MISSION PLANNING FOR THE ASTRO OBSERVATORY
"Mission Planning" is the process whereby the astronomical observations desired by the science teams are turned into an actual operational plan and observing schedule. Hence, it is composed of science planning, where the scientists devise programs of investigation and select the best astronomical objects to carry out the program, and Operational Implementation, where a team of engineers and technical staff produce the computer files and paper work that make the plan happen. Clearly, the link between these two aspects of mission planning is crucial for the success of a mission like Astro. It is a complicated process that involves the interaction of the individual science teams with each other, the science teams with NASA/Marshall planners, the NASA/Marshall planners with NASA/Johnson personnel (who are responsible for the space shuttle operations and safety), and mission management personnel.
The mission planning process worked on Astro-1, but in a difficult way that resulted in a lot of stress, especially with the launch delays that affected that mission. A special Mission Planning Team was constructed after Astro-1, composed of knowledgeable people from all aspects of mission planning, to address the issue. The results were tangible: communications were greatly improved, the process was streamlined, and made more flexible at the same time. In addition, new software was generated that not only made "pre-mission" planning more efficient, but allowed tremendous flexibility for real-time replanning during the Astro-2 mission itself.
Below we describe a few key aspects of mission planning for our "Observatory on a Shuttle."
The astronomical observations planned for each Astro mission were selected by the teams that dedicated years to building and preparing the instruments, and/or Guest Investigators, who were scientists that proposed to (and were selected by) NASA to have the right to use the telescopes. Hundreds of hours were spent by astronomers huddled in meetings selecting the highest priority observations to be scheduled. Massive databases on all of the potential targets were put together to assist in making these decisions. The programs of investigation were selected mainly by the scientific interests of the people involved, with consideration also given to the strengths of each of the telescopes in the Astro package. Also crucial to this activity was having computer programs that "simulate" what the instrument may observe for a given target, based on the known information and assumptions. Finally, when a launch date was known, the availability of the targets were assessed, including such things as: a) Is the object too close to the sun for observation? b) What is the target's visibility with respect to the daylit and night time portions of each orbit? c) Does the target's visibility change very much with time or is it stable with respect to the assumed orbit? Only after all of these and many other issues were addressed could a selection of potential targets be assembled into a science timeline.
(There are a number of interesting differences in planning observations for a shuttle-based telescope such as HUT versus a free-flying telescope such as the Hubble Space Telescope. Click here to see a brief description of some of these effects.)
The science timeline is basically an orderly "wish list", where a sequence of proposed science observations is laid out in an efficient manner, using basic information about the target visibilities and space shuttle constraints. It is the bridge between the science planning and operational implementation phases of mission planning. The Astro science teams plan this timeline using special software, meshing the interests of all three science teams together into one timeline of observations. This is the primary "product" delivered to the mission planners at NASA/Marshall Space Flight Center.
It is no small task to choreograph some 400 pointings of the space shuttle at nearly 300 objects of interest to one or more of the scientists. Suffice it to say that the science timeline is used as a starting point, and then NASA mission planners turn it into a dazzling variety of supporting information to make it all work. Guide stars and roll angles for the Instrument Pointing System are selected for each observation. A "maneuver timeline" for the shuttle is generated. Gimbal angles to point the shuttle's communications antenna at one of the Tracking and Data Relay Satellites are calculated. A "thermal profile" for the shuttle is generated, along with many other safety-related checks. Detailed crew schedules and crew procedures are generated, right down to a minute by minute timeline for some activities. The list goes on and on. Some of this work must be done in conjunction with NASA's Johnson Space Center in Houston, which is responsible for the operation of the Shuttle while in orbit. This process takes several months of concerted effort by hundreds of people prior to a mission!
Meanwhile, the instrument teams are busy planning the detailed instrument set-up procedures for each planned observation. There are several important deliveries of information back and forth between the MSFC planners and the instrument teams, all of which have to occur on schedule to keep the mission planning process flowing properly. By the time of launch, all of the necessary supporting materials are in place to support the operational phase of the mission.
Given this description, it should be obvious that anything causing disruption of these timelines of events could potentially cause great difficulty for mission operations. If the shuttle failed to achieve the anticipated orbit or launched two hours late due to a weather delay, the detailed visibilities of the astronomical objects of interest all change. Because of this, procedures for what is called "real-time replanning" were implemented, whereby the timelines are regenerated in 12 hour time blocks during the mission. It was this real-time replanning capability, in conjunction with the pre-mission work to assemble all of the supporting information, that led to the tremendous scientific return from the Astro missions.