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SPACE FLIGHT OPERATIONS
Encounter Phase
The term "encounter" is used in this chapter to indicate the high-priority data-gathering period of operations for which the mission was intended. It may last a few months or weeks or less as in the case of a flyby encounter or atmospheric probe entry, or it may last a number of years as in the case of an orbiter. Encounter operations are typically carried out from the Space Flight Operations Facility at JPL, Buildings 230 and 264.
Flyby OperationsAll the interplanetary navigation and course corrections accomplished during cruise result in placement of the spacecraft at precisely the correct point, and at the correct time to carry out its encounter observations and obtain any planned gravity assist. A flyby spacecraft has a limited opportunity to gather data. Once it has flown by its target, it cannot return to recover lost data. Its operations are planned years in advance of the encounter, and refined and practiced in the months prior to the encounter date. Sequences of commands are prepared by the flight team to carry out operations in various phases of the flyby, depending on the spacecraft's distance from its target. During each of the six Voyager encounters, the phases were titled observatory phase, far encounter phase, near encounter phase, and post encounter phase. They may have different names for different spacecraft, but many of the functions most likely will be similar. In a flyby operation, observatory phase (OB) is defined as the period when the target can be better resolved in the spacecraft's optical instruments than it can from Earth-based instruments. This phase generally begins a few months prior to the date of flyby. OB is marked by the spacecraft being completely involved in making observations of its target, and ground resources are completely operational in support of the encounter. This phase marks the end of interplanetary cruise phase, during which time ground system upgrades and tests are conducted, and spacecraft flight software modifications are implemented and tested. Far encounter phase (FE) includes time when the full disc of a planet can no longer fit within the field of view of the instruments. Observations are designed to accommodate parts of the planet rather than the whole disc, and to take best advantage of the higher resolution available. Near encounter phase (NE) includes the period of closest approach to the target. It is marked by intensely active observations by all of the spacecraft's science experiments, including onboard instruments, and by radio science investigations. It includes the opportunity to obtain the highest resolution data about the target. Radio science observations include ring plane measurements during which ring structure and particle sizes can be determined, celestial mechanics observations which determine the planet's or satellites' mass, and atmospheric occultations which determine atmospheric structures and composition. Observations must be planned in detail many months or years prior to NE, but precise navigation data may not be available to program accurate pointing of the instruments until only a few days before. Late updates of stored parameters on the spacecraft can be made to supply the pointing data just in time. OPNAVs, may be an important navigational input to the process of determining values for late parameter updates. Some observations of the target planet or its environs may be treated as reprogrammable late in the encounter, in order to observe features which had not been seen until FE. During the end of FE or the beginning of NE, a bow shock crossing may be identified through data from the magnetometer, plasma and plasma wave instruments, as the spacecraft flies into a planet's magnetosphere and leaves the solar wind. When the solar wind is in a state of flux, these crossings may occur again and again as the magnetosphere and the solar wind push back and forth over millions of kilometers. (Traditionally, PIs are not above wagering on the time and distance from a planet where these crossings will take place). Post encounter phase (PE) begins when NE completes, and the spacecraft is receding from the planet. It is characterized by day after day of observations of a diminishing, thin crescent of the planet just encountered. This is the opportunity to make extensive observations of the night side of the planet. After PE is over, the spacecraft stops observing its target planet, and returns to the activities of cruise phase. DSN resources are relieved of their continuous support of the encounter, and they are generally scheduled to provide less frequent coverage to the mission. After encounter, instrument calibrations are repeated to be sure that any changes in the instrument's state are accounted for.
Planetary Orbit InsertionThe same type of highly precise interplanetary navigation and course correction for flyby missions are also applied during cruise for an orbiter spacecraft. This process places the spacecraft at precisely the correct location at the correct time to enter into planetary orbit. Orbit insertion requires not only the precise position and timing, but also controlled deceleration. As the spacecraft's trajectory is bent by the planet's gravity, the command sequence aboard the spacecraft fires its engine(s) at the proper moment, and for the proper duration. Once the retro-burn has completed, the spacecraft has been captured into orbit by its target planet. If the retro-burn fails, the spacecraft will continue to fly on past the planet. It is common for the retro-burn to occur on the far side of a planet as viewed from Earth.
Once inserted into a highly elliptical orbit, Mars Global Surveyor will continue to adjust its orbit via OTMs near periapsis which will decelerate the spacecraft further, causing a reduction in the apoapsis altitude, and establishing a close circular orbit at Mars. Galileo used a gravity assist from a close flyby of Jupiter's moon Io to decelerate, augmenting the deceleration provided by the 400 N rocket engine. Thereafter, additional OTMs over a span of two years will vary the orbit slightly to choreograph multiple encounters with the Galilean satellites and the magnetosphere.
System Exploration and Planetary MappingAt least two broad categories of orbital operations may be identified. Exploring a planetary system includes making observations of the planet and the satellites and rings, etc., in its neighborhood. On the other hand, mapping a planet obtains data mainly from the planet's surface. Galileo will be exploring the entire Jovian system, including its satellites, rings, magnetosphere, the planet, and its environment. At Saturn, Cassini will accomplish a similar exploratory mission, exploring planet's rings and environs, and the large satellite Titan with its atmosphere. Magellan, a planetary mapper, covered 98% the surface of Venus, in great detail, using SAR imaging, altimetry, radiometry, and gravity. Mars Global Surveyor will map the surface of its planet also, using imaging, altimetry, spectroscopy, and gravity survey. An orbit of low inclination at the target planet is well suited to a system exploration mission, because it provides repeated exposure to satellites orbiting within the equatorial plane, as well as adequate coverage of the planet and its magnetosphere. An orbit of high inclination is better suited for a mapping mission, since the target planet or body will rotate fully below the spacecraft's orbit, providing eventual exposure to every part of the planet's surface. In either case, during system exploration or planetary mapping, the orbiting spacecraft is involved in an extended encounter period, requiring continuous or nearly continuous support from the flight team members, the DSN, and other institutional teams. [an error occurred while processing this directive] |