On a fall evening in 2022, scientists at Johns Hopkins University’s Applied Physics Laboratory were busy working on the final stages of a planetary defense mission. As one of the team leaders, Andy Rivkin, was preparing to appear on NASA’s live broadcast of the experiment, his colleague posted a photo of a pair of asteroids. Didymos, which is half a mile wide, and a smaller asteroid orbiting it. It’s called Dimorphos, which fell about 7 million miles from Earth.
“We could see Didymos and this little dot in the right spot where we expected Dimorphos to be,” Rivkin recalled.
After the interview, Rivkin joined numerous scientists and guests to watch the mission’s finale on several large screens. As part of the Asteroid Deflection Mission, called DART, a spacecraft was approaching Dimorphos to take increasingly detailed images of its rocky surface.
Then, at 7:14 p.m., the approximately 1,300-pound spacecraft collided head-on with the asteroid.
Within minutes, mission team members from Kenya and South Africa posted telescope photos showing a bright plume of debris.
Over the next few days, researchers continued to observe the dust cloud and found that it had morphed into a variety of shapes, including clumps, spirals, and two comet-like tails. They also calculated that the impact slowed Dimorphos’ orbit by about a tenth of an inch per second, a proof of concept that spacecraft, also called kinetic impactors, can target and redirect asteroids farther from Earth.
Ron Ballouz, a planetary scientist at the lab, said what you often see in movies is “kind of a last-ditch effort, what we like to call the final phase of planetary defense.” But if dangerous objects can be detected years in advance, other technologies, such as kinetic shock devices, could be used, he added.
When deflection is necessary, scientists must change the speed of dangerous objects, such as asteroids or comets, so they don’t end up at the same place and time as Earth as they orbit the sun. Rivkin said this results in a change of at least seven minutes in arrival time. For example, if an object the size of Dimorphos was predicted to hit Earth 67 years from now, the deceleration delivered by DART would be enough to add up. Up to seven minutes, he added.
Shorter lead times allow researchers to use a combination of deflections in multiple directions, larger spacecraft, or increased speed depending on the hazardous object. “DART is designed to validate technology, and adjustments will inevitably be needed in certain situations,” Rivkin said.
Researchers use data from DART and smaller experiments to predict the amount of bias through computer simulations.
“What you often see in movies is ‘some kind of last-ditch effort, what we call the final phase of planetary defense.’”
Scientists are also focusing on the type of asteroid Dimorphos appears to be: so-called “debris piles.” This is because these types of objects are thought to be made up of many rock chunks.
In fact, scientists believe that most asteroids the size of Dimorphos or larger are debris piles. As scientists continue to learn more about the debris pile, they will be able to make better predictions about the changing direction of an asteroid or comet. And in 2026, new missions will arrive on Didymos and Dimorphos to collect more data to fine-tune computer models.
In the meantime, researchers are trying to learn as much as possible about the unwelcome cases in which an asteroid or comet turns out to be a threat to Earth and requires a quicker response.
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Scientists first About 50 years ago, many asteroids were suspected to be debris piles. Their model showed that when larger asteroids collide with each other, the collisions break apart fragments, which can then recombine to form new objects.
But it wasn’t until 2005 that scientists saw their first pile of debris, asteroid Itokawa, when a spacecraft visited and took pictures. Then in 2018, they saw another asteroid called Ryugu, and later that year they saw another asteroid, Bennu. DART’s cameras also showed that Didymos and Dimorphos were likely the same variant.
“It’s one thing to talk about a pile of rubble, but it’s another thing to see a pile of rubble from a truck up close,” said William Bottke, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. said.
Scientists suspect there is a large amount of empty space between the rocks in the rubble pile. They believe the pile is held together by very weak forces, mostly gravity. This means that it can break apart more easily than a single rock, an asteroid. This was evident at Dimorphos, as DART unearthed approximately over 10,000 tons of material. The debris plume acted like a rocket thruster, slowing the asteroid by providing additional thrust in the opposite direction. So while the asteroid’s empty space may have absorbed some of the DART impact, the explosion of debris increased the amount of deflection, with estimates ranging from about two to five times what the spacecraft would displace.
But Sabina Raducan, a planetary scientist at the University of Bern in Switzerland, warned that caution must be exercised if kinetic impactors are to be used on small debris piles.
