Nearly one year ago, NASA flung the DART spacecraft into the asteroid Dimorphos at 14,000 miles per hour. It was the first test to see whether they could slightly deflect a space rock’s trajectory using a high-speed collision, a technique that could be used to protect Earth from future killer asteroids. It worked. But now they’re trying to figure out the details of the crash. And if people have to defend earthly life from a potential asteroid impact, those details will surely matter.
Scientists are starting by studying the ejecta, boulders, and numerous smaller bits the strike cast off. They predicted there would be debris, but they didn’t know exactly what to expect. After all, compared to stars and galaxies, asteroids are tiny and dim, so it’s hard to ascertain their density and composition from afar. When you strike one, will it simply bounce? Will the probe thud into it and create a crater? Or if the asteroid is brittle, will slamming a craft into it risk creating space shrapnel that is still big enough to threaten Earth?
“This is exactly why we needed to do an in-space test of this technology. People had done laboratory experiments and models. But how would an actual asteroid, of the size we’re concerned about for planetary defense, react to a kinetic impactor?” says Nancy Chabot, the DART coordination lead and a planetary scientist at Johns Hopkins University’s Applied Physics Laboratory, which developed the craft in partnership with NASA.
Many asteroids appear to be “rubble piles,” dirt, rocks, and ice loosely held together, rather than something hard and dense like a billiard ball. The asteroid Ryugu, visited by the Japanese space agency’s Hayabusa2 in June 2018, and the asteroid Bennu, which NASA’s OSIRIS-REx took samples from in 2020, both count as rubble piles. A new study published in July in Astrophysical Journal Letters shows that Dimorphos appears to be built like that too, which means that an impact is likely to create a crater and to fling off debris on or near the asteroid’s surface.
To figure out what happened after the crash, David Jewitt, a University of California, Los Angeles astronomer, and his colleagues used the Hubble Space Telescope to zoom in repeatedly on Dimorphos. The combined deep observations allowed them to discern objects that are otherwise too faint to see. A few months after the DART probe’s impact, they found a swarm of about three dozen boulders not seen before—the largest of which is 7 meters in diameter—slowly drifting away from the asteroid. “It’s a slow-speed cloud of shrapnel from the impact that’s carrying away a significant amount of mass: about 5,000 tons in boulders. That’s quite a lot, considering the impactor itself was only half a ton. So it blew out a tremendous mass in boulders,” Jewitt says.
Other researchers, including the DART team, have also been investigating the cloud of rocks thrown off by the spacecraft’s swift punch. Chabot and her colleagues published a study in Nature earlier this year, also using Hubble photos, imaging the ejecta. They showed that at first the pieces flew off in a cone-shaped cloud, but over time, that cone turned into a tail, not so different from a comet’s tail. That finding also means that models of the behavior of comets could be applied to impactors like DART, Chabot says.
Dimorphos was never a threat to Earth, but details like these would matter in a real asteroid deflection scenario. Boulders and smaller ejecta would have to be knocked out of the way, along with the rest of the asteroid, in order to spare the planet. Or let’s say the asteroid wasn’t spotted until it was very close to Earth, and its trajectory couldn’t be altered enough to avoid a crash. Could it at least be pulverized into boulders small enough to burn up in Earth’s atmosphere? “Is it better to be shot by a high-velocity rifle bullet or a bunch of pellets from a shotgun?” asks Jewitt. “The answer is: The shotgun is better, because the smaller boulders are more likely to be cushioned or dissipated by the impact with the atmosphere.”
And nations should avoid cluttering those spots with mechanical detritus, which could complicate future missions. Like campers heading into the backcountry, it’s important to think carefully about what you pack with you and what you take out, Birk says.
India’s success doesn’t mean the end of the race toward the moon’s south pole, but it does boost India’s standing. “This will certainly contribute to its status as a rising power with technological prowess. What’s happening in space is a reflection of what’s happening geopolitically on Earth,” says Cassandra Steer, an expert on space law and space security at the Australian National University in Canberra. And while Roscosmos suffered a setback, this isn’t the end of their moon program either, or their role in the new lunar competition. The Soviets beat the US at every stage of the 20th-century space race, Steer says, except for the landing of astronauts on the moon. Next, Russia intends to collaborate with China on a lunar research station.
Over the past decade, only China’s space program has achieved considerable success landing spacecraft on the moon, including its Chang’e 3, 4, and 5 missions in 2013, 2019, and 2020. India’s Chandrayaan-2 and Israel’s Beresheet lander failed in 2019, and Japan’s Ispace lander failed this April.
