Proposed Solution

The B612 goal is to actually change the orbit of an asteroid in order to demonstrate that humankind can indeed protect the Earth from future asteroid impacts.  The determination to conduct such a mission is born of our conviction, based on experience, that the best way to learn how to do something is to do it.  Anyone who has learned to ski knows that real learning begins after you’ve put down the books, returned the rented videos, and gotten out on the slopes.  You may very well fall down. In fact you probably aren’t learning as rapidly as you might unless you are falling down on occasion.

Our preferred solution to deflecting an incoming near Earth asteroid (NEA), for reasons that are clarified below, is to rendezvous and "dock" with it at either its North or South Pole, realign the asteroid’s spin vector to a preferred direction and then push it (gently and for a long time) until we’ve changed its speed enough to miss the Earth. This scenario is very direct. It makes the key assumption, well supported by statistics and common sense that we will know of a pending impact many years ahead of time. The strange logic of NEA detection is that we will either know that an impact is pending several decades ahead of time, or that we will be hit by an asteroid we don’t know about with no warning at all! The critical importance of the detection program is that as you detect and track more and more NEAs you increase the former category and reduce the latter.

Because of the likely warning time of decades, we can utilize a deflection technique that is virtually universal, i.e., it does not depend strongly on the specific characteristics of the asteroid.  A NEA-Tug will handle virtually any asteroid.  The specifics of docking and attachment will vary slightly, but this is a minor challenge compared with the uncertainties and problems associated with many of the other possible deflection techniques.  These alternatives will be discussed on another page.

An additional, but very important advantage of the NEA-Tug technique, it that all of the technologies and techniques needed to deflect an Earth-bound asteroid are also extremely useful for enabling both in situ scientific research and future asteroid resource exploitation.  In addition, the power and propulsion technologies involved are key to future deep space missions.  Given that asteroid deflection missions will be inherently few and far between, the fact that the technologies that enable it are also necessary for other key applications is of great importance.

With the deflection system technologies defined, we’ll move on to questions of how we propose to use this NEA-Tug to meet our goal.  The basics of launch and rendezvous are well known.  All the orbital mechanics are very well known.  In a sense, this mission is conceptually very simple.  The primary challenges are engineering (new power and propulsion technologies), docking and attaching to the asteroid (the phase with the greatest unknowns), and to some extent, the public policy issues inherent in committing to and executing the mission.  All of these elements will be discussed on other pages.

In parsing the B612 goal there are a few questions in addition to how we would move an asteroid that need to be answered.  E.g., what asteroid would we move?  What does “in a controlled manner” mean?  Why 2015?  We’ll deal with these before going on.

The NEA that we will choose (we haven’t done that yet) will be “representative” of the class of potential impactors, i.e., it will have the key characteristics of NEAs that we might have to deflect later under operational conditions.  So what does that mean since there are many asteroid types, sizes, rotation rates, etc.?  Here we need to keep in mind our primary objective, i.e., to demonstrate the key capabilities necessary to move ahead to an operational system.

For example, it is not critical at all to choose an asteroid that would require a large amount of fuel just to get to it. In the case of an actual deflection mission the specific orbit of the asteroid will determine the fuel requirements for rendezvous. The capability to carry larger amounts of fuel, however, is not significant and can be met simply by launching with additional fuel and taking more time to rendezvous with the asteroid. So, for our purposes, we’ll choose an asteroid that’s “convenient” in terms of the fuel required and the time to get to it.

Another selection criterion that’s essential is that we pick an asteroid that does not, in the foreseeable future, threaten the Earth, or come even close to doing so.  We want to demonstrate the capability to deflect asteroids away from the Earth, not convert one that’s a non-threat into a real threat! Of course this is an easy condition to meet since we know of no asteroid that currently threatens the Earth or would do so after our modest B612 maneuvers.

