asteroids

Smashing into an asteroid shows researchers how to better protect Earth

asteroids, jpl, NASA, Space, syndication / Ryan Harris / May 15, 2024

Connecting with a fastball —

Slowing down an asteroid by just one-tenth of a second makes all the difference.

Theo Nicitopoulos, Undark Magazine – May 15, 2024 2: 54 pm UTC

Enlarge / Riding atop a SpaceX Falcon 9 rocket, NASA’s Double Asteroid Redirection Test, or DART, spacecraft sets off to collide with an asteroid in the world’s first full-scale planetary defense test mission in November 2021.

On a fall evening in 2022, scientists at the Johns Hopkins University Applied Physics Laboratory were busy with the final stages of a planetary defense mission. As Andy Rivkin, one of the team leaders, was getting ready to appear in NASA’s live broadcast of the experiment, a colleague posted a photo of a pair of asteroids: the half-mile-wide Didymos and, orbiting around it, a smaller one called Dimorphos, taken about 7 million miles from Earth.

“We were able to see Didymos and this little dot in the right spot 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 big screens: As part of an asteroid deflection mission called DART, a spacecraft was closing in on Dimorphos and photographing its rocky surface in increasing detail.

Then, at 7: 14 pm, a roughly 1,300-pound spacecraft slammed head-on into the asteroid.

Within a few 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 discovered 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, proof-of-concept that a spacecraft—also called a kinetic impactor—could target and deflect an asteroid far from Earth.

The final five-and-a-half minutes of images from the DART spacecraft as it approached and then intentionally collided with asteroid Dimorphos. The video is 10 times faster than reality, except for the last six images.

NASA/Johns Hopkins APL/YouTube

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

If a deflection were necessary, scientists would need to change the speed of a hazardous object, such as an asteroid or comet, enough that it doesn’t end up at the same place and time as Earth as they orbit the Sun. Rivkin said this translates into at least a seven-minute change in the arrival time: If a Dimorphos-sized object were predicted to collide with Earth 67 years from now, for instance, the slow-down that DART imparted would be just enough to add up to the seven minutes, he added.

With less lead time, researchers could use a combination of multiple deflections, larger spacecrafts, or boosts in speed, depending on the hazardous object. “DART was designed to validate a technique, and specific situations would inevitably require adapting things,” said Rivkin.

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

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

In fact, scientists think that most asteroids the size of Dimorphos and larger are rubble piles. As scientists continue to learn more about rubble piles, they will be able to make better predictions about deflecting asteroids or comets. And in 2026, a new mission will arrive at Didymos and Dimorphos to collect more data to fine-tune the computer models.

In the meantime, researchers are trying to learn as much as possible in the unwelcome case an asteroid or comet is discovered to be a threat to Earth and a more rapid response is necessary.

Scientists first suspected that many asteroids are rubble piles about 50 years ago. Their models showed that when larger asteroids smashed into one another, the collisions could throw off fragments that would then reassemble to form new objects.

It wasn’t until 2005, though, that scientists saw their first rubble pile: asteroid Itokawa, when a spacecraft visited it and photographed it. Then, in 2018, they saw another called Ryugu, and later that year, one more, asteroid Bennu. DART’s camera also showed Didymos and Dimorphos are likely of the same variety.

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

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Outdoing the dinosaurs: What we can do if we spot a threatening asteroid

asteroids, Features, near earth object, near-earth asteroids, Science / Ryan Harris / May 10, 2024

We'd like to avoid this. — Enlarge / We’d like to avoid this.

Science Photo Library/Andrzej Wojcicki/Getty Images

In 2005, the United States Congress laid out a clear mandate: To protect our civilization and perhaps our very species, by 2020, the nation should be able to detect, track, catalog, and characterize no less than 90 percent of all near-Earth objects at least 140 meters across.

As of today, four years after that deadline, we have identified less than half and characterized only a small percentage of those possible threats. Even if we did have a full census of all threatening space rocks, we do not have the capabilities to rapidly respond to an Earth-intersecting asteroid (despite the success of NASA’s Double-Asteroid Redirection Test (DART) mission).

Some day in the finite future, an object will pose a threat to us—it’s an inevitability of life in our Solar System. The good news is that it’s not too late to do something about it. But it will take some work.

Close encounters

The dangers are, to put it bluntly, everywhere around us. The International Astronomical Union’s Minor Planet Center, which maintains a list of (no points award for guessing correctly) minor planets within the Solar System, has a running tally. At the time of the writing of this article, the Center has recorded 34,152 asteroids with orbits that come within 0.05 AU of the Earth (an AU is one astronomical unit, the average distance between the Earth and the Sun).

These near-Earth asteroids (or NEAs for short, sometimes called NEOs, for near-Earth objects) aren’t necessarily going to impact the Earth. But they’re the most likely ones to do it; in all the billions of kilometers that encompass the wide expanse of our Solar System, these are the ones that live in our neighborhood.

And impact they do. The larger planets and moons of our Solar System are littered with the craterous scars of past violent collisions. The only reason the Earth doesn’t have the same amount of visible damage as, say, the Moon is that our planet constantly reshapes its surface through erosion and plate tectonics.

It’s through craters elsewhere that astronomers have built up a sense of how often a planet like the Earth experiences a serious impact and the typical sizes of those impactors.

Tiny things happen all the time. When you see a beautiful shooting star streaking across the night sky, that’s from the “impact” of an object somewhere between the size of a grain of sand and a tiny pebble striking our atmosphere at a few tens of thousands of kilometers per hour.

