asteroids

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Radiation should be able to deflect asteroids as large as 4 km across

asteroids, astronomy, Nuclear weapons, planetary defense, planetary science, radiation, Science, z-machine / /
Image of a large, circular chamber covered filled with a lot of mechanical equipment, all of which is lit by blue internal glows and covered with massive, branching trails of lightning.

Enlarge / Sandia National Labs’ Z machine in action.

The old joke about the dinosaurs going extinct because they didn’t have a space program may be overselling the need for one. It turns out you can probably divert some of the more threatening asteroids with nothing more than the products of a nuclear weapons program. But it doesn’t work the way you probably think it does.

Obviously, nuclear weapons are great at destroying things, so why not asteroids? That won’t work because a lot of the damage that nukes generate comes from the blast wave as it propagates through the atmosphere. And the environment around asteroids is notably short on atmosphere, so blast waves won’t happen. But you can still use a nuclear weapon’s radiation to vaporize part of the asteroid’s surface, creating a very temporary, very hot atmosphere on one side of the asteroid. This should create enough pressure to deflect the asteroid’s orbit, potentially causing it to fly safely past Earth.

But will it work? Some scientists at Sandia National Lab have decided to tackle a very cool question with one of the cooler bits of hardware on Earth: the Z machine, which can create a pulse of X-rays bright enough to vaporize rock. They estimate that a nuclear weapon can probably impart enough force to deflect asteroids as large as 4 kilometers across.

No nukes! (Just a nuclear simulation)

The Z machine is at the heart of Sandia’s Z Pulsed Power Facility. It’s basically a mechanism for storing a whole lot of electrical energy—up to 22 megajoules—and releasing it nearly instantaneously. Anything in the immediate vicinity experiences extremely intense electromagnetic fields. Among other things, this can be used to heavily ionize materials, like the argon gas used here, generating intense X-rays. These served as a stand-in for the radiation generated by a nuclear weapon.

For an asteroid, the researcher used disks of rock, either quartz or fused silica. (Notably, they only did one sample of each but got reasonably consistent results from them.) Mere mortals might have stuck the disk on a device that could register the force it experienced and left it at that. But these scientists were made of sterner stuff and decided that this wouldn’t really replicate the asteroid experience of floating freely in space.

To mimic that, the researchers held the rock disks in place using thin pieces of foil. These would vaporize almost instantly as the X-ray burst arrives, leaving the rock briefly suspended in the air. While gravity would have its way, the events triggered by the radiation evaporating away a bunch of the rock would be over before the sample experienced any significant downward acceleration. Its movement during this time, and thus the force imparted to it by the evaporation of its surface, was tracked by a laser interferometer placed on the far side of the disk from the X-ray source.

With all that set, all that was left was to fire up the Z machine and vaporize some rock.

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An asteroid wiped out the dinosaurs, not a comet, new study finds

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It came from outer space —

Analysis of ruthenium isotopes showed the impactor was a carbonaceous-type asteroid.

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Artist impression of a large asteroid impacting on Earth such as the Chicxulub event that caused the end-Cretaceous mass extinction, 66 million years ago.

Enlarge / Artist impression of a large asteroid impacting on Earth, such as the Chicxulub event that caused the end-Cretaceous mass extinction 66 million years ago.

Mark Garlick

Some 66 million years ago, an errant asteroid wiped out three-quarters of all plant and animal species on Earth, most notably taking down the dinosaurs. That has long been the scientific consensus. However, three years ago, Harvard astronomers offered an alternative hypothesis: The culprit may have been a fragment of a comet thrown off-course by Jupiter’s gravity and ripped apart by the Sun.

Now an international team of scientists have reaffirmed the original hypothesis, according to a new paper published in the journal Science. They analyzed ruthenium isotopes from the Chicxulub impact crater and concluded the impact was due to a carbonaceous-type asteroid, likely hailing from beyond Jupiter.

As previously reported, the most widely accepted explanation for what triggered that catastrophic mass extinction is known as the “Alvarez hypothesis,” after the late physicist Luis Alvarez and his geologist son, Walter. In 1980, they proposed that the extinction event may have been caused by a massive asteroid or comet hitting the Earth. They based this conclusion on their analysis of sedimentary layers at the Cretaceous-Paleogene boundary (the K-Pg boundary, formerly known as the K-T boundary) found all over the world, which included unusually high concentrations of iridium—a metal more commonly found in asteroids than on Earth. (That same year, Dutch geophysicist Jan Smit independently arrived at a similar conclusion.)

