Extinction asteroid (10-100 km) impact avoidance

Question

With the present "state-of-the-art" of space technology what is possible to do to avoid an Earth impact by rocky bodies 10 km to 100 km in diameter?

Is there any international project to develop a planetary defense system in the near future?

Answer 1

The threat posed by such near-Earth objects can be illustrated by the most famous extinction-level asteroid, the one responsible for the Chixclub crater of the Yucatán Peninsula. This bolide, at least 10 km wide, is almost universally credited with the demise of the dinosaurs.

Defensive measures against such objects entirely depends upon finding it in time to do something about it. Should we discover an asteroid on an impact trajectory only months before D-Day, we could do little except hunker down. From the wiki on Asteroid impact avoidance:

“REP. STEWART: ... are we technologically capable of launching something that could intercept [an asteroid]? ... DR. A'HEARN: No. If we had spacecraft plans on the books already, that would take a year ... I mean a typical small mission ... takes four years from approval to start to launch ...”
—Rep. Chris Stewart (R,UT) and Dr. Michael F. A'Hearn, 10 April 2013, United States Congress(2)

Most deflection efforts for a large object require from a year to decades of warning, allowing time to prepare and carry out a collision avoidance project, as no known planetary defense hardware has already been developed.

The best place to go for data on dangerous NEOs is NASA/JPLs Sentry Risk Table. This upcoming Thanksgiving, when the news media starts talking about 2007 VE191, you can bet they'll be interviewing someone from the Sentry team (fortunately this object has only a 1 in 63,000 chance of striking Earth).

As mentioned by TildalWave in his comment above, the B612 Foundation is a private nonprofit dedicated to planetary defense against near-Earth object (NEO) impacts. In addition to publicizing the rate and magnitude of the problem posed by planetary impactors, it assisted the UN in establishing the International Asteroid Warning Network. The B612 Foundation is also designing and building (with Ball Aerospace) a privately financed asteroid-finding space observatory, the Sentinel Space Telescope, to be launched in 2017–2018. From a heliocentric orbit it will use a supercooled infrared detector to survey space for potentially dangerous NEOs. An interesting article from their site by one of Sentinel's mission scientists, including a video simulation, can be found here.

There are various other surveys and databases trying to keep track of threats summarized here.

If we discover an extinction-size asteroid bearing down on Earth, and we have time to do something about it, should we call Bruce Willis? The pros and cons of the Armageddon approach, boring a hole deep into the asteroid and dropping in a nuke in order to shatter it, are pretty well addressed in Is it (or why is it) worse to break up a asteroid on a collision course with Earth? There may be more utility in a well-timed and positioned surface blast or even a standoff explosion.

Other collision avoidance strategies include:

• Kinetic impact, essentially ramming it with a spacecraft
• Gravity tractor, hovering a heavy spacecraft near an asteroid to gravitationally deflect it
• Conventional rocket engine attached to the asteroid
• Solar sails
• Laser ablation

Answer 2

A practical way to deflect large asteroids could be kinetic impact with a smaller asteroid. That smaller asteroid would in turn be deflected to collide with the larger asteroid using either a spacecraft that landed on it and used e.g. ion thrusters, or it could be done via a kinetic impact using a yet smaller asteroid. In the latter case, the accuracy of the deflection may be a problem, but it's fun to speculate about the possibility of having to deflect only a very small asteroid to make it hit a larger one which in turn will then hit an even larger one etc. etc...

Suppose that the Earth is going to be hit in $\tau = 2$ years time by an asteroid of size $D_a$, and that on average impacts by asteroids of size larger than $D_a$ happen once every $T = $ million years. To make the asteroid miss the Earth requires a change in its velocity of the order of $\frac{R}{\tau}$ where $R$ is the Earth's radius. Then from conservation of momentum, you can derive that diameter of the smaller asteroid $D_s$ that needs to collide with the asteroid we want to deflect is given by:

$$D_s = \left(\frac{R}{\tau v_s}\frac{\rho_a}{\rho_s}\right)^{\frac{1}{3}}D_a$$

where $v_s$ is the relative velocity of the small asteroid w.r.t. to the large one and $\rho_s$ and $\rho_s$ are the densities of the large and small asteroids, respectively. If we take $v_s = 30$ km/s, then for the assumed figures this works out as approximately $D_s = 0.015 D_a$.

The question is then what the typical closest approach of an asteroid with diameter $D_s$ to the large one is within a time frame of the order of $\tau$, as this determines by how much we need to deflect a smaller asteroid to hit the large one. The cumulative size distribution of asteroids $N(D)$ decays approximately as a power law:

$$N(D)\sim D^{-1.32}$$

The number of asteroids with diameters larger than $D_s$ is thus larger than the number of asteroids with diameters larger than $D_a$ by a factor of 256. If the Earth will on average collide with an asteroid of size $D_a$ or larger once every $T = $ million years, then a closest approach of an asteroid of diameter larger than $D_s$ within Earth's radius of the large asteroid will happen once every $\frac{T}{256}\approx 4000$ years. The typical closest approach within a two year period will thus be of the order of $r$ such that $\left(\frac{r}{R}\right)^2 = 2000$, so the small asteroid will typically have to be deflected by a distance of the order of 45 earth radii to make it hit the large one.

While the deflection needed is 45 times larger, the mass of the smaller asteroid is much smaller, about 300,000 times smaller, so it's much more feasible to achieve than deflecting the large one directly.