- Objectives:
- Upon completing this chapter, you will be able to describe major factors involved in flyby operations, planetary orbit insertion, planetary system exploration, planet mapping, and gravity field surveying. You will be able to describe the unique opportunities for science data acquisition presented by occultations, and problems involved. You will be able to describe the concepts of using aerobraking to alter orbital geometry or decelerate for atmospheric entry, descent and landing.
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 Operations
All 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. 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 the plans are 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.
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In a flyby operation, observatory phase (OB) begins 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. Ground system upgrades and tests have been completed, spacecraft flight software modifications have been implemented and tested, and the encounter command sequences have been placed on board.
Far encounter phase (FE) begins 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 during NE include ring plane measurements during which ring structure and particle sizes can be determined, celestial mechanics observations that determine the planet's or satellites' mass, and atmospheric occultations to determine atmospheric structures and composition.
While observations must be planned in detail many months or years prior to NE, precise navigation data may not be available to command accurate pointing of the instruments until only a few days before the observations execute. So, late updates to stored parameters on the spacecraft can be made to supply the pointing data just in time. Some observations of the target planet or its environs may be treated as reprogrammable late in the encounter, in order to observe features that 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, the plasma instrument and plasma wave instrument 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.
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 during PE.
After encounter, instrument calibrations are repeated to be sure that any changes in the instrument's state are accounted for.
Planetary Orbit Insertion Operations
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The same type of highly precise interplanetary navigation and course correction used for flyby missions also apply 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 places the spacecraft in the correct attitude, and 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 were to fail, the spacecraft would continue to fly on past the planet as though it were a flyby mission. It is common for the retro-burn to occur on the far side of a planet as viewed from Earth, with little or no data available until well after the burn has completed and the spacecraft emerges from behind the planet, successfully in orbit.
Once inserted into a highly elliptical orbit, Mars Global Surveyor continued
to adjust its orbit via aerobraking (discussed later in this chapter) near periapsis to 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 were used to vary the orbit slightly and choreograph multiple encounters with the Galilean satellites and the magnetosphere.
System Exploration or Planetary Mapping
At least two broad categories of orbital operations may be identified: system exploration and planetary mapping. Exploring a planetary system includes making observations of the planet, its atmosphere, its satellites, its rings, and its magnetosphere during a tour typically a few years or more in duration, using the spacecraft's compliment of remote-sensing and direct-sensing instruments. On the other hand, mapping a planet means concentrating observations on the planet itself, using the spacecraft's instruments to obtain data mainly from the planet's surface.
Galileo explored the entire Jovian system, including its satellites, rings, magnetosphere, the planet, its atmosphere, and its radiation environment. At Saturn, Cassini will accomplish a similar exploratory mission, examining the planet's atmosphere, rings, magnetosphere, icy satellites, and the large satellite Titan with its own atmosphere. Magellan, a planetary mapper, covered 99% the surface of Venus in great detail using SAR imaging, altimetry, radiometry, and gravity. Mars Global Surveyor is mapping the surface of its planet also, using imaging, altimetry, spectroscopy, and a gravity field survey.
An orbit of low inclination at the target planet (equatorial, for example) 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 (polar, for example) 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 dependably regular support from the flight team members, the DSN, and other institutional teams.
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