Orbiter Deflection Maneuver Status July 27
A press release on ODM is available.
Early this morning (27 July), the Galileo Orbiter performed the first sustained firing of its 400-Newton main engine. A real-time telemetry link was maintained throughout the burn, providing engine performance data from both changes in the spacecraft's radio frequency (doppler data) and accelerometer data. The burn lasted five minutes, eight seconds and was autonomously terminated, as planned, by Attitude Control's on-board velocity estimate from the accelerometers. Early analysis shows that the change in velocity was only slightly below the ideal (61.0 meters/second versus 61.8 meters/second). The Orbiter's course has now been changed to miss Jupiter by approximately 230,000 km.
With this change, the Orbiter is now targeted for a flyby of Jupiter's volcanic moon Io on December 7. Io's gravity assist will carry the spacecraft onto an overflight of the Probe's entry site for Probe data collection. Later that day, the 400-N engine will be fired again for the Jupiter Orbit Insertion burn (JOI) which permanently places the Orbiter into Jovian orbit.
Between now and arrival at Jupiter, some small "trajectory correction maneuvers" using the 10-N thrusters will probably be performed to fine-tune the flight path. The earliest, TCM-26, is scheduled for late August.
The ODM sequence is still running. From now through July 30, the spacecraft will be spun-down and returned to Earth-point for resumption of normal cruise sequences. ODM burn data that was stored in the on- board memory will be read-out for further ground analysis starting Friday morning.
The Galileo mission continues, but it is now two big steps closer to its goal!
Previously, all the propulsion system components upstream of the 400-N engine had been exercised and shown to operate nominally. The Wake-Up Burn was a final check of the entire system prior to committing to the five minute ODM Main Burn. It's roughly equivalent to starting your car's engine to check it the day before a long road-trip. (Recall that road service is roughly 500 million miles away.)
The fastest real-time data is currently sampled every 100 seconds, too slow to get much insight into changes during a 2 second burn. The tape recorder collects telemetry much faster, but it captures far more information than is required for engineering analysis; large amounts of data would need to be sent down, tying up days of Deep Space Network (DSN) track time, just to collect a few samples of interest. To overcome this, the spacecraft's onboard computer has been programmed to grab a handful of selected measurements from the telemetry stream and store them in spare memory for read-out later. This data has already been returned from the Wake-Up Burn and is providing valuable insights into the engine's operating characteristics. The same technique will be used for the ODM Main Burn.
The Wake-Up Burn was programmed to cut-off at exactly two seconds. In contrast, the ODM Main Burn will use accelerometers to estimate when the required speed change (62 meters/second) has been accumulated before the flight software commands the 400-N engine to stop. For added protection, the software uses back-up timers to unconditionally halt the burn should accelerometer data fail any of several reasonableness checks.
The 2-sec Wake-up Burn (WUB) executed just after midnight Sunday night (i.e., 12:38 am ERT/PDT) and the 5-minute main burn is scheduled for just after midnight Wednesday night.
Last night's turn was intentionally designed to stop 1.35 degrees short of the desired attitude for ODM. At this "off-ODM" attitude, the spacecraft can verify its attitude based on star positions (celestial reference). The Star Scanner can readily sight many bright stars, all easily distinguishable and with good geometries. At the desired ODM attitude, the observable stars are not so well situated and may yield an ambiguous estimate of the spacecraft's attitude. degrees) and was the first turn performed after the mass property changes due to probe release.
The desired ODM attitude will be achieved by way of small gyro-based pointing corrections. These pointing corrections will take the spacecraft from the celestially-referenced attitude to an attitude without celestial references. This type of pointing correction induces some pointing errors. The pointing errors are minimized by keeping each pointing correction small (i.e. less than one degree). Hence, we "leapfrog" our way to the correct attitude, by first performing a large initial turn that relies on stars, followed by a series of small final turns that do not rely on celestial references.
Tonight (July 21), the spacecraft performs gyro-based pointing corrections to the ODM attitude and spins back up to 10.5 rpm.
The spacecraft had been spun up to 10.5 rpm in order to assure stability of the Probe upon release. The 400-N engine burns next week also require a 10.5 rpm spin-rate for Orbiter stability during thrusting. The reason for transitioning to 3 rpm in between these two events is that Probe Release and the ODM burn are performed at different spacecraft attitudes. To turn to the proper attitude, Attitude Control must determine spacecraft pointing through observations of the stars around it; above 3 rpm, the Star Scanner sees the stars sweep by too quickly for the on-board computer to identify them and compute an attitude. Furthermore, at higher spin-rates the Orbiter is more resistant to turning (gyroscopic effects) and requires more propellant to change attitude.
Why do so many activities fall so early in the morning? For critical events like spin-downs, we want to observe the activity over two 70-m DSN antennas at different sites; this assures the event will be monitored even if one ground antenna is down for repairs. Given Galileo's position in the sky, the best overlap occurs when the Goldstone antenna is watching the spacecraft set as the Canberra antenna is watching it rise. This happens around midnight PDT currently, but will change as Galileo (along with Jupiter) moves along the ecliptic. It's all geometry.
Tomorrow morning (yes, starting around midnight), the Orbiter will be commanded toward the proper attitude for the ODM burn.
The pilot valve starts and stops the 400-N engine by directing helium pressurant to pneumatically controlled engine valves. With the pilot valve closed (its current condition), springs on the engine valves prevent fuel and oxidizer from entering the combustion chamber. Opened, the pilot valve directs helium to lift the engine valves, releasing propellant for combustion. With the pilot valve pressurized, the 400-N engine is ready for operation.
Propellant tank repressurization verifies helium will still flow through the regulator and check valves after remaining closed for over 18 months of interplanetary cruise. Helium flow allows the system to control propellant pressures as fuel and oxidizer are expended through the ODM burn.
Early the next morning (20 July), the orbiter will be spun down from 10.5 rpm, where it had been for probe release, to its nominal 3 rpm. Over the following days, the ODM sequence will turn the orbiter to the ODM burn attitude and spin it back up to 10.5 rpm (required for 400-N operation).
The next activity affecting the 400-N engine will be a 2 second "wake-up burn" at 12:38 am ERT (Earth Received Time), Monday, 24 July. The ODM main burn starts at the same time Thursday, 27 July.
Over the weekend, following successful completion of probe release activities, the ODM sequence (Orbiter Deflection Maneuver) was uplinked to Galileo. This two week sequence will ultimately modify the orbiter's velocity by 62 meters/second , assuring a trajectory which overflies Jupiter's satellite Io and the probe entry site.
ODM marks the first in-flight use of the 400-N main engine. Up to the main burn on 27 July, the sequence will exercise components leading to the engine so that ground analysts can verify their operation.
The first step was passed early this morning when the 400-N latch valves were each opened and closed 25 times, demonstrating that the engine's propellant supply can be accessed and isolated for a burn.
The next step (Wednesday, 19 July) is pressurization of the 400-N engine pilot valve. Once pressurized, it will be used to direct helium to pneumatically controlled engine valves near the engine chamber. In this way, we may start and stop the flow of fuel and oxidizer into the engine and control the burn.