Raducan and her team used computer models to apply DART impact results to various debris piles. This is the first time such a study has been conducted. The results, published in The Planetary Science Journal, show that a spacecraft the size of DART crashing at a speed of about 3.7 miles per second could shatter a pile of debris less than 80 meters in diameter into several pieces. As a result, some rocks can impact the Earth, potentially causing injury and damage.
Raducan wrote in a follow-up email that despite DART’s success, similar scenarios may not always be optimal for all asteroids.
Instead, she added, a successful deflection may require adjusting the size or speed of the spacecraft.
“‘It’s one thing to talk about a pile of rubble, it’s another thing to see up close a pile of rubble thrown out of a truck.’”
The material’s potential for disintegration may also be related to comets. These objects are similar to asteroids, except that they contain ice such as water or carbon dioxide. When a comet passes close to the sun, these materials can turn into gases and act like rocket boosters, pushing the comet faster. So if researchers aim to drop a comet on a collision course with Earth, they must consider the possibility that it may have exposed or buried ice, which could change its velocity and require additional deflection.
Rivkin said that while comet impacts with Earth are relatively rare compared to asteroids, “there’s definitely a lot more to track.”
Also complicating matters is that some objects classified as asteroids may have ice embedded in them.
“Things are getting very dark,” Bottke said. “We saw that asteroids have tails.” Similar to those found in comets.
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Scientists As part of a planetary defense mission led by the European Space Agency, in collaboration with the Japan Aerospace Exploration Agency, a spacecraft called Hera is eagerly awaited when it is scheduled to arrive in the Didymos system in late 2026. It will place two small satellites there and together they will begin to study the pair of asteroids up close. In particular, researchers are hoping to finally be able to measure Dimorphus’ mass. This will allow for more refined estimates of how much pressure the spacecraft exerted and the explosion of its debris. Hera and its satellite will also make measurements that will allow scientists to calculate the density and strength of Dimorphos, which can be used in impact models.
The Hera mission will allow scientists to determine what effect DART had on Dimorphos. Preliminary measures suggest the asteroid was so weak that the impact likely changed its shape rather than cratered it. “I really want to see the results,” Raducan said. “Is it a fireball or not?”
“An explosion or tsunami from the impact of an asteroid like Bennu could cause fatalities and damage on a regional or continental scale.”
The new shape may have altered Dimorphos’ orbit around Didymos. Hera will help scientists better understand the response of kinematic impacts to asteroids with one or more moons by allowing them to check. It is currently estimated that about 16% of near-Earth asteroids larger than about 650 feet in diameter are binary star systems, or two systems. The Earth is thought to have suffered a double blow 458 million years ago that left behind the Lockne and Målingen craters in Sweden.
Hera and its satellites will also collect measurements of Didymos’ material properties, which will also help advance scientists’ knowledge of its debris pile and bending. Rivkin said he was able to quickly see Didymos as DART passed him quickly.
Meanwhile, researchers are busy analyzing surface samples of asteroid Bennu, which NASA’s spacecraft OSIRIS-REx returned to Earth in the fall of 2023. The findings will help researchers better understand the material properties of asteroids. At approximately 1,600 feet across, Bennu is the most dangerous object known (as of May 14, 2024), with a 0.037% chance of hitting Earth on September 24, 2182.
Ballouz noted that an explosion or tsunami from such an impact could cause fatalities and damage on a regional or continental scale. He added that Bennu remains a hazard and if a deflection is deemed necessary, multiple kinetic impacts would be necessary due to its large size. The observations and measurements made since OSIRIS-REx observed Bennu up close through 2021, along with sample return results, will be critical to planning a kinetic impactor mission to the asteroid if needed. Additional spacecraft missions may also be organized to re-study the asteroid or collect more samples, which could provide more information for impact models.
It’s never good news to hear that a potential threat to the Earth has been discovered, but unlike some natural disasters that occur without warning, knowing the possibility in advance at least allows scientists to take action.
“It’s important for people to recognize that shocks have affected the planet in the past and that this is likely to happen in the future as well,” Ballouz said. “There should also be general awareness that there are people studying these aspects of how we interact with space.”
This article was originally published on Undark. Read the original article.
Image credit: SpaceX A Falcon 9 rocket launches the Double Asteroid Redirection Test (DART) spacecraft in the first full-scale planetary defense test. (NASA/Bill Ingalls)
