removal of cosmic-ray events can be problematic since the target object is moving relative to the background stars and can thus be mistaken for spurious events by many cosmic-ray detection algorithms.
As mentioned in Chapter 4, exploiting the ATD/NTOT’s eccentric orbit to obtain very long exposures is an important technology demonstration of this mission. The feasibility of this test will depend on additional study of the extent to which cosmic-ray events can be handled, either by reducing them through shielding of the chip (although at least one side cannot ever be shielded), by finding intelligent ways to analyze data contaminated with such events, or by implementing continuing improvements in CCD technology.
The planned ATD/NTOT mission is defined as a build-to-cost mission and not a build-to-specification mission. This constraint has important implications. If such a plan is continued, great care must be taken to ensure that the mission schedule does not slip, since time slips always result in higher costs and, if the costs are capped, a loss of capability. However, it is important that early in the mission, the capabilities be well defined so that all of the necessary pieces can be completed. A plan should also be assembled that dictates which specifications are most important in the event of a schedule slip, thus providing a mission with the most value for the budget.
Some of the astronomical projects discussed in this report can be accomplished with the baseline telescope and instruments. However, some important scientific projects and technology demonstrations will require enhancements, for example, an optical framing camera. Great care must be taken in deciding to add such instruments, since their addition has implications for the mission’s cost and schedule. In today’s cost-constrained NASA, it is not reasonable to perturb the budget greatly in order to add an instrument of the cost of the WFPC2. NASA and/or DOD should investigate alternatives, such as a university instrument of lower than class A or an instrument procured in the mode of Clementine or an instrument derived very directly from a camera currently being developed. It is also important that NASA and the astronomical community understand the short time scale needed for the delivery of such instruments and that the reasonableness of achieving timely production of such an instrument be considered.
If scientific investigations are to be an important part of the ATD/NTOT mission, then scientists will need to be included early so that appropriate decisions can be made regarding details such as filters. Such early inclusion means that the selection process for scientists will have to come early in the mission and be expedited.
Traditionally in NASA programs, the interval is long from formulation of a Research Announcement to the point when the personnel start work. The short development schedule anticipated for the ATD/NTOT does not allow a lengthy selection procedure if the goal of including scientists early is to be met. Similarly, the finite lifetime of the mission requires that personnel be used efficiently throughout the mission. Thus, consensus will have to be reached on how to achieve early involvement of scientists, in order to minimize any impacts on the delivery schedule and maximize the flexibility of the team to react to problems. Since such an approach is consistent with many of the goals being espoused within NASA today, the ATD/NTOT mission would provide an excellent opportunity to evaluate this new paradigm.
1. Space Studies Board, National Research Council, Lessons Learned from the Clementine Mission, National Academy Press, Washington, D.C., in preparation.