Planning Activities for a Planetary Lander

Beagle 2, the ill-fated probe intended for the surface of Mars, was designed to operate within tight resource constraints. The constraint on payload mass, the desire to maximise science return and the rigours of the hostile Martian environment combine to make it essential to squeeze high performance from the limited energy and time available during its mission. One of the tightest constraints on operations is that of energy. On Beagle 2, energy was stored in a battery, recharged from solar power and consumed by instruments, the on-board processor, communications equipment and a heater required to protect sensitive components from the extreme cold over Martian nights. These features of Beagle 2 are common to all deep space planetary landers.

The performance of the battery and the solar panels are both subject to variations due to ageing, atmospheric dust conditions and temperature. Nevertheless, with long periods between communication windows, a lander can only achieve dense scientific data-gathering if its activities are carefully planned and this planning must be performed against a nominal model of the behaviour of battery, solar panels and instruments. The state of charge of the battery of the lander falls within an envelope defined by the maximum level of the capacity of the battery and the minimum level dictated by the safety requirements of the lander. This safety requirement ensures there is enough power at nightfall to power the heater through night operations and to achieve the next communications session.

All operations change the state of battery charge, causing it to follow a continuous curve within this envelope. In order to achieve a dense performance, the operations of the lander must be pushed into the envelope as tightly as possible. The equations that govern the physical behaviour of the energy curve are complex, but an approximation of them is possible that is both tractable and more accurate than a discretised model of the curve would be. As in the refinery domain, any approximation has a cost: the coarser the approximation of the model, the less accurately it is possible to determine the limits of the performance of a plan.

In this paper we refer to a simplified model of this domain, which we call the Planetary Lander Domain. The details of this model are presented in Appendix C, and discussed in Section 4.3.