NASA has started the contractor selection process for its lunar surface thermal control system study that could find a solution to the biggest hurdle in its plans to return to the Moon: stopping propellant loss.

For rocket engine efficiency and mass savings NASA would prefer to use cryogenic propellants for its Constellation programme's lunar vehicles, instead of the heavier storable fuels and oxidisers.

Keeping hydrogen or methane and oxygen in their liquid states requires a very low constant temperature.

But in space the Sun's radiation and heat transmitted through the spacecraft's own structure can raise temperatures causing propellants to vaporise. This increases propellant tank pressure, which is reduced by vapour release.

The study will focus on a conceptual lunar lander ascent module that uses liquid oxygen and methane with gaseous helium, stored at the liquid methane's temperature, for tank pressurisation.

NASA says: "A lunar lander may be required to operate for as long as 219 days with a surface stay of 210 days." Three thermal control systems will be compared.

The agency will award contracts on 3 June this year and all of the study's results will have been presented by early June 2010. The study has two phases and the second phase has two options to be exercised.

Phase one is to evaluate and compare the three systems for a polar mission of 210 days.

Phase two's options are to analyse the systems from low Earth orbit (LEO) to lunar landing, and then analyse it from LEO to a short surface stay of days or weeks up to and including the lunar ascent burn.