Should Planetary Defence Take Center Stage?

Throughout the Solar System, planets and moons bear the scars of a past fraught with collisions. The Moon, Mercury, and Mars are so scarred from these impacts that craters overlap one another on their surfaces. Earth was subject to the same bombardment, though most of its impact scars disappeared over time due to active geology.

But some are still visible, and we know how catastrophic some of these impacts were for life.

Our Solar System is a calm, sedate place compared to what it used to be. Back in Earth’s early ages, there were far more rocks moving around and far more collisions. Episodes like the Late Heavy Bombardment 4 billion years ago illustrate that fact.

The Granddaddy of impacts on Earth is the Chicxulub Impact Event, the strike that spelled doom for the dinosaurs and about 75% of all plant and animal species in existence at the time. The Chicxulub impactor, which was almost certainly an asteroid rather than a comet, was about 180 kilometres (110 miles) in diameter and 20 kilometres (12 miles) in depth. If something that massive struck Earth now, it would end human civilization.

The threat of a massive asteroid impact has decreased with time, but the risk will never be zero. Image Credit: NASA/Don Davis

Even though the Solar System has calmed down, there’s still a non-zero chance of a large asteroid striking Earth, triggering an extinction, and wiping out our civilization. Astronomers predict that a 1 km (0.62 mi) diameter asteroid will slam into Earth once every 440,000 years on average. That’s only a fraction of the size of the Chicxulub impactor, but an asteroid that size would still have an explosive impact force of 46,300 Megatons of TNT and would leave a crater 13.6 km (8.5 mi) in diameter. Depending on the circumstances, it could also trigger massive forest fires and even tsunamis. That would be devastating.

But the largest rocks are the easiest to spot, and we know where most of them are and how they orbit the Sun. The risk we face comes mostly from undetected smaller bodies. Rocks too small to wipe out civilization—but large enough to wipe out a city, a country, to kill large numbers of people and render large amounts of land uninhabitable and non-arable—are still out there.

The Vredefort Crater in modern-day South Africa is the largest impact structure on Earth. It was created about 2 billion years ago by the largest object to impact Earth since the Hadean Eon. Now it’s covered with farmland. Image Credit: Left: NASA, Right: Google Earth.

There’s a concerted effort to find all the rocks that could threaten Earth and one day mount a defence against them. They’re called Near Earth Objects or NEOs. In 2005, the US Congress directed NASA to find 90% of NEOs more than 140 meters (460 feet) across that come within 30 million miles (48 million kilometres) of our planet’s orbit. The effort falls under the auspices of NASA’s Planetary Defense Coordination Office (PDCO.)

The effort to detect dangerous space rocks doesn’t generate the same excitement as other endeavours and missions. Sweeping cosmic images from the James Webb Space Telescope generate the kind of enthusiasm that unseen asteroids will never generate. And the spine-tingling landing of the Perseverance Rover on the surface of Mars is far more captivating than a statistical count of small space rocks.

But a lot hangs in the balance when it comes to asteroids, Earth, collisions, and affairs here on Earth. When we dig below the headlines, the effort to catalogue asteroids and figure out how to protect Earth from deadly impacts is its own intriguing story. One of the emerging characters in that story is NASA’s Neo Surveyor.

“NEO Surveyor represents the next generation for NASA’s ability to quickly detect, track, and characterize potentially hazardous near-Earth objects.”

The NEO Surveyor is a small space telescope with only one mission: to hunt down and catalogue asteroids that threaten Earth. The NEO Surveyor just passed a critical review, and the spacecraft is transitioning into the final design-and-fabrication phase. The spacecraft’s mission is to locate the hardest-to-find Near Earth Objects, the ones that have so far evaded our asteroid-hunting efforts.

When it comes to detecting NEOs, the low-hanging fruit has been picked. We know where the large ones are and where their orbits take them. Ground-based telescopes have proven effective at finding many of them. What’s left are the difficult-to-spot rocks.

“NEO Surveyor represents the next generation for NASA’s ability to quickly detect, track, and characterize potentially hazardous near-Earth objects,” said Lindley Johnson, NASA’s Planetary Defense Officer at PDCO. “Ground-based telescopes remain essential for us to continually watch the skies, but a space-based infrared observatory is the ultimate high ground that will enable NASA’s planetary defence strategy.”

A white paper for NEO said that the mission would cost about $600 million. For that price, it would help find the most elusive asteroids. The white paper says it’ll find smaller NEOs that “… constitute a newly recognized threat regime that cannot be efficiently detected from the ground.” According to the initial white paper, the NEO Surveyor will detect about:

85% of all > 100-meter diameter NEOs 70% of all > 60-meter diameter NEOs 50% of all > 50-meter diameter NEOs

NASA says that space rocks smaller than 25 meters (about 82 feet) will most likely burn up as they enter the Earth’s atmosphere and cause little or no damage.

