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(meteorobs) Excerpts from "CCNet 106/2000 - 19 October 2000"
------- Forwarded Message
From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet, 19 October 2000
Date: Thu, 19 Oct 2000 10:35:28 +0100
CCNet 106/2000 - 19 October 2000
--------------------------------
(1) TEXAS FIREBALL IDENTIFIED AS RUSSIAN SATELLITE RE-ENTRY
Mark Kidger <mrk@ll.iac.es>
(2) WEEKEND METEORS
NASA Science News for October 18, 2000
(3) EROS IS NASA'S LITTLE PRINCE
Larry Klaes <lklaes@bbn.com>
(4) NEAR AT EROS: DANCING THE TANGO
Ron Baalke <baalke@zagami.jpl.nasadot gov>
[...]
=================================================================
(1) TEXAS FIREBALL IDENTIFIED AS RUSSIAN SATELLITE RE-ENTRY
>From Mark Kidger <mrk@ll.iac.es>
Dear Benny:
The bright fireball seen over Texas, Oklahoma and Kansas on the 13th has now
almost certainly been id
entified as the re-entry of the Proton booster of a
Russian Glonas satellite. The groundtrack of the boos, calculated by Alan
Pickup
http://www2.satellite.eu.org/sat/seesat/Oct-2000/0235.html
agrees well with the estimated groundtrack of the fireball.
Curiously, a NORAD spokeperson has suggested that the fireball may have been
an early Orionid meteor, but this now seems highly unlikely.
More detailed information on the event can be found at
http://meteors.com/cometlinear/index.shtml
Unlike the Los Alamos fireball reported on CCNet recently, this event is
almost certainly man-made.
Mark Kidger
=================================================================
(2) WEEKEND METEORS
>From NASA Science News <snglist@lyris.msfc.nasadot gov>
NASA Science News for October 18, 2000
http://science.nasadot gov/headlines/y2000/ast18oct_1.htm?list20392
October 18, 2000 -- Last Friday, the 13th of October, thousands of
high-school football spectators were gathered in outdoor stadiums across the
Midwestern US when something happened to distract even diehard fans from
action on the field below.
With no warning, a fiery meteor as bright as the full Moon streaked over
Texas, Oklahoma, and Kansas around 7:30 p.m. local time. It was a halftime
show from the heavens!
"At first I thought it was a high-flying aircraft with its landings light
on," recounts Alex Leslie, who saw the meteor from a Hill City, Kansas,
football game. "As it passed almost directly south of us, it separated into
about five burning points, mainly white but some green hues, too. A 'smoke
trail' lingered for some time after the object passed. It took about five
minutes to cross the entire sky."
Emergency phone lines were jammed with UFO reports soon after the sighting,
but in this case the strange lights were not from outer space. The object
that slowly burned across Texas skies had left Earth only hours before.
Earlier on Friday a trio of Glonass satellites lifted off from the Baikonur
Cosmodrome in Kazakstan aboard a single Russian Proton rocket. Glonass is
the Russian equivalent of the American Global Positioning System. The
successful launch added three new satellites to the Glonass array and,
unintentionally, triggered the Friday night sky show over Texas.
"In my opinion, the [Texas fireball] was the re-entry of the Proton rocket's
4th-stage casing," says Alan Pickup, a satellite decay expert who works at
the United Kingdom's Astronomy Technology Centre at the Royal Observatory in
Edinburgh. "It was a cylinder 3.7m in diameter and 4m long that weighs some
800 kg."
"The object had passed through perigee (closest approach to Earth) at 7:19
p.m. Central Daylight Time (00:19 UT) when it was over the eastern Pacific
en route to the Mexico coast. It would have passed 3.1 degrees west of
Abilene, Texas, at 7:25 p.m. (00:25 UT) and almost directly over Lubbock,
Texas, 19 seconds later. Its track continued over Oklahoma and Kansas
towards Lincoln, Nebraska, which it would have reached at about 7:27 p.m.
local time were it still in orbit."
But there is a real meteor shower in store this weekend for erstwhile star
gazers inspired by Friday's fiery display.
Earlier this month our Earth's orbit carried our planet into a diffuse
stream of dusty debris from Halley's comet. Until now we've been in the
rarefied outskirts of the debris field, but we're heading for denser parts.
Tiny bits of Halley dust that burn up in our planet's atmosphere will
produce a meteor shower, called the Orionids, that peaks this weekend,
October 21st and 22nd. Orionid meteors won't be nearly as bright as a
decaying Proton rocket shell, but the display should be nonetheless
pleasing.
