A rare asteroid fastball hazard is turning scientists into sluggers

This article originally appeared on Dark.

On a fall evening in 2022, scientists at the Johns Hopkins University Applied Physics Laboratory were completing the final stages of a planetary defense mission. As Andy Rivkin, one of the team leaders, prepared to appear on NASA's live broadcast of the experiment, a colleague posted a photo of a pair of asteroids: the 800-meter-wide Didymos and, orbiting it, a smaller one. called Dimorphos, about 7 million miles from Earth.

“We could see Didymos and this little speck in the right place where we expected Dimorphos to be,” Rivkin recalled.

After the interview, Rivkin joined a crowd of scientists and guests to watch the mission's finale on several large screens: As part of an asteroid deflection mission called DART, a spacecraft approached Dimorphos and photographed its rocky surface in increasing detail.

Then, at 7:14 p.m., a spacecraft weighing about 1,300 pounds collided head-on with the asteroid.

Within minutes, members of the mission team in Kenya and South Africa posted images from their telescopes showing a bright plume of debris.

In the days that followed, researchers continued to observe the dust cloud and found that it had morphed into various shapes, including clumps, spirals and two comet-like tails. They also calculated that the impact slowed Dimorphos' orbit by about a tenth of a centimeter per second, a proof-of-concept that a spacecraft – known as a kinetic impactor – could target and deflect an asteroid far from Earth.

Ron Ballouz, a planetary scientist at the lab, noted that what is often seen in movies is a “kind of last-ditch effort, what we like to call a final phase of planetary defense.” But if dangerous objects can be detected years in advance, other techniques such as a kinetic impactor can be used, he added.

If a deflection were necessary, scientists would calculate the speed of a dangerous object, such as a asteroid or comet, enough that it does not end up at the same place and time as Earth as they orbit the Sun. Rivkin said this translates into a change of at least seven minutes in the arrival time: for example, if an object the size of Dimorphos were predicted to collide with Earth in 67 years, the delay caused by DART would be just enough to allow to the seven-minute mark, he added.

With less lead time, researchers could use a combination of multiple deflections, larger spacecraft or speed increases, depending on the dangerous object. “DART is designed to validate a technique and specific situations inevitably require adjustment,” Rivkin said.

Researchers use data from DART and smaller-scale experiments to predict the amount of deflection using computer simulations.

What is often seen in movies is a “kind of last-ditch effort, what we like to call a final phase of planetary defense.”

Scientists are also focusing on the type of asteroid that Dimorphos appears to be: a “rubber pile,” as they call it, because these types of objects are thought to be made of clumps of many rocks.

In fact, scientists think that most asteroids the size of Dimorphos and larger are debris clusters. As scientists learn more about debris piles, they will be able to make better predictions about the deflection of asteroids or comets. And in 2026, a new mission will arrive in Didymos and Dimorphos to collect more data to refine the computer models.

In the meantime, researchers are trying to learn as much as possible in the untoward event that an asteroid or comet is discovered to pose a threat to Earth and a faster response is necessary.


Scientists first suspected that many asteroids were debris clusters about fifty years ago. Their models showed that when larger asteroids collided, the collisions could throw off fragments that would then reassemble to form new objects.

It wasn't until 2005 that scientists saw their first mess, asteroid Itokawa, when a spacecraft visited and photographed it. Then in 2018 they saw another planet called Ryugu, and later that year another, asteroid Bennu. DART's camera also showed that Didymos and Dimorphos are likely the same species.

“It's one thing to talk about rubble piles, and another to see what looks like a pile of rocks dumped from a truck up close,” said William Bottke, a planetary scientist at the Southwest Research Institute in Boulder, Colorado.

Scientists suspect that rubble piles have large amounts of empty space between their rocks. They believe that these stacks are connected by very weak forces, mainly gravity, meaning they can fall apart more easily than an asteroid made of a single boulder. This was evident at Dimorphos, when DART estimated that more than ten thousand tons of material were excavated. The debris plume in turn acted like a rocket engine and provided an extra push in the opposite direction, slowing the asteroid. So while the asteroid's voids may have absorbed some of the DART impact, the debris explosion increased the amount of deflection, with estimates ranging between two and five times as much as the force of the spacecraft alone.