Every few years or so, an object 10 meters across hits us; when it does, it delivers energy roughly equivalent to that of our earliest atomic weapons. Thankfully, most of the Earth is open ocean, and most impactors of this class burst apart in the upper atmosphere, so we typically don’t have to worry too much about them.

The much larger—but thankfully much rarer—asteroids are what cause us heartburn. This is where we get into the delightful mathematics of attempting to calculate an existential risk to humanity.

At one end of the scale, we have the kind of stuff that kills dinosaurs and envelops the globe in a shroud of ash. These rocks are several kilometers across but only come into Earth-crossing trajectories every few million years. One of them would doom us—certainly our civilization and likely our species. The combination of the unimaginable scale of devastation and the incredibly small likelihood of it occurring puts this kind of threat almost beyond human comprehension—and intervention. For now, we just have to hope that our time isn’t up.

Then there are the in-betweeners. These are the space rocks starting at a hundred meters across. Upon impact, they release a minimum of 30 megatons of energy, which is capable of leaving a crater a couple of kilometers across. Those kinds of dangers present themselves roughly every 10,000 years.

That’s an interesting time scale. Our written history stretches back thousands of years, and our institutions have existed for thousands of years. We can envision our civilization, our ways of life, and our humanity continuing into the future for thousands of years.

This means that at some point, either we or our descendants will have to deal with a threat of this magnitude. Not a rock large enough to hit the big reset button on life but powerful enough to present a scale of disaster not yet seen in human history.

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What would the late heavy bombardment have done to the Earth’s surface?

asteroids, craters, Earth science, geology, Impacts, planetary science, Science, tsunamis / Ryan Harris / January 26, 2024

Under fire —

Early in Earth’s history, bombardment by enormous asteroids was common.

Alka Tripathy-Lang – Jan 26, 2024 6: 38 pm UTC

Image of a projection of the globe, with multi-colored splotches covering its surface. — Enlarge / Each panel shows the modeled effects of early Earth’s bombardment. Circles show the regions affected by each impact, with diameters corresponding to the final size of craters for impactors smaller than 100 kilometers in diameter. For larger impactors, the circle size corresponds to size of the region buried by impact-generated melt. Color coding indicates the timing of the impacts. The smallest impactors considered in this model have a diameter of 15 kilometers.

Simone Marchi, Southwest Research Institute

When it comes to space rocks slamming into Earth, two stand out. There’s the one that killed the dinosaurs 65 million years ago (goodbye T-rex, hello mammals!) and the one that formed Earth’s Moon. The asteroid that hurtled into the Yucatan peninsula and decimated the dinosaurs was a mere 10 kilometers in diameter. The impactor that formed the Moon, on the other hand, may have been about the size of Mars. But between the gigantic lunar-forming impact and the comparatively diminutive harbinger of dinosaurian death, Earth was certainly battered by other bodies.

At the 2023 Fall Meeting of the American Geophysical Union, scientists discussed what they’ve found when it comes to just how our planet has been shaped by asteroids that impacted the early Earth, causing everything from voluminous melts that covered swaths of the surface to ancient tsunamis that tore across the globe.

Modeling melt

When the Moon-forming impactor smashed into Earth, much of the world became a sea of melted rock called a magma ocean (if it wasn’t already melted). After this point, Earth had no more major additions of mass, said Simone Marchi, a planetary scientist at the Southwest Research Institute who creates computer models of the early Solar System and its planetary bodies, including Earth. “But you still have this debris flying about,” he said. This later phase of accretion may have lacked another lunar-scale impact, but likely featured large incoming asteroids. Predictions of the size and frequency distributions of this space flotsam indicate “that there has to be a substantial number of objects larger than, say, 1,000 kilometers in diameter,” Marchi said.

Unfortunately, there’s little obvious evidence in the rock record of these impacts before about 3.5 billion years ago. So scientists like Marchi can look to the Moon to estimate the number of objects that must have collided with Earth.

Armed with the size and number of impactors, Marchi and colleagues built a model that describes, as a function of time, the volume of melt this battering must have produced at the Earth’s surface. Magma oceans were in the past, but impactors greater than 100 kilometers in diameter still melted a lot of rock and must have drastically altered the early Earth.

Unlike smaller impacts, the volume of melt generated by objects of this size isn’t localized within a crater, according to models. Any crater exists only momentarily, as the rock is too fluid to maintain any sort of structure. Marchi compares this to tossing a stone into water. “There is a moment in time in which you have a cavity in the water, but then everything collapses and fills up because it’s a fluid.”

The melt volume is much larger than the amount of excavated rock, so Marchi can calculate just how much melt might have spilled out and coated parts of the Earth’s surface with each impact. The result is an astonishing map of melt volume. During the first billion years or so of Earth’s history, nearly the entire surface would have featured a veneer of impact melt at some point. Much of that history is gone because our active planet’s atmospheric, surface, and tectonic processes constantly modify much of the rock record.

Balls of glass

Even between 3.5 and 2.5 billion years ago, the rock record is sparse. But two places, Australia and South Africa, preserve evidence of impacts in the form of spherules. These tiny glass balls form immediately after an impact that sends vaporized rock skyward. As the plume returns to Earth, small droplets begin to condense and rain down.

<span class= — Enlarge / Spherule bed from impact S3 in drill core. Here, S3’s spherule beds were deposited in deep enough water to not be diluted by other detritus.

Nadja Drabon, Harvard

“It’s remarkable that we can find these impact-generated spherule layers all the way back to 3.5 billion years ago,” said Marchi.

What would the late heavy bombardment have done to the Earth’s surface? Read More »