NEO’s mission parameters have evolved since the white paper. NASA now estimates a $1.2 billion price tag and a launch no later than 2028. NEO Surveyor will conduct a five-year baseline survey to find at least two-thirds of the near-Earth objects larger than 140 meters (460 feet). These rocks are large enough to cause major regional damage if they strike Earth. And while we already know where many of those are, there are two types of asteroids in that size range that are very difficult to detect: bright and dark. Most of them are hidden in the Sun’s glare, and that’s where NEO is designed to ferret them out. The NEO Surveyor is built to find them with its single instrument: a 50 cm (20 in) telescope that sees in two infrared wavelengths.

The NEO Surveyor will be neighbours with the James Webb Space Telescope. The JWST is in a halo orbit at the L2 point, and the Surveyor will orbit at the L1 point. From that vantage point, it’ll do what even our most powerful ground-based telescopes weren’t built to do: search for asteroids in the glare of the Sun, one of their last hiding places.

Large, deadly asteroids are still out there, though the pace at which we find them has slowed. In January 2022, American astronomer Scott Sheppard discovered a kilometre-sized asteroid named 2022 AP7. It was the largest asteroid discovered in eight years and is a potentially hazardous object (PHO.) Critically, Shepard found 2022 AP7 in the Sun’s glare, exactly where NEO Surveyor will look. Sheppard found it using an unlikely tool, the Dark Energy Camera. Even though 2022 AP7 poses no threat to Earth—at least not for centuries—an object this large could potentially cause a mass extinction if it strikes Earth. And while it’s good news that we found it, it’s also unnerving. How many more are there?

NEO Surveyor should find more of them if they’re out there. NASA forecasts that the telescope will find tens of thousands of new NEOs as small as 30 m (98 ft) in diameter. As it does so, it’ll also fulfill its secondary science objectives: to detect and characterize about one million asteroids in the main belt, thousands of comets, and to identify NEOs that could be targeted for exploration, either human or robotic.

Not only do we need to protect Earth from asteroid strikes, but we also need to learn more about these ancient rocks. NEO Surveyor can help by identifying asteroids that are good targets for robotic exploration. This image is an artist’s concept of NASA’s OSIRIS-REx spacecraft as it readies itself to touch the surface of the asteroid Bennu. Credits: NASA/Goddard/University of Arizona

NEO Surveyor isn’t the only telescope hunting asteroids. Sometime in 2023, the Vera Rubin Observatory will see its first light. Its powerful wide-angle camera will capture twin 15-second exposures in succession and will cover the entire southern sky every three nights. That imaging strategy will find anything transient in the night sky, and a 2016 paper in the Astronomical Journal says the VRO could detect 62% of the NEO population of size 140 meters or greater.

We’re strengthening our asteroid-detecting capabilities, and the popular press has caught on. There are now regular headlines regarding asteroids passing close to Earth. In the panicky press, headlines cry wolf and overstate the danger, inviting us to click and become titillated with misinformation. But the real threat isn’t these harmless asteroids that pass by so frequently that they’re little more than background noise. The real threat is the ones on a collision course.

Hopefully, when an asteroid is on a collision course with Earth, we’ll have plenty of advance warning. At least, that’s the plan. And thanks to the NEO Surveyor and other efforts, we should have sufficient notice.

But what happens then? We still don’t know for sure, but scientists are working on it.

The astronomy community holds conferences to game the detection of threatening asteroids. They’re called Planetary Defence Conferences; the last one was held (virtually) in 2021. The next one is in April 2023.

The Conference asks teams of scientists to respond to a fictional asteroid detection scenario. At the 2021 Conference, an asteroid named PDC 2021 was detected on April 21st. Over the following days, more information was revealed to the participants to simulate what would happen as astronomers gathered more data during an asteroid’s approach:

The most likely potential impact occurs on October 20, 2021 – just 6 months away. The probability of that impact is low, about 1 chance in 2500, but that’s after only two days of tracking. The asteroid’s size could range anywhere from as small as 35 meters to as large as 700 meters. When first detected, the asteroid is about 0.38 au (57 million kilometres or 35 million miles) from Earth. It’s travelling at about 5 km/s (3 mi/s or 11,000 mph.) The asteroid is too distant to be detected by radar and will not come within radar range until its potentially impacting approach in October. Astronomers continue to track the asteroid every night after discovery, and the impact probability steadily increases.

When the Conference begins, the impact probability is at 5% and increasing. The final mock radar observations revealed that the incoming asteroid was 105 meters across and would strike an area bordering Germany, the Czech Republic, and Austria. What could be done?

The fictional asteroid strike in the PDC 2021 Conference struck Europe. The shaded regions in this image show where the impact is most likely to occur. There is a 99% chance the impact will be located within the outer contour, 87% inside the middle contour, and 40% inside the central dark red region. Image Credit: Google Earth/CNEOS

Not much. All the conference attendees could come up with was Civil Defense and Emergency Preparation. All we could do was evacuate people and somehow try to harden and protect the most critical infrastructure. Nobody’s satisfied with that.

