This week in rocket history, we look at one of the biggest remote sensing satellites ever launched — Envisat.

Envisat was an Earth-observing remote sensing satellite built and operated by ESA. Launched in 2002, it was operational until 2012 when it suffered an anomaly that resulted in it going into a safe mode that it was never recovered from. This is its story.

The history of the Envisat satellite starts back in 1990 with a related project called the Polar Platform, an uncrewed remote sensing satellite designed to be visited by ESA’s planned Hermes spaceplane. The goal of Hermes was to give ESA a crewed spaceflight capability. In addition to the goal of visiting the Polar Platform, it was also planned to visit a crewed ESA space station called Columbus and NASA’s Space Station Freedom, which later became the International Space Station (ISS).

Because of budget overruns and the availability of inexpensive Russian space hardware after the fall of the Soviet Union, the Hermes program, including all the various missions, was canceled; however, that wasn’t the end for some components of the Hermes program. The Columbus station became the Columbus module on the ISS, and the instruments from the Polar Platform became two standalone satellites called Envisat and MetOpSat, short for Meteorological Operational Satellite.

The development of Envisat’s payload instruments started in 1993 following the cancellation of the Polar Platform. The design of the satellite bus, optimized to maximize the payload mounting area while being able to fit inside the fairing of an Ariane 5, started in 1995. A total of ten instruments were selected, which accounted for over two metric tons of the eight-metric-ton spacecraft which was 10.5 meters tall, or about as long as a telephone pole, making it the largest European satellite launched at the time.

The instruments consisted of two radars, two radiometers, three spectrometers, and an interferometer. In addition to these, there were two instruments designed to make it easier to track the satellite in orbit — the DORIS or Doppler Orbitography and Radiopositioning Integrated by Satellite and a Laser Retro Reflector. As it turned out, these two would end up being very useful.

To save money, Envisat used the same service module – the part that provides power and propulsion – as another remote sensing satellite, SPOT 4. Using a completely independent service module reduced risk in development because the payload module and the service module could be developed and integrated separately and only combined shortly before launch.

Envisat was launched on its planned five-year mission on March 1, 2002, aboard the eleventh Ariane 5 rocket. The launch was successful, which was a relief for everyone because over the course of the previous ten Ariane 5 launches there had been one complete failure and two partial failures, including the launch before Envisat. Despite these initial growing pains, Ariane 5 has gone on to become one of the most reliable rockets in the world, which serves as a reminder that space is hard.

Immediately after launch, the satellite autonomously activated its solar panel release mechanism and pointed towards Earth. It raised its orbit the next day and unfolded its big radar on the fourth day after launch. The payloads were powered up throughout the rest of March and early April 2002. Commissioning of the instruments took place between September and December 2002, with the start of routine operations occurring on January 1, 2003.

The satellite completed its planned mission in 2009 and was extended through to 2013. However, on April 8, 2012, control of the satellite was lost for no immediately obvious reason.

On April 15, 2012, the high-resolution optical imaging satellite Pléiades took a picture of Envisat to help ground controllers diagnose an external cause to the disruption, but even with this, controllers were unable to determine a cause. One possible reason may have been a short circuit that put the satellite in safe mode, but we’ll probably never know for certain.

Envisat’s mission officially ended on May 9, 2012.

It will take at least 150 years for the spacecraft to decay from its nearly 800-kilometer orbit. A mission called E.Deorbit was planned by ESA to deorbit Envisat, but it was canceled in 2018.

Though it ended, Envisat more than doubled its expected lifespan and delivered ten terabytes of data on the Earth’s land, atmosphere, oceans, and ice caps for disaster and climate change monitoring. It was replaced by the Sentinel series of satellites.

More Information

Envisat (ESA)

Envisat mission page (ESA)

Hermes (Astronautix)