NEAR EARTH ASTEROIDS
Impact Threat Comes with Silver Lining
ASTRONOMY MAGAZINE
22 JULY 2002
Of all the reports in recent years about possible asteroid or comet
impacts, one published in Science magazine this April stands out for
having by far the highest likelihood of a collision found to date: a
1-in-300 shot. And with a diameter of 0.7 mile (1.1 kin), the asteroid
1950 DA could cause global devasta- tion if it does strike Earth. Yet
the impact date remains so far off (there is no possi- bility of it
hitting before March 16, 2880) that not only are astronomers not
worried, but the find may even prove a scientific bonanza — and a threat
that contains the seeds of its own solution. The most amazing aspect of
this forecast, made by a
14-member team led by Jon Giorgini of NASA’s Jet Propulsion Laboratory,
is that such a long-term pre- diction was possible at all. With most
near-Earth aster- oids, the uncertainties build up over time so that
mean- ingful forecasts can’t be made more than about a century ahead.
But with asteroid 1950 DA, several factors help to pin down its future
path. First, as its name indicates, it was discovered more than a
half-century ago, provid- ing a longer-than-usual baseline of
observations. Second, its orbit is inclined to the ecliptic, reducing
the number of perturbing encounters with planets and other asteroids.
And finally, it has a never-before-seen kind of gravitational reso-
nance that tends to force it back toward its original orbit anytime a
perturbation tugs it one way or the other. There’s another unusual
twist to the story of this possible doomsday asteroid: The greatest
uncertainty in its future motion comes from the way it reradiates
absorbed solar radiation, an obscure phenomenon called the Yarkovsky
effect. This arises because energy absorbed on the sunward side radiates
away on the dark side, imparting a tiny momentum to the asteroid that
depends on its exact spin rate and orientation, composition, and even
surface color — none of which are known. The bad news is that this
means it may be impossible to improve estimates of the impact odds until
very near the encounter date, although close-up examination by
spacecraft could help. The good news is that the same effect might be
harnessed someday to gently nudge this or any other asteroid aside if it
does turn out to be on a collision course. According to a study by
University of Arizona planetary scientist Joseph Spitale, simply
altering the surface reflectivity, for example by dropping white chalk
or black carbon powder on the surface or using a huge reflective Mylar
sheet to wrap the asteroid like a giant baked potato in aluminum foil,
could be enough to change its path (though it might take centuries).
The prospect of using a silvery coating to save us from destruction may
be the silver lining in this potentially threatening dis- covery. —
DAVID L. CHANDLER Radar reveals the menacing asteroid 1950 DA. steve,
Ost,,
22 ASTRONOMY JULY 2002 V
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