Feb 02 2016

NASA – Defending Earth

asteroid-threat-global-action-plan-101109-02NASA recently announced that it has created a Planetary Defense Coordination Office (PDCO). The purpose of this new office is to defend the Earth from alien invasion.

OK, no, but the name does sound like that, doesn’t it?

The purpose of the office is to coordinate efforts to defend the Earth from Near Earth Objects (NEOs) – essentially comets and asteroids on a collision course with the Earth. The director is Lindley Johnson, who is currently the NEO program executive, which is an obvious fit with the new office.

NEOs do pose a threat to our civilization. You don’t have to be Bruce Willis to imagine how devastating it could be for a large rock traveling faster than a bullet to hit the Earth. Of course, the answer is that it could fall anywhere along a spectrum from nothing to wiping out our entire species. Obviously NASA is more concerned with the latter.

There are many NEOs, mostly asteroids whose orbits cross the orbit of Earth or bring them close. Objects pass close to the Earth all the time, in fact.

Perhaps more worrisome are objects that are visitors to the inner solar system. These are mostly comets, either periodic or one-time comets, barrelling in from the outer solar system. These are more worrisome because they can be more difficult to predict and they are traveling, relative to the Earth, much faster than asteroids moseying across our orbit.

The probability of getting hit by a large asteroid or comet is very low over the short term. According to NASA:

A working group chaired by Dr. David Morrison, NASA Ames Research Center, estimates that there are some 2,100 such asteroids larger than 1 kilometer and perhaps 320,000 larger than 100 meters, the size that caused the Tunguska event and the Arizona Meteor Crater. An impact by one of these larger meteors in the wrong place would be a catastrophe, but it would not threaten civilization. However, the working group concluded that an impact by an asteroid larger than 1-2 kilometers could degrade the global climate, leading to widespread crop failure and loss of life. Such global environmental catastrophes, which place the entire population of the Earth at risk, are estimated to take place several times per million years on average.

So it may be 100,000 years before the new PDCO is called into action. Or it could be in 10 years. We can’t predict.

Well actually, we can, to some extent. We can look for NEOs, catalogue them, map their orbits, and predict which ones will come dangerously close to the Earth and when. In fact, that is one of the projects that will keep PDCO busy. They will also be working in coordination with the UN and European Space Agency (ESA):

That UN subcommittee has formalized the International Asteroid Warning Network (IAWN) and a Space Mission Planning Advisory Group (SMPAG) of U.N. member states that have space agencies.

In addition to finding all the potential threats, PDCO and its international partners will be developing the technology to deflect asteroids (otherwise, what’s the point). It certainly makes sense to have this technology ready to go. It is quite possible we will find out the Earth is going to get hit with little warning. Even a 10 year warning is possibly not enough, if we are starting from scratch.

Even if the technology is fully developed, a few years lead time would be nice.

The ESA is planning a possible Asteroid Impact & Deflection Assessment mission (AIDA), which will send up two spacecraft. The first will ram into an asteroid, and the second will monitor the effects. NASA is still deciding if PDCO will be part of this mission.

Crashing a fast-moving rocket into an asteroid is a crude, but effective, method for deflecting an asteroid. It’s all about momentum – a heavy rocket at high speed would impart a lot of momentum to even an asteroid. The more lead time we have (and this is why surveillance is critical) the greater change we can make in the orbit of the asteroid. Even a slight change over four years can be enough to turn a hit into a miss.

While this seems like a simple method, it’s still rocket science. We need to have the ability to quickly launch one or more rockets with accuracy and predict the result. We could just as easily turn a near hit into a guaranteed hit, or have the asteroid miss on this pass then come around and hit us in 20 years.

We also need to develop methods for fine-tuning. Smashing stuff into an asteroid makes a big change, but then we may want to give the asteroid a nudge. There are a variety of possible mechanisms for this. We could use a gravity tractor, a ship that stays close to the asteroid and uses its engines to slowly pull it by gravity in one direction.

A nuclear explosion could also be used, not to blow up the asteroid but change its course. Blowing it up would likely not be a good idea – then we would have a swarm of smaller asteroids still on a collision course, perhaps causing even more damage.

There are more exotic ideas, such as painting one side of the asteroid white so that solar pressure will push it. This is very tricky, however, as most asteroids are tumbling with respect to the sun.


I, for one, feel a tiny bit safer knowing that the space agencies of the world are working together to identify and develop the technology to deflect large asteroids. While the short term probability is low, I think of it like insurance for our civilization.

It always a judgement call – when you advocate for taking action to prevent a low probability but very damaging event. How low a probability, and how devastating an event?

I think that the investment of resources we would need to make to protect ourselves is relatively tiny, and it buys us a measure of insurance (of course, no guarantee) against a world-wide catastrophic event. Even if we prevent only a small meteor from hitting a city and killing thousands or tens of thousands of people, and causing trillions of dollars of damage, that is worth it and probably cost-effective.

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