3.12.2020

CLEARSPACE SA/EUROPEAN SPACE AGENCY

The European Space Agency (ESA) today finalized a contract to launch a mission in 2025 that will be the first to capture and dispose of a piece of orbiting space junk. The ClearSpace-1 mission, built by Swiss startup ClearSpace, will home in on a piece of debris the size of a washing machine, grapple it with a four-armed claw, and escort it down to a lower orbit where the duo will enter the atmosphere and burn up.

Darren McKnight, a space debris expert at the technology company Centauri, applauds ESA for being one of the few agencies to take action. But he is concerned by the slow progress in removing orbital debris, which he believes will increasingly threaten working satellites and astronauts. “If we don’t get started soon, we’re going to be in big trouble,” he says. “We need to take baby steps fast.”

The space around Earth is becoming increasingly crowded because satellites have traditionally been left in orbit when their useful life ends. In higher orbits they can remain there for hundreds, if not thousands, of years. Discarded rocket stages are another important source of space junk, if not nudged down into the atmosphere after use. 

The 5500 launches over the 60 years of the space age have left 23,000 objects larger than a grapefruit in orbit. There are many millions of smaller objects that can’t be tracked. At the speeds things move in low-Earth orbit, even a collision with a stray bolt can be catastrophic. But big objects are most concerning, because they can collide and create cascades of smaller collisions. That’s what happened in 2009, when a working Iridium communications satellite collided with a dead Russian military satellite, generating thousands of new pieces of trackable debris and many more smaller ones. Two years later, the International Space Station had to move to avoid debris from the crash; in 2012 some passed within 120 meters.

ClearSpace is starting with something easy. In 2013, one of ESA’s Vega rockets launched a payload of two satellites. The 112-kilogram payload adapter that attached ESA’s PROBA-V Earth-observing satellite to the launcher has remained in orbit ever since, between 664 and 800 kilometers up. It is now in ClearSpace’s crosshairs. “It’s a simple structure, like a small satellite,” says Muriel Richard-Noca, chief engineer of ClearSpace.

The challenge is designing an imaging system that can quickly and autonomously characterize the object before the claw grapples it, says Luisa Innocenti, head of ESA’s Clean Space Office. “You don’t know how it is moving and the only way to know is to go up and look,” she says. Innocenti says she and her colleagues have also argued over the capture technique: Grabbing with a claw requires a close approach, whereas snaring in a net can be done from a safe distance—but has to work the first time. Richard-Noca says ClearSpace opted for the claw because you can have multiple attempts. “You can rehearse the whole procedure. It gives us the flexibility we need for this first mission,” she says.

Other missions are testing out similar ideas with debris they create themselves. An EU-funded mission called RemoveDEBRIS, designed by the University of Surrey, flew in 2018 and tested a harpoon and a net on small targets it deployed. Its test of a drag sail, to speed its descent into the atmosphere, failed to deploy, so it is making slower progress to re-entry. In March 2021, the Japanese company Astroscale plans to launch a privately funded mission called ELSA-d. During the mission, a servicer craft will release a target craft with a ferromagnetic docking plate and capture it using magnets. It’s an option if future satellite designers adopt the idea, but not for legacy junk.

ClearSpace has grander plans. “We will steadily move to larger and more ambitious targets,” says ClearSpace CEO Luc Piguet, as well as missions that can dispose of multiple targets to reduce costs.

That’s going to be necessary, says Hugh Lewis, who models space debris at the University of Southampton. He points out that ClearSpace-1 will spend €100 million (€86 million from ESA) to clean up a little over 100 kilograms of space junk. At that cost Envisat—a defunct, bus-size, 8000-kilogram, Earth-observing, ESA satellite—would be prohibitively expensive to bring down, even though Lewis says it is “the riskiest object in orbit.”

At the International Astronautical Congress in October, McKnight presented an analysis of the 50 most concerning pieces of debris in low-Earth orbit. He asked 11 teams around the globe to draw up their top 50 list based on criteria including mass, expected orbital lifetime, and proximity to working satellites. He then compiled a composite list. The first 20 items on the list are Soviet and Russian rocket stages launched between 1985 and 2007 and left in crowded operational orbits. Each one is heavier than an elephant and as big as a school bus. These giant objects, he says, are more of a threat than the swelling fleets of relatively small internet communications satellites that have been launched by the likes of SpaceX with plans and capabilities for deorbiting. “Clusters of dead things are more hazardous than constellations of live things,” he says.

But, experts say, although technology is beginning to be developed, political will is lacking. The policy of space agencies, McKnight says, is “‘study, wait, and hope,’ while we want ‘monitor, characterize, and act.’” Almost all the debris on McKnight’s top 50 list was put there by government space agencies. “It’s a superwicked problem,” Lewis says. “The people trying to solve the problem are also the ones causing it.”

Quelle: AAAS