Private space flight photo
Illustration of NASA's DART spacecraft showing images of the asteroids Dimorphos (left) and Didymos (right) acquired by DART. It is believed that debris asteroids like Dimorphos are bound together by very weak forces, especially gravity, making them easier to break apart than a single boulder. Visual: NASA/Johns Hopkins APL/Joshua Diaz

However, Sabina Raducan, a planetary scientist at the University of Bern in Switzerland, warned that caution should be exercised if kinetic impactors are ever to be used on smaller debris piles.

Raducan and her team used a computer model to apply the results of the DART impact to different debris piles – the first time such research has been done. The results, which were published in The Planetary Science Journal, show that a DART-sized spacecraft traveling at the speed at which it collided, about 6 km per second, could break a piece of rubble less than 80 meters in diameter into many pieces. Some boulders can in turn strike the earth, potentially causing injury and damage.

Raducan wrote in a follow-up email that despite DART's success, a similar scenario may not always be optimal for all asteroids.

Instead, she added, a spacecraft's size or speed may need to be adjusted for a successful deflection.

“It's one thing to talk about piles of rubble, and another to see what looks like a pile of rocks dumped from a truck up close.”

The possible disintegration of materials could also be related to comets. These objects are similar to asteroids, except they contain ice such as water or carbon dioxide. When comets pass close to the Sun, these materials turn into gases, which can act like a rocket booster and push the comet faster. So as researchers aim to knock a comet off its crash course with Earth, they must consider the possibility of ice being exposed or buried, which could change its speed and possibly require further deflections.

Rivkin said cometary collisions with Earth are relatively rare compared to asteroids, but there are “certainly a lot of additional things to keep an eye on.”

Also complicating matters, some objects classified as asteroids may also contain buried ice.

“Things are getting very murky, though,” Bottke said. “We have seen asteroids develop tails” similar to those in comets.


They are scientists eagerly awaiting late 2026, when a spacecraft named Hera, as part of a planetary defense mission led by the European Space Agency, in collaboration with the Japan Aerospace Exploration Agency, is expected to arrive at the Didymos system. There it will deploy two smaller satellites, and together they will study the pair of asteroids up close. In particular, researchers are looking forward to finally being able to measure Dimorphos' mass, which will help them better refine their estimate of the force given by the spacecraft and the debris explosion. Hera and satellites 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 also allow scientists to see what DART has done with Dimorphos. The interim measures suggest the asteroid is so weak that the impact changed shape instead of leaving a crater: “I really want to see the outcome,” Raducan said. “Is it a crater or not?”

The explosion or tsunami resulting from the impact of an asteroid like Bennu could cause fatalities and damage on a regional or continental scale.

A new shape may in turn have changed Dimorphos' orbit around Didymos. Hera will allow scientists to check this, helping them better understand the response of kinetic impacts to asteroids with one or more moons. Currently, it is estimated that about 16 percent of near-Earth asteroids larger than about 650 feet in diameter are binary stars, or systems of two. It is believed that the Earth suffered a double whammy 458 million years ago, creating 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 debris piles and deflections. Rivkin said they only got a quick look at Didymos as DART sped past.

In the meantime, researchers are analyzing samples from the surface of asteroid Bennu that a NASA spacecraft called OSIRIS-REx returned to Earth in the fall of 2023. The results will help researchers better understand the asteroid's material properties. The approximately 1600 meter wide Bennu is the most potentially dangerous object known (from May 14, 2024), with a 0.037 percent chance of hitting Earth on September 24, 2182.

Ballouz noted that the explosion or tsunami resulting from such an impact could cause fatalities and damage on a regional or continental scale. He added that if Bennu remains a hazard and if deflections are deemed necessary, it would require multiple kinetic impacts due to its large size. The observations and measurements from the time OSIRIS-REx closely observed Bennu, which took place until 2021, along with the results of the sample returns, would be invaluable for planning kinetic impactor missions to the asteroid, if necessary. Additional spacecraft missions could also be organized to study the asteroid again or collect even more samples, to help further inform the impact models.

It's never good news to hear about discoveries of potential threats to Earth, but knowing about the possibility in advance can at least help scientists take action, unlike some natural hazards that occur without warning.

“It is important that people are aware that impacts have affected the Earth in the past and that this possibility exists in the future,” Ballouz said. “There should also be a general awareness that there are people studying this aspect of how we interact with space.”

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