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(meteorobs) Excerpts: CCNet, 66/2000 - 8 June 2000
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From: Benny Peiser <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet, 8 June 2000
Date: Thu, 8 Jun 2000 15:23:23 +0100
CCNet, 66/2000 - 8 June 2000
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(1) THE DOG STAR'S BONE?
The Guardian, 8 June 2000
[...]
(5) NEW ROTATIONAL PERIODS OF 18 ASTEROIDS
C. Blanco et al., *)UNIVERSITY OF CATANIA
[...]
(7) SATELLITE COLLISION RISK
A. Rossi et al., CNR,IST CNUCE,AREA RIC PISA
[...]
=======================================================
(1) THE DOD STAR'S BONE?
>From The Guardian, 8 June 2000
http://www.guardianunlimited.codot uk/science/story/0,3605,329559,00.html
Duncan Steel looks at an astronomical anomaly among the asteroids
Thursday June 8, 2000
Astronomers often speak of the cosmic zoo, reflecting the exotic variety
of phenomena to be seen in the deep space. Pulsars, colliding galaxies
and superluminous quasars are just a few of the bizarre images their
telescopes pick up. But right here in the solar system, we still have a
few surprises in store. Such as an asteroid shaped like a dog's bone.
Cartoonists tend to draw asteroids as rocky spheres pockmarked by
craters. They're halfway correct. Asteroids not only slam into the moon
and planets producing craters, but they're also holed themselves by
smaller projectiles.
But spherical? Only the very largest asteroids - bigger than a couple of
hundred miles across - have sufficient gravity to pull themselves into a
completely rounded shape. Smaller bodies are expected to be irregular in
profile. But just how irregular?
Our vantage point is so far from the main belt between Mars and Jupiter
that only the largest asteroids can be resolved. Using the Hubble Space
Telescope, we find that the behemoths such as Ceres (at 600 miles
diameter the largest asteroid), Pallas and Vesta are close to being 7938
6285 spherical, but we know little about the smaller rocks.
While on its trip to Jupiter Nasa's Galileo spacecraft returned close-up
photos of Gaspra and Ida, showing these to be angular and elongated,
like pebbles. No surprises there. Similarly, since the NEAR-Shoemaker
space probe entered orbit around asteroid Eros in February, it has sent
back vivid photographs of that body indicating the anticipated rounded
but non-spherical shape, scattered with craters.
Another way of profiling an asteroid's shape is to use radar. Over the
past few years Nasa researchers led by Steve Ostro at the Jet Propulsion
Laboratory in California have taken the briefly available opportunities
to get radar bounces from various small asteroids whizzing close by the
Earth, revealing the expected rocky shapes. Some look like peanuts, some
like potatoes, others like footballs in need of a pumping up.
But the huge numbers of asteroids in the main belt, more than 100
million miles away, were too distant, out of the range of their radar
equipment. Too far away, that is, until the recent multi- million dollar
upgrade of the vast radar located at Arecibo in Puerto Rico was
completed, funded by the US National Science Foundation. This has
dramatically improved its sensitivity, making observations of more
distant objects feasible.
The Arecibo radar employs a dish 1,000 feet across, fixed in place in a
natural bowl-shaped valley to the north of the island. It is not
steerable in the way that most radio telescopes, such as that at Jodrell
Bank, can be directed to any point in the sky. So astronomers using
Arecibo need to choose targets that happen to pass overhead during their
observation runs.
The first main belt asteroid to be selected was Kleopatra. This object,
discovered in 1880, was not thought to be remarkable in any way. It was
just a suitably located target on which to test the new radar system.
Data from optical telescopes indicated a size of about 100 miles, and
its colour indicated a metallic composition. Because metals are good
reflectors of radio waves, a strong echo seemed likely.
This was a complicated experiment. Kleopatra was so far away - further
away than the sun from the earth - it took the radar pulse 19 minutes to
make the round trip. A strong echo was obtained, but it varied in a way
which initially perplexed the astronomers. As more data were collected,
it became possible to build up a picture of the overall shape of the
asteroid. It looks somewhat like a dumb-bell, 135 miles long and 58
miles wide. "With its dog bone shape, Kleopatra has the most unusual
shape we've seen in the solar system," commented Ostro.
The best guess is that the surprising shape may be the result of some
phenomenal inter-asteroid collision many aeons ago. The lumps thrown off
at high speed in that collision may then have made their way to our
planet, some of them ending up in museum collections of meteorites. Many
meteorites have nickel-iron compositions similar to the presumed make-up
of Kleopatra, making it a candidate parent body for at least some of
these samples. So maybe we've got some of the meat off the dog's bone
available for study in our laboratories.
Alternatively, rather than Kleopatra originally having been more
rounded, and disfigured by a cataclysmic collision, perhaps it gradually
accumulated debris to attain this form.
Another member of the team, Scott Hudson of Washington State University,
suggests that the asteroid "may once have been two separate lobes in
orbit around each other with empty space between, subsequent impacts
filling in the area between the lobes with debris." This idea has appeal
because the radar echoes also indicate Kleopatra to be somewhat porous,
rather than one solid lump of metal.
The shape of Kleopatra remains a mystery for the time being. If it tells
us anything, it's that we still have much to learn about asteroid
origins and collisions. We can expect nothing less than the unexpected
as more members of the asteroid zoo are unveiled in the coming years.
* Duncan Steel researches asteroids and comets at the University of
Salford.
Copyright 2000, The Guradian
=======================================================
(5) NEW ROTATIONAL PERIODS OF 18 ASTEROIDS
C. Blanco*), M. DiMartino, D. Riccioli: New rotational periods of 18
asteroids. PLANETARY AND SPACE SCIENCE, 2000, Vol.48, No.4, pp.271-284
*)UNIVERSITY OF CATANIA,IST ASTRON,VIALE A DORIA 6,I-95125
CATANIA,ITALY
The results of photoelectric observations of 18 main-belt asteroids are
discussed. The V-band lightcurves, the B-V colors, and the values of
the synodic rotational period are presented. There exists no previous
determination of the period for 13 of them. (C) 2000 Elsevier Science
Ltd. All rights reserved.
=======================================================
(7) SATELLITE COLLISION RISK
A. Rossi*), G.B. Valsecchi, P. Farinella: Collision risk for high
inclination satellite constellations. PLANETARY AND SPACE SCIENCE,
2000, Vol.48, No.4, pp.319-330
*) CNR,IST CNUCE,AREA RIC PISA,VIA ALFIERI 1,I-56010 PISA,ITALY
We assess the collision hazard for a constellation of telecommunication
satellites such as IRIDIUM, arising from the possible chance impact
break-up of one of the satellites. The resulting swarm of fragments
will orbit the Earth at about the same altitude as the surviving
satellites, but will gradually spread due to orbital perturbations, so
as to make possible impacts with satellites staying on orbital planes
different from that of the parent satellite. We find that at
intermediate fragment masses of the order of 1 kg, sufficient to
trigger subsequent catastrophic impacts, the self-generated collision
hazard for the constellation satellites exceeds the background level
due to the overall debris population for several years. This is true,
in particular, when differential precession of the orbits leads the
fragments to encounter satellites revolving around the Earth in the
opposite sense, resulting both in higher impact speeds and in enhanced
collision probabilities. We estimate that there is about a 10% chance
that a first-generation fragment will trigger subsequent disruptive
collisions in the constellation within a decade. (C) 2000 Elsevier
Science Ltd. All rights reserved.
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