washington

A faulty solar panel on the US Mars Global Surveyor spacecraft has led mission managers to suspend normal operations, and may jeopardize their goal of mapping the entire planet at high resolution.

Project managers raised the craft's orbit to a safe altitude on 12 October after one of two electricity-producing panels moved unexpectedly while experiencing drag in the martian atmosphere.

Since arriving at Mars last month, the spacecraft has been repeatedly dipping into the atmosphere to lower and circularize its elliptical orbit, a technique known as ‘aerobraking’. But on 6 October the atmospheric density suddenly doubled. The solar panel, which engineers had thought was stuck open at 20 degrees from its intended latched position, moved first to the latched position, then past it.

Engineers at the Jet Propulsion Laboratory in Pasadena, California, and Lockheed Martin Astronautics in Denver, Colorado, which built the spacecraft, will spend two weeks or more trying to duplicate the solar array's behaviour using a replica on the ground to try to understand the problem.

Mars Global Surveyor is meant to end up in a nearly circular orbit averaging 378 km above the surface of Mars. Mapping had been due to begin in March and to last for 687 days, or one full martian year. Even if no more aerobraking is allowed, it may be possible to use the spacecraft's fuel to lower its altitude, but it is unlikely that the optimum mapping orbit could then be achieved. Science instruments continue to collect data while engineers are studying the solar panel problem.

If the craft remains in an elliptical orbit, it will be too far from the surface of Mars for high-resolution imagery, so observations of the planet's southern hemisphere will suffer, according to Michael Malin of Malin Space Science Systems in San Diego, California, which built the spacecraft's camera system.

Some damage has already been done. The plan was for the craft to pass over the martian equator at the same time every day, two o'clock in the local afternoon. Due to the hiatus in aerobraking, that orbit is no longer possible, says Malin, so other options are being considered.

If crossing the equator shifts too much towards midday, the shadows become shorter, and the definition of surface features would be washed out in the photographs. Malin says a mid-morning equator crossing is preferable for imagery, but is not ideal for thermal measurements of the surface, which are better done in the afternoon after the planet has warmed up.