  Moduli stabilisation is explored in the context of low-energy heterotic
$M$-theory to show that a small value of the cosmological constant can result
from a balance between the negative potential energy left over from stabilising
the moduli and a positive Casimir energy from the higher dimensions.
Supersymmetry breaking is induced by the fermion boundary conditions on the two
branes in the theory. An explicit calculation of the Casimir energy for the
gravitino reveals that the energy has the correct sign, although the size of
the contribution is close to the edge of the parameter range for which the
calculation is valid.
