STARDUST Status Report
June 25, 1999
Earlier this week, flight sequence SC006 started with the STARDUST
spacecraft at 1.6 AU from the Sun and 0.7 AU from the Earth, increasing
about 0.01 AU per day. All subsystems are performing normally. The
spacecraft attitude is off-pointed 20 degrees from the Sun, exposing only
20% of the Max Planck Institute Cometary and Interstellar Dust Analyzer
(CIDA) collector to the interstellar dust stream. The collector area of
CIDA exposed to the stream has been decreasing as the need to point the
solar panels more directly toward the sun has increased as the spacecraft
recedes from the sun. During flight sequence SC007, the spacecraft will be
pointed directly at the sun to collect sufficient energy to operate the
spacecraft and the CIDA collector will be totally blocked by the dust shield
from the interstellar dust stream.
The flight team at Lockheed Martin Astronautics completed testing
commands to turn on the High Gain Antenna (HGA) for the first time.
Currently communications with the spacecraft have been through the Low
Gain (LGA) and the Medium Gain Antennae (MGA) which uses Solid State Power
Amplifier 1 (SSPA 1). The HGA uses SSPA 2 which will be powered on for
the first time in flight along with the HGA. This means that SSPA 1
will be powered off and will be left off until the next communications
session using the MGA next Thursday. SSPA 1 has been powered off
previously, during safing and to dissipate trapped charges.
SSPA 1 has received attention since its performance has had small,
but explained variations during the last few months. The SSPA's have
only one commandable gain state, high, which uses about 55 watts of
power and draws 1.9 amps of current. Over time, many hours to days, a
trapped charge builds raising the temperature of SSPA 1 at which time
SSPA 1 protection circuitry drops its current to 1.5 amps, with an
associated drop in gain of 2-3 dB. The trapped charge can be
dissipated and an annealing process occurs to bring SSPA 1 back to
normal operations by powering it off, for many days or more being
preferable. It is not known if SSPA 2 will exhibit this same
The drop in gain in SSPA 1, up until recently, has not been a problem.
But as the spacecraft continues to journey far beyond the orbit of Mars
and as the distance to Earth increases, every dB of gain counts.
During last week's commanding of the Solar Array Switching Unit (SASU)
changing from series to parallel connections, we had only a margin of
0.5 dB, dangerously close to losing lock on the downlink signal. This
has lead the flight team to consider powering off SSPA 1 between
communications sessions to dissipate the trapped charges and then
turning it back on just before communications start so that the SSPA
will stay in its high gain state through the communications pass and
provide an extra few dB of margin.
The STARDUST Outreach made a presentation to NEWMAST, a national group of
K-12 teachers, and also to the San Diego Astronomy Association.
For more information on the STARDUST mission - the first ever comet sample
return mission - please visit the STARDUST home page: