Killer Asteroids: Can We Track Them?

near-earth-asteroid-image-nasaWhen I was at LoneStarCon in San Antonio back in 2013, I attended a session in which a NASA researcher talked about the role the Ames Research Center plays in the Near Earth Object Program (NEO). The NEO Program tracks those asteroids, and several comets, whose orbital paths around the solar system bring them close enough to Earth to merit attention.

In technical terms, an asteroid is considered an NEO if its closest pass to the sun is less than 1.3 Astronomical Units (AU), where 1 AU is defined as the approximate distance between the Earth and the Sun. As of October of this year, the program has tracked over 15,000 of these.

neos-e1460572562406-580x431There’s good news in what they found so far: as of October 25, 2016, the program has identified and is tracking 875 asteroids larger than 3,300 feet in diameter (0.62 miles). These brutes account for about 95 % of the so-called “planet killer” asteroids (the one that killed the dinosaurs and just about everything else 65 million years ago was 6 miles in diameter).  This severity of impact is estimated to occur only about once every billion years, and we can track those bodies well enough to give us several years warning to do something about it. All good.

The NEO Program is focusing now on the next lower category of asteroids, those bigger than 500 feet in diameter, i.e. larger than a football field. They’re tracking more than 7,500 of these “nation busters” to date, which represent about 40% of the estimated total. The NEO Program’s objective is to raise that percentage to 90.

The NEO has a special subcategory of these asteroids, innocuously referred to as PHAs, which it tracks more closely. These are of more than scientific interest because PHA stands for Potentially Hazardous Asteroids. These objects larger than 500 feet in diameter have orbital paths that bring them closer than 4.6 million miles to Earth. The NEO website refers to these as “threatening close approaches” since “closer than” also includes some which might actually strike the Earth. As of 2016 there are 1,737 known PHAs.

asteroids_passing_earthThe NASA Ames Center researcher’s talk on that September day in 2013 was especially timely, since that previous February, Earth actually had a close encounter with a smallish asteroid in a size category on which the NEO Program was not yet deeply focused.

On February 13, 2013, the so-called Chelyabinsk meteor exploded in the sky over the city of Chelyabinsk in Russia. This “meteor”, actually a near-earth asteroid, was about 60 feet long, weighed 26 million pounds, and was travelling 60,000 feet per second, more than 50 times the speed of sound, when it entered the atmosphere. The explosion shattered windows and damaged buildings, injuring 1,200 people. Its explosive force was almost 30 times more powerful than the first atomic bomb. Fortunately, no one was killed.

Chelyabinsk was lucky in more than one way. The path of the asteroid through the atmosphere was fairly shallow, about 16 degrees from horizontal, basically a glancing blow. If the bolide had come in seconds later, and therefore at a steeper angle closer to vertical, one estimate was that the shock wave blast would be an order of magnitude stronger, maybe 300 times the force of an atomic bomb. There’s no question the destruction would have been devastating. The angle of the asteroid’s passage through the atmosphere that day was a roll of the dice.

cherlyabinsk_asteroid-580x365Theoretical physicist Dr. Michio Kaku has labeled these small but potentially deadly asteroids “city busters”. Two things about them are troubling:

First, because of their smaller size they lie below the current instruments’ tracking threshold of larger than 500 feet. They are hard to find and harder to track, so we can’t be sure where and how many they are. Second, the Chelyabinsk meteor approached Earth from sunward, that is, on an orbit that came at us from the direction of the Sun. This region is in the NEO Program’s so-called “blind spot”, because none of our current tracking systems can discern objects while pointed at the Sun.  So, in spite of our technology and vigilance, the asteroid explosion over Chelyabinsk caught everyone by surprise.

Progress is being made. The Chelyabinsk meteor spurred NASA’s Center for NEO Studies at the Jet Propulsion Laboratory in California to develop a computer program called Scout that can use sightings recorded by astronomers around the world to enhance the probability of identifying and tracking these smaller asteroids.

Before Scout, if an astronomer identified an object that might be an asteroid, they would post that information on the Minor Planet Center (MPC) website in Cambridge, Massachusetts. Other astronomers could use that posting to search for it and post any additional information they gleaned. That coordination of data sources could take many hours.

An early version of Scout went online in 2015. The software automatically surveys the MPC website; when a new object is posted, it calculates the object’s possible paths in about 10 minutes. If any of those trajectories are potentially hazardous, Scout emails and texts astronomers to do follow-up observations and confirm the hazard. This significantly shortens the confirmation time and therefore increases the window of time to issue an alert.

Those asteroids coming at us from the sunward blind spot, however, remain largely invisible. The NASA Ames researcher said that a highly sensitive tracking instrument stationed at one of the Earth-Sun Lagrange points and looking sideways at the asteroid’s orbit would have been required to discover it. But we’re at least a decade away from launching such a technology, so for now our tracking situation for these is vulnerable.

In 2013, NASA administrator Charles Bolden was asked what NASA could do if we discovered a large asteroid heading for Earth. He answered, “If it’s coming in three weeks, pray.” Since some of these smaller asteroids give us no warning at all, in the near-term the prospects for detecting and deflecting a “city buster” are still pretty low.

Here’s the link to the NEO Program’s excellent website.

Copyright 2016 Dandelion Beach LLC Images: NASA, JPL-Caltech, ESA
Cherlyabinsk Image Credit: Tuvix/Youtube

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