The 66-million-year-old Cretaceous-Paleogene (K-Pg) boundary layer at Stevns Klint in Denmark.

Enlarge / The 66-million-year-old Cretaceous-Paleogene (K-Pg) boundary layer at Stevns Klint in Denmark.

Philippe Claeys

Since then, scientists have identified a likely impact site: a large crater in Chicxulub, Mexico, in the Yucatan Peninsula, first discovered by geophysicists in the late 1970s. The impactor that created it was sufficiently large (between 11 and 81 kilometers, or 7 to 50 miles) to melt, shock, and eject granite from deep inside the Earth, probably causing a megatsunami and ejecting vaporized rock and sulfates into the atmosphere.

This in turn had a devastating effect on the global climate, leading to mass extinction. In 2022, scientists suggested that one reason so many species perished while others survived may have been because the impact occurred in the spring (at least in the Northern Hemisphere), thereby interrupting the annual reproductive cycles of many species.

In 2016, a scientific drilling project led by the International Ocean Discovery Program took core samples from the crater’s peak ring, confirming that the rock had been subjected to immense pressure over a period of minutes. A 2020 paper concluded that the impactor struck at the worst possible angle and caused maximum damage. It has been estimated that the impact would have released energy over a billion times higher than the atomic bombs dropped on Hiroshima and Nagasaki in 1945.

Asteroid or comet?

Harvard’s Avi Loeb and his then-undergraduate student Amir Siraj challenged the asteroid-as-impactor hypothesis in a 2021 paper, proposing instead that the impact was caused by a special kind of comet—originating from a field of debris at the edge of our solar system known as the Oort cloud—that was thrown off course by Jupiter’s gravity toward the Sun. The Sun’s powerful tidal forces then ripped off pieces off the comet—akin to what happened to the comet Shoemaker-Levy 9 when it crashed into Jupiter in 1994—and one of the larger fragments of this “cometary shrapnel” eventually collided with Earth.

Loeb and Siraj’s analysis was based on numerical simulations to calculate the flux of long-period comets in our solar system. They found that events like the one described above should happen frequently enough and produce enough sufficiently large fragments to result in a significantly higher impact rate of Chicxulub-sized impactors than the background comet or asteroid populations. They argued that their comet hypothesis would also explain the Chicxulub impactor’s unusual composition of carbonaceous chondrite—rare for asteroids but more common for long-period comets—which is consistent with an Oort cloud origin rather than the main asteroid belt.

This latest paper addresses that latter point in particular. Mario Fischer-Gödde of the University of Cologne in Germany and his co-authors took samples from the K-Pg boundary layer from a site at Stevns Klint in Denmark and analyzed the ruthenium isotopes via plasma mass spectrometry. They did the same for samples taken from the sites of five other known asteroid impacts over the last 541 million years, as well as ancient Archean samples (between 3.5 to 3.2 billion years old).

Fischer-Gödde et al. concluded that the ruthenium signatures in the K-Pg samples were a close match to asteroids known as carbonaceous chondrites, so the impact most likely resulted from a C-type asteroid that hailed from the outer Solar System. They were able to rule out the possibility of a comet impactor proposed by Loeb and Siraj since the ruthenium data was inconsistent with that hypothesis. Most of the other samples had ruthenium isotope signatures consistent with salicaceous (S-type) asteroids from the inner Solar System, although the ancient Archean samples were also consist with a C-type asteroid.

Science, 2024. DOI: 10.1126/science.adk4868  (About DOIs).

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NASA shuts down asteroid-hunting telescope, but a better one is on the way

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Prolific —

The NEOWISE spacecraft is on a course to fall out of orbit in the next few months.

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Artist's illustration of NASA's Wide-field Infrared Survey Explorer spacecraft.

Enlarge / Artist’s illustration of NASA’s Wide-field Infrared Survey Explorer spacecraft.

Last week, NASA decommissioned a nearly 15-year-old spacecraft that discovered 400 near-Earth asteroids and comets, closing an important chapter in the agency’s planetary defense program.

From its position in low-Earth orbit, the spacecraft’s infrared telescope scanned the entire sky 23 times and captured millions of images, initially searching for infrared emissions from galaxies, stars, and asteroids before focusing solely on objects within the Solar System.

