Tomorrow is Asteroid Day, an annual event to highlight the potential risks that near-Earth objects (NEOs) such as comets and asteroids pose to Earth.
Launched in 2014 by Queen guitarist and astrophysicist Brian May and film director Grigorij Richters, the event was backed by prominent scientists across the globe, including Stephen Hawking, Neil deGrasse Tyson and Bill Nye.
Now in its fourth year, Newsweek has spoken to Paul W. Chodas, Manager of the Center for Near-Earth Object Studies at NASA’s Jet Propulsion Laboratory about the risk asteroids pose to the planet.
What are near-Earth objects?
We affectionately call these NEOs. They are asteroids and comets that are on orbits which bring them into the inner solar system, in the general vicinity of the Earth. To be nerdy and precise, they are small bodies whose orbits bring them to within 1.3 astronomical units of the Sun, where 1 astronomical unit is the average Earth-Sun distance. Keep in mind that asteroids and comets orbit the Sun just like the planets.
Why is it important to monitor the sky for them?
It’s important to monitor NEOs because one of these small bodies might be on a collision course with the Earth! Fortunately, it’s a very small chance.… But scientists think that the dinosaurs became extinct because of the collision of a large 10km (6.2 miles) asteroid 65 million years ago. And NEOs continue to pose a hazard—just a few years ago, in 2013, a 20-meter asteroid entered the atmosphere over Chelyabinsk, Russia, causing an explosion high in the atmosphere of nearly half a megaton, resulting in thousands of injuries just from the shock wave.
How many have we found so far, and how many more are thought to be out there?
Over 16,000 NEOs have been found so far, if we count all sizes. Objects smaller than about 50 meters in size will almost certainly disintegrate in the atmosphere without producing much damage if they are on a collision course. Roughly 12,000 known NEOs are larger than 50 meters in size, but there are a lot more to find. The numbers of NEOs go up exponentially as we consider smaller and smaller sizes, but the hazard is much lower for smaller asteroids.
What is currently being done to identify more NEOs?
NASA operates several telescopes dedicated to searching for NEOs. The two largest programs operate out Tucson Arizona and Haleakala in Hawaii. Over the last 19 years, NASA-funded search programs have found over 90 percent of the known NEOs. But there’s more work to do. The U.S. Congress has assigned NASA the goal of finding 90 percent of the NEOs larger than 140 meters in size, of which there are probably about 20,000. So far we have found only about a third of these.
How worried should we be about NEOs?
We should be concerned about the possibility of an asteroid impact, but not overly worried. The chances of a large NEO heading toward the Earth are very small, but the consequences of a collision could be very large. In the worst case, we could have an impact that leads to a global catastrophe, but that would take an asteroid larger than about 1km in size. The NASA program has already found over 90 percent of those objects, and because we track them very closely, we know that none of them can impact the Earth over the next few centuries. Smaller asteroids such as Apophis could lead to regional disasters, and we have found a smaller percentage of that population, but again, the chances that an asteroid of this size might impact the Earth within our lifetime are very small, less than one in a few thousand.
Does a NASA have a plan for if a large asteroid is on course to hit Earth?
Yes. The first and most important step of NASA's plan is to find that asteroid early enough to do something about it. The impact could be averted if we have years or decades of warning. Several space agencies have the capability to launch a spacecraft that could, for example, collide with the asteroid far away from the Earth and many years before its potential impact. We would only need to change the velocity of the asteroid by a centimeter per second or so to make it miss the Earth, as long as we perform that mission early enough.
Other techniques for deflecting an asteroid have been proposed as well, like ion-beam deflection, gravity tractors, or even a nuclear explosion to vaporize part of the surface. There are some current proposals to actually build spacecraft that could test out some of these technologies.
What's the worst case scenario?
It’s very unlikely, but possible, that a large, kilometer-sized asteroid on a collision course with Earth is found only months or just a few years before impact. That wouldn’t provide enough time for deflection, and in some cases, large asteroids are harder to deflect. This scenario is highly unlikely: Our asteroid search programs have found most of these asteroids, but if an asteroid is on an unusual orbit, it may have been missed. The longer we search, the less likely this situation might actually happen.
What can we learn from asteroids?
We can learn about the formation of the solar system because the distribution of asteroid orbits, especially in the outer solar system, has left evidence of the planet orbits moved around in the early solar system. By learning about the composition of asteroids, we can learn about the original constituents that formed our own planet, and maybe even how they brought compounds essential for the development of life on Earth. For example, it is thought that comets brought much of the water to the Earth when it was young.