The Decay of ASCA's
OrbitASCA
was launched into a roughly circular orbit with an initial altitude
of 570 km, and an orbital period of 96.1 minutes.
There were no consumables on-board ASCA. Thus, the
lifetime of the satellite was determined by the inevitable aging of
the components and by the decay of the satellite orbit. The former
took the form of slow degradation in the performance of, for
example, the on-board batteries, or a sudden breakdown of
components, such as gyroscopes. Although ASCA experienced its
share of these (for example, two of the five gyroscopes showed
erratic behaviors), the satellite remained healthy and functional
overall. The end of the ASCA mission was the result of the
decay of its orbit, and a large geomagnetic storm.
The plot above shows the altitude of the ASCA orbit since
launch. ASCA did not travel in a complete vacuum at the
altitude of 550 km. There was enough (though very tenuous)
atmosphere such that the friction dragged down satellites gradually.
As ASCA dropped to lower altitude, it encountered
increasingly denser parts of the atmosphere; moreover, the density
of the upper atmosphere increased towards the end of the mission due
to increased solar activity. Thus, the rate of orbital decay also
increased.
A large solar flare erupted on July 14, 2000, with an extremely
strong solar proton flux. When these charged particle reached the
Earth, it triggered a strong geomagnetic storm that lasted for a few
days starting on July 15. The geomagnetic storm caused a sudden
increase in the density of the atmosphere around ASCA. This
acted like a sudden gust of wind on a car crossing a bridge: the
attitude control system of ASCA could not keep up with the
buffeting of the satellite, and it entered a safe hold mode. The
satellite stared spinning slowly, with the solar panels pointed
somewhat away from the sun.
Despite heroic effort by the Japanese operations team, they could
not regain control of the satellite attitude. If they could slow
down the spin, and point the solar panel to the Sun, normal
operation could have resumed; however, the battery was far from
fully charged, which made it impossible to slow down the spin.
In the mean time, the orbit of ASCA kept decaying. Finally, the
satellite re-entered the atmosphere at 05:21 UT on March 2, 2001,
ending one of the most successful missions in the history of X-ray
astronomy.
This page created by Dr. Koji
Mukai (USRA) at the U.S. ASCA Guest Observer Facility.
This file was last modified on Monday, 28-May-2001 17:15:23
EDT
Curator: Michael Arida (SP Sys); arida@milkyway.gsfc.nasa.gov
HEASARC
Guest Observer Facility
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