On-Orbit Serviceable Spacecraft Make a Comeback
Ever wonder about the future of space science? Hop inside a time machine that transports you back 40 years and you may get a good idea about where things are headed. History, it would seem, has a funny way of repeating itself.
Serviceable spacecraft like the NASA-developed Multi-Mission Modular Spacecraft (MMS) and, of course, the iconic Hubble Space Telescope that NASA conceived and developed in the 1970s with servicing in mind are once again de rigueur. (The serviceable MMS shouldn’t be confused with NASA’s Magnetospheric Multiscale mission, which also goes by the MMS acronym.)
Case in point: As required by Congress in a law passed in 2010 and then amended five years later, NASA is requiring that proposed flagship astrophysics missions support servicing, even if their orbits are up to a million miles away. The agency also released a Request for Information (RFI) seeking ideas for a spacecraft design that it could use for both its proposed Asteroid Redirect Mission (ARM) and as a vehicle for refueling a government satellite in low-Earth orbit.
“The 40-year cycle is starting all over again,” said Benjamin Reed, deputy project manager of the Satellite Servicing Capabilities Office (SSCO) at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. SSCO personnel developed all of the technologies to service Hubble and is now creating and demonstrating next-generation servicing technologies, including validation experiments on the International Space Station. “It worked with Hubble. It would be crazy not to think it would work again. It’s back to the future.”
And according to him, NASA is well along the way to realizing that future even if it means servicing spacecraft positioned tens of thousands of miles away from terra firma.
What’s Different Today
Satellite servicing certainly isn’t new as evidenced by the five Hubble servicing missions and the 1984 repair of the Solar Max mission that used the MMS serviceable satellite bus.
What’s different today is where these operations are expected to occur in the future and the technology needed to ensure success. Robotic spacecraft likely operated with joysticks by technicians on the ground would carry out the hands-on maneuvers, not human beings using robotic and other specialized tools, as was the case for low-Earth-orbiting Hubble and Solar Max.
“Goddard knows how to service satellites in low-Earth orbits,” said Michael Kienlen, an SSCO engineer who is studying servicing beyond low-Earth orbit. “However, future flagship missions, including the WFIRST-AFTA (Wide-Field Infrared Survey Telescope-Astrophysics Focused Telescope) and other future observatories should operate in more distant orbits.”
WFIRST-AFTA, which NASA plans to equip with an 8-foot (2.4-meter) mirror and a slitless spectrometer and imager, will study dark energy, the mysterious form of energy that permeates all of space and accelerates the expansion of the universe, while providing cosmic surveys. It also will carry a coronagraph that will allow the observatory to image giant exoplanets and debris disks in other solar systems.
Other conceptual missions that various groups currently are studying in preparation for the 2020 Astrophysics Decadal Survey also could operate in more distant orbits.
Although still in the conceptual stage, these missions may carry very large primary mirrors that would allow scientists to study cosmological targets with greater resolution and sensitivity. One possible scientific objective would be to find Earth-size exoplanets [This links to the story about the VNC] in the habitable zone in our solar neighborhood and then identify chemicals in their atmospheres that may indicate the presence of life.
To achieve these ambitious goals, WFIRST and the other conceptual observatories ideally would operate from Sun-Earth L2 (SEL2), a thermally stable sun-Earth orbit roughly a million miles away.
Due to concerns that technologies might not exist to service SEL2-orbiting missions, some have recommended that the observatories fly in geostationary orbit (GEO), roughly 10 percent of the distance to the moon.
Robotic Servicing Achievable in SEL2
“GEO may not an option for these missions because of the thermal-stability requirements,” Kienlen said. “One stumbling block is the perception that there is not a plausible scenario for servicing satellites at SEL2. We don’t want to force all future flagship missions to a lesser-performing orbit because they are required to be serviceable. So, we will figure out how to take servicing to them.”
“It’s not like we have to reinvent the wheel. We have never stopped developing servicing technologies. The difference today is that future flagship missions are required to be serviceable,” added Julie Crooke, a Goddard engineer, astrophysics technical manager, and member of a team that has been studying one of several future mission concepts. “With appropriate technology investments, we are on a clear path to demonstrating a servicing capability far from low-Earth orbit,” she said.
Beth Keer, who heads SSCO’s Advanced Concepts Office, agrees. “We have demonstrated robotic refueling on the space station. It’s one of the stepping stones along the way to making robotic servicing the way of the future.”
Robotic Refueling
Now in the second phase of its on-orbit demonstration aboard the International Space Station, NASA’s Robotic Refueling Mission (RRM) is using the Canadian Space Agency’s two-armed robotic handyman, Dextre, to show how future robots could service and refuel satellites in space.
One of those tools, VIPIR, short for Visual Inspection Poseable Invertebrate Robot, is a robotic, articulating borescope equipped with a second motorized, zoom-lens camera that would help mission operators who need robotic eyes to troubleshoot anomalies, investigate micrometeoroid strikes, and carry out teleoperated satellite-repair jobs. NASA successfully demonstrated VIPIR’s capabilities earlier this year. During RRM’s third phase, the SSCO team plans to demonstrate the transfer of xenon, a colorless, dense noble gas potentially useful for powering ion engines.
RRM, however, is only one piece of SSCO’s ongoing efforts to making servicing a tried-and-true capability for future missions.
ROSE and Restore
To be easily serviceable, regardless of its orbit, the satellite itself must be specially designed to accommodate repairs. For example, NASA’s MMS serviceable satellite bus featured a modular design that made it easy for astronauts to install a new attitude control system when the original failed on Solar Max.
Though not modular like MMS, Hubble did support on-orbit servicing on a component level. Like opening a door, astronauts literally would pull out an instrument before reinserting the new one into the same cavity a job made easier with the observatory’s 76 handholds. However, Hubble’s lack of modularity meant that NASA had to develop special tools and procedures specifically for nearly each component and task.
“Although Hubble servicing was extremely successful, the missions were complex and required a highly orchestrated combination of robotic and astronaut activities,” observed Dino Rossetti, of Conceptual Analytics in Glen Dale, Maryland, in a paper submitted at an American Institute of Aeronautics and Astronautics conference in September.
Modularity is key, and SSCO is taking it to a new level, Keer said.
The organization now is developing the Reconfigurable Operational spacecraft for Science and Exploration (ROSE), a low-cost spacecraft concept that seeks to build on the success of MMS. The organization’s overriding goal is long-term affordability and servicing at a system level, which would make ROSE highly flexible for medium-size missions, Keer said.
“We view ROSE as a pathfinder for future missions,” Reed added.
Repair Craft Needed
Also needed is a robotic servicing vehicle. Reed said his organization has focused on developing that capability, as well. For the past few years, the organization has pursued a notional mission called Restore, which would be capable of refueling satellites in both geostationary and low-Earth orbits.
Key servicing technologies, he said, are baselined for NASA’s proposed asteroid mission, ARM, which involves the capture of a large boulder from the surface of a near-Earth asteroid and moving it into a stable orbit around the moon for astronaut exploration. The same type of spacecraft also could refuel a government satellite in low-Earth orbit, as called for in NASA’s RFI.
“I can imagine a fleet of Restores,” Reed said. “A single servicer could refuel and service WFIRST and another future observatory at the SEL2 orbit. Another could be parked in another orbit for other servicing tasks, such as helping assemble a 65-foot segmented mirror in space, he said.
“We’re taking all we learned over the past many years, robots and humans working together,” Keer added. “Our attitude here is you have to have one foot in the future. We expect to be on the cutting edge. Servicing at more distant orbits, such as SEL2, is coming,” she said.
