Likelihood Low of Asteroid Colliding With Earth

By Daniel Stolte, University Relations – Communications | June 29, 2015
Do asteroids deserve their nasty reputation? On the occasion of Asteroid Day, Eric Christensen, director of the UA's Catalina Sky Survey, talks about the odds and consequences of an asteroid wreaking havoc on Earth.
If there were a way to label an asteroid after its discoverer, one out of every two space rocks tumbling about in our neighborhood of the solar system would have a big red-and-blue "A" on it. And that's because of 12,700 known near-Earth asteroids, 5,800 were discovered through the Catalina Sky Survey, an asteroid detection program founded in 1998 at the University of Arizona's Lunar and Planetary Laboratory.
To make sure humanity doesn't go the way of the dinosaurs, the Asteroid Day initiative has chosen June 30 to raise awareness — and funds — for asteroid detection programs. UANews spoke with Eric Christensen, director of the Catalina Sky Survey and staff scientist at LPL, about the real and imagined dangers of incoming rocks from outer space.
If we are to believe the Asteroid Day initiative, a mere 1 percent of about 1 million asteroids capable of destroying a city have been discovered. Is that true?
Statements like these have a grain of truth, but are extremely misleading at the same time. Population models suggest that there are about 1 million "Near-Earth Objects," or NEOs, down to around 50 meters in diameter, and yes, we have only seen about 1 percent of them. But the majority of these, while classified as NEOs because they can approach the Earth to less than 45 million kilometers — more than 100 times the distance to the moon — pose zero risk of impact. Zero. Their orbits just do not intersect the orbit of the Earth. Even for the small subset of these million objects that can potentially impact the Earth, they will strike a random place on the planet when their time comes, hundreds or thousands or millions of years down the road. Only a small fraction of the Earth’s surface is populated, so it’s just very unlikely they will impact directly over a city. We’ll most likely end up with an airburst over the ocean, or another Tunguska-like event that just knocks down a bunch of trees.
Where do we stand with respect asteroids vs. Earth? Are we doomed?
NASA was directed by Congress in 1998 to find 90 percent of asteroids measuring 1,000 meters or more. The impact of an object that size would have global consequences, potentially extinctions. It would lift up a tremendous amount of pulverized rock and water vapor into the atmosphere, causing an effect similar to nuclear winter, where the amount of sunlight that reaches the Earth would be diminished for several years, severely disrupting the global food chains. NASA-funded surveys like the Catalina Sky Survey; the Spacewatch Project, which was initiated at the UA in 1984; and other surveys have discovered almost 13,000 NEOs. The original goal to find the 1-kilometer objects has been met, giving us the confidence to essentially rule out a civilization-ending impact in the foreseeable future. The remaining risk posed by smaller NEOs continues to drop as more and more of those objects are discovered. NASA is now mandated to push the search down to smaller sizes, to NEOs measuring 140 meters or larger, which is about the size of a football stadium. An impact by an asteroid of that size would have significant regional consequences, potentially affecting an area the size of a small country or so. We believe that our inventory of objects in that category is only about 25 percent complete, so there is still significant work to be done.
Where do asteroids come from?
Asteroids are primitive remnants from the birth of the solar system. Most of the asteroids we know about harmlessly orbit the sun in relatively stable orbits in the main asteroid belt between Mars and Jupiter. Scientists have cataloged over 800,000 main-belt asteroids, but the full population down to small sizes likely reaches into the hundreds of millions. Under the influence of Jupiter's gravity, some asteroids are nudged into the inner solar system, where they become NEOs. On timescales of millions of years, NEO orbits are unstable, and most of them end up smashing into a planet or evolving back into more distant orbits.
What are the actual risks of a devastating asteroid impact?
Impacts of 1-kilometer asteroids happen maybe once per half-million years. Impacts of objects in the 140-meter range statistically happen every 10,000 years or so. We are not talking about events that happen on timescales of human lifetimes here, which makes the real risk a little difficult to understand. Risk is expressed in terms of predicted losses to human life and infrastructure, integrated over millions of years. Some people will spin these numbers into comparisons that sound like an asteroid is likely to kill you or someone you love, and that it's something you need to have a visceral fear of. I try to gently steer people away from that. The chances of a major impact within our lifetime or the lifetimes of our children is extremely low, but it’s easy to focus on the drastic consequences rather than the tiny probabilities.
The individuals behind Asteroid Day call for discovery efforts to be stepped up a hundredfold. Is that necessary to keep us safe?
