Two different data systems were also needed for the task, one with optical data and one with radar data, but both gave the same answer: 11 hours and 23 minutes.
The alteration of an eclipse
The optical data comes from ground-based observatories around the world, including the telescopes at South Africa’s Las Cumbres Observatory (LCO) and the Southern Chile Astrophysical Research Telescope. One limitation of optical telescopes is that, due to the distance and small size of the Didymos-Dimorphos system, the two objects are seen as a single bright point. The asteroids are 1.2 km apart and Didymos, the larger of the two, is only 780 meters across.
Ground-based optical telescopes can’t tell the two apart, but that doesn’t mean Dimorphos is invisible to our eyes. Didymos’s brightness temporarily decreases by 10% each time Dimorphos passes in front of him. It is through these eclipses that astronomers were able to know the orbital period of Dimorphos before carrying out the test and what has allowed them to determine it. now. That Dimorphos passes in front of Didymos from our perspective on Earth is a purely fortuitous action and one of the reasons key for the that this system was chosen for the DART test.
Optical observatories around the world made continuous observations on the asteroid system. “Since the orbital period was about 12 hours, having telescopes in South Africa as well as in Chile, about six hours apart, meant we could capture times when Dimorphos was behind or ahead of Didymos and we couldn’t see from Chile,” said Tim Lister, an astronomer at the LCO. “This really helped pin down the new period and the change caused by the DART impact.”
Detection of ‘weak radar echoes’
The radar data comes from NASA’s Goldstone Planetary Radar and NSF’s Green Bank Observatory in West Virginia. Unlike optical telescopes, radar “can get different signals from both objects directly,” Chabot said.
Radar images from the two observatories, taken each night for a total of two weeks, were combined to create before-and-after views of this binary asteroid system. This allowed astronomers to measure the “difference between where Dimorphos is observed and where it would have been with its original orbit,” as NASA explained in a blog post. release.
“The Green Bank Telescope’s large collecting area makes it extremely sensitive and makes it a perfect receiving station to detect these faint radar echoes,” explained Jim Jackson, director of the Green Bank Observatory, in a statement. “These radar measurements” were key to determining “how dramatic the event really was by detecting changes in its orbit around Didymos and definitively establishing its deviation.”
The “two independent methods” provided “the same answer,” he explained. Chabot referring to Dimorphos’ new orbital period of 11 hours and 23 minutes. However, much work remains to be done.
the beginning of the beginning
Much is still unknown about the effect of the experiment. DART was a resounding success, but it’s clear that scientists still have much to learn about kinetic impactors and the art of deflecting asteroids.
For example, astronomers must refine their estimates of Dimorphos’s mass, shape, density, and surface composition. This will help them understand how the DART spacecraft transferred its momentum to its target and how the resulting effects contributed to the observed orbital change.
During a press conference last week, Tom Statler, NASA’s DART program scientist, said that recoil from debris that blasted off the asteroid’s surface was a major factor in the orbital shift. This is probably a consequence of the physical composition of Dimorphos, since it is a rubble-pile asteroid rather than a compact rock. Statler also wondered if Dimorphos is now wobbling as a result of the impact. Astronomers are now watching the asteroid system closely to refine their preliminary estimates and to detect any further changes.
The European Space Agency is already preparing a follow-up mission to see the asteroids up close. The HERA probe, scheduled for launch in 2024, will observe Dimorphos in late 2026 and return images and other data to help us better understand the effects of the DART mission. A robust planetary defense system against asteroids will not be built overnight, but it is important work that has already begun.
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