Astronomers, planetary scientists and astrophysicists have long known about the risks posed to humanity by rogue asteroids. Over the last ten years, an international cohort of scientists and amateur observers decided to do what they could to prevent it: scanning the skies searching for objects hurtling through space that could potentially spell doom for life on Earth.
It sounds like anxious work, but someone has to do it. One of those asteroid hunters is Eric Christensen. Based at the Mount Lemmon Observatory outside of Tuscon, Arizona, he's been working full-time on trying to find killer space rocks since 2003.
First thing is that they aren't really killer. "No, I don’t feel anxious about asteroids or about my job," he says, reassuringly. "Asteroids are not something that we need to fear, necessarily."
As a result of the global efforts of the Spaceguard and NEOWISE sky survey programs, today more than 90% of massive near-Earth asteroids over a kilometer across have been discovered and mapped, which is great news! Having only a 10% chance of being wiped out by an extinction-sized asteroid that we didn’t even know was coming gives us pretty terrific odds of not dying horribly en masse. However, of the near-Earth asteroids that aren’t extinction-sized but still massive enough to do terrible damage (up to 140 meters in diameter), only 25% are currently tracked and cataloged.
Since 2012 the global effort to map and study near-Earth objects has received much more generous funding that it had previously. Christensen explains that NASA's funding dedicated to asteroid study increased from $4 million annually to $50 million. The result has been a decrease in anxiety over collision events, because not knowing what was out there at all was a lot worse.
"Asteroid impacts obviously happen and we’ve seen them even in our lifetimes on small scales," said Christensen. "It’s a fact of living on a planet whizzing around the Sun that we’re in a pretty busy area."
In 2013 a twenty meter-wide asteroid exploded over the Ural region of Russia with the force of between 20 and 30 times that of the bomb dropped on Hiroshima. The Chelyabinsk meteor was brighter than the Sun in broad daylight, and before it blew up in a superbolide explosion in the atmosphere, no one saw it coming.
"In near-Earth space there are millions and millions of objects. Just saying that conjures up an image that we’re in danger and we’re going to get hit, but that’s not the reality," said Christensen. "Small asteroid impacts happen on human timescales, but the really large impacts that are going to cause significant life or ecosystem destructions happen on the tens to hundreds of millions of years timescales."
There's more than a thousand near-to-Earth asteroids we know about that are over a kilometer wide (you can unwind by pursuing them all in the Sentry Risk Table.) Hunting down the remaining threats is the work of dozens of people around the world hoping to stave off Armageddon, including Christensen.
Where exactly asteroids came from (or what they want with us) is an area of active inquiry. Asteroids are made up mostly of rock, a small number containing heavy minerals like nickel and iron, and some are much less dense, made of ice and dust debris. They travel at enormous speeds of up to 25 miles per second. Most are agreed to be the result of collisions in the very early solar system among planets that no longer exist, at the time when the planets we now know in our system were forming. Some are the smaller chunks of collisions between those leftover larger asteroids. Some are the size of a fist, many are the size of a house, and some are massive objects, like Ceres, big enough at 945km in diameter to be classified as a dwarf planet.
Ceres and most of the asteroids in our solar system are thankfully in a quite stable orbit far, far away from us in the asteroid belt between Mars and Jupiter. But in the primordial solar system, the inner planets, including Earth, were bombarded by asteroids and meteorites, leaving ancient craters like the two most massive impact sites ever found, in the Australian outback, and the Chicxulub crater in the Gulf of Mexico, thought to be responsible for the extinction of the dinosaurs 66 million years ago.
Later in our distant past, the gravity of Jupiter’s enormous mass began to slingshot most asteroids and planet debris out of the inner solar system to where they are now in orbit, traveling around the Sun in the asteroid and Kuiper belts. Some were sent even further, to the most distant reaches of the solar system, to the Oort cloud, where comets don’t obey orbit trajectories and zip around in whichever direction they feel like. Disturbances to the Oort cloud from nearby nebulae can dislodge comets and send them careening over an incomprehensibly vast distance towards the Sun.
In sending this material into the farther reaches of the solar system, Jupiter acted essentially as a point guard for Earth, allowing us to evolve mostly free of cataclysmic run-ins with errant asteroids capable of extinguishing all life on our planet. Though Jupiter is also occasionally responsible for tugging asteroids out of their orbit and sending them into the Sun, and even more occasionally, right into us.
Extinction-level impact events over the history of Earth are rare, estimated on available data to occur only every 200 million years. It's the smaller ones, that hit on average every million years with a force strong enough to wreak major disruption to life, that the people trying to find them all are worried about.
But, as we know from emotionally manipulative disaster movies, the principle question is what we could actually do if an asteroid were to pose a serious threat to life as we know it.
Christensen says, "Depending how much time you have before an impact, say if you have a few years before, you’re likely to have to do something pretty drastic if it’s a larger asteroid. But if you have decades to plan a mitigation mission then you can get away with exerting smaller forces further in advance to try and change the course of an asteroid. There are a lot of interesting ideas and some crazy ideas, but it’s a fairly active area of research."
These plans include NASA's newly formed Planetary Defense Coordination Office, which would co-ordinate among existing emergency services agencies any action required in the aftermath of a collision. The more out-there strategies include everything from gravity tractor beams, focussing heat energy via solar sails, pushing an asteroid off course with lasers, and the strategy which worked so well for Bruce Willis, nuclear detonation. The International Academy of Astronautics will hold its next biennial Planetary defense conference in 2017 where the latest research in the field will be presented.
The bad news: there are some parts of the sky that we can't observe. "The whole sky is not being surveyed," said Christensen. "There is some lack of dedicated survey resources in the southern hemisphere, you know, right now we can’t get down to the poles. So there’s 25% of the nighttime sky that’s not available to us."
Which doesn't seem to totally go with the 'nothing to fear' attitude, but let's choose to believe an expert on this so everyone can sleep tonight.
I ask if there isn't an element to Eric Christensen's job that must be very satisfying, like playing a cosmic game of Where's Waldo, looking for tiny needles in a giant haystack.
"You know, it is," he says. "I think that’s maybe something that’s pretty unique in planetary science. Our observers at the telescope on an average night will find a new near-Earth object, maybe the first time that anyone has seen it, and that’s a new discovery. We find about 600 objects per year. On a clear night you’ll generally find a couple. On a great, clear night maybe ten or fifteen."
So you see shooting stars every single time you go to work?
“Haha, yes,” he laughs. “Exactly.”
Elmo is a writer with Real Future.