Envisat imaged by sharp-eyed Pleiades

Envisat pictured by the upward-looking Pleiades spacecraft

Engineers have managed to get the first detailed photographs of the crippled Envisat platform.

Europe's <link> <caption>flagship Earth observation satellite</caption> <url href="http://www.esa.int/SPECIALS/Envisat/index.html" platform="highweb"/> </link> shut down unexpectedly over the Easter weekend and is not responding to any commands.

The French Space Agency (Cnes) tasked its high-resolution <link> <caption>Pleiades satellite</caption> <url href="http://www.astrium-geo.com/pleiades/" platform="highweb"/> </link> to image Envisat while passing some 100km from the stricken mission.

The pictures confirm the flagship has not suffered any obvious impact damage.

All of its key structural parts appear to be intact.

"We are very grateful to <link> <caption>Cnes for the Pleiades pictures</caption> <url href="http://smsc.cnes.fr/PLEIADES/GP_mission.htm" platform="highweb"/> </link> , and to the <link> <caption>German Fraunhofer Institute</caption> <url href="http://www.en.vvs.fraunhofer.de/members/fraunhofer-fhr/" platform="highweb"/> </link> which has given us some excellent ground radar images. Both show amazing details," Prof Volker Liebig, the director of Earth observation at the European Space Agency (Esa), told BBC News.

The Pleiades imaging spacecraft was launched in December. Its agility is exceptional

Controllers were hoping Envisat might have got itself into "safe mode", a trouble-shooting configuration that ailing spacecraft are programmed to adopt and which helps a recovery team re-establish communications. Unfortunately, this has not happened.

"We'll keep trying for sometime," Prof Liebig said. "We've still got a list of things we want to do, but I cannot offer any relief; we haven't yet been able to revive the satellite."

Just getting the Pleiades images represents a remarkable achievement.

The French satellite, which was only launched in December, orbits the Earth at an altitude of 695km. That is some 70km below Envisat's orbit.

To see its wayward target, Pleiades would have had to swivel to look up and across. The separation in the image at the top of this page was about 100km.

<link> <caption>Pleiades carries control moment gyros</caption> <url href="http://www.bbc.co.uk/news/science-environment-16301914" platform="highweb"/> </link> that give it exceptional agility, but that capability was designed for picking out targets on the surface of the Earth - not for snapping other, overflying satellites.

Budget impasse

Envisat was launched in 2002 and is the biggest non-military Earth observation spacecraft ever put in orbit.

It has been at the forefront of European Earth science endeavours for a decade, monitoring the land, the oceans, Earth's ice cover and its atmosphere.

The mission, which has so far cost about 2.5bn euros (£2.1bn), is already five years beyond its planned lifetime, but Esa had hoped to keep it operating until 2014.

Envisat's data has been in high demand

This would have given the agency time to run it alongside some of the scheduled replacements, and to cross-calibrate the data.

The first of these is <link> <caption>Sentinel 1</caption> <url href="http://www.esa.int/esaLP/SEM097EH1TF_LPgmes_0.html" platform="highweb"/> </link> , which is supposed to take over the radar duties of Envisat. It should be launched next year.

Sentinels 2 and 3 will image changes on the land and over the oceans, and they should follow in early 2014.

However, whether the replacement missions actually get to launch on time is currently an open question.

The European Commission, which owns the Sentinels, has not as yet agreed an operational budget for them.

And Esa, which has procured the spacecraft for the EC and will manage them in orbit, is reluctant to send the satellites to the launch pad until the necessary funds are in place.

A prolonged impasse, combined with a premature end to the Envisat mission, would be immensely damaging to the scientific projects and operational services, such as meteorology, that have come to rely on the old spacecraft's data.

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