In fact, until China made its first landing, the moon had arguably been neglected for decades. NASA ended its Apollo mission in 1972, and the USSR’s Luna-24 mission in 1976 was the last successful lunar landing. That could mean limited institutional memory, especially for Russia, making it tough to develop and deploy new moon missions, Metzger says.
Over the past few decades, Russia has been trying to resuscitate its program, but with little success. Roscosmos has Luna-26 and Luna-27 planned for 2027 and 2029, as the agency aims to bring an orbiter and a larger lander to the moon. But their limited funding, thanks to sanctions following the Ukraine invasion, means these followup missions will likely be delayed, Zak says. And if the space agency decides to overhaul their propulsion system design after investigating the failure of Luna-25, that could be another reason for delays, he adds.
NASA has fared better with its Artemis program, which last year sent the uncrewed Artemis 1 to orbit the moon and is aiming for a crewed landing in 2026. But the program has faced its own challenges: NASA plans on using a SpaceX Starship lander, though, as its abortive test flight in April shows, Starship clearly has a long way to go. More than half of the 10 cubesat satellites deployed by Artemis 1 experienced technical glitches or lost contact with Earth, including the Japanese Omotenashi probe, which was unable to land on the moon as planned.
NASA has increasingly relied on commercial partners in a bid to boost the speed and lower the price of moon exploration—moving some of the costs onto businesses, rather than taxpayers. But these companies, too, are new players in the space race. In late 2024, NASA plans to send its Viper rover on an Astrobotic lander, though that company’s first moon lander, meant to demonstrate the technology, hasn’t even launched yet. NASA has also charged Firefly Aerospace, Intuitive Machines, and Draper with delivering a variety of payloads to the lunar surface over the next couple years.
In the meantime, nations like India, Japan, and Israel have begun moon programs from scratch. India next plans to collaborate with Japan on the Lunar Polar Exploration rover, which would launch no sooner than 2026.
“We have set the bar now so high. Nothing less spectacular than this is going to be inspiring for any of us in the future,” said Shri M. Sankaran, director of ISRO’s U R Rao Satellite Centre, speaking on today’s telecast. “We will now be looking at putting a man in space, putting a spacecraft on Venus, and landing on Mars. Those efforts have been ongoing for years. This success today will inspire us and spur us to take those efforts even more strongly to make our country proud again and again and again.”
Updated 8/23/2023 12:00 pm ET: This story was updated to correct the ISRO chief’s name.
That has created a problem. Around 2010, soon after meeting this big, new predator that could outcompete and eat them, South Florida’s mammal populations collapsed. Large and medium-size mammals have been scarce for almost a decade, leaving mostly smaller mammals, like rodents.
Some ecologists thought the pythons would become victims of their own success. “They were supposedly out of food,” says Paul Taillie, a wildlife ecologist at the University of North Carolina at Chapel Hill. But Taillie’s research has shown that pythons just switched to eating the smaller mammals instead, causing those populations to drop too. In 2021, Taillie reported disappointing proof that mammals were not bouncing back. “There’s exceedingly little sign of any mammal activity” in South Florida, he says.
The only resistant species has turned out to be black rats—but they’re also invasive. Black rats arrived in the Americas from Europe centuries ago onboard the ships of explorers and colonizers. They’re resistant because they reproduce a lot and don’t compete with the pythons or large mammals for food: They can scavenge carcasses and eat plants, insects, and scraps from humans. This is the reason they thrive all over the world.
So can anything curb the python’s takeover? First, there are teams like Kirkland’s, which employ contractors to track and capture the snakes year-round. Every capture and kill follows ethics guidelines and federal laws about transporting illegal pets. “They need to be respected as the beautiful living creatures that they are,” Kirkland says. “They’re here through no fault of their own.”
And for six of the past 10 years, Florida has tried to educate the public about invasive species and the folly of keeping pythons as pets, thanks to the Florida Python Challenge, a 10-day event for amateur python hunters, in partnership with the state’s wildlife agency. Participants catch the snakes, which they euthanize. This year, at least 840 participants registered for a shot at $17,500 in prizes. The tally for this year’s hunt hasn’t been released yet, but each of the last two hunts yielded over 200 captures. “It really does a lot to educate the public,” Kirkland says, “to teach about the importance of why you shouldn’t allow an invasive exotic pet to get out.”
But scientists also want to know if the nonhuman denizens of the Everglades are pushing back against the python—specifically, to see if pythons have their own “prey naivete.” Could other species be preying on young pythons?
To answer this question, in 2020 and 2021 a team of USGS researchers implanted 2- to 3-foot-long pythons with radio transmitters and released them back into Big Cypress National Preserve. The transmitters tracked movements down to a 3-meter radius, and each transmitter had a “mortality sensor” that was triggered if the animal hadn’t moved in 24 hours.