I think there is a false sense of urgency to find every potential impactor as soon as possible. Pushing the survey completeness to smaller diameters will incur greater and greater costs, to address smaller and smaller risks. I see no reason to try and find every last one of the millions of 10-meter NEOs that will harmlessly explode in the Earth's atmosphere should they ever come our way. It would be nice to detect a few of these prior to impact, and CSS has demonstrated that this is actually possible with our current set of telescopes. But finding every single 10-meter NEO in the solar system would be a multibillion-dollar effort, and if the ultimate goal is to protect and improve human lives, then there are a lot of other things we could be doing right now with that kind of money that have immediate and guaranteed benefits for society.
How are asteroids discovered?
The business of discovering asteroids is pretty routine: We use wide-field telescopes to scan the skies, night after night, looking for things that move. Ten years ago, CSS was discovering NEOs at a rate of 300 per year. Last year, we discovered more than 600. Most of that improvement has been due to more sophisticated software. We are currently replacing the cameras in our two survey telescopes to cover more sky, and we have refurbished a one-meter telescope next to the one on Mount Lemmon, which will be mostly used for follow-up observations. One of the great benefits of searching for NEOs is that there is a tremendous amount of insight from incidental science. For example, we have detected hundreds of thousands of the main belt asteroids. Having a complete catalog of main belt asteroids allows planetary scientists to probe the development, evolution and dynamics of the solar system.
How long will it take to find the remaining objects of 140 meters or more?
At the current pace, it would probably require several more decades. NASA was given this mandate in 2005, with a deadline of 2020, but the funding necessary to complete the job has not yet been entirely allocated. With the current suite of one- to two-meter class telescopes, we are not optimized to efficiently detect the smaller asteroids at the necessary rate. The smaller they are, the fainter they are. To find the smaller objects with small telescopes, you have to wait until they are close to the Earth and favorably placed. Part of this is a waiting game — the fainter you can go, the more opportunities you have for discovery and the faster the work goes. In order to complete the goal on a timescale of 10-15 years, it would require significant new assets to be developed, including an infrared space-based survey telescope and additional large ground-based telescopes.
Could there be larger asteroids or comets out there that we don't know about?
It's possible, but we have a reasonably good understanding of the flux of long-period comets into the inner solar system. Remember that near-Earth space is a very big place and that Earth makes for a tiny target. So the risk of an impact with a long-period comet is low, only about 1 percent of the risk represented by asteroid impacts. Considering that we have found about 95 percent of the one-kilometer-plus sized objects, and every single one has been shown unambiguously to not be dangerous for the next 100 years, I would argue that the risk from the last remaining few percent of large asteroids is very small. I'm not saying the risk is zero — after all, that's why we are doing this work. But the risk has to be framed in a responsible way. I think we can do so without resorting to calling them "city killers," or expressing their hypothetical impact energies in terms of Hiroshima bombs.
Where does the UA-led OSIRIS-REx mission fit in with asteroid science?
The OSIRIS-REx mission combines a tremendous amount of asteroid science into one mission. The sample-return aspect of the mission is perhaps the most exciting, but we’ll also get direct measurements of asteroid surface properties, and we get a chance to study some of the very subtle perturbing forces that act on asteroids. When we take telescopic observations of asteroids from Earth, we can make inferences about their size and composition, but the only way to really know is to go there, study it up close and bring back a sample to analyze in our laboratories. This closes the gap between what we know about meteorites and what we infer about asteroids from telescopic observations. The mission also has implications for planetary defense. Bennu, the target asteroid, is one of the most hazardous objects we know of, though "hazardous" in this case means it has less than a 0.04 percent chance of impact in 160 years.
What are the chances of successfully diverting a rock in space that's headed our way?
Usually, an object that is going to hit the Earth will almost hit the Earth many times before the final impact. One of the most hazardous asteroids we know about is Apophis, whose impact predictions are very sensitive to the previous encounters with Earth. Let's say we discovered an object tomorrow that could hit the Earth in 2050. We might only estimate at discovery time that there is a 1-in-10,000 chance of impact, but with repeated observations that number can shrink or grow. So the question is, at what point do you launch a mission to better characterize the object or try to mitigate the threat? There are several feasible ideas being explored that could potentially divert an asteroid enough to avoid an impact. The simplest of these is the "kinetic impactor" approach, which is just a fancy way to describe a spacecraft that would run into an asteroid at high speed. An asteroid’s orbit doesn’t need to be drastically altered in order to avoid an impact — the arrival time at Earth just needs to be sped up or slowed down by a few minutes. As long as a potentially impacting asteroid is discovered and characterized with several decades of lead time, there is a good chance that we will be able to divert it.

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