Op-ed | A Not-so-final Servicing Mission
The Hubble Space Telescope is one of the greatest tools ever designed for science. It has studied questions far beyond the reach of any other instrument. With its three deep field exposures alone, it has seen farther at more wavelengths than any ground-based telescope, and it revolutionized our understanding of the early universe. For decades, some scientists have predicted that adaptive optics would “soon” render it obsolete. Hubble has proved them wrong every time.
Twenty-five years after launch, the queue of scientists hoping for a shot of Hubble telescope time extends around the proverbial block. Those at the end of the line may be disappointed. The final Hubble servicing mission was launched in May 2009 and, absent the space shuttle, we are told Hubble cannot be repaired again.
Without another servicing mission, Hubble will inevitably fail, most likely early in the next decade. But is the shuttle really necessary to mount a repair mission?
Scientists had hoped to operate Hubble at the same time as the James Webb Space Telescope, currently planned for launch in late 2018. In spite of its giant 6.5-meter primary mirror, and its being advertised as a “successor” to Hubble, it is not a successor. Webb is optimized for infrared wavelengths extending just into the visible range (orange), while Hubble operates at ultraviolet and visible wavelengths, extending just into the near-infrared. The two instruments are complementary, with a slight overlap. There is no successor to Hubble; no comparably large space telescope designed to observe at visible or ultraviolet wavelengths is anywhere near launch.
Continuing technological and management problems with the JWST may well delay that instrument’s debut beyond Hubble’s ability to survive without further maintenance. If so, a tremendously valuable yet low-cost scientific opportunity to simultaneously examine the same objects at high resolution across an extremely broad range of optical and infrared frequencies will slip away.
Hubble was designed for servicing in orbit, and five repair flights were conducted by space shuttle astronauts, including the initial one to install extra lenses to compensate for a flawed primary mirror. (Confusingly, NASA counts four maintenance missions, because mission three was split into two flights, servicing missions 3A and 3B.)
Any future repair mission will most likely need to replace failed gyroscopes and reboost the telescope into a higher orbit. The gyroscopes spin at 19,200 revolutions per minute on gas bearings to measure and counter telescope drift, providing an extremely still platform for observations. According to the European Space Agency, they are the best available, but having moving parts they are prone to failure.
Hubble’s orbit is low enough to encounter odd particles of air at the top of the atmosphere. Friction with these causes the orbit to slowly decay. Without a boost, it will re-enter early in the 2020s. Replacing instruments with more up-to-date cameras would be nice, but is not essential to keeping Hubble in business and productive.
In terms of resources, the ability to carry cargo and to conduct spacewalks is essential. Excess propellant or a propulsion module is required for the reboost. We will soon have at least one spacecraft with all of these characteristics.
The SpaceX Falcon 9-launched Dragon v2, or “Crew Dragon,” is a NASA-subsidized, privately owned, human-rated capsule in advanced development. If the rest of its development goes well, it could fly as early as 2016 and be operational in late 2017. It will have every one of the required characteristics. Crucially, it has what amounts to a miniature version of the space shuttle’s payload bay — an unpressurized “trunk.”
The trunk is the cylindrical adapter between the spacecraft and its Falcon 9 launch vehicle, which remains with the Dragon capsule until just before re-entry. The freighter version of the Dragon has already carried a number of large experiments in its trunk for mounting on the exterior of the International Space Station. If the commercial crew trunk is used the same way, it should be able to easily carry multiple 11-kilogram Hubble rate sensor units, each about the size of a breadbox and carrying two gyroscopes. The telescope has a total of six gyroscopes; three are required for normal observations.
The Crew Dragon also has powerful rocket engines mounted around its perimeter to escape an exploding rocket. If those engines are not needed in an emergency, their fuel is available for use in orbit.
SpaceX, manufacturer of both the Dragon and the Falcon 9, did a very preliminary, informal study of using Crew Dragon with a robot arm to deorbit Hubble, or to repair and reboost the telescope. This was part of a wider SpaceX PowerPoint presentation on using Crew Dragon to service satellites, publicly released in March 2010 just before the first Falcon 9 launch.
At the time, the Crew Dragon design was envisioned as being closer to that of the Dragon commercial cargo spacecraft than is the case today, but its gross characteristics were similar to today’s specifications.
