[meteorite-list] Mars Odyssey MOI Sequence Of Events
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:48:13 2004
Here are the sequence of events for today's Mars orbit insertion burn by
Mars Odyssey. All times are in Pacific Daylight Time (UTC - 7 hours) and
in Earth Receive Time. One-way light time from Mars to Earth is about
8 1/2 minutes.
All times p.m., PDT:
4:56 About two-and-a-half hours before the engine firing, commands are
sent to fire small onboard thrusters to "desaturate" or unload the
momentum of the spinning reaction wheels. These devices are similar
to gyroscopes and are used to control the spacecraft's orientation -
its positioning in three axes - so that it is facing the desired
direction. The desaturation readies the reaction wheels for the task
of turning Odyssey into the proper pointing position for the orbit
insertion engine firing.
7:06 Catalyst bed heaters, called "catbed" heaters, are turned on to warm
the "catbeds" to an operational temperature for more efficient
operation of the reaction control thrusters. These small jets control
the spacecraft's position in three axes: pitch, yaw and roll. They
will fire intermittently during the main engine firing to keep the
spacecraft on an even keel.
7:12 The lines from the fuel and oxidizer tanks to the main engine (which
were vented of any residual gas before launch) are now filled by
opening pyrotechnic valves. Five minutes later, the fuel and oxidizer
tanks are pressurized by opening additional pyrotechnic valves to
ensure an even and steady flow of fuel and oxidizer during the engine
operation. This is important to provide smooth combustion within the
engine for stable thrust and steady deceleration of Odyssey.
(Propulsion system valves for filling the lines and pressurizing the
tanks are activated when small pyrotechnic charges are electrically
ignited to open valves in tubing about the diameter of a pencil. Each
charge breaks open a seal and creates a clear line to allow the
pressurant - helium gas - to flow into the tanks. gasses to flow
freely into the empty tubes.)
7:18 Telecommunications channels are switched from the spacecraft's
high-gain antenna to the medium gain antenna for transmission of a
carrier signal to the Earth, and to the low-gain antenna for receipt
of commands from the Earth. These antennas are less powerful, but
they can receive and send signals from through a wider arc than the
high-gain antenna. Only the carrier signal -- which includes no
telemetry -- will be transmitted from now until the spacecraft has
emerged from behind Mars into full view of the Earth. At that time,
the spacecraft will have completed its engine firing and been
captured by the Martian gravity into a large elliptical orbit around
7:19 The 70-meter (230-foot) diameter antenna of the Deep Space Network
complex in Goldstone, Calif., locks on to Odyssey's carrier signal.
Receipt of this signal will allow ground controllers to gauge the
spacecraft's motion by the changes in the carrier frequency. These
changes, called Doppler shift, will occur as the spacecraft's
velocity changes during the main engine firing. Reaction wheels now
turn the spacecraft to face the proper direction in preparation for
the engine firing.
7:26 Ignition of the main engine starts for the Mars orbit insertion.
7:36 The Deep Space Network loses the spacecraft signal as Odyssey passes
7:36 Still behind Mars and incommunicado, the spacecraft enters Mars'
shadow and darkness for two minutes.
7:39 Odyssey reaches periapsis - the lowest point in its first orbit of
Mars, at an altitude of about 328 kilometers (203 miles). It is still
out of reach of Earth ground stations.
7:45 The main engine firing is now complete.
7:49 Still out of touch with the Deep Space Network, Odyssey's reaction
wheels turn the spacecraft to point the high-gain antenna toward
Earth. Fault protection software that was turned off during the
important orbit insertion phase orbit insertion firing is turned on
again. (Fault protection software overrides normal spacecraft
operations when an unexpected event occurs on the spacecraft, and
directs the spacecraft to stop what it's doing, place itself in a
safe state and orient itself to await further commands from Earth.
Fault protection software is turned off during important
single-opportunity events to prevent such an override from
interfering with accomplishing these events.)
7:56 From Earth's point of view, Odyssey emerges from behind Mars, and the
Deep Space Network antennas seek to lock on to the spacecraft's
8:00 Odyssey's propellant, oxidizer and pressurant tanks are mechanically
isolated with pyrotechnically activated valves so the propellant
tanks will not be overpressurized.
8:01 Odyssey turns on its telemetry and begins transmitting data at 40
bits per second. The Deep Space Network will take several minutes to
synchronize their equipment with the pattern in the telemetry because
of the slow rate at which the data is being received. Once the Deep
Space Stations' equipment has locked on to the signal, the messages
from Odyssey will be forwarded to JPL.
Received on Tue 23 Oct 2001 11:48:32 AM PDT