People have a bottomless appetite for all things space these days. Some space news is truly mind-blowing, like the first image of a black hole last year or this year’s time lapse of said black hole’s dancing shadow. Then there’s news of the less mind-blowing variety. Second only to full coverage of every supermoon are headlines of near (but harmless) misses by asteroids.
However, while a supermoon isn’t too much more breathtaking than a run-of-the-mill full moon — those harmless near-misses actually do hint at something more significant. As our planet plows through space, its orbit inevitably crosses the orbits of other inhabitants of the solar system. Among this group are asteroids of all sizes. Most of these are so small they’d be vaporised by the atmosphere, but others are big enough to impact the surface and do serious damage.
Infamously, an asteroid the size of a city crashed into Mexico’s Yucatán Peninsula some 66 million years ago. The blast was enormous. It sent mega-tsunamis racing across the ocean and dug a roughly 93-mile-wide crater, throwing much of that material into the atmosphere where it blocked and dimmed the sun for years. Many scientists believe the Chicxulub impact was the prime culprit behind the extinction of the dinosaurs and 75% of life on the planet at the time.
There’s been no repeat performance in the last 66 million years, and the day-to-day risk of a major impact is very, very low. But it’s a near certainty that another large object will at some point in the future collide with our planet—unless we do something about it.
Luckily, while the last killer space rock dropped out of the sky with no warning, we have a few tools the dinosaurs didn’t. In addition to telescopes to chart potentially hazardous asteroids, we can visit and, theoretically, divert an asteroid’s course before it reaches us.
Now, the world’s space agencies are teaming up to take planetary defence beyond theory. This month, the European Space Agency (ESA) approved and funded their part of the Asteroid Impact Deflection Assessment (AIDA), a joint mission with NASA and other space agencies to, for the first time, attempt to alter the orbit of a sizable asteroid in deep space.
The asteroid in question poses no threat to Earth — rather, it’s a test case for how we may deflect a hypothetical future asteroid that does pose a risk. The missions aim to yield the first hard data in humanity’s quest to avoid the fate of the dinosaurs.
Desperately seeking NEOs
Of course, to divert an asteroid, you have to find it first.
In 1998, NASA launched a program to chart 90% of all the asteroids and comets in our neighbourhood — known as near-Earth objects (NEOs) — larger than a kilometre in diameter. NASA hit this mark in 2010, but by then, the space agency had already been re-tasked to find 90% of all NEOs larger than 140 meters by the end of 2020. The new mandate would include objects that can wreak significant global havoc—as in the case of Chicxulub — but also smaller strikes that would nonetheless do serious damage to the regions they impact.
To date, we’ve charted 9,334 NEOs larger than 140 meters (including comets). But that number is only a little more than a third of the total estimated population of 25,000. While current telescopes have done a great job, a space-based, infrared telescope would speed progress. So, it’s good news that, after years awaiting approval, the NEO Surveillance Mission (previously NEOCam) won first funding to kick the search into high gear with such a telescope. The mission aims to round out our list of 90% of NEOs bigger than 140 meters within a decade of launch.
Of course, it doesn’t end there. Once we’ve found a killer asteroid or comet, it’d be nice to be able to do something about it—like, you know, gently (or not so gently) take it by the elbow and usher it from our planet’s path. So, how does one move a mountain in space?
All it takes is a (maybe nuclear) nudge
There are many ideas about how to avoid asteroidal Armageddon.
Most well-known of these is the nuclear option, where we’d detonate a nuclear explosive (or multiple explosives) near, on, or below an asteroid’s surface. Likely we’d send robotic spacecraft to do the job (as opposed to a crew of oil rig roughnecks). Other far-out ideas include building and launching a lens-like spacecraft that focuses sunlight onto an asteroid’s surface, thus creating a makeshift propulsion system from vaporised dust. Or we might launch a space tug or a spacecraft whose mass would slowly alter an asteroid’s course over many years.
Just how successful these ideas would be in practice depends on cost, technological capability, the size and composition of the interloper and, critically, how much time we have.
With enough time and if the asteroid is small enough, there may be a fairly simple solution: whack it with a fast moving projectile. In this approach, called a “kinetic impactor,” we’d intentionally crash a spacecraft (or a series of them) into a hazardous asteroid. An object’s momentum is its mass times velocity, so even a small object can impart significant momentum on a larger object if its relative velocity is high enough.
But how much would this approach bend a space mountain’s orbit? AIDA is about to put theory to the test.
Impacting asteroids before they impact us
Next summer, NASA plans to launch the Double Asteroid Redirection Test (DART) spacecraft on a Falcon 9 rocket. DART will travel to the 780-meter asteroid Didymos, where, in the fall of 2022, it will smack into Didymos’s 160-meter moonlet Dimorphos at well over 14,000 miles per hour. The collision will take place some seven million miles from Earth, which is close enough for ground-based telescopes to record the expected transfer of momentum and subsequent several-minute change to Didymoon’s orbital period.
This might sound like a risky manoeuvre, but the asteroid pair don’t threaten Earth, Heli Greus, product assurance and safety manager for ESA’s Hera mission, told Digital Trends.
“Nobody has to panic,” Greus said.
The ESA will then launch its Hera spacecraft in 2024 to observe the aftermath of the impact in more detail and at close range. As ESA notes in its description of the Hera mission, “By the time Hera reaches Didymos, in 2026, Dimorphos will have achieved historic significance: the first object in the Solar System to have its orbit shifted by human effort in a measurable way.”
While initial ground telescope measurements may give an approximation of how much DART deflected Dimorphos, they’ll be hamstrung by distance and the expected dust plume. Hera will do its own “crash scene investigation,” measuring the exact change in orbital period and studying the crater in detail. It will also map the asteroid’s surface and interior structure.
This data will allow scientists to validate and refine their models. In the future, then, we might discover a new threat, calculate its orbit over the next century, and discover there’s a scheduled rendezvous with Earth. Scientists would consult their charts, built on Dimorphos data, and suggest with greater confidence what it would take to deflect the space rock.
The future of planetary defence
To be clear, this is only the beginning. Kinetic impactors are the simplest solution within our reach. In its 2007 report to Congress, NASA characterised them as “the most mature approach” for some deflection scenarios, especially for objects made up of a “single small, solid body.”
Other options are either more complex, less effective, or still controversial. In the same report, however, NASA suggested a nuclear blast was the most effective approach studied, but that it also carried with it greater development and operations risks. It’s more than likely that the specific context of any future threats would determine our course.
To underscore the point, though, the risk of a collision with a large asteroid on any given day is very low. What’s more, our awareness of exactly what’s lurking out there has advanced by leaps and bounds — especially when it comes to the largest, most dangerous objects in our neighbourhood. We were flying blind just a few decades ago.
In coming years, thanks to new projects like the NEO Surveillance Mission and AIDA, our understanding and capability will advance even more, further mitigating the risk.
It’s a huge task, but for the first time in the history of the planet, one that’s within our grasp. As the old quote goes (attributed in various forms to Larry Niven, Carl Sagan, and astronaut Don Pettit), “If the dinosaurs had a space program, they’d still be here.”