The study envisioned three crew members (versus the seven astronauts Crew Dragon is theoretically capable of carrying) with an undetermined number of spacesuits. Equipment for extravehicular activity would be stored in the trunk, along with a small propulsion module. The module would attach to the docking device that was mounted on Hubble during the final shuttle servicing mission. Under one scenario, the propulsion module would be used to safely deorbit Hubble into a terrestrial ocean — and that would be that.
A more interesting scenario would use the robot arm and/or spacesuited astronauts to change out failed equipment, and possibly to install a new instrument or two. The propulsion module would then boost Hubble into a higher orbit, where it would continue operations. With regular servicing flights, Hubble could conceivably remain active and productive indefinitely.
In December 2014, David L. Akin of the University of Maryland’s Space Systems Laboratory presented the results of his study of maintaining Hubble using the Crew Dragon or Sierra Nevada Corp.’s Dream Chaser commercial crew spaceplane. He considered mounting an inflatable airlock in the nose cap of Dragon to improve the efficiency of spacewalks, although this would make it impossible to form a rigid dock to Hubble. The trunk could hold relatively long and robust robot arms — with additional joints, one design might be as capable as the shuttle’s Canadarm. If two smaller arms were carried, one could form a “soft dock” and create a path for astronauts to use to get to the telescope, while the other manipulated cargo and tools.
SpaceX does not provide much information to the public about its plans, or even details of nearly operational vehicles. However, Akin told me, “As I understand it, the avionics on Dragon are capable of operating in a vacuum,” meaning the capsule could be evacuated to allow spacewalks without the inflatable airlock. He added, “SpaceX is also developing their own spacesuit in house.”
Akin said he did not have enough details about the other winner of NASA’s commercial crew competition, Boeing’s CST-100, to evaluate it for a Hubble maintenance mission. It is not clear that the CST-100 would be able to carry the required cargo, although that might be launched on a separate flight. Akin concluded that the loser in NASA’s commercial crew competition, Sierra Nevada’s Dream Chaser mini-shuttle, would offer some advantages over Dragon. At a March 17 press conference to reveal the automated cargo version of Dream Chaser, intended to win a commercial cargo contract to supply the International Space Station, Sierra Nevada mentioned using a crewed version to maintain Hubble. Unfortunately, that idea remains a political and technical long shot. Dream Chaser would have to win a cargo contract over the incumbent commercial cargo contractors (SpaceX and Orbital ATK) and find the financial wherewithal to adapt that to the crewed version — all before Hubble succumbs to the atmosphere. However, Akin concludes that Hubble servicing is “definitely feasible” from either Dream Chaser or Dragon.
Lockheed Martin has proposed a spacecraft called Jupiter for the next round of space station commercial cargo contracts. According to the company, “The Jupiter spacecraft builds upon the design of MAVEN, now in orbit around Mars, and OSIRIS-REx, currently under construction for an asteroid sample return mission,” with the addition of a robot arm. After its first launch with an attached cargo module, Jupiter would be left in orbit to dock with new, separately launched modules, which would deliver cargo to the space station — rather like standardized shipping containers are loaded aboard seagoing freighters — or to conduct satellite maintenance missions. This vehicle, with its robot arm, pressurized module and ability to carry cargo, would be an ideal companion to one of the commercial crew vehicles in supporting a Hubble repair.
Neither NASA nor SpaceX has unambiguously revealed the cost of a Crew Dragon mission, but it is clear that a Dragon Hubble maintenance flight would cost much less than a space shuttle servicing mission. Science writer Govert Schilling last year suggested a crowdfunding campaign to raise some of the money, a strategy he said some at the Space Telescope Science Institute were considering. Space analyst Tim Kyger suggested SpaceX should negotiate doing it “on their own dime. “It might be salvage,” he said, and then SpaceX could own Hubble outright and rent it out to scientists.
With two commercial crew vehicles being prepared for flight, in addition to NASA’s Orion spacecraft, we are entering a period when human spaceflight will be relatively routine and low-cost.
There is no longer any excuse to wastefully abandon major assets — built and placed in orbit at great expense — and let them fall into the ocean. After refurbishing Hubble, NASA should also consider using Orion to maintain the JWST, utilizing a docking ring that was added to the telescope for just such a contingency.
As we approach Hubble’s 25th anniversary on April 25, it is time to start planning for the observatory’s next 25 years.
Donald F. Robertson is a freelance space industry journalist based in San Francisco. He is a shareholder of Orbital ATK, a company mentioned in this article. For further examples of his work, see www.DonaldFRobertson.com. Follow him on Twitter: @DonaldFR