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.

11 responses so far

11 thoughts on “NASA – Defending Earth”

  1. Kawarthajon says:

    “…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…”

    Hmm, sounds similar to something I’ve heard a lot of prominent politicians denying lately. Oh yeah, global warming! Hopefully, meaningful action will be taken on anthropogenic global warming too, which is an impending and devastating disaster.

  2. DS1000 says:

    @Kawarthajon, the only difference is that there are no vested interests in keeping asteroids on their trajectory like there are for continued and inefficient use of fossil fuels. From that perspective, I don’t think it’d become a political issue.

  3. carbonUnit says:

    ” We could just as easily to a near hit into a guaranteed hit,” I presume you meant “as easily TURN a near hit into a guaranteed hit,”

    While I do believe the other mechanisms are to be preferred, it seems that breaking up an incoming asteroid has it’s advantages. Mostly, if you break it up early enough, a lot of the mass that would have hit will now miss, because the asteroid is now spread out like a shotgun blast. Smaller pieces are also more easily dealt with by the atmosphere, to a point anyway. A dozen Chelyabinsk meteors would seem to be preferable to a single 12x meteor.

    I imagine that Chelyabinsk sized meteors and up are the level at which we’d want defenses to be workable. (Imagine the damage that thing would have done if it had entered more vertically and not expended most of its energy high in the sky.) Unfortunately, small stuff like that is going to be hard to spot, so a defense system will have to be watching closely for small stuff nearing Earth. It would have to be able to react in a matter of a few days or even hours. The good news is that a Chelyabinsk sized object is something we can deal with, given some notice.

  4. I previously asked that question too, about breaking up an asteroid. The answer was that mutual gravity would probably keep it together, not continuously spreading out like a shotgun blast, and so we would be hit by a swarm. Each individual strike would do less damage, but the chance of hitting a city would go way up. The cumulative damage may be worse. Obviously individual results will vary.

  5. carbonUnit says:

    That’s somewhat surprising given that the escape velocity of a small asteroid much be pretty close to zero. I can sort of see this for a big one. An explosion which attempts to break up an asteroid would have to be powerful enough to lift the pieces out of its gravity well.

  6. daedalus2u says:

    The effect that a nuclear explosion in space would have to a nearby asteroid is mostly determined by the radiation. Virtually all of the energy of a nuclear explosion comes out as hard gamma and X-rays. These travel line-of-sight and are absorbed very well by the asteroid surface. This heats that surface up, and depending on the energy flux heats it beyond the boiling point. That is the surface of the asteroid ablates off.

    If you heat the surface to 10,000 degrees with X-rays, it will ablate off at that temperature, thousands of tons of surface can be ablated off this way. An important question is can the structural integrity of the asteroid survive this kind of a momentum impulse.

    Another way to ablate surface is with a pulsed laser. A solar powered laser can be used to shoot high energy pulses at the surface which ablate material off and generate momentum. The momentum of the laser is essentially zero, all the momentum comes from the ablated asteroid. The solar powered laser could be in orbit around the asteroid (but getting it into an orbit is tricky because usually it requires a high delta V). This is the most sure way because it is the cumulative effect of the many pulses that produce the change in the asteroids orbit. Because you have your device in orbit around it, you are continuously monitoring it.

    The nuclear explosion technique doesn’t require matching the orbit of the asteroid, but then the timing of the explosion and precise trajectory (of both asteroid and projectile) is important. If there is a 10 km/s velocity difference, then 10 milliseconds represents a 100 meter difference in position. Depending on the details of the direction the error(s), it could shatter the asteroid (too close), or miss it (too far), or nudge it in the wrong direction.

    I think it is doubtful that the position of the asteroid will be known to within a few hundred meters from telescope observations. What would be optimal is to deposit a beacon on the asteroid, either a retro-reflector or (better) and active receiver/transmitter. Then the precision of the asteroid position can be refined to sub-millimeter accuracy. That is really what you want so you can continuously monitor how well the deflection is going.

    With that kind of precision, you could start to think of putting asteroids into capture orbits around the Earth. A few km diameter asteroid in Earth orbit would be a fabulous resource. Oxygen could be produced from the asteroid and that would reduce fuel requirements from Earth.

  7. NASA and ESA will also be coordinating with the Sentinel Mission, https://b612foundation.org/sentinel/, proposed by retired astronauts Ed Lu and Rusty Schweickart, amongst many other worthies. Their goal is to launch a space-based telescope capable of identifying all objects “larger than a lump of coal” and on a near-earth trajectory. So private science got to this party early, NASA is a welcome but late arrival.

    PS B612 Foundation, the parent org for Sentinel, also has some cool merch with which you can show/give support. I have their tee-shirt with a pic of a T.Rex looking through a telescope and saying “All clear, guys!” Cute but poignant.

  8. Update: seems private science may not have managed to get the funds needed to perform the mission, which may be why NASA had to step up: http://www.theverge.com/2015/9/29/9417595/nasa-b612-foundation-asteroid-contract-termination
    May or may not be comforting to know there’s still a need for publicly funded Big Science Projects!

  9. Robert Christ says:

    Blast a hole into it as long and narrow as possible with a nuke which is gauged by how many fractions of the size of Hiroshima rather than times larger it is. Spray the sides of the hole with water which should help hold the material together once it’s frozen. Wrap a cable with teeth similar to the chain on a chainsaw all the way around the object. Anchor a motor or motors into the object to drag the cable. Use it to drag material over the hole after an explosive has been placed inside. Keep blasting material out into space until you have reached the desired effect. Seems to me shouldn’t be much more difficult than any of the other theories postulated. It would be a matter of cost and having infrastructure in place and ready to go, instead of developing some type of exotic tech that would not have anywhere near the effectiveness of an approach like this. As well as job security for Bruce Willis.

  10. Damlowet says:

    @ Robert Christ, that seems like a convoluted and technically improbable solution. Consider if the object you are trying to manipulate is randomly tumbling, what then?


  11. DLC says:

    There are any number of rock-pushing scenarios out there, but most of them ignore the magnitude of energy required, and the fact that to move in space you need a reaction mass to push. One good fictional look at it might be Arthur C Clarke’s “The Hammer of God”.

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