||Earth-Impactor Mitigation Methods
by Germano D'Abramo (*) - copyright
The need to avoid the impact of an asteroid with the Earth has
led to what is now known as mitigation strategy. There are
two basically different approaches to the problem: the change of
the asteroid's orbit (deflection) or the asteroid fragmentation
and its dispersal.
The fragmentation procedure appears to be risky and in some
cases even impossible for at least two reasons:
- it would require huge amounts of nuclear explosive to be
put in orbit and this cannot be done without some risk
for the Earth environment.
- given our current knowledge, it is very difficult to
predict the right amount of energy required to completely
fragment and disperse the asteroid, and even if we
succeed in this operation we cannot exclude that the
great bulk of the asteroid's fragments falls on the Earth
anyway (see actual issue of
T.S. "We only need a little, gentle kick ..."
by Andrea Carusi.
Moreover, the fragmentation procedure is impossible in some
cases, for example:
- the asteroid is too big for the available nuclear
explosive here on the Earth (it is estimated that in
order to fragment an asteroid 0.1, 1.0 and 10 km wide
nuclear charges are necessary with energies of the order
of some Kt, Mt and Gt (dict.),
- in order to produce optimum fragmentation, the charge
should be buried in the asteroid, with obvious (currently
insuperable) technical difficulties.
Can a nuclear explosion be used to
deflect an impactor?
Concerning the deflection procedure, it suffices to aptly
modify the orbital velocity of the impacting body along its
revolution around the Sun. For the sake of simplicity, we could
say that the velocity change must lead the asteroid gain ground
(or lose it, depending on if we increment or decrement its
orbital velocity) with respect to the motion of the Earth during
the time span between the application and the predicted epoch of
collision of an amount of at least one Earth radius. Therefore,
it is clear that the longer the warning time before the epoch of
the impact, the less the magnitude of the velocity change
required for the same deflection. Namely, in most cases it is
exactly an inverse proportion between warning time and magnitude
of velocity change, if we obviously ignore such peculiar cases
like that described by A. Carusi in this issue of T.S..
The velocity change can be impulsive or steady. It is
impulsive if it is delivered "instantaneously'', in one
solution. The velocity change is steady when a constant thrust is
applied to the asteroid for a longer time span, which could be
even equal to the warning time before the impact.
Within the known mitigation strategies there are the following
- Asteroid-Asteroid collisions: this method consists
in changing the orbit of a small harmless asteroid so
that it will collide with a larger asteroid on collision
course with the Earth and change its orbit.
- Nuclear explosives: nuclear explosions could be
used in two different ways: (1) as a stand-off explosion
at some distance from the asteroid surface; the flash
produced by the explosion vaporises the exposed side of
the asteroid. The surface material will therefore
instantaneously spall away delivering an impulse to the
rest of the asteroid (see image above). (2) as an
explosion directly on the asteroid surface; such
explosion excavates a huge crater and ejects its material
away from the asteroid. In this case, the deflecting
impulse is provided by the recoil from the ejected mass.
As for the fragmentation and dispersal strategies, some
perplexities arise in these cases: such ``energetic''
approaches to deflection seem to be very risky because
they are not so much controllable.
Among the steady mitigation methods there are:
- Chemical, electric or nuclear propulsion:
conventional chemical propulsion system, electrical or
nuclear fission engines could be attached to the
asteroids and fired when the thrust vector points to the
desired direction (due to the asteroid's own rotation).
- Laser systems: in this case the asteroid surface
is irradiated by an high energy laser beam (ground-based
or space-based). This beam would vaporise surface
material which will stream away from the asteroid
- Mass drivers: mass drivers are devices which has
to be installed on the asteroid surface. They excavate
and accelerate away from the asteroid gravitational field
small mass packages. The recoil produced by this
expulsion provides the thrust required to deflect the
asteroid from its pristine orbit.
- Non-gravitational forces: this method consists,
for instance, in covering the asteroid surface partially
or completely with some high-reflectivity material (e.g.
white powder, see
T.S. number 5: "The sweet solution" by Andrea
Milani). This material would enhance the thrust given
by the solar radiation on the asteroid surface.
Nevertheless, the effectiveness of this approach is
rather scanty, even with very small asteroids (e.g. of
the order of 10 meters), and pretty long warning times
are needed to reach a sensible deflection.
- Mirrors and solar sails: for what concern solar
mirrors, they essentially act like laser system. A
suitable mirror, orbiting around the asteroid, collects
solar radiation and focuses it onto the asteroid surface.
This high energy concentration vaporises the surface
material creating a thrusting stream. A different way to
take advantage of solar radiation is to create huge
mirror sails and to attach them to the asteroid. In this
case, the thrust needed for deflection is provided by the
solar light pressure. It is pretty clear that this
strategy suffers, more than others, from many technical
problems; for example, the sail area has to be at least
in the order of many square kilometers to provide a
a mean of deflection?
By the way, all these methods were never tested or applied for
mitigation actions. They were only theoretically handled.
Besides, as already mentioned, the actual accomplishment of some
of these mitigation methods seems out of reach even in the
distant future, due to their insurmountable technical problems.
Obviously, the hope is that we would never need such remedies,
but, as the old saying goes, hope the best, get ready for the