When it
comes to 22-million-ton asteroids, the small stuff, it turns out, can make a
huge difference in a potentially disastrous path toward Earth.
Using
limited observations and lots of high-end computer modeling, astronomers have
gotten a better handle on the limitations of asteroid-track forecasting in a
new study of a potentially threatening asteroid called
Apophis. In this high-stakes game of Whack-a-Cosmic-Mole, just knowing
exactly what it is you don't know can be useful.
Apophis'
chance of hitting
our planet in 2029 is now slim to none, but astronomers will have to wait
four to six years before they can predict what it might do during a second pass
in 2036. A team of scientists arrived at the conclusion after accounting for
small influences like the solar wind, gravitational drag of smaller asteroids
and human error.
The rock is
between 690 and 1080 feet (210 and 330 meters) wide. Until better measurements
lower the uncertainties, its predicted path could be severely off, potentially
thwarting proactive efforts to change its course.
Uncertain
future
In 2004,
astronomers gave Apophis a frightening one-in-37 chance of striking
the Earth in 2029, which later measurements from Puerto Rico's giant
Arecibo radio observatory knocked down to a one in 45,000 likelihood.
Apophis is
expected to make a comfortably distant encounter of about 30 million miles (47.9 million
kilometers) with Earth in 2036. But its first path-bending pass in 2029—about
three Earth diameters of our home—and other unknowns could greatly alter the
asteroid's second approach seven years later.
In their
study, to be detailed in an upcoming edition of the journal Icarus,
astronomers examined hundreds of potential courses with computer simulations,
then hashed out course-altering uncertainties after the first encounter. The largest
such source of error is the sun's ability to push small asteroids around with
solar radiation; in Apophis' case, up to 18.6 million miles (30 million
kilometers) or about 2,350 Earth diameters off-course.
Other
sources of course prediction error include:
- Small
uncertainties in planetary masses and position, up to 11.5 Earth diameters
of error.
- Earth's
imperfect spherical shape, up to 1.5 Earth diameters of error.
- Gravitational
influence of small asteroids, up to 1.7 Earth diameters of error.
Astronomers
said looking at the rock via telescope is necessary to cut out most of the
error, but won't be possible until 2011 or 2013, when Apophis comes out of
hiding from behind the sun. Noting the shape, colors and rotation, not just
current position, should cut out up to 97 percent of the uncertainty about the
asteroid's future track.
A little
goes a long way
Until then,
scientists are dreaming up of cost-effective ways to redirect Apophis before
2029, should the need arise.
One recently proposed
solution calls for strapping on a small reflective patch to the rogue body. By
adding a 130-by-130 foot (40-by-40 meter) section of material that acts like a
solar sail, Apophis could be pushed away by at least one Earth radius, or about
4,000 miles (6,400 kilometers)—more than enough to avoid cataclysm.
The reflective material doesn't have to be stuck in one place, either. Spreading a shiny mesh of carbon fibers over Apophis might nudge it away from Earth's path for good.
But a lack
of good observations, astronomers warn, could turn such proactive deflections
into disaster.
"Without
such performance margin," the authors said, "the deflection action
would instead create an unpredicted outcome or a new hazard."
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