Selecting Targets for a Given Launch Assumption

Visibility of targets from low earth orbit is dependent on not only the launch time of year, but also on the specific orbital parameters assumed and various planning constraints such as the size of the solar avoidance zone or earth limb-angle viewing constraints. Because launch dates have a way of shifting around unexpectedly in the Shuttle world, it is important to have targets available year round. The list of all potential targets for any time of year is called the Program Target List (PTL); once selected, GI targets will be added to this file as the first step of the planning process. If good 1950 epoch coordinates ($\sim 1''$)are available at this point it will simplify things downstream in the process, although later updates are possible.

When a launch date and time and an assumed orbit are made available, the PTL can be culled down to a list of targets with acceptable visibilities. Science team members (again, including GI's) prioritize this list and assign requested observing times to those objects that are under consideration for observation. The finalized list is called a ``Mission Target List", or MTL, and this file provides planners with the information necessary to plan a science timeline.

The problem of creating a ``science plan" (SCIPLAN) boils down to arranging the potential targets into a sequence of observations that are consistent with the target visibilities and all the other constraints arising from other considerations (including STS thermal and communications constraints, for instance). Software has been developed to calculate these visibilities and to insert objects into the observation sequence, checking constraints and leaving time for slews between the various targets. Producing this timeline is no small task and has not been fully automated because of the large number of ``gray" constraints (i.e., constraints that one might be willing to violate for one target, but not another).

For Astro-2, we intend to modify the ``SCIPLAN" process to include a concept called ``block scheduling". The mission will be broken into integral orbit blocks and the blocks will be assigned to the teams in relation to the total time each team (including its GI's) is assigned for the mission. (At present it is envisioned that blocks will be two orbits long.) This is done before the SCIPLAN planning begins. Each team can then plan ``their" blocks as they desire, and only the block interfaces need to be negotiated. (In reality, recall that each of the three UV telescopes is observing every possible target, so this is really just a planning aid.) As needed during the mission, each team can shuffle or replan their blocks with relative impunity, in order to maximize their instrument's scientific return. HUT GI observations will simply be scheduled in the ``HUT" blocks, like any other HUT target. Once the SCIPLAN file has been finalized, the MSFC Payload Activity Planner (PAP) team and the instrument teams work in parallel on various activities.

Since the nominal launch plan for Astro-2 calls for a night launch into a 28$^\circ$ inclination circular orbit, to first order, the night visibility of a given target is what one would expect to see from a ground-based observatory at a northern latitude of +28$^\circ$.Many targets can also be effectively observed during orbital daylight. The only daylight targets that are absolutely excluded are those within the 45$^\circ$ solar avoidance zone. Targets within 25$^\circ$ of the moon are also nominally excluded. Targets with substantial amounts of both day and night visibility are classified as ``Night into Day" or ``Day into Night", depending upon when they rise sufficiently high above the earth limb. During daylight, targets must be >10$^\circ$ above the earth limb, and during night this constraint is relaxed to 5$^\circ$.Since only half the available observing time is during orbital night, however, the scheduling strategy tries to preserve orbital night for faint targets or for extended sources, concentrating on bright point sources during orbital day.

While targets near the pole of the shuttle orbit can have nearly continuous visibility, these opportunities are rare since the orbital pole shifts as the earth rotates underneath the shuttle. More typical visibilities are closer to 2000 s, and, once allowance is made for acquisition times, most observations have only 1500-1800 s of actual science data. Maneuver times can also have a substantial impact on the time available for scientific observations, especially if targets are far apart on the sky. The shuttle can slew at 0.2$^\circ$ per s, so the maximum maneuver time is 900 s. Average maneuvers are on the order of 400-600 s. One potential time-saving feature for targets that lie within the 20$^\circ$ maneuvering cone of the IPS is to perform an IPS slew. These are faster (0.5$^\circ$ per s), and they are particularly suitable for clusters of bright sources that require only a few hundred seconds of integration time.