Impact Response

What will we do if we find an incoming impactor?

Asteroid 2008 TC3 is the only asteroid that was ever discovered before it hit Earth.

The first thing that will happen is that we will communicate this information to the appropriate scientific clearing house (the Minor Planet Center, MPC). They will distribute all the information to the various groups around the world that perform the calculations that will determine when and where the impact will take place. There are already systems in place between the MPC, NASA and JPL to report the information to the appropriate national and international authorities.

One of the actions ATLAS will take upon identifying an impactor is to notify you if you sign up for our ATLAS alerts on Facebook, Twitter or email (sign up here).

It is important to realize that ATLAS is not the system that responds to the asteroid threat - it is simply the system that detects an asteroid threat. Just like geophysicists set up earth monitoring sites and report earthquakes to disaster relief agencies, ATLAS will detect the incoming asteroids and report them. ATLAS is not the system that will then respond to the asteroid threat.

What happens after ATLAS detects an incoming asteroid depends on two factors:

1) how big the object is and

2) how much time there is before impact.

As you would expect, the bigger the object and the smaller the warning time the more critical the situation. It is also important to note that at this time the most likely amount of warning time is a few days. We simply do not know where most of the dangerous asteroids are and we do not the location of the next impactor.

In general, asteroids smaller than about 30 to 50 meters (or yards) in diameter will cause dramatic explosions high up in the atmosphere (many tens of kilometers or miles) but not induce significant damage on the ground. This isn't always true as illustrated by the Tunguska and Meteor Crater events but its a good rule of thumb. These `small' impactors are far more common than the large impactors and there is about a 1 in 10 chance of such an impact happening this century. ATLAS could provide a few days warning for these types of impact and the most appropriate response would be to evacuate the impact region of people and take measures to protect buildings and other infrastructure.

If we were to discover an asteroid that was going to hit the Earth that was in the 50-300 meter (or yard) diameter size range humankind's response would depend on the exact scenario. An object on the large end of that size distribution that was going to impact on land would be a major threat to life and property. But if were to hit in the ocean it might not be so bad. Also, objects that are hundreds of meters (or yards) in diameter will typically be discovered many years or even decades before impact so we will have a long time to develop an appropriate response - maybe asteroid deflection.

One thing we do not want to do is blow it up...

Why we do not blow up an incoming asteroid?

This is really pretty simple.

What would you rather be hit by - a bullet or shrapnel? At least with a bullet you could imagine getting out of the way. It's much more difficult to get out of the way of shrapnel and every piece can still be lethal.

Deflecting an asteroid

Believe it or not, astonomers believe it is possible to stop an incoming asteroid from hitting the Earth.

Imagine the asteroid like a rifle bullet - except billions of times more massive and moving about a hundred times faster (the typical speed of an asteroid is about 30 km/s or about 20 miles second). A long range rifle shot requires tremendous skill and takes into account the motion of the target, gravity pulling the bullet downwards, the heartbeat of the person at the trigger, and even the wind speed and direction. The slightest change in windspeed or twitch of the trigger finger is the difference between a bull's-eye and a wide miss.

The same idea applies to an asteroid with the Earth's name on it. If we can give the object a small nudge long before the predicted impact we can make it miss the Earth entirely. The less time we have before impact the bigger the nudge has to be.

So the issue really comes down to how we nudge an asteroid. There are many ideas for how to do this ranging from whimsical (some might say comic) to the possible. Rather than going over all the possibilities we will point you to several good sources for different options available, for instance, on Wikipedia or a summary of a study performed by one of our colleagues, Andrea Carusi, or run this Google query.

One of the problems with deflecting an asteroid is that scientists can't really tell you the internal structure of an asteroid. Until relatively recently we used to think that asteroids were basically large chunks of rock. But now we think that most of them are sort of flying piles of rocky rubble held together by very weak gravity and the mechanical forces of just being packed together. Trying to deflect a big chunk of rock is different from the situation of deflecting a rubble pile. To get an idea of the problem imagine trying to deflect a basketball coming at you or a giant pillow. If you push the basketball away it will move where you tell it too. But the pillow will merely deform its shape and keep coming at you.

Artist's concept of the Deep Impact Spacecraft at Comet Tempel 1.

Considering that we can't know the internal structure of an asteroid without actually going to it in the first place and that we on the ATLAS team are a little anxious about launching conventional or nuclear weapons we have a preference to the scenario known as a 'gravitational tractor'. This is not a gravitational tractor beam like you might have heard about if you were a fan of the original Star Trek show! The figure on the left shows an artist's conception of the gravitational tractor. The idea is being promoted by the B612 Foundation that is lobbying NASA and the government to actually demonstrate the technology of measurably deflecting an asteroid's trajectory. At the risk of picking a favorite scenario we like the idea because it is non-explosive, uses a well-understood technology, and is non-contact and therefore independent of the asteroids' internal structure.

