Learning about how the atmospheric structure of these bodies changes over time scales of months and years is necessary for understanding the complex relationships involved in surface-atmosphere volatile exchange, as well as the processes that control their basic atmospheric structure. For example, one model predicts that the structure of Pluto’s middle and lower atmospheres is controlled by CH4, whereas another predicts that CO is more important. The more we learn about these atmospheres from remote observations, the more effectively we can plan investigations to be conducted by future spacecraft missions to these bodies.

The structure of Pluto’s atmosphere has been probed at only one time—by a stellar occultation in 1988. The structure of Triton’s atmosphere has been probed twice—during Voyager 2’s encounter in 1989 and by a stellar occultation in 1993.

If the technology demonstration of the ATD/NTOT’s maneuverability (see Chapter 4) proves successful, then it would be possible to carry out a program of 1 to 2 atmospheric probes of these bodies per year, based on their (present) average rate of stellar occultations. This rate could be higher or lower, depending on the orbit of the telescope and the maneuverability demonstrated (i.e., how much, how accurately, and how often) during the testing phase of the ATD/NTOT mission.

The occultation rate is proportional to the fraction of time during an orbit when Triton or Pluto is visible. Also there would be a gain in the number of potential occultations that is proportional to the diameter of the ATD/NTOT’s orbit (projected onto the Earth’s polar axis) relative to the diameter of the Earth. For a Molniya orbit this factor is 2.6.

The ability to undertake this observing program does not depend on any of the enhancements suggested in Chapter 2. Rather, the decision should depend on the availability of resources to pay for the significant extra operational costs entailed in maneuvering the spacecraft to a new orbit or to a different position in its current orbit. It is also unclear to the task group whether or not the spacecraft would be usable for other astronomical tasks while the maneuvering was taking place. If not, the observational efficiency, defined as time collecting scientific data divided by elapsed time, would be extremely low. Low efficiency is not inherently bad, but it represents a value judgment that cannot be made until more is known about the maneuverability of the spacecraft.

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