The satellite, called the Mission Extension Vehicle-1 (MEV-1), will demonstrate technology capable of prolonging the lifespan of obsolete satellites.
The 2,300kg MEV-1 was originally developed by US aerospace company Orbital ATK, which was purchased by defence giant Northrop Grumman in 2018. The spacecraft is based on Orbital ATK’s GEOStar-3 platform, typically used for geostationary telecommunications satellites.
“Normally, these spacecraft carry a communications payload but for this particular spacecraft we removed that communications payload and replaced it with a rendezvous and docking system,” said Joe Anderson, vice-president of business development and operations at Space Logistics Services, a subsidiary of Northrop Grumman, which is responsible for the project.
The MEV uses components developed by Orbital ATK for the Cygnus vehicle, which delivers cargo to the International Space Station (ISS). Some elements of the system, however, are completely new. Orbital ATK (and later Northrop Grumman) spent 10 years developing the technology using giant robots manipulating spacecraft mock-ups at the company’s Rendezvous, Proximity Operations and Docking Laboratory.
Docking process
“Unlike Cygnus, which, after approaching the ISS, is captured by a robotic arm and berthed to the berthing mechanism of the station’s Harmony module, MEV-1 has to dock completely autonomously,” said Anderson.
“Moreover, MEV has to be able to dock with satellites, which, unlike the ISS, are not fitted with docking or berthing mechanisms.”
The engineers managed to overcome the absence of a dedicated docking port by taking advantage of two design features – the so-called liquid apogee engine and the launch adapter ring. According to Anderson these two features are present on 80% of geostationary satellites currently in orbit.
The liquid apogee engine is used to raise the orbit of a geostationary satellite from the geostationary transfer orbit, where it’s left after the separation from a rocket, to the geostationary orbit, a circular orbit at 36,000km where satellites appear to be suspended above a certain spot on Earth. The launch adapter ring helps the satellite separate from the rocket.
“We have a capture mechanism, which consists of a probe that enters the liquid apogee engine, and three stanchions which press against the launch adapter ring,” said Anderson. “First the probe enters the engine through the engine’s cone. Once it passes through the narrow throat into the inner chamber, it deploys its ‘fingers’. The probe is then retracted, pulling the two vehicles together.”
Push-pull tension
As the two spacecraft touch, the three stanchions on the MEV press against the launch adapter ring.
“We end up with a very simple push-pull tension where we are pulling at the centre of that liquid apogee engine and we are pushing against the launch adapter ring,” said Anderson. “That’s what clasps the two vehicles together.”
From that point onward, the MEV takes over the attitude and orbit control of the combined vehicle stack.
MEV-1’s first customer will be the IS901 satellite of the US satcoms operator Intelsat. Since the technology has not yet been proven in orbit, the operators of the two spacecraft will conduct the experiment in the so-called graveyard orbit – a disposal orbit a few hundred kilometres above the geostationary ring, where satellites are disposed of at the end of their life in order not to clutter the geostationary orbit.
Lengthy manoeuvre
MEV-1 relies on its electric thrusters to raise its orbit after launch to match that of IS901. The manoeuvre will take three months. Before the servicing operation begins, MEV-1 will circle around IS901 a few times and inspect it with its sensors. It will then position itself 80m behind IS901 and wait for a command from the ground control team.
“The approach will be very slow,” said Anderson. “It will take us two hours to get from the 80 metres to the docking. We need to make sure that all the control systems behave as expected and that the dynamics work just as we have modelled them in our simulations.”
After the docking, MEV-1 will take IS901 back to the geostationary orbit, where it will maintain its position for the next five years, serving Intelsat customers on Earth. Once the useful life of IS901 is truly over, MEV-1 will push it back to the graveyard orbit. After that, the servicing craft will undock from the satellite and move to another customer.
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