Extraterrestrial Debris (Expanded Version)

The remote possibility that an uncontrolled re-entry of orbital debris, also called space debris, could endanger civil airspace falls far outside normal experience. Space debris is defined at the international level as “all man-made objects, including fragments and elements thereof, in Earth orbit or re-entering the atmosphere, that are non-functional.”¹

Before initiating controlled spacecraft re-entries, U.S. programs must demonstrate that the probability of human casualty from the “surviving” debris — that is, debris not rendered harmless by atmospheric demise (objects being consumed by burning) — will not be greater than 1 chance in 10,000, says the U.S. National Aeronautics and Space Administration (NASA).²

In January, a de-orbiting spacecraft inspired the Russian Federal Space Agency (ROSCOSMOS) and Eurocontrol to direct the world’s attention to orbital debris re-entry issues such as reasonable preparedness, mitigations and limiting human casualty risk.³ The issues somewhat paralleled those surrounding the April 2010 eruption of the Eyjafjallajökull volcano in Iceland. The eruption caused the progressive shutdown of airspace across the European continent, disrupted trans-Atlantic aviation (see, “Clearing the Air” and prompted governments and airlines in many countries to revisit dozens of issues assumed to have been addressed adequately (see “Safety News April 2011”).

Working group reports from the July 2011 meeting of the International Volcanic Ash Task Force, under the auspices of the International Civil Aviation Organization, show that many opportunities for improvement were discovered and addressed. Equivalent gaps regarding orbital debris re-entry events — beyond alerting the aviation community — have not been identified, based on a brief ASW review of space agency summaries of known risks and mitigations. These summaries lacked a level of detail encompassing aircraft vulnerability during flight, protection of civil airspace or explicit risks to civil aviation but provided the following insights.

The United Nations and other organizations today advocate global cooperation, political will and timely investments by states in spacecraft “design to demise,” spacecraft disposal (by controlled re-entry or repositioning to safer orbits) and active removal of orbital debris by, so far, theoretical systems. In January 2010, for example, the Space Debris Mitigation Guidelines of the Committee on the Peaceful Uses of Outer Space — a U.N. publication — noted that this objective must involve not only space but also the “risk of damage on the ground, if debris survives Earth’s atmospheric re-entry.”

A number of documents outline the high-level issues and specify mitigation procedures required by national authorities. For example, according to the Inter-Agency Space Debris Coordination Committee (IADC), a forum for space agencies from 11 nations and Europe, “If a spacecraft or orbital stage is to be disposed of by re-entry into the atmosphere, debris that survives to reach the surface of the Earth should not pose an undue risk to people or property. This may be accomplished by limiting the amount of surviving debris or confining the debris to uninhabited regions, such as broad ocean areas. … The operator of the system should inform the relevant air traffic and maritime traffic authorities of the re-entry time and trajectory and the associated ground area.”⁴

NASA notes that “using materials that tend to demise [limiting the number and size of orbital debris fragments that survive] upon re-entry remains one of the more important strategies in reducing the debris risk to persons on the Earth.”⁵

European Alert of 2012

Eurocontrol placed its Network Management Directorate on standby status, then alert status, “for the possible uncontrolled re-entry of the Russian satellite, Phobos Grunt, into Europe’s busy airspace,” according to its summary of the January event. Eurocontrol said, “Aiming to collect soil samples from Phobos, one of the moons of Mars, [this spacecraft] failed to leave Earth orbit after its launch on 9 November [2011].

“[ROSCOSMOS] announced that it was expected to fall somewhere on Earth between Saturday and Monday, 14–16 January 2012. But they could not predict when — neither date nor time — or where this re-entry would happen, as it was affected by many changing factors, such as solar [space] weather and the spacecraft’s orientation.

On that day, Eurocontrol facilitated a coordination process for countries that were considering closing their airspace, conducted a teleconference for European air navigation service providers (ANSPs) and aircraft operators to discuss the Russian request, and arranged access to a website providing real-time tracking of this spacecraft’s re-entry.

“A number of ANSPs issued a NOTAM warning operators of the potential hazard but given the uncertainty as to the area of possible re-entry, no further action [such as closing airspace or grounding aircraft] was proposed,” Eurocontrol said. “The satellite landed in the Pacific Ocean, some distance away from the Chilean coast.”

Controlled vs. Uncontrolled

European Space Agency (ESA) scientists in 2009 explained the potential interactions between functional/non-functional spacecraft and the multiplication of orbital debris. “Only 6 percent of the [objects tracked by a U.S. program, about 600] are operational spacecraft, while 38 percent can be attributed to decommissioned satellites, spent upper stages [of rockets] and mission-related objects (launch adapters, lens covers, etc.). The remaining 56 percent originates from more than 200 in-orbit fragmentations. … These are assumed to have generated a population of objects larger than 1 cm [0.4 in in diameter] on the order of 600,000.”⁶

They also explained how, other than by failure to propel a spacecraft into an intended orbit or by traveling beyond the Earth’s gravity, even intact spacecraft ultimately experience an uncontrolled re-entry. “Satellites launched into low Earth orbit are continuously exposed to aerodynamic forces from the tenuous upper reaches of the Earth’s atmosphere,” ESA said. “Depending on the altitude, after a few weeks, years or even centuries, this resistance will have decelerated the satellite sufficiently so that it re-enters into the atmosphere. At higher altitudes — i.e., