Understanding the power balance at the surface of the nucleus is essential
to study the chemical and physical evolution of a comet. Therefore, we pres
ent a detailed energy budget analysis for the surface of a model comet in t
he orbit of 46P/Wirtanen, target comet of the European space craft mission
Rosetta, for a variety of parameters and assumptions. We will show that for
a fast spinning Jupiter-family comet such as 46P/Wirtanen with a rotation
period of about 6 h, a fast rotator approximation underestimates the effect
ive energy input. This yields lower gas fluxes from the surface. For an 100
% active, non-dust covered surface we obtain a water gas flux on the order
of about 1.5 x 10(28) molecules s(-1) at perihelion, assuming a radius of 6
00 m. The calculated gas flux of water is within the order of measured valu
es for comet 46P/Wirtanen. But our calculated values are maximum gas fluxes
at noon-not averaged over one cometary day or taking the lesser insolation
at the polar areas into account. Therefore, we conclude that either the ra
dius of comet 46P/Wirtanen may be much larger than the accepted value of 60
0 m. A radius in the order of 2 km seems more likely to explain the measure
ments. Or, an other possibility could be that water-ice particles are blown
off from the surface like dust particles. This may also increase the effec
tive surface area of sublimation. (C) 1999 Elsevier Science Ltd. All rights
reserved.