We’re reaching the point where we can mount some defence against asteroids on a collision course with our world. But only if we know where these asteroids are and when they’ll impact Earth. NEO Surveyor and other endeavours will handle that. But what do we do next when one comes for us? Evacuate people and clutch our pearls? That would be a devastating, demoralizing failure for all of humanity.

It’s critical that we learn more about asteroids. They’re not all the same, and we need to know more about their composition, what types there are, and what methods will work best to defend ourselves. Though Hollywood (and Russia) might lean on massive explosive devices to protect Earth, others think that’s a crude and possibly ineffective method. Deflection might be easier, cheaper, and more reliable. And probably safer.

NASA launched its DART (Double Asteroid Re-Direction Test) spacecraft in November 2021. DART’s mission was to test the deflection of asteroids. It explored how a spacecraft could deflect an asteroid with a given impact mass. Its target was Dimorphos, the small moon of a larger asteroid named Didymos.

On 26 September 2022, the 610 kg (1,340 lb) DART probe crashed into Dimorphos. It struck Dimorphos in the opposite direction of its rotation, and the moon’s orbital speed dropped slightly, reducing the radius of its orbit around Didymos. It also modified Didymos’ trajectory and blasted some ejecta into space.

NASA planned the impact so that multiple ground-based observatories could monitor the results, including the cloud of ejecta emitted after the impact. In 2024, the ESA will launch the HERA spacecraft to visit Didymos/Dimorphos. HERA will arrive in 2026 and perform detailed observations of the moon and how the impact affected its surface.

We’ve sampled asteroids and comets, and now we’ve sent spacecraft to impact with both. These efforts teach us more about them and how to prepare for one with our name on it. But we’ve still got a long way to go. And while we hope we’ll have enough advance warning to prepare and launch a kinetic impactor to deal with the threat, what happens if we don’t have enough time for that?

Philip Lubin is a Physics Professor at the University of California, Santa Barbara. Lubin is developing an idea called PI-Terminal Defense for Humanity. PI stands for “Pulverize It!” and outlines a potential response to an asteroid that comes for Earth on short notice. In that situation, we may not have time to launch an impactor. Instead, we should have specialized spacecraft available on short notice, waiting to be launched when needed.

Lubin presented his PI Planetary Defence idea at the 2021 Planetary Defence Conference. “So far, humanity has been spared large-scale catastrophe as was visited upon our previous tenants, but counting upon being “lucky” is a poor strategy in the longer term,” Philip Lubin said in 2021.

Lubin’s idea is to launch a spacecraft that would pulverize a larger asteroid into smaller chunks. Those chunks might still head for Earth, but they’d be smaller and more likely to burn up in the atmosphere and be rendered harmless. Depending on the asteroid’s size and the point of impact, some could reach the surface and cause serious damage. But widespread destruction and even extinction could be avoided.

The Pulverize It (PI) impactor would feature an array of rods, all of which would penetrate the asteroid. Some of the rods would fracture the asteroid, and some would contain explosives. The result is a cloud of debris headed for Earth rather than one solid chunk of rock. It could save us.

In the Pulverize It scenario, an asteroid heading for Earth would be struck with an impactor that had an array of rods, some with explosives. The asteroid, or comet, would be broken into smaller pieces that pose less of a threat. Image Credit: Lubin/Experimental Cosmology Group, UCSB.

PI impactor (s) could wait in orbit somewhere or even on the surface of the Moon until they’re needed. They’re nimble enough to act in scenarios where we have only days, maybe weeks, rather than months advance notice of an impending asteroid strike. They could also act as backups in the event of a kinetic impactor that failed or missed its target.

Lubin’s Pulverize It idea has merit, enough merit that NASA made it a Phase One awardee in the NASA Innovative Advanced Concepts (NIAC) program.

We’ll never be able to reduce the risk of asteroid strikes to zero. Asteroids follow orbits that are mostly predictable but never completely. Everything in space is moving, and sometimes a larger body can perturb a smaller body, changing its orbit and sending it on a collision course with Earth or another body. It’s like Jefferson’s words about Liberty: “Eternal vigilance is the price of Liberty.” We can never become complacent to the threat of an impact.

For any single one of us, the likelihood of living during the approach of a deadly asteroid is very low. But for humanity as a whole and the rest of life on Earth, the probability rises as the time span under consideration lengthens.

While many of humanity’s worst problems and catastrophes are self-inflicted, nature is also full of threats. Those threats will always exist, and we can use our big brains to prepare. We’re approaching the point where we can not only detect nature’s asteroid threat but protect ourselves.

And it’s only a matter of time before history repeats itself and one comes for us.