Rusty's Blog

Rusty’s Planetary Defense – Deflect, Or Duck And Cover? | Part Two


Alright, so just what is it that I think you need to know or should know about asteroid impacts?  Well there are several rather distinctive, and not so obvious aspects to preventing the potential damage of impacts.

The most important goal of all planetary defense efforts is to prevent the loss of life.  Preventing damage to property is also important, but clearly secondary.  The first step in being able to prevent any kind of damage is to have early warning of a future impact.  No way to protect anything if we don’t know what’s coming at us!

Early warning means knowing that an asteroid is headed toward an impact with us and then, based on the specific information we have, taking action to protect life, and perhaps property as well.

Predicting an impact ahead of time is based on a sequence of actions beginning with the discovery of the asteroid using telescopes.  Generally these are optical telescopes on the ground, but there some are space telescopes in use as well, and there will be more capable space telescopes in the future.  The most productive system of telescopes used to date is that run by NASA (  This search program is often referred to as the Spaceguard Survey.

The nice thing about asteroids, or any object in space, for that matter, is that they are well behaved, i.e., they follow predictable paths.  It’s important to remember that asteroids, like comets and planets, orbit the Sun, and as long as an asteroid is not close to any other large object (like 99.99999+% of the time!) it follows a very predictable path around the sun… i.e. its orbit.  So the trick is getting enough tracking information on an asteroid, once it is discovered, that its orbit can be calculated and therefore its position can be predicted at any desired time in the future.

The asteroids of particular interest to us (we planetary defense folk) are those which happen to be in orbits that cross the orbit of planet Earth.  Now recall that we’re dealing with three dimensions here so unlike two orbits that cross on a flat piece of paper, the dangerous asteroids are those where this orbital intersection is real in all three dimensions.  These asteroids (millions of them!) are called near-Earth asteroids, or NEAs.  Mostly we talk about near-Earth objects (NEOs) where we also count the occasional near-Earth comet.  (“if it looks like a duck, hangs out with ducks, and quacks like a duck… it’s a duck!”)

If we discover an asteroid and track it a bit, we can then calculate its orbit and predict whether, in the next 100 years or so, it and the Earth will be in that three dimensional intersection at the same time!  If so, that’s a collision.  <duh>

The goal, of course, is that if we know about this pending collision far enough ahead of time (think 15-20 years) then we could use our space technology to intercept and deflect the asteroid from that predicted impact.  That’s the idea.  It’s also the ideal!

There are lots of blanks, of course, to be filled in here.  This is not (at all!) some slam-dunk.  Among the sticky issues is the fact that there are imperfections in the telescopic measurements (no such thing as a perfect measurement) and therefore uncertainty in the predicted orbits and therefore in the future positions of the asteroid.  There is also the challenge of deflecting an asteroid (i.e. changing its orbit enough to avoid an impact) with our current space technology.  These and other realities I’ll deal with in future blogs, but for now let’s assume that we have an adequate handle on these issues.

So if we can predict a future impact, and deflect it, what’s to worry?  Well recall that I said above that we’d need 15-20 years of early warning.  So what if we don’t have that much warning time?  Aye, there’s the rub!

At this point (to be revisited in the future) let’s say that with “adequate” early warning, we can prevent future asteroid impacts.  OK, so for that category of future asteroid impacts we can protect both life – and property.  If we can make the asteroid miss the Earth entirely then there’s no loss!  And that’s our ideal planetary defense goal.

Fine.  But what about situations where we have inadequate future knowledge?  And what would those situations be?  Well, a couple of things.  We might discover a sizable asteroid, figure out its orbit, and find that it has an impact with the Earth with too little time to mount a successful deflection campaign.  But this is a highly improbable situation since even for the smallest, most populous asteroids that would cause damage on the surface, they only impact (on average) once every 50 years.  (Think the Chelyabinsk impact)  And to happen to find “the one” of those that is going to hit in say 7 years isseriously improbable.  Nevertheless possible.

