Orbital Debris Mitigation Strategies
Collisions Increase; Explosions Decrease
Upper stage rocket explodes, spewing out chunks of debris. Credit: European Space Agency.
The Earth is encircled with human-made orbital flotsam. Space junk comes in all shapes and sizes, such as abandoned satellites and leftover booster stages.
Discarded space hardware includes a variety of launch vehicle upper stages, left on orbit after they are spent. Add to this mix abandoned spacecraft that are no longer functioning. Toss in for good measure separation bolts, lens caps, momentum flywheels, nuclear reactor cores, clamp bands, auxiliary motors, launch vehicle fairings, and adapter shrouds.
Given hypervelocity speeds in space, even the most minuscule of rubbish could create havoc on impact with a functioning satellite.
As of today, on-orbit explosions have been the primary source of debris. Explosions can occur when propellant and oxidizer inadvertently mix, residual propellant becomes over-pressurized due to heating, or batteries become over-pressurized. See Figure 1.
While the threat of orbital debris to operating spacecraft is very real, there are encouraging steps being taken by many spacefaring companies and countries. For example, deployment procedures can be designed to prevent ejection of objects. Tethered lens caps and bolt catchers for explosive bolts are examples of preventive design.
To avert explosions, space hardware that stores energy can be passivated at the end of its useful life. Passivation is a practice of ridding a spent rocket stage of any residual stored energy left on-board, typically taking the form of venting leftover propellants or pressurants, or controlled discharging of any batteries or momentum wheels.
According to The Aerospace Corporation’s Center for Orbital and Reentry Debris Studies, “As the debris mitigation measure of passivation becomes more commonly practiced, it is expected that explosions will decrease in frequency. It may take a few decades for the practice to become implemented widely enough to reduce the explosion rate, which currently stands at about four per year.”
Underscored in a report issued earlier this year by the U.S. Defense Advanced Research Projects Agency (DARPA), there is a lengthy history of trying to curb orbital debris. As pointed out in DARPA’s Catcher’s Mitt Final Report, to help control the future growth of orbital debris, most spacefaring nations have adopted measures to limit the creation of new orbital debris.
In 1995 NASA was the first space agency in the world to issue a comprehensive set of orbital debris mitigation guidelines. Two years later, the U.S. Government developed a set of Orbital Debris Mitigation Standard Practices based on the NASA guidelines. Other countries and organizations, including Japan, France, Russia, and the European Space Agency (ESA) have followed suit with their own orbital debris mitigation guidelines.
In 2002, after a multi-year effort, the Inter-Agency Space Debris Coordination Committee (IADC), comprised of the space agencies of 10 countries as well as ESA, adopted a consensus set of guidelines designed to mitigate the growth of the orbital debris population.
In February 2007, the Scientific and Technical Subcommittee of the United Nations’ Committee on the Peaceful Uses of Outer Space (COPUOS) completed a multi-year work plan with the adoption of a consensus set of Space Debris Mitigation Guidelines very similar to the IADC guidelines. The guidelines were accepted by the COPUOS in June 2007 and endorsed by the United Nations in January 2008.
Currently accepted mitigation measures include limiting the use of explosive bolts and other disposable deployment mechanisms, limiting orbital lifetimes of retired payloads and spent rocket bodies to 25 years, and venting unused propellant at the end of operations.
While the United Nations 2008 Report on Space Debris discusses these guidelines’ contribution to a slower growth in the space debris population, “these efforts have only slowed the overall growth in the amount of space debris, not halted it,” the DARPA report states. “To date, on-orbit explosions have been the primary source of debris. Nevertheless, collisions are expected to be the leading source within the next few decades.”
In June of last year, the Obama administration issued a National Space Policy that among many other things requires the United States to “lead the continued development and adoption of international and industry standards and policies to minimize debris, such as the United Nations Space Debris Mitigation Guidelines … pursue research and development of technologies and techniques, through the Administrator of NASA and the Secretary of Defense, mitigate and remove on-orbit debris, reduce hazards, and increase understanding of the current and future debris environment.”
The French Connection
Dutifully aware of the menacing problem of orbital debris – and not contributing to an already cluttered situation – is French launch provider, Arianespace. It was founded in 1980 as the world’s first satellite launch company. Since its creation, the enterprise has chalked up a notable roster of boosting spacecraft for a variety of nations.
The spaceport used by Arianespace – also known as the Guiana Space Center – is a strategically-located facility in French Guiana. That locale makes it ideally situated for missions into geostationary orbit. The technical performance of its launch vehicles and a substantial order book have made Arianespace the world leader in satellite launch services for the last few years, with a market share exceeding 60 percent.
“Arianespace takes the issue of orbital debris mitigation seriously and actively works with the French Government to ensure that our launch systems respect international agreements whose goal is to reduce space debris,” said Clayton Mowry, President of Arianespace, Inc., with responsibility for managing Arianespace’s customer, industry and governmental relations at the company’s U.S. affiliate.
Mowry told Imaging Notes that under the French Space Operation Act and policy, Arianespace authorization to proceed for each mission and with each launch system is associated with mission optimization strategies that take into account debris mitigation planning.
The latest version of the Ariane 5 booster is called the Ariane 5 ECA, for Cryogenic Evolution type A. This powerful launch system is qualified to use the propellant tanks’ residual pressure and altitude control systems to lower the upper stage apogee altitude prior to a final passivation procedure, Mowry noted. “This practice prevents the upper stage from crossing into the geostationary region populated by communications satellites.”
Nicholas Johnson, chief scientist for Orbital Debris at the NASA Johnson Space Center in Houston, Texas flags the most important actions to mitigate the growth of debris from a launcher perspective:
- passivate the orbital stages;
- leave low Earth orbit stages in short-lived orbits, e.g., orbits from which they will re-enter within 25 years.
“Both of these actions are recommended by the IADC and the UN. Some launch service providers do a good job in both areas; some do not,” Johnson told Imaging Notes.
Johnson spotlighted the recent June launch of Aquarius SAC-D. The Aquarius mission is the first satellite mission specifically designed to provide monthly global measurements of how sea water salinity varies at the ocean surface, and was developed in an international partnership with Argentina’s space agency, Comisión Nacional de Actividades Espaciales. “For the SAC-D mission, the Delta 2 second stage performed a depletion maneuver that moved the stage from an orbit with a lifetime of more than 20 years to one with a lifetime of only a month or two,” Johnson said.
The primary focus of the Secure World Foundation is on space sustainability – the ability of all humanity to continue to use outer space for peaceful purposes and socioeconomic benefit over the long term. Indeed, unsafe or irresponsible actions by one actor can have long-term adverse consequences for all.
Given the growth of using outer space by an increasing number of governmental and non-governmental actors, steps to deal with the worrisome issue of orbital debris – today and into the future – are welcome news. That being said, there are many more steps that must be taken to tackle this environmental problem of humankind’s own making.