Envisat - hope remains to regain contact

The European Space Agency remains hopeful that it can regain contact with its Envisat Earth observation satellite, which unexpectedly stopped sending data to the ground on 8 April.

Attempts to regain contact have so far been unsuccessful, but the spacecraft - one of the most sophisticated of its type and invaluable in such efforts as quantifying climate change - is in a stable orbit, ESA has determined by optical, radar and laser observation.

But earlier this week France's Pleiades Earth observation satellite passed within about 100km of Envisat and was successfully turned to gather images of Envisat that are being analysed along with ground-based observations to determine whether its solar panels are oriented to the Sun. If they are, ESA says, Envisat may have enough power to have entered a safe mode - and, possibly, enough power for re-establishing communication with Earth.

Information on Envisat's orbit is being provided by the US Joint Space Operations Center. In addition, multiple laser ranging stations on the ground are providing information to verify the stability of the satellite's orbit.

More than 4000 projects in over 70 countries have been supported with Envisat data, and even if contact is regained, many will have been affected by the loss of continuity of data. Should Envisat remain out of contact, a contingency agreement with the Canadian Space Agency will see some of the users assisted by data from its Radarsat.

But in any case Envisat, launched in 2002 with a planned life of five years has already exceeded its expectations. However, its current difficulties make the launch next year of the first in a series of replacements that much more urgent.

These Sentinel satellites are seies being developed for Europe's Global Monitoring for Environment and Security (GMES) programme and will provide the data needed for information services to improve the management of the environment, understand and mitigate the effects of climate change and ensure civil security. ESA describes the Sentinal series missions thus:

Sentinel-1 is a polar-orbiting, all-weather, day-and-night radar imaging mission for land and ocean services. The first Sentinel-1 satellite is planned for launch in 2013.

Sentinel-2 is a polar-orbiting, multispectral high-resolution imaging mission for land monitoring providing, for example, imagery of vegetation, soil and water cover, inland waterways and coastal areas. Sentinel-2 will also deliver information for emergency services. The first Sentinel-2 satellite is planned for launch in 2013.

Sentinel-3 is polar-orbiting, multi-instrument mission to measure variables such as sea-surface topography, sea- and land-surface temperature, ocean colour and land colour with high-end accuracy and reliability. The first Sentinel-3 satellite is planned for launch in 2013.

Sentinel-4 is a payload that will be embarked upon a Meteosat Third Generation-Sounder (MTG-S) satellite in geostationary orbit scheduled to be launched in 2019. Sentinel-4 is dedicated to atmospheric monitoring.

Sentinel-5 is a payload that will be embarked on a MetOp Second Generation, satellite, also known as Post-EPS, to be launched in 2020. Sentinel-5 is dedicated to atmospheric monitoring.

Sentinel-5 Precursor satellite mission is planned to launch in 2015, thereby avoiding data gaps between Envisat (Sciamachy data in particular) and Sentinel-5. This mission will be dedicated to atmospheric monitoring.