We must ask where we are and whither we are tending.
-- Abraham Lincoln
Once the spacecraft and the radar system were ready for the mission objectives, we were prepared to get under way with the mission timeline, or itinerary (see Figure 9-1).
The alignment of Earth and Venus dictated that Magellan be launched between April 28 and May 29, 1989, with the best launch occurring on May 5. (The advantage of a May 5 launch date would become apparent during the VOI maneuver, when the fixed performance of the SRM would be the exact value required to put Magellan into the desired orbit.) The Mission Design Team had skillfully maximized the number of possible launch days so that delays caused by weather or shuttle problems would not prevent a launch. The next few paragraphs attest to the wisdom of this planning effort.
The Space Shuttle Atlantis was moved to Launch Pad 39B at Cape Canaveral, Florida, on March 22, 1989, and the Magellan/IUS combination was installed in the cargo bay on March 25.
In preparation for launch activities, Magellan team members from JPL who were part of the launch team assumed their command posts at the Kennedy and Johnson Space Centers. During the preceding months, they and representatives from these other two NASA centers underwent extensive training for the launch event. Other team members remained at JPL to monitor the spacecraft's state of readiness as it rested inside the shuttle's cargo bay. Magellan team members across the country shared the excitement as the launch status remained green (for "go"). Five people who couldn't be more ready were the STS-30 Atlantis astronauts: Captain David M. Walker, Commander; Colonel Ronald J. Grabe, Pilot; and Mission Specialists Mary L. Cleave, Ph.D., Major Mark C. Lee, and Norman E. Thagard, M.D.
The countdown began April 24 and proceeded smoothly toward an April 28 launch. On the 28th, however, with the countdown at Launch -31 seconds, the automatic ground software system detected a shuttle problem and the countdown came to a halt. A hydrogen recirculation pump that cooled the shuttle engines prior to firing developed a short and stopped. The launch on this day was scrubbed.
After careful review of the pump problem and of a second problem involving abnormal venting of a hydrogen circulation line, the launch team selected May 4 for the next attempt.
The countdown resumed, starting this time at Launch -2 days, and proceeded smoothly. But May 4 did not dawn as a likely day for a launch. The sky was overcast, and strong crosswinds (greater than 12 knots) blew across the runway at the Kennedy Space Center's emergency landing site. No one was surprised when a hold for weather was called at Launch -5 minutes.
Fortunately, a 64-minute launch window had been designed for May 4. After 59 anxiety-filled minutes, the winds dissipated and the clouds parted just enough for launch at 2:46:59 p.m., eastern daylight time (see Figure 9-2), only 5 minutes before the end of the launch window for that day. The shuttle slowly rose out of the billows of steam and accelerated toward the low clouds. It went briefly out of sight and then reappeared for a few seconds, framed in a blue window amid the clouds. It was truly picture perfect.
The Space Shuttle Atlantis compensated for the delay in launch by yaw steering into the correct orbit plane. After five revolutions around the Earth at an altitude of 296 kilometers (160 nautical miles), Magellan was slowly deployed from the shuttle (see Figure 9-3). Sixty minutes later, with the solar panels extended as shown in Figure 9-4, the IUS ignited its two SRMs in rapid succession and propelled the spacecraft on very nearly the precise trajectory to Venus. After firing its attitude-control thrusters for a small course correction, the IUS separated from Magellan and used its remaining fuel to move away from the spacecraft.
Figures 9-3 and 9-4 are two photographic mementos the astronauts brought back to the Magellan team.
The original May 1988 launch period would have allowed Magellan to reach Venus 4 months later via a Type-I trajectory, meaning that from launch to destination, the spacecraft would have traveled less than 180 degrees around the Sun. There was a similar opportunity in the October 1989 launch period initially set aside for Magellan but sub-sequently assigned to the Galileo mission to avoid further delays in its launch.
