Conference 7.286::space

868.0. "ESA's OLYMPUS Satellite" by VERGA::KLAES (Quo vadimus?) Wed Sep 15 1993 16:35

Article: 70616
Newsgroups: sci.space
From: MAILRP%[email protected]
Subject: message from Space Digest
Sender: [email protected]
Organization: [via International Space University]
Date: Thu, 26 Aug 1993 15:22:18 GMT
 
Press release nr. 40-93
Paris, 26 August 1993
 
OLYMPUS: End of mission
 
As indicated in the press release of 17 August, 1993, service from the
Agency's experimental OLYMPUS satellite was interrupted during the
night of 11/12 August when, for reasons which are not yet understood,
the satellite lost Earth pointing attitude and began spinning. This
event, and the subsequent recovery actions, used the last few
kilograms of fuel remaining on the satellite. An assessment of the
situation indicated that it would be impossible to re-establish service. 
It has therefore been decided that the Olympus mission should be 
terminated and the satellite removed from the geostationary orbital ring. 
 
Olympus entered its fifth year of operations in July of this year.
Throughout its mission, the satellite payloads were used by a wide
range of European and Canadian Organisations to develop and test
advanced satellite communications technologies and provide new
services. The results of these were the subject of an international
Conference held in April 1993, in Seville, Spain, which brought
together 300 scientists, engineers, educators and broadcasters, who
reported on their achievements using the satellite. The proceedings of
this conference, which contain the approximately 130 papers which were
presented, are now being distributed. 
 
Olympus carried four separate payloads, a two channel high power
direct broadcasting payload operating at 18/12 GHz, a four channel
12/14 GHz Specialized Services Payload, a 20/30 GHz payload for
advanced communications experiments, and a 12/20/30 GHz beacon package
for propagation experiments. 
 
The Ka band (20/30 GHz) transponders have been exploited extensively
by numerous organisations to demonstrate the use of small terminal
networks and for video conferencing and satellite news gathering
applications including daily transatlantic transmissions. These have
shown a high degree of service availability at these new frequencies.
In addition, this payload was used to constitute the geostationary end
of a data relay link between the Inter- Orbit Communications (IOC)
terminal mounted on the Agency's EURECA spacecraft and ground
controllers and experimenters during that highly successful recently
completed mission. 
 
Olympus has also been instrumental in the development of new
applications, such as distance learning, data distribution and new
commercial services. In the distance learning field alone, over 100
organisations, in 12 countries have used Olympus to develop training
courses which are now part of the established satellite based
educational infrastructure. Several of these operations have now been
transferred to the EUTELSAT space segment. 
 
In the broadcast field, Olympus was the initial test bed for a number
of satellite broadcast programmes, which are now running on a
commercial basis, including RAISAT and the BBC World Service. It was
also used for experimental high definition TV broadcasts, to assist in
the development of that new technology. 
 
Olympus provided valuable opportunities to collect propagation data
for hundreds of scientists across Europe and North America. The 20/30
GHz data, in particular, forms a major part of the statistical
information available globally. 
 
It will be remembered that control of OLYMPUS was accidentally lost in
the summer of 1991 and, following an uncontrolled orbital drift around
the world, in a frozen state at temperatures of some minus 60 degree
Celsius, the satellite was the subject of a spectacular recovery
action which allowed it to be put back into service again. The
recovery at that time, however, used a large amount of fuel, and
little was left on-board to complete the intended five year mission.
By instituting new procedures to conserve fuel, it was nevertheless
hoped that OLYMPUS could complete its nominal five year mission in
early 1994 and then still have a reserve amount of fuel that could be
used to re-orbit the satellite, i.e. remove it several hundred
kilometres from the geostationary orbital ring. This re- orbiting at
the end of a geostationary satellite mission is ESA policy in order to
eliminate the probability that a satellite left to drift in that orbit
could later strike or interfere with another satellite. 
 