Another, more likely scenario, is that we could happen to be looking in just the right direction that we see an asteroid just before it hits!  Literally we’re talking about looking up the approach path and seeing an asteroid on “final approach”, something like standing at the end of a runway and seeing an airplane about to land (hopefully not crash!)

Ironically this is not as improbable as it may seem, because impacting asteroids approach the Earth from two preferential directions.  In the last weeks and months prior to impact an asteroid will appear in the sky in either the approximate direction of the Sun, or from the anti-solar point (or opposition).  Those that hit at night will approach from near opposition, whereas those that hit in the daytime come from near the Sun.  Think Chelyabinsk.  It occurred in Russia at 9:20 in the morning coming out of the sky within just a few degrees of the Sun.


So you should know that NASA is today funding the development of the ATLAS system (, a set of small telescopes that will preferentially look in the direction of opposition (it can’t look at the Sun!) to watch for any asteroids on final approach to an impact.  It will see lots of asteroids, but mostly below the size where they will do any damage.  But every once in awhile ATLAS (or other similar systems) will find one headed for impact that can do damage – and we can thereby generate a warning.

What do we do when we don’t have enough time for a deflection but still we know that an impact is about to occur?  We take “civil defense” actions, i.e. we do what we can to prevent the loss of life.  Think hurricane or storm warning.  We head for the basement until it’s over, or we evacuate – head out of town.  No way to save buildings, etc.  But we can save lives.  And this is the value of short term early warning.

So to summarize, you should know that with long term early warning, i.e. with an impact predicted 15-20 or more years in the future we aim to deflect it (more to come on that) and prevent the impact entirely.  And if we have a prediction of an impact in 2 days, or a month, we can evacuate or otherwise take simple measures to minimize the threat to life.

In summary then, you should know that long term early warning is underway and in the long term once we have a “full” inventory of NEOs that can do serious damage, we can either deflect them, or if they’re pretty small, evacuate or take other protective measures to avoid the loss of life.  What you need to know is until that full inventory of NEOs is in hand (more on what we will know and when we will know it in a later blog) we will have soon a short term warning system that can provide a few days or weeks of warning so that you can take action to protect yourself.

The action party in discovering a threat, and in the instance of an adequate long term warning, deflecting the NEO (i.e. preventing the impact) is government(s).  The action party in the instance of a short term warning being issued is you!  And what you should do is to follow the instructions and guidance provided by the local disaster response agency in your area.  Once, of course, that “system” is in place (see my prior blog, Chelyabinsk Warning – One Day)

Final word here… since I know it’s on your mind… the good people of Chelyabinsk could not have had a near-term warning of the impact 2 years ago.  For several reasons.  First it was a daytime impact and the asteroid came “from out of the Sun”, so even an operational ATLAS system could not have seen it ahead of time.  Furthermore it was only 20 meters in diameter and the long term warning telescopes can find only an extremely low percentage of NEOs this small.  (While we’ve found over 95% of the NEOs larger than 1 km in diameter, we’ve found much less than 1% of the objects of Chelyabinsk size)  Therefore the Chelyabinsk object was outside our capability to predict ahead of time.  Still, because of the nascent ATLAS system, in the near future we should be able to have a couple of days warning for Chelyabinsk-sized objects that will hit at night.  That’s about 50+% of the total… better than none!

Well and good, but how many of these guys are out there and how often do then hit?


Tags : asteroid dayasteroidsduck and coverrusty schweickartrusty's planetary defense blog
Rusty Schweickart

The author Rusty Schweickart

Russell L. (Rusty) Schweickart is a retired business and government executive and from 2002-2015 served as Chairman of the Board and Chairman Emeritus of B612 Foundation. The organization, a non-profit private foundation, champions the development and testing of spaceflight concepts to protect the Earth from future asteroid impacts.

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