Galileo FAQ - More Antenna
"I bet that you could free the high gain antenna by ........"
Over the years, we've received and read many ideas about how to free the high gain antenna (the current strategy is delineated in the accompanying telecommunications strategy fact sheet). Galileo's engineers are a creative lot, so just about every idea suggested below--no matter how unusual--was included on a long list of proposed solutions. That doesn't mean that everything is tried, because the Galileo engineers working on the antenna anomaly are guided by two basic principles:
Keeping these basic rules in mind, here's a thorough look at some of the more frequently suggested ideas (and some of the more unusual ones):
What is really needed isn't just a gravitational tug, but a differential tug-- the "tug" would have to be much stronger on the antenna ribs than on the entire spacecraft. At Galileo's closest approach to Jupiter (on December 7, 1995, when the spacecraft was located 3 Jovian radii from the planet's cloud tops), the giant planet could only muster a pull of .0000001 G's (i.e. one-ten- millionth of Earth's gravity) on the antenna.
So why didn't we give it a try anyway, since the spacecraft will never again be so close to Jupiter? On December 7, 1995, all the project's attention and resources were focused on the probe relay and the Jupiter Orbit Insertion burn, activities that were absolutely essential to the mission's success. Trying to attempt something nonessential (like unsticking the HGA) on Arrival Day would have been "imprudent." Plus, "reorienting" the spacecraft to put the maximum amount of gravitational tug on the antenna would have meant that the coummunications antenna wasn't pointing towards Earth; we wouldn't have been able to get real-time telemetry or be able to command the spacecraft during the day's critical events.
Galileo's engineers thought about this shortly after first discovering that the antenna hadn't opened. As the ribs are lightweight, and the spacecraft is well- damped and quite stable, it's unlikely that the thrusters could generate enough force to unstick the ribs. There are several difficulties involved in trying:
Radiation by itself would have no significant mechanical effect on the antenna. In fact, the thermal impact that Jupiter would have on the spacecraft is insignificant compared with the thermal effects caused by the electric heaters mounted near the high gain antenna. Engineers already have used spacecraft heaters and altered the radio transmitter power level to cool the stuck antenna rib area, to no effect.
The engineering team also used thermal cycling, or "cold/warm turns" (i.e. turning the antenna alternately towards the sun and then 165 degrees off sun) to unstick the antenna. To take full advantage of this method, the antenna would be left in the on or off-sun position long enough to reach a near-steady-state temperature. Unfortunately, this repeated thermal cycling did not free the ribs.
Now, this is a suggestion that would be sure to drain the color from the Project Manager's face! Aside from the tremendous danger of damaging the spacecraft and the antenna (which was not built to survive being knocked about by large heavy objects) while trying something like this, there are numerous technical problems involved. For example: since the probe has no maneuvering capability of its own, only the orbiter would be able to maneuver about to the correct position. The HGA and the probe are at opposite ends of the spacecraft, so the orbiter would have to turn rapidly head-over-heels to align the Probe and the antenna. The spacecraft can't make this turn quickly enough, so the orbiter would then have to go chasing after the probe. And the probe's alignment would possibly change after being used as a hammer--it could then well plunge into Jupiter's atmosphere without having its heat shield oriented correctly, causing the probe to rapidly burn up without completing its mission.
Galileo normally spins about its central axis at about 3.15 revolutions per minute: its top planned spin rate is a whopping 10.5 revolutions per minute (for comparison, consider that a carousel usually turns at 7 or 8 rpm, not even fast enough to unseat a toddler). Nevertheless, the spacecraft was spun up to 10.5 rpm in 1992 in an attempt to open the antenna. In addition, the antenna was "hammered"--the antenna's deployment drive motors were pulsed on and off as close as feasible to the resonant frequency of the stuck ribs. The motors have been pulsed over 15,000 times as of September, 1994. No luck--the ribs remain stuck. There will be one last attempt to "hammer" the antenna in March of 1996 (see the next suggestion).
Galileo has also been put in high spin mode for the probe release, the orbit deflection maneuver, and the Jupiter orbit insertion burn. The upcoming Perijove Raise maneuver burn will also be done with the spacecraft in high spin.
If the spacecraft's attitude control software was modified, Galileo could be spun no faster than about 13.5 rpm, at which point the long magnetometer boom would be on the verge of "ripping off." Although there's a "safety net" of flight software that should prevent the spin rate from accidentally getting too high (currently set to about 11.5 rpm), it's far too risky to the spacecraft's health to even consider raising the spin rate higher than 10.5 rpm. Even if the boom didn't get "ripped off," structural damage is thought likely. Loss of the boom doesn't mean that we "only" lose several science experiments--the mag boom is essential for spacecraft stable dynamic operation. In either case, the safety of the spacecraft would be put at substantial risk by increasing Galileo's spin rate, with little prospect of freeing the ribs.
The antenna's status was closely monitored following major "jarring" events such as the probe release (causing a mechanical "shock"), the orbit deflection maneuver, and the Jupiter orbit insertion burn (causing a "vibration"). There will be one last main engine major burn--the perijove raise maneuver in March of 1996. After that, engineers will make one final attempt to take advantage of all of the cumulative shaking and jostling of the antenna, and will once again "hammer" the antenna by pulsing the antenna's drive motors on and off. Again, the pulsing will be done near the resonant frequency of the stuck ribs.
Yes, using the High Gain Antenna (HGA) may buy a bit of a performance gain. Antenna tests show that there's a 1 decibel gain in X-band (high power) performance over the Low Gain Antenna (the LGA, or what is currently being used for communications). But, there are several catches. For example (and this is far from a complete list):
There is nothing in Galileo's telemetry information that suggests improper CRM operation. During development, precautions were taken to eliminate CRM-spoke jamming. But, there is no way to know if the antenna alignment spokes are caught in the CRM.
The CRM release was performed by firing redundant pyro events several times during the launch sequence. Launch telemetry indicated that the pyros fired properly, and that unlatch had occured as planned. The pyro CRM release commands were sent again from the ground after the antenna failed to open on the off chance that the launch telemetry was erroneous, and that both CDM pyros hadn't fired during launch. Indications are that the pyros did fire properly at launch.
The force imparted by the scan platform (which holds the remote sensing science instruments) would have to be transferred from the despun section of the spacecraft, through the spin bearing connecting the spun and despun sections, to the antenna. The resulting force transferred through to the ribs would be very small. A similar idea would be to rock the spin bearing assembly back and forth. Once again, there wouldn't be enough transfer of dynamic force to have any effect on the stuck ribs.