In terms of what size asteroid, we get into some specific and interesting characteristics of these little worlds.  One of the biggest unknowns we face is the structural strength of these objects.  There is substantial evidence that we are seeing a population dominated by objects that are very loosely held together and, in many cases, are quite porous.  These asteroids have been commonly described as “rubble piles”.  However, this observation applies primarily to NEAs greater than about 150 meters in diameter.  Below that size it appears that a high percentage of the population is more substantially held together.

How is this known?  By looking at the rotation rate of the population.  Specifically by plotting the rotation rate vs. the size.  As you can see from the diagram above, none of the asteroids above 200 meters has a high rotation rate, whereas below that size high rotation rates are common.  It turns out that if you place a rock on the "equator" of a typical asteroid greater than 200 meters in diameter, it would fly off if the rotation rate were higher than delineated by the line in the graph above.  I.e., it appears that these larger asteroids barely hang together at the speed they rotate.  Their surface gravity is so low that the centrifugal forces would exceed the “weight” of an object on the surface if they rotated much faster.

Given the implicit structural weakness of asteroids greater than ~200 meters in diameter, we want to make certain that we select one in this class.  Clearly the challenge of reorienting and accelerating an asteroid classified as a rubble pile is greater than dealing with a solid object.  This is therefore, a key characteristic in choosing our particular NEA.

While detailed knowledge of the structural characteristics is unknown, it seems clear that many of these NEAs are very loosely consolidated and porous.  This is known not only by reference to the rotation vs. size relationship above, but also by the measured density of several of the asteroids which happen to be binaries.  To the surprise of many, it turns out that about 15% of the NEAs scanned directly by radar to date are asteroid pairs, not single objects.  In this configuration one can calculate the mass of the larger body and, given the size by radar as well, compute the density.  This turns out in many cases to be as low as 2.2 gr/cc and even lower, i.e., only twice as dense as water.  Yet we know that the individual rocks and mineral grains of which the asteroids are composed have a specific gravity of 3.5 or more.  The only way for this to occur is if there is open pore-space internal to the asteroid.

It is one of the clear advantages of the low thrust, long period deflection technique that despite the potentially tenuous nature of these objects the small forces we will apply will almost certainly not threaten their structural integrity.  A typical value for the force we will apply in reorientation and acceleration is about 10 Newtons, or about 2.25 pounds!  If this force is distributed over an area of several square meters by a contact “footprint” the resultant stresses on the asteroid will be extremely low.

What about the phrase, “in a controlled manner”?  This simply means that we will not consider our mission accomplished if we happen to go out there, push on an asteroid and simply move it.  What we clearly want to demonstrate is that we can move an asteroid in a very specific way; that the entire operation is fully under control; reorientation and acceleration.  Furthermore, to demonstrate convincingly that the technologies we employ are capable of performing the deflection function operationally, we want to push the demonstration asteroid long enough (i.e., change its velocity enough) that, simply by adding more fuel, we would be able to cause it to pass by the Earth at a safe distance were it actually headed toward Earth in the first place.  Again, demonstrating the capability to meet this challenge in a realistic way is essential to the B612 mission.

Finally, why did we choose 2015 as the target date?  This is clearly a judgment call, but we believe, from the assessment we’ve done, that the technologies and techniques required can be developed to meet that timeframe.  Everyone in the engineering world knows that selecting too long a timeline does not generally end up with a better system.  Conversely it does generally mean that the system developed is more costly.  At the other extreme, having too short a timeline will guarantee failure and waste of money.  The challenge is to select a reasonable timeline that enables the job to be done, but not with so much slack that the better continually replaces the good enough. 

It must be remembered that this is a demonstration mission.  Its purpose is to clearly demonstrate to the general public and to public officials that the Earth can be protected from this cosmic challenge.  No one knows when we will detect a NEA headed for an impact with Earth, but it could be at any time.  We believe it would be irresponsible and inexcusable for the world to face such a frightening situation without a confident answer that we can prevent it.  This scenario will occur; it is only a matter of how soon.  Our proposed answer is to be ready for it as soon as possible by already having demonstrated the capability.  To do otherwise would be unthinkable.