Radar reveals asteroid force

R. A. Hettinga rah at shipwright.com
Sat Dec 6 08:29:12 PST 2003

More important, a way to calculate an asteroid's mass, and thus its density
and probable content.

This is probably the most important discovery in asteroid mining in a very
long time.





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Radar reveals asteroid force
5 December 2003

Astronomers in the US and the Czech Republic have observed a tiny
non-gravitational force called the Yarkovsky effect in an asteroid for the
first time. The team also notched up another first by measuring the mass of
an individual asteroid. The results will lead to better tracking of the
orbits of asteroids in the future (S Chesley et al. 2003 Science 302 1739).

The basic idea behind the Yarkovsky effect is that the surface of an
asteroid is heated by the Sun during the day, and then cools off during the
night. This means that the asteroid tends to emit more heat from its
"afternoon side", and less from the opposite side. The recoil from the
afternoon side is therefore larger as well, and the overall effect is that
a tiny, non-gravitational force acts on the asteroid.

The acceleration caused by this force is also tiny, but over millions of
years its effect could become large enough to displace an asteroid from its
natural orbit around the Sun. This might be enough to push the asteroid
inwards from the "main belt" - which lies between Mars and Jupiter - and
towards the Earth.

Steven Chesley of the Jet Propulsion Lab (JPL) in California and colleagues
at JPL, Charles University in Prague, the Arecibo Observatory in Puerto
Rico and the University of California at Los Angeles have now studied how
the orbit of asteroid 6489 "Golevka" has changed over a period of 12 years.
They analysed radar data taken using telescopes at Arecibo and the
Goldstone Observatory in California, and found that the Yarkovsky effect
has changed Golevka's orbit by about 15 kilometres since 1991. Chesley and
co-workers also calculated that the asteroid is just 530 metres in
diameter, weighs about 210 billion kilograms and has a bulk density of 2.7
grams per cubic centimetre.

"Never before has the mass of a small solitary asteroid been measured,"
Chesley told PhysicsWeb. "This allows us to determine the object's density,
which provides clues to its internal structure. In this case it suggests
that the asteroid has a heavily fractured interior."

The US-Czech team now plans to study other asteroids and hopes that its new
method will eventually become routine when tracking these objects. The
astronomers also believe that measuring the strength of the Yarkovsky
acceleration is the only way to determine the mass and density of small
(sub-kilometre) asteroids from Earth.

R. A. Hettinga <mailto: rah at ibuc.com>
The Internet Bearer Underwriting Corporation <http://www.ibuc.com/>
44 Farquhar Street, Boston, MA 02131 USA
"... however it may deserve respect for its usefulness and antiquity,
[predicting the end of the world] has not been found agreeable to
experience." -- Edward Gibbon, 'Decline and Fall of the Roman Empire'

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