The event which occurred on the night of August 11 made it impossible
to achieve the planned five years of operations. When the satellite
lost earth pointing, an automatic mode change was initiated, but the
on-board control loops were not immediately successful in despinning
and reorienting the satellite. The firing of thrusters caused an
orbital drift and used much of the remaining fuel. Actions then had to
be taken by the spacecraft controllers on the ground to over-ride the
automatic functions and manually control the satellite. These actions
were successful, but the amount of fuel assessed to still be available
would not have been sufficient to  re-establish three-axis controlled
attitude of the satellite, stop its orbital drift, return it to its
proper position in the geostationary ring and still retain a margin
for eventual withdrawal from the geostationary orbit. It was therefore
decided to initiate re-orbiting at once. In order to achieve this, it was 
first necessary to despin the satellite to less than about 2 deg/second. 
 
The re-orbiting process has been initiated and is being carried out
several hours per day. The satellite is now in an orbit with
respectively an apogee some 195 km, and a perigee some 390 km below
geostationary. It was decided to re-orbit to a lower rather than
higher altitude due to the fact that the incident itself caused an
orbital perturbation in this direction, and, consequently, there would
not have been sufficient fuel to retransfer the spacecraft to a higher
orbit. This activity will continue for several more days, in order to
reach the lowest possible orbit. Several end-of life tests will then
be conducted with the satellite, after which steps will be taken to
deplete the pressurant remaining in its tanks. Once the satellite is
in this "safe" configuration, it will be turned off and its mission
will have come to an end. 
 

Article: 3575
From: [email protected] (DAVE CRAVOTTA)
Newsgroups: clari.tw.space,clari.local.sfbay
Subject: Leonids meteor shower to light up the sky
Date: Fri, 12 Nov 93 13:43:11 EST
 
	SAN FRANCISCO (UPI) -- The annual evening light show of the Leonids
meteor shower begins Monday and lasts five nights.

	Meteors, also known as shooting stars, are flashes of light caused by
meteoroids, particles orbiting the sun that glow from the friction of
hitting Earth's atmosphere.

	Leonids are so called because they appear to radiate from the
direction of the constellation Leo, which is under the Big Dipper's
bowl. Optimum viewing time is late at night.

	Many meteoroids are the size of sand grains and smaller, but a few
range up to tens of yards or meters in diameter.

	``You don't want to use binoculars or a telescope because they'll
restrict your view,'' said Bing Quock of the Morrison Planetarium in San
Francisco. ``Meteors can appear anywhere in the sky.''

	Quock advised people to ``dress warmly, bring a lawn chair or spread
out a blanket and be patient. Meteors are best seen away from the glow
of city lights.''

	The Leonids are expected to peak at 4 p.m. EST Nov. 16, but the best
view will be from places in darkness at that hour, such as Moscow or the
Persian Gulf region.

	``The Leonids are among the fastest showers because they hit the
Earth head-on at 71 kilometers per second (44 miles per second),'' Quock
said.

	``This is a good year for the Leonids. The moon is going to be a
fairly slender crescent,'' so it won't reflect light that would block
the view of the meteors.

	Leonids particles come from the Tempel-Tuttle comet, which orbits the
sun once every 33 years. As it nears the sun, the comet's frozen, dirty
head heats up. Some of its ice changes to gas, releasing particles as a
trail of meteoroids that orbit the sun.

	Earth passes through this trail every year in November.

	In 1999, when the comet reaches the point in its orbit closest to the
sun, more particles will be released and astronomers expect the Leonids
shower to become a storm.

	During the last storm in 1966, 40,000 meteors per hour flashed during
a 40-minute period.

	This year the Perseids, a meteor shower that peaks in August, was
stronger than expected and hit a European communications satellite
called Olympus, causing it to malfunction, according to Phillip Anz-
Meador of Lockheed Engineering & Science Co. in Houston.

	Astronomers reported 350 meteors per hour.

	But the Leonids should pose no danger to spacecraft, said Andrew
Potter, chief of the space science branch at NASA's Johnson Space Center
in Houston.

	``We haven't worried about the Leonids. It'll be a normal shower,''
he said.

	Anz-Meador predicted the Leonids will flash ``about 10 per hour,
which is about equal to the normal background rate of shooting stars
seen every night of the year.''