Earth passes close to the orbit of Halley's comet twice a year, once in May
and again in October. Although the comet itself is very far away --
presently beyond the orbit of Jupiter -- tiny pieces of Halley are still
moving through the inner solar system. These particles are leftovers from
Halley's close encounters with the Sun every 76 years; each time the comet
returns, solar heating evaporates about 6 meters of ice and rock from the
nucleus. The debris particles, usually no bigger than grains of sand,
gradually spread along the comet's orbit until it is almost uniformly filled
with tiny meteoroids. When these meteoroids strike Earth's atmosphere they
produce the Orionid meteors in October and the eta Aquarid meteors in May.
No matter where you live, the best time to see Orionid meteors will be
during the hours before dawn on October 20th through 23rd. Rural observers
should enjoy as many as 20 shooting stars per hour. During this year's broad
peak, centered approximately on Oct. 21st, the light of the waning quarter
Moon will make faint meteors hard to spot; pre-dawn observers on the 22nd
and 23rd may have better luck with diminishing moonlight.
Orionid meteoroids are fast. They hit the atmosphere at a head-spinning
velocity of 90,000 mph. There's no danger, though, because the tiny specks
of dust disintegrate well above the stratosphere. True to their name, the
Orionids will appear to stream from the constellation Orion, which is high
in the southern sky before dawn. The best place to look for meteors is not,
however, directly toward Orion. Orionids can appear anywhere in the sky,
with tails that point back to the shower's radiant above Orion's left
shoulder. Experienced meteor watchers suggest looking 90 degrees away from
the constellation -- that's usually the best direction to watch Orionids fly
by. Also, try to choose a dark area of the sky away from the bright Moon.
Even if the usually-reliable Orionids fail to produce a pleasing show,
there's still plenty to see. October's glittering pre-dawn sky includes
Jupiter, Saturn, and the brightest star of all, Sirius -- all near to the
Orionids radiant. Waking up early for this weekend's meteors is a no-lose
proposition!
=================================================================
(3) EROS IS NASA'S LITTLE PRINCE
>From Larry Klaes <lklaes@bbn.com>
>From Boston Globe, 17 October 2000
http://www.boston.com/dailyglobe2/291/science/Eros_is_NASA_s_Little_Prince+.
shtml
By Chet Raymo, Globe Correspondent, 10/17/2000
Antoine de Saint-Exupery's "Little Prince" lived on an asteroid scarcely
larger than himself. As readers of the childhood classic will remember, his
companions were a sheep and a rose, and some baobab seedlings that he
carefully weeded, lest they grow into giant trees that would split his tiny
world. The asteroid had three volcanoes, two of which were active, and all
of which the Little Prince assiduously cleaned.
A charming little world, but of course scientifically implausible. An
asteroid the size of the Little Prince's would not have enough internal heat
to cause volcanic activity, nor enough gravity to hold an atmosphere. Water
too would be absent, and surface temperatures would be either too hot or too
cold for comfort.
For his journey to Earth, the Little Prince took advantage of the migration
of a flock of wild birds, to which he attached himself with cords. On a
world as small as his, birds might well provide enough propulsive power to
effect an escape - if only there were air in space in which to fly.
Where children fly in their imaginations, NASA takes us in reality. On Feb.
17, 1996, the NEAR, or Near Earth Asteroid Rendezvous spacecraft was
launched on a four-year voyage to the asteroid, Eros. Not a flock of wild
birds but a Delta rocket with nine strap-on boosters lifted the car-sized
craft away from Earth and sent it on its way.
Eros is not so far away. It doesn't circle in the asteroid belt between Mars
and Jupiter, but on an eccentric orbit that takes it nearly as close to the
sun as Earth, and out just past Mars. Eros was the first near-Earth asteroid
to be discovered, and the second biggest. It is a potato-shaped chunk of
rock about the size of Martha's Vineyard. Not as small as The Little
Prince's world, but small enough to circumnavigate in a brisk day's walk.
NEAR's voyage to Eros took four years. A three-year journey was planned, but
the first attempt to put the spacecraft into orbit around the asteroid
failed. An extra year's travel gave engineers time to trim their skills and
calculations. And it allowed NEAR to rendezvous with an asteroid named for
the god of love on Valentine's Day 2000.
An object as small as Eros doesn't have much gravity to hold a spacecraft in
orbit. The Little Prince would weigh about an ounce on Eros, and he could
launch a stone into space with a swing of his arm. NEAR is bound to Eros by
a slender gravitational thread, and slipping the spacecraft into the thrall
of the asteroid was a tour de force of remote navigation.