However, the positions of Earth and Venus during the late-April to late-May 1989 launch period required a Type-IV trajectory (see Figure 9-5). This meant that the spacecraft would travel between 1-1/2 to 2 times around the Sun (slightly more than 540 degrees) and that it would arrive at Venus on August 10, 1990. While it dictated a longer cruise duration (15 months), the Type IV actually had the advantages of reductions in launch energy and Venus approach speed.
Since launch, Magellan has traveled more than 1-1/2 times around the Sun at an average speed of 113,600 kilometers per hour (71,000 miles per hour) relative to the Sun and has logged over 1.261 billion kilometers (788 million miles). Three trajectory-correction maneuvers (TCMs) have kept the spacecraft on track for the correct aim point and arrival time at Venus. The TCMs were executed on May 21, 1989, and on March 13 and July 25, 1990.
Magellan's Type-IV trajectory and the resultant Venus arrival date brought about some changes in the basic mapping plan developed for the 1988 mission.
Superior conjunction (where the Sun is positioned between Venus and the Earth) will now occur during the primary mapping mission, instead of at the end. The result is that up to 18 days of mapping data will be lost around November 2, 1990, because radio interference from the Sun will make it impossible to communicate with the spacecraft. Fortunately, the missing data can be recovered in early July 1991, if the mission is extended for additional 243-day mapping cycles.
The trajectory also dictates an approach over the north pole; this will result in a mapping swath from north to south, the reverse of that planned for the 1988 mission.
The word "cruise" conjures images of leisure, spare time, and relaxation. It is true that people who work on interplanetary missions usually take some time after launch to reflect on what it has taken to get that far and on what lies ahead to ensure a successful mission. But it's the "what lies ahead" that makes this period of reflection indeed brief.
Magellan team members have been occupied with two primary tasks during the cruise to Venus. The first was to fly the spacecraft and evaluate the performance of its various subsystems and components in the actual space environment. Ground test chambers are the next best thing to being there, but they cannot completely simulate interplanetary conditions. The second task was to plan and prepare for the activities that will occur in Venus orbit.
The cruise period has not been a time of leisure for the spacecraft either.
Magellan has traveled farther from the Sun than Earth's orbit (149,669,000 kilometers or 93,000,000 miles) and has approached to within 104,640,000 kilometers (65,400,000 miles) of the Sun, 2,880,000 kilometers (1,800,000 miles) closer to the Sun than the orbit of Venus. This changing environment allowed us to characterize the thermal responses of various parts of the spacecraft over a range of temperatures as these parts faced toward or away from the Sun. Knowing these responses is referred to by spacecraft engineers as "having a model." The ability to refine and validate the thermal model means that we will be better able to predict the thermal response of the spacecraft once it is in Venus orbit.
Similarly, the spacecraft power models, both for input from the solar arrays and output from the batteries, were validated as we performed cruise activities that required varying power output.
A series of "guide-star" calibrations was carried out during cruise to determine precisely how the star scanner responds to the set of stars we plan to use for accurate spacecraft pointing during the prime mapping mission. These calibrations are called STARCALs.
Magellan is a three-axis-stabilized craft that relies on three reaction wheels to provide attitude (pointing) control (see Chapter 4). Four gyroscopes provide the information required to determine the attitude. Because extremely high pointing accuracy is required to successfully capture radar reflections from the planet's surface, several calibrations were conducted on the gyroscopes to provide a thorough understanding of their orientation and behavior.
Two types of gyroscope calibrations were conducted to correct two possible error sources. The Scale Factor Calibration (SFCAL) allows correction of the difference between the amount the spacecraft thinks it has turned and the amount it has actually turned in a large-angle excursion. The Attitude Reference Unit Calibration (ARUCAL) allows correction of the offset of the axes of the gyroscope assembly relative to the star scanner reference frame. During flight, this offset can change from the amount measured before launch.
Pointing of the HGA was calibrated to assure its accuracy while performing the dual functions of radar mapping and telecommunications. This activity is called an HGACAL.