The basic idea of the gravitational tractor is to make use of Newton's third law of motion which states for every action (or force) there is an equal and opposite reaction (or force). When a spacecraft is going around the Earth in an orbit the Earth's gravity tugs on the spacecraft to keep it in orbit. But the spacecraft also gravitationally tugs on the Earth. The difference is that the spacecraft has a negligible mass compared to the mass of the Earth. Also, for a spacecraft that is in orbit it's force on the object averages out to zero over a single orbit.

With a gravitational tractor the idea is to have as massive a spacecraft as possible get as close as possible to the asteroid and then maintain the separation and orientation of the spacecraft using the onboard propulsion. In this way the tiny gravitational influence of the spacecraft on the asteroid can be exploited over a long period of time and it will gradually tug the asteroid in the direction of the spacecraft and be used to pull the object off it's impact trajectory.

Is it really possible? Has it been done before?

Illustration of gravity tractor.

Absolutely. Sending spacecraft to rendezvous with and orbit asteroids and comets has been done on numerous occasions like the Japanese Hayabusa (MUSES-C) mission and NASA's NEAR Shoemaker spacecraft. Both those missions touched down on their respective asteroids - Itokawa and Eros. NASA's Deep Impact spacecraft actually slammed a dishwasher-size 370 kg (about 820 pounds) `smart impactor' made mostly of copper into Comet Tempel 1. There is a good description of the mission on Wikipedia. Even though the comet took a direct hit at about 10 km/s (about 6 miles per second) from the NASA-made impactor that left a crater on the surface it did not have a measurable effect on the orbit of the comet because it was still a very small nudge to a very large comet. The energy of that impact was equivalent to nearly 5 tons of TNT! So to nudge an asteroid or comet off course would take a lot more energy.

Thus, even though no asteroid has yet been measurably nudged off course through interaction with a human-made object, we have demonstrated the ability to orbit and even hit an asteroid or comet with a spacecraft. Thus, the technology to build a gravitational tractor to deflect an incoming asteroid from hitting the Earth is definitely within the scope of reality.

Impactor danger rating systems

Torino Scale.

Rating the danger due to a potential asteroid impact is different from rating earthquakes, tornadoes and hurricanes. Most of us are familiar with the Richter scale for earthquakes, the Fujita scale for tornadoes, and the Saffir-Simpson scale for hurricanes. But all three of these rating systems measure the danger or intensity of a disaster that is or was taking place - when all the data is in hand to measure its effects. The situation with asteroid impact risk is more like predicting the future risk of earthquakes in California, tornadoes in Kansas, or hurricanes in Florida. Seismologists and meteorologists can predict the probability of these events but not the specifics.

Our ability to predict an asteroid impact depends on how well we know the object's orbit around the Sun and this (mostly) depends on how much time has elapsed between the first and last observation of the object. The longer the time span the better we know the orbit and the more confident we can be of the prediction of an impending impact.

With most of the asteroids we discover we can very quickly establish that they will not hit the Earth at any time in many thousands to millions of years. But with some asteroids we only know the orbit well enough to predict the probability of impact - i.e. at the time of writing there is about a 1 in 1,700 chance that asteroid 2011 AG5 will hit the Earth on Feb 5th, 2040. So the odds are that it will not hit the Earth and that as we learn more about the asteroid's orbit we will re-run the calculations and determine that the Earth is safe because we have refined the object's trajectory.

If an asteroid is going to hit the Earth then the amount of damage it does depends on massive it is and how fast it's moving. Bigger and/or faster objects pack more of a wallop. Scientists can be very accurate when it comes to predicting the impact speed but it is much harder to know how massive the object is because we don't know just from looking at it whether it is mostly rock, mostly ice, or mostly space between chunks of either. So again, we have to make estimates of how massive the object might be.

Thus, providing an asteroid impact danger scale is not simple. Despite the difficulties scientists have come up with two popular scales for assessing the danger: The Palermo scale and the Torino scale. The Palermo scale is something only a scientist could love while the Torino scale is much simpler. On the Torino scale, of about 8500 asteroids that are Near Earth Objects, only 2 have a Torino scale rating of 1 - all the rest are zero. The scale increases from a no-danger rating of zero to a global devastation rating of 10. The Tunguska impact in 1908 would have ranked 8 on the Torino scale while the impact 65 million years ago that wiped out the dinosaurs and 85% of life on the planet would have been ranked 10.