De-orbit mission to remove huge dead satellite

May 3rd, 2018, Published in Articles: EngineerIT

Year after year, the traffic orbiting the earth increases, whilst space debris poses an ever greater threat for satellites and crews of space vehicles. The European Space Agency (ESA) wants to combat this problem and, as part of the “Clean Space” initiative, is launching the e.Deorbit mission with the first objective of removing the inactive Envisat satellite. This is the heaviest civil unmanned satellite in space; it weighs 8,2 ton and, together with its solar panels, is more than 25 m long.

After decades of launches, earth is surrounded by a halo of space junk of more than 17 000 trackable objects which threaten working missions with possible collision. The only way to control the debris population across key low earth orbits is to remove large items such as derelict satellites and launcher upper-stages. Such uncontrolled multi-ton items, besides being collision risks, can also explode due to leftover fuel or partially charged batteries heated up by orbital sunlight.

Fig. 1: Earth is surrounded by a halo of space junk of more than 17 000 tractable objects.

Just weeks after celebrating its tenth year in orbit, communication with the Envisat satellite was suddenly lost on 8 April 2012. Following rigorous attempts to re-establish contact and the investigation of failure scenarios, the end of the mission was declared on 21 May 2012. As there were no signs of degradation before the loss of contact, the team has been collecting other information to help understand the satellite’s condition. These include images from ground radar and the French Pleiades satellite. With this information, the team has gradually elaborated possible failure scenarios. One is the loss of the power regulator, blocking telemetry and telecommands. Another scenario is a short circuit, triggering a “safe mode” – a special mode ensuring Envisat’s survival. A second anomaly may have occurred during the transition to safe mode, leaving the satellite in an intermediate and unknown condition.

The outstanding performance of Envisat over the last decade led many to believe that it would be active for years to come, at least until the launch of the follow-on Sentinel missions. With ten sophisticated sensors, Envisat has observed and monitored Earth’s land, atmosphere, oceans and ice caps and witnessed the gradual shrinking of Arctic sea ice.

Notwithstanding its huge scientific contribution, it is of major concern that large objects can be launched into orbit without ensuring enough failsafe or deorbit mechanisms. Space agencies have only recently decided to deal with the problem of orbiting debris. ESA recently announced that in future, all its satellites will be equipped with systems facilitating automatic deorbit. The current international policy is that satellites and second stage launcher debris have to deorbit within 25 years. With the prevalence of launches, this is clearly a policy due for serious reconsideration. As far as second stage launcher debris is concerned, SpaceX has the answer with the successful development of a reusable second stage which, after delivering its payload, returns to earth.

ESA is responsible for removing Envisat, hence the e.Deorbit mission which in itself is a major technological challenge owing to the size and unknown rotation of Envisat. In 2016/7, work was conducted by two competing international consortia headed by OHB and Airbus in parallel. ESA has now selected for the next stage of the mission, the one headed by Airbus.

Fig. 2: The 25 m long Envisat to be removed from orbit.

The chosen solution consist of a so-called chaser satellite that will capture Envisat using a robotic arm. The robotic arm will perform a grappling on Envisat’s launch adapter ring and the engines of the chaser satellite will be subsequently activated, and both will be directed to the Earth’s atmosphere where they will partially burn up and their remains will fall into the Pacific Ocean.

The clamping mechanism has to ensure a high level of rigidity of the connection with Envisat, in order to resist the considerable forces acting on the device during manoeuvers. ESA selected Sener’s clamping mechanism as part of the solution. Sener is a private engineering and technology business group founded in 1956 in Spain.

Mechanisms are one of group’s specialties in space, having produced more than 275 systems for satellites and space agencies such as NASA, ESA, JAXA and Roscosmos. The company is one of the leading partners in the European Space Agency’s science programmes.

The company is known in South Africa for its renewal energy projects. With its South African headquarters in Sandton the company has been involved in the 50 MW Bokpoort CSP plant in Upington and the Kathu Solar Park.

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