ORBITING ROBOTS MAY BE USED TO FIX AND FUEL SPACE SATELLITES

 

The Landsat 7 satellite orbited Earth every 99 minutes or so for more over 20 years, collecting photographs of nearly all of the planet's surface every 16 days. It was one of numerous craft that examined how the world was changing, revealing melting glaciers in Greenland, the expansion of shrimp farms in Mexico, and the level of deforestation in Papua New Guinea. However, Landsat 7's useful life came to an end when it ran out of fuel. Regular service has not been possible in space.

However, NASA now has a potential solution for such weakened satellites. The organisation wants to send a robot into orbit in a few years and position it to within grabbing distance of Landsat 7. The robot will catch it and refill it in mid-flight using a mechanical arm.

If the mission is successful, it will be the first time a satellite has been refuelled in space. And this mission is only one of a slew of public and private initiatives aimed at using robots to repair and improve the billions of dollars in satellites in orbit.

These initiatives may eventually lead to better and cheaper satellites that reduce the cost of Internet and cell phone networks, improve weather forecasts, and provide unprecedented views of planetary change and the universe. They may even pave the way for a new phase of in-orbit construction, with armies of robots erecting satellites, space stations, and even Mars-bound rockets.

At the moment, there are around 4,852 operational satellites in orbit, which perform critical communications, remote sensing, and other functions. Almost all were built with the understanding that if something went wrong, there was no way to fix it. Most satellites also require fuel to modify their orbits on a regular basis. They may become so much space junk once that's gone, contributing to the already massive stream of material around the earth.

"Imagine you're going to buy a car tomorrow," says Brian Weeden, president of the Consortium for Execution of Rendezvous and Servicing Operations, an industry trade group (CONFERS). "And you have to remember that you'll never be able to add additional gas to it." You'll never be able to replace the oil. You'll never be able to maintain or repair anything. You must also use it over the next ten years. Now, how much do you think that car will cost and how complicated do you think it will be? "We've been doing exactly that with satellites."

Engineers develop backup systems and cram as much fuel as they can into satellites to keep them running as long as feasible. All of this over-engineering raises the costs of producing and launching satellites, which can reach $500 million for a modern communications satellite.

Almost all space construction and maintenance has so far relied at least in part on humans, including Hubble Space Telescope repairs and the International Space Station's construction. However, because sending humans into space is very expensive, the drive to construct robots capable of doing the task has accelerated in recent years.

"What we'd really like to have is some method of having a robotic mechanic in space that can fix satellites when they break," says Carl Glen Henshaw, chief of the US Naval Research Laboratory's robotics and machine learning branch.

To the rescue, robots

Researchers have made strides toward this goal during the last few decades. A pair of specially manufactured craft docked in orbit and transferred fuel in a NASA demonstration effort in 2007. More recently, in 2020, Northrop Grumman successfully launched two "mission extension vehicles" that hooked themselves to two commercial satellites and boosted them into new orbits, each with its own motors and fuel.

Two new missions, set to debut later this decade, will take servicing to the next level. Semiautonomous robots with mechanical arms will be used in the demonstration projects to feed fuel to orbiting satellites and even do minor repairs.

Henshaw is working on Robotic Servicing of Geosynchronous Satellites, a mission supported by the US Defense Advanced Research Projects Agency (DARPA). It would be the first time a robotic vessel has succeeded to catch a satellite that was not specifically built to dock with it if it succeeds in a demonstration slated for 2024. In the Annual Review of Control, Robotics, and Autonomous Systems, Henshaw and his colleagues looked at some of the issues that come with servicing satellites with space robots.

There are a number of issues like this. Existing satellites lack the markers, known as fiducials, that would allow a robot to visually coordinate itself with a moving satellite. There aren't any fixtures for the robot to grab onto. And the parts of a satellite that do protrude, like as antennae and solar panels, are usually too delicate to handle.

Another issue is the robot's and Earth's time difference. Distance and signal processing cause a communication delay of several seconds between a robot in geosynchronous orbit, some 35,000 kilometres above Earth, and its controllers on Earth. As a result, the robot will have to take care of the most important jobs on its own.

On the good side, the work can build on existing robotic arms in orbit, such as the two that are now on the International Space Station.

Henshaw and his colleagues aim to use one of the thousands of old, dormant satellites "parked" in out-of-the-way orbits for a demonstration mission. Using cameras and a laser range finder, a robot would match orbits with the satellite and manoeuvre to within roughly two metres. When the robot is close enough, one of its two arms will grip an aluminium ring that previously held the satellite to the launch vehicle.

The other robotic arm would be able to poke and prod solar cells or antennae that have failed to unfold properly, an issue that Henshaw claims occurs every two or three years. It would also be able to attach new sensors, such as more powerful transmitters, cameras, or antennas, to the outside of satellites.

NASA intends to deploy an even more ambitious robot beyond 2025. The OSAM-1 (On-orbit Servicing, Assembly, and Manufacturing) robot would first take care of a complicated refuelling procedure for an existing satellite. Then it would show that it can construct entirely new structures in space.

OSAM-1's first mission will be to launch Landsat 7. The US Geological Survey launched the satellite into low-Earth orbit, roughly 700 kilometres high, in 1999. Its mission has since been taken over by more advanced satellites. It does, however, provide scientists with an opportunity to try robotic refuelling.

"Twenty-some years ago, technicians filled up the satellite in preparation for flight, and they never believed anyone would touch that interface again," says NASA's OSAM-1 Project Manager Brent Robertson.

According to Robertson, OSAM-1 will cut through a layer of insulation, sever two wires, and remove a bolt before connecting a hose and pumping in 115 kg of hydrazine fuel. (Watch the video here.)

Although the immediate goal is to repair and service current satellites, in-orbit assembly and manufacture could be more crucial in the long run.

OSAM-1, for example, will take a separate robot called the Space Infrastructure Dexterous Robot (SPIDER) on a separate mission to demonstrate its ability to assemble items in space. The first mission for SPIDER will be to assemble a seven-piece, three-meter antenna that it will carry into orbit.

OSAM-1 will also demonstrate its ability to produce structural components from the ground up, using a process comparable to 3D printing to create strong yet lightweight composite beams out of spools of carbon fibre and other fabrics. These beams could be linked together to construct structural components of a spacecraft or other orbiting structures.

If the planned missions are successful, robotics might usher in a new era of space construction that is currently unaffordable — fuel stores, space mining activities, larger space stations for space tourism, and even Mars-bound vessels built in orbit.

"We want to show that we can construct these things." "This has never been done before," Robertson says. "You can bring your own stuff or have material supplied to you if you have the ability to assemble things in space." You can also construct considerably larger structures."