Nineteen young pythons died during the study period. Team members waded into the swamp to find out exactly where and how. They snooped for every sign imaginable: paw prints, fur, bite marks, scrapes, and scat. Dead snakes and transmitters turned up in soil, in trees, and underwater. The team brought any carcasses they could find back to the lab for necroscopies. Twelve of the 19 cases had enough evidence to point to a killer, according to results published earlier this year in a study titled “Natives bite back.”
Physics questions are the most fun when people don’t immediately agree on the answer. What feels intuitive or obvious—sometimes isn’t. We can argue over the solution for hours of entertainment, and we might even learn something in the end.
Here’s one of these seemingly obvious questions that’s been around a long time: Suppose a large rock is on a boat that is floating in a very small pond. If the rock is dumped overboard, will the water level of the pond rise, fall, or remain unchanged?
Go ahead and debate it with your friends and family. While you convince them that your answer is correct, here is a picture of my boat with a rock in it:
Photograph: Rhett Allain
OK, it’s not actually a boat, it’s part of a plastic bottle. Also, the “rock” is a lead weight and the “pond” is a beaker. But this way we can see what happens to the water level when we drop an object into it.
When a boat is floating on water, two forces are acting on it. First, there is the downward-pulling gravitational force, which is equal to the mass of the boat and everything on it (m) times the gravitational field (g = 9.8 newtons per kilogram). We often call this product the “weight.”
The other force is the upward-pushing buoyancy interaction with the water. Two things are true about this buoyancy force. First, if the boat is floating, then the upward buoyancy must be equal to the weight of the boat. Second, the buoyancy force is equal to the weight of the water displaced by the boat.
We can calculate this buoyancy force by taking the volume of the water displaced (Vd) and using the density of water (ρw) along with the gravitational field (g).
So much on this planet depends on a simple matter of density. In the Atlantic Ocean, a conveyor belt of warm water heads north from the tropics, reaching the Arctic and chilling. That makes it denser, so it sinks and heads back south, finishing the loop. This system of currents, known as the Atlantic Meridional Overturning Circulation, or AMOC, moves 15 million cubic meters of water per second.
In recent years, researchers have suggested that because of climate change, the AMOC current system could be slowing down and may eventually collapse. A paper published yesterday in the journal Nature Communications warns that the collapse of the AMOC isn’t just possible, but imminent. By this team’s calculations, the circulation could shut down as early as 2025, and no later than 2095.
That’s a tipping point that would come much sooner than anyone thought. “We got scared by our own results,” says Susanne Ditlevsen, a statistician at the University of Copenhagen and coauthor of the new paper. “We checked and checked and checked and checked, and I do believe that they’re right. Of course, we might be wrong, and I hope we are.” But there’s vigorous debate in the scientific community over just how quickly the AMOC might decline, and how best to even figure that out.
It’s abundantly clear to researchers that the Arctic is warming up to four and a half times faster than the rest of the planet. Arctic ice is melting at a pace of about 150 billion metric tons per year, says Marlos Goes, an oceanographer from the University of Miami and NOAA’s Atlantic Oceanographic and Meteorological Laboratory who was not involved with the new paper. Greenland’s ice sheet is also rapidly declining, injecting more freshwater into the sea. That deluge of freshwater is less dense than saltwater, meaning less water sinks and less power goes into the AMOC conveyor belt.
The consequences would be brutal and global. Without these warm waters, weather in Europe would get significantly colder—more like that of similar latitudes in Canada and the northern United States. “In model simulations, the collapse of the AMOC cools the North Atlantic and warms the South Atlantic, which may result in drastic precipitation changes throughout the world,” Goes says. “There would be changes in storm patterns over the continental areas, affecting the monsoon systems. Therefore, a future AMOC shutdown could bring massive migration, impacting ecological and agricultural production, and fish population displacement.”
Ditlevsen did her team’s calculation by using measurements of Atlantic sea surface temperatures as a proxy for the AMOC. These readings go all the way back to the 1870s, thanks to measurements taken by ship crews. This meant researchers could compare temperatures before and after the start of the wide-scale burning of fossil fuels and the ensuing changes to the climate.
Because the AMOC system involves warm water heading north from the tropics, if the circulation is slowing down, you’d expect to find cooler temperatures in the North Atlantic over time. And indeed, that’s what Ditlevsen’s group found, once they compensated for the overall warming of the world’s oceans due to climate change. “When it is established that the sea surface temperature record is the fingerprint of the AMOC, we can calculate the early warning signals of the forthcoming collapse and extrapolate to the tipping point,” says University of Copenhagen climate scientist Peter Ditlevsen, coauthor of the new paper. (The Ditlevsens are siblings.)
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