Since April, NEAR has been orbiting just above the lumpy surface of Eros,
sending back stunning pictures of a gray and lifeless world without air or
water (http://near.jhuapldot edu). No sheep or baobabs, but lots of impact
craters and scattered boulders. We are catching a glimpse into the early
history of the solar system, when scattered dust and gas was gathered into
larger and larger chunks of rock, some of which would eventually coalesce to
form the planets, and others that were destined to drift through space like
gloomy Flying Dutchmans.
That's exactly the impression one gets from the NEAR photos of Eros. It's as
if a manned boat had pulled aside the spectral galleon of seafaring myth,
hailing without answer for a human response. The lifeless asteroid sails on
in dusty solitude, grimly colorless and deathly silent, reminding us of just
how extraordinary and rare is life in the universe.
A recent issue of the journal Science brought us the first summary reports
from NEAR. We have the mass, shape, rotation rate, elemental composition,
and even something of the asteroid's internal structure. The potato-shaped
rock named after the god of love is no longer just a speck of light in a
telescope; it is now a world as exactly accessible to our imaginations as
the Little Prince's domicile.
Astronomer's have charted about 250 near-Earth asteroids, and there may be
as many as 1,000 with a diameter of a half-mile or more. A few of these are
probably destined to collide with Earth at some time in the future, as other
asteroids have done in the past. If something like Eros came smashing our
way, we would be in big, big trouble indeed. Fortunately, Eros is on an
orbit that will keep it safely out of the way.
But NASA scientists plan to get our licks in first, anyway. Next year, two
days before the first anniversary of the Valentine's Day rendezvous, NEAR
will be caused to collide with Eros while the scientists listen - one last
attempt to extract information from an asteroid that has been clobbered
enough in its long history to withstand a gentle assault from Earth.
Chet Raymo is a professor of physics at Stonehill College and the author of
several books on science.
This story ran on page F2 of the Boston Globe on 10/17/2000.
) Copyright 2000 Globe Newspaper Company.
=================================================================
(4) NEAR AT EROS: DANCING THE TANGO
>From Ron Baalke <baalke@zagami.jpl.nasadot gov>
http://near.jhuapldot edu/news/sci_updates/00oct17.html
NEAR Shoemaker Science Update
October 17, 2000
Dancing the Tango
NEAR is now eight months into its year-long rendezvous
with Eros. We have seen Eros from as low as 35 km orbit
for about ten days back in July, but then returned to
higher altitude. For the last five weeks, we have been
mapping Eros from 100 km orbit, but we are now preparing
for our closest descent yet, a 6 km flyover scheduled
for October 25, 2000. Why has NEAR Shoemaker performed
this elaborate tango with Eros? One answer is that we
chose one of the most attractive partners on the dance
floor, and we have to pay the price if we wish to dance
very close - it costs extra fuel and requires frequent
maneuvers. But another reason for dancing both up close
and farther out is that we scientists want it that way.
The design of the mission has been the result of a
complicated interplay between science and engineering
requirements. To start with, NEAR Shoemaker was designed
as a simple spacecraft, with fixed antennas, fixed solar
panels, and fixed instruments. This simplicity makes for
a more reliable and robust spacecraft, but it also
places operational constraints on the mission. The
spacecraft must always keep its solar panels pointed at
the sun - it cannot survive even for an hour without
solar power, not only because the electronics would stop
operating, but also because instruments and subsystems
would be damaged by cold and the fuel would quickly
freeze. The instruments must be pointed at the asteroid
in order to acquire data. The main antenna must be
pointed at Earth to send data back to Earth at high
rate, although the spacecraft status can be monitored
and the spacecraft can be tracked at lower data rates
using other antennas, even when the main antenna is not
pointed at Earth. If we had designed a more complex
spacecraft, we could have lifted many or all of the
operational constraints, but it would have cost more.
And indeed these restrictions complicate the day-to-day
operations of the spacecraft, but it turns out that the
ever-vigilant enforcement of simple rules is a task
better suited for computers than for humans, and this
task is largely automated for NEAR. Paradoxically, the
use of a simple spacecraft leads to an overall
simplification of mission operations, despite
operational restrictions, because there are fewer
options to be studied.