Wising up to NEOs

The Wide-field Infrared Survey Explorer, or WISE, spacecraft launched in December 2009 on a mission originally designed to last seven months. After WISE completed checkouts and ended its primary all-sky astronomical survey, NASA put the spacecraft into hibernation in 2011 after its supply of frozen hydrogen coolant ran out, reducing the sensitivity of its infrared detectors. But astronomers saw that the telescope could still detect objects closer to Earth, and NASA reactivated the mission in 2013 for another decade of observations.

The reborn mission was known as NEOWISE (Near-Earth Object Wide-field Infrared Survey Explorer). Its purpose was to use the spacecraft’s infrared vision to detect faint asteroids and comets on trajectories that bring them close to Earth.

“We never thought it would last this long,” said Amy Mainzer, NEOWISE’s principal investigator from the University of Arizona and UCLA.

Ground controllers at NASA’s Jet Propulsion Laboratory in California sent the final command to the NEOWISE spacecraft on August 8. The spacecraft, currently at an altitude of about 217 miles (350 kilometers), is falling out of orbit as atmospheric drag slows it down. NASA expects the spacecraft will reenter the atmosphere and burn up before the end of this year, a few months earlier than expected, due to higher levels of solar activity, which causes expansion in the upper atmosphere. The satellite doesn’t have its own propulsion to boost itself into a higher orbit.

“The Sun’s just been incredibly quiet for many years now, but it’s picking back up, and it was the right time to let it go,” Mainzer told Ars.

Astronomers have used ground-based telescopes to discover most of the near-Earth objects detected so far. But there’s an advantage to using a space-based telescope, because Earth’s atmosphere absorbs most of the infrared energy coming from faint objects like asteroids.

With ground-based telescopes, astronomers are “predominantly seeing sunlight reflecting off the surfaces of the objects,” Mainzer said. NEOWISE measures thermal emissions from the asteroids, giving scientists information about their sizes. “We can actually get pretty good measurements of size with relatively few infrared measurements,” she said.

The telescope on NEOWISE was relatively modest in size, with a 16-inch (40-centimeter) primary mirror, more than 16 times smaller than the mirror on the James Webb Space Telescope. But its wide field of view allowed NEOWISE to scour the sky for infrared light sources, making it well-suited for studying large populations of objects. One of the mission’s most famous discoveries was a comet officially named C/2020 F3, more commonly known as Comet NEOWISE, which became visible to the naked eye in 2020. As the comet moved closer to Earth, large telescopes like Hubble were able to take a closer look.

“The NEOWISE mission has been an extraordinary success story as it helped us better understand our place in the universe by tracking asteroids and comets that could be hazardous for us on Earth,” said Nicola Fox, associate administrator of NASA’s science mission directorate.

What’s out there?

The original mission of WISE and the extended survey of NEOWISE combined to discover 366 near-Earth asteroids and 34 comets, according to the Center for Near-Earth Object Studies. Of these, 64 were classified as potentially hazardous asteroids, meaning they come within 4.65 million miles (7.48 million kilometers) of Earth and are at least 500 feet (140 meters) in diameter. These are the objects astronomers want to find and track in order to predict if they pose a risk of colliding with Earth.

There are roughly 2,400 known potentially hazardous asteroids, but there are more lurking out there. Another advantage of using space-based telescopes to search for these asteroids is that they can observe 24 hours a day, while telescopes on the ground are limited to nighttime surveys. Some hazardous asteroids, such as the house-sized object that exploded in the atmosphere over Chelyabinsk, Russia, in 2013, approach Earth from the direction of the Sun. A space telescope has a better chance of finding these kinds of asteroids.

WISE, and then the extended mission of NEOWISE, helped scientists estimate there are approximately 25,000 near-Earth objects.

“The objects (NEOWISE) did discover tended to be overwhelmingly just dark, [and] these are the objects that are much more likely to be missed by the ground-based telescopes,” Mainzer said. “So that, in turn, gives us a much better idea of how many are really out there.”

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Smashing into an asteroid shows researchers how to better protect Earth

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Connecting with a fastball —

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

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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.

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

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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 / /

Under fire —

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

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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.

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.” data-height=”2560″ data-width=”1735″ href=”https://cdn.arstechnica.net/wp-content/uploads/2024/01/Image2-IMG_1552-scaled.jpg”><span class=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.” height=”1446″ src=”https://cdn.arstechnica.net/wp-content/uploads/2024/01/Image2-IMG_1552-980×1446.jpg” width=”980″>

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.

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