4.4 Spacecraft Constraints

Several operational constraints must be satisfied to ensure the safety of the spacecraft and scientific instrument.

(1)

The spacecraft may not be slewed within 45 degrees of the sun ( > 135 degrees) or more than 152 degrees from the sun ( < 28 degrees), so that the sun is always in the field of view of the Fine Sun Sensor. The former restriction also prevents sunlight from falling within the telescope tube. These constraints apply to observing as well as maneuvering. Similarly, the spacecraft is kept very close (< 1°) to optimum roll with respect to the sun.

(2)

The OBC normally operates with a temperature in the vicinity of 52 degrees Celsius. Observing targets in the "Hot OBC Beta" region (between about 65 < < 85 degrees) may cause the computer to overheat, during the winter months when the earth is near perihelion. If the OBC temperature reaches the maximum operating limit (58.3 degrees), the spacecraft must be slewed to a target outside the hot zone. The "Hot OBC Beta" zone has precisely defined boundaries (see Table 3.1) which are allowed to vary from month to month to provide maximum observing flexibility. NOTE, typically this does not impose any problems on the science program.

(3)

At high and low Betas, the solar arrays present a smaller collecting area and therefore produce less power. As of June 1991, Betas greater than approximately 112 degrees and less than about 30 degrees the batteries will discharge significantly. (Note: This power-positive Beta zone is gradually contracting with time due to solar array degradation. The letter accompanying your skymap will give the zone's current limits.) The number of times per year that the batteries may be discharged below specific levels (excluding shadow seasons) is limited. See Section 3.3 for details.

(4)

Twice each year, in late summer and winter, the IUE's orbit carries it through the earth's shadow once each day for about three weeks. During shadow passages, which can last up to 80 minutes, no observations or maneuvers can be performed due to insufficient battery capacity.