In any case, the spacecraft design constrains NEAR
Shoemaker to fly in an orbit plane that is within about
30 degrees of perpendicular to the line from Eros to the
Sun. The spacecraft can then keep its solar panels
pointed at or close enough to the Sun at all times,
while for 16 hours a day it keeps the instruments
pointed at Eros for data taking, and for 8 hours a day
it points the main antenna at Earth for data
transmission. This tight constraint on the orbit plane
at Eros, plus the constraint that the orbit is flown at
a particular time, already fairly well settle three of
the six parameters required to specify an orbit
completely. In some sense the orbit is now almost
halfway designed, although in real life our engineers
determine these parameters to 10 decimal places. Those
of us who don't actually have to fly the spacecraft can
afford to take a more relaxed attitude. The remaining
three orbit parameters deal with how low and how high
the orbit goes, and precisely where it dips low.
That is where science comes in, although even at this
point operational constraints are still critical. Some
of our science operations are best performed at higher
altitudes, while others require that the spacecraft be
at low altitude. For instance, our imaging team desires
to map the whole asteroid under a variety of lighting
conditions and from a series of orbit radii,
specifically 200 km, 100km, and 50 km. In addition, the
team requires both monochrome (black-and-white) and
color imaging, and the ideal lighting conditions for the
one are not ideal for the other. For monochrome images,
we prefer the sun to be low in the sky so that shadows
accentuate the structures, whereas for color images we
prefer the sun to be higher to reduce the shadowing. In
addition, there are seasons on the asteroid. Until late
June of this year, portions of the southern hemisphere
never came into sunlight at all. The reverse is now true
in the southern hemisphere summer, when the north polar
region is always in the dark. Beyond all these
requirements, the x-ray and gamma ray teams need to have
the spacecraft in low orbits of 50 km or less as long as
possible to achieve the highest possible signal-to-noise
ratio. Also the laser rangefinder team obtains the
highest resolution and measurement accuracy in the low
orbits, and the study of the asteroid's interior
structure, through determination of its gravity and
magnetic fields, achieves the highest sensitivity in the
low orbits.
So there are many science tasks that require low orbits,
but there are also science tasks that require high
orbits, and in both cases, the spacecraft is required to
fly over all parts of the asteroid at the altitudes in
question. In addition, particular solar illumination
geometries are often required, such as for color and
spectral observations as well as the x-ray measurements.
Hence the choices of how low to fly, and where, and
when, are complicated, and we really needed to spend a
full year at Eros. Moreover, there are engineering
requirements which derive from orbit stability. The
spacecraft cannot be put into an orbit that would be so
unstable that we could not predict with sufficient
accuracy where it would be a week later, or so unstable
that, if for any reason we could not contact the
spacecraft or correct its orbit for a week, it would
crash or escape from the asteroid. Furthermore, we avoid
orbits that would require excessive fuel expenditures or
corrective maneuvers more often than once a week.
Finally, we cannot send the spacecraft into an orbit
that would carry it into the shadow of the asteroid,
where the Sun would be eclipsed and the spacecraft would
die.
Generally speaking, the orbits we need to worry about
are the low orbits. As we discussed earlier (April 18,
2000), the irregularity of an object's shape produces
greater and greater distortions of its gravity field,
the closer one approaches to the object. At large
distances from any object, its gravity field becomes
monopolar and spherical, so higher orbits tend to be
better behaved in terms of being more like ordinary
elliptical orbits. There is a caveat, which is that the
gravity from the object must remain the biggest force
field around; if we get too far from the object, then we
also have to worry about other forces like solar gravity
and radiation pressure, and orbits become complicated
again. For Eros, too high in this sense means above
about 1000 km. Hence the 200 km orbits are fairly stable
and ordinary - that is, not too close and not too far.
The 50 km orbits, on the other hand, are close enough to
be strongly perturbed by the irregular shape of Eros.
The most serious disturbance is that the orbit plane is
continually torqued around (that is, it precesses), so
it would quickly violate the operational constraint we
started with unless we perform maneuvers to correct the
orbit. In other words, we need to fire the rocket
engines to keep the orbit plane within the allowed
angles to the line from Eros to the Sun. It turns out
that the precession rate depends on the orbital
inclination to the Eros equator as well as the orbital
radius.
The upshot is, there are only certain times of year when
NEAR Shoemaker can fly in 50 km orbits or lower, without
using too much fuel or putting the spacecraft at too
much risk. Even so, we have no choice but to get up
close to Eros to make the measurements we need. This is
why we are dancing a tango with Eros, sometimes close,
and sometimes far. Like the real tango, our dance with
Eros has been exciting, full of mystery, and much hard
work - and more is still to come. Our closest view of
the surface to date is eight days away.
Andrew Cheng
NEAR Project Scientist
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