Another high-precision task was determining the desired orbit and its timing. Useful SAR images can be obtained only if the exact range from the spacecraft to the planet's surface is known throughout each mapping pass. Because Magellan's orbit will be highly elliptical, the range to the surface will change every moment and require frequent adjustments to the radar commands. Accurate calculation of the needed adjustments is totally dependent on precise knowledge of the orbit.
The orbit-determination task relies on a navigation technique called "differenced Doppler," which involves measurements of the spacecraft's signal using tracking antennas at the Spain and California (and sometimes the Australia and California) Deep Space Network (DSN) complexes. Obtaining these measurements during cruise refined the techniques, verified the procedures to be used in orbit, and assured us that the differenced Doppler approach will provide sufficient orbit-prediction accuracy to guarantee good radar-data collection.
Other major ground test activities that involved interaction with the spacecraft were the Mapping Readiness Tests carried out at the DSN sites; these tests verified that the DSN is primed to support mapping operations. Magellan will send 1.8 gigabits of data back to Earth during every orbit. Because the data will be stored on tape recorders during each mapping pass and overwritten with new data during the next pass, there will be only one chance during each orbit to send the data to a DSN station. Additionally, the timeline allows the station only one minute to lock on the spacecraft's signal before the data flow begins. The DSN's lockup and recording operations must occur without a hitch to avoid gaps in the Magellan Venus map. Results of the Mapping Readiness Tests verified that lockup can occur within one minute and validated the operational procedures for capturing all of the data from the spacecraft.
In December 1989, the radar electronics were turned on for the first time since before launch. Both the radar system and the hardware passed muster. This test paved the way for a more complicated test performed in May 1990, when the radar and the spacecraft were put through their paces for more than three days. The spacecraft turned through the intricate series of maneuvers it will perform orbit after orbit as it maps the planet. At the same time, the radar system issued its complex series of mapping commands. This period of simulated mapping operations allowed us to verify many spacecraft and ground procedures and much of the mapping software that will drive Magellan once it is in orbit around Venus.
Magellan has also performed some routine "housekeeping" activities. Star scans were performed daily to allow correction for the normal drift in spacecraft pointing, and the reaction wheels were desaturated twice daily to eliminate the momentum accumulated from small torques to the spacecraft caused by the Sun's radiation. These two activities are discussed in more detail in Chapter 4.
Familiarity with the spacecraft's in-flight characteristics gained during the first few months following launch allowed us to take a critical look at our plans for both the in-orbit checkout (IOC) and mapping phases and revise them where needed.
In-orbit checkout, a thorough examination of the spacecraft and the radar, will be the first event after achieving Venus orbit. Final planning for this activity took almost a year. Assembling the requirements for in-orbit tests, resolving conflicts between requirements, negotiating a fundamental plan, and working out the operational and procedural details was an intense effort conducted in parallel with the activities involved in flying the spacecraft.
Final planning for the primary mapping mission was also achieved during this period. The prime mission involves three distinct types of geometry: nonocculted mapping, the superior conjunction phase, and the apoapsis occultation phase. Each type places different constraints on the mapping plan, and each was analyzed and updated separately.
The results of the planning efforts for IOC and mapping are described in Chapters 10 and 11, respectively.
Placing a spacecraft into a precise orbit around a planet millions of miles away, checking out its equipment and subsystems to make sure they are working properly, and pronouncing the spacecraft ready for mapping operations are responsibilities and pressures definitely a cut above those we face on a daily basis. But the Magellan team will perform this scenario throughout the VOI maneuver and the IOC phase. As with any well-orchestrated production, an intensive period of rehearsal has been essential.
The eight-member Mission Engineering Team has devoted a portion of the cruise period to developing and conducting operational readiness tests and various training exercises, including simulated anomalies, that have tested and evaluated our performance in carrying out the major mission functions mentioned above. These dress rehearsals allowed us to refine our procedures and techniques as we went through the actual processes and interfaces (some of which are complex and time critical) that will be required. Now we feel we are ready for the real thing.
Chapter 10 - In Orbit At Last!
The Magellan's Venus Explorer Guide
