Now's the day and now's the hour. -- Robert Burns
The period from TCM-3 on July 25 to Venus arrival on August 10 includes a tightly linked sequence of activities. Results of the TCM maneuver led to updating the precise timing and attitude of the SRM ignition to optimize the burn that will insert Magellan into the desired orbit.
When the spacecraft is approximately 13 hours away from Venus, the pull of the planet's gravity will gradually increase the spacecraft's velocity relative to the planet from 15,984 kilometers per hour (9,990 miles per hour) to 38,944 kilometers per hour (24,340 miles per hour). At approximately 9:30 a.m. (Pacific daylight time) on August 10, 1990, the trajectory will bring Magellan over the north pole of Venus (see Figure 10-1). As the spacecraft dives toward periapsis at 10 degrees N latitude, it will disappear behind the planet. Three minutes later, the SRM will ignite and begin its 84-second burn, braking Magellan to a speed of 29,600 kilometers per hour (18,500 miles per hour) relative to Venus. Once slowed, the spacecraft will be captured by Venus' gravitational field and transferred into a highly elliptical orbit around the planet.
Ground controllers would prefer the lines of communication to be open between them and the spacecraft during such a critical event. However, the SRM burn and VOI maneuver will take place "behind" the planet, as viewed from Earth (see Figure 10-2), thus preventing communication until the spacecraft emerges some 30 minutes later.
The shuttle/IUS launch combination allowed Magellan to carry only one SRM. As it approaches Venus, the spacecraft will be traveling too fast to achieve a circular orbit (which is ideal for radar mapping) from the deceleration provided by the single SRM burn. The planet will therefore be mapped from an orbit that is highly elliptical.
Before jumping into 8 months of full-time mapping operations, the mission-critical activities of the IOC phase will be conducted over a period of 22 days, from August 10 through August 31.
The first item on the agenda is reconfiguration of the spacecraft from an interplanetary cruiser to a planetary orbiter. This involves separating the spacecraft from the SRM and allowing it to drift away. The separation is accomplished by firing four pyrotechnic separation nuts and allowing springs to push apart the spacecraft from the SRM at a slow rate of only 0.9 meters per second (2.6 feet per second). The spacecraft's reconfiguration also requires changing the fault-protection logic from that which is appropriate for cruise to that which is suitable for orbital operations.
It takes several days to generate the first set of commands for IOC activities; the generation of these commands follows the collection of navigation tracking data that tell ground operations personnel what orbit the spacecraft actually achieved. It is important to acquire good orbit data prior to starting IOC operations, because the success of so many of the IOC activities requires that the orbit be accurately determined.
The first set of sequences will include a series of commands to the spacecraft to point the HGA at Earth, increase the telemetry rate from 40 to 1,200 bits per second, and play back the data recorded during the VOI burn. These data will allow Magellan navigators to reconstruct the orbit-insertion event and assist them in designing the orbit-trim maneuver (OTM) scheduled for August 28. If, at that time, the spacecraft is already in the targeted orbit, the OTM will be deleted from the sequences and mapping operations will begin August 29. If the OTM is required, mapping operations will begin September 1.
Calibrations conducted during cruise indicate the offset between the onboard gyroscopes and the star scanner reference frame. Any change in offset between the HGA and the star scanner, resulting from the shock of the VOI burn, will be determined by performing a HGACAL on August 19. This calibration involves passing the signal from the HGA back and forth across a DSN receiving station immediately after the spacecraft performs a star calibration.
Four health and calibration tests of the radar system will be conducted during IOC. (Note that the nonsequential numbering of the tests resulted from changes in plans that included a decision to not reuse any numbers that were associated with discontinued tests.)
Radar Test 1 (August 15) is a stepwise power-up of the various components of the radar subsystem, which leaves the radar in the standby mode. From this point onward, the plan is to never turn off the radar.
Test 2.5 (August 16) presents the first opportunity to collect radar reflections from the planet. The radar will be operated in a mode whereby the timing commands will be changed in a very coarse manner so that fundamental timing calibrations can be performed. As a result, Test 2.5 will not provide full swaths (or strips) of data, but rather a number of "framelets" down the length of the mapping passes.
Test 3 (August 22) will provide the first full-length strips of data. This test will use the look-angle profile that will be used for the mapping phase, as well as radar-control commands that vary continuously as orbit altitude and look angle change through the mapping pass. The data from Test 3 should provide strips that can be mosaicked into the first large image.
While Test-3 data are being analyzed, Test 5 will be performed August 24. This test is intended to measure the performance envelope of the SAR by testing combinations of look angle and radar-timing commands that should push the limits of the ground software to process the data into images. While this test is not expected to produce data for the final Magellan Venus map, it will provide invaluable information to support data interpretation.
Several "Recovery Days" (August 14, 21, and 29) have been scheduled in case anomalous conditions occur. In such a situation, the plan would be to correct the anomaly and resume the preplanned IOC schedule. The recovery days would give ground personnel a chance to accomplish that goal. Alternatively, they would provide ground personnel with some welcome breathing space if IOC proceeds as planned.
At the end of IOC, the spacecraft should be in the targeted mapping orbit, the parameters of which are given in Table 10-1. The orbit is nearly fixed inertially in space (i.e., Venus will rotate under the spacecraft) and will not precess appreciably over the length of the 243-day mapping cycle.
Now it's time to start collecting data for the Magellan Venus map.
Table 10-1. The Magellan mapping orbit ------------------------------------------------- Parameter Value ------------------------------------------------- Periapsis altitude, km (miles) 257 (172) Apoapsis altitude, km (miles) 8,000 (5,000) Periapsis latitude 10 degrees N Orbit peroid, hours 3.15 Inclincation (relative to 85.3 Venus's equator), degrees -------------------------------------------------
Chapter 11 - Mapping the Veiled Planet
The Magellan's Venus Explorer Guide