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L-5 Unstable

On 26 November, the last remaining solar array drive on Landsat 5 began to exhibit unusual behaviour. The device is one of many single points of failure on the venerable spacecraft, which has been in orbit since March 1984.

By coincidence, when a group of distributors of data from the Australian Centre for Remote Sensing met last week, one of the items on the agenda was contingency plans for a catastrophic failure of the spacecraft.

L-5's solar array drive points the solar panels on the spacecraft at the sun. The arrays provide electricity for the spacecraft's on-board batteries, which power all its systems. If the array does not point at the sun, the batteries will drain, and the communications link between the spacecraft and controllers on the ground will fail.

In order to keep the batteries working as long as possible, all non-essential systems on the craft, especially its imaging systems, have been shut down.

The drive that has developed problems is a back-up; the original failed in January 2004.

The Landsat community around the world has long feared this eventuality. The spacecraft has exceeded its design life by 16 years. More than 125,000 images have been recovered from a network of downstations scattered across the globe.

In 2003, there was a partial failure of the thematic mapper on the other operational craft in the Landsat fleet, Landsat-7. Since then, L-5 has carried the only viable instrument of this type that is available to most researchers around the world.

Meanwhile, officials at ACRES are maintaining a watching brief on L-5. They recently completed a mission to find operational sensors in space that might make a viable replacement. The research turned up few candidates. Most sensors do not produce Landsat-like data, and of those that do, data supply is uncertain.

Adam Lewis is the Leader of the Spatial Information Access and Remote Sensing Group at Geoscience Australia, which is responsible for ACRES. He says that in the event of the total failure of either in-service spacecraft, data from the other Landsat and from various satellites in the Spot constellation would be used in the short term.

In the longer term, the best candidate appears to be the China Brazil Earth Resources Satellite series.

China and Brazil have committed to a continuous stream of these satellites, which will ensure data for the next decade at least. Although there are operational problems with the current CBERS-2 satellite, the launch of CBERS-2B is planned for October 2006. CBERS-3 is also funded, and slated for launch in 2008.

A recent meeting of space ministers form the two countries reaffirmed support for CBERS-4 in the 2010 time frame, but this has not been budgeted.

ACRES officials also expressed great interest in the Disaster Monitoring Constellation. The DMC consists of five micro-satellites owned by different countries. All were built by Surrey Satellite Technology. The satellites contain a multispectral sensor that can deliver 32 metre pixels. The constellation could offer daily revisits over anywhere in Australia.

In the very long term (beyond 2009), ACRES is pinning its hopes on a follow-on mission by the US. This might be Landsat 8, but equally, could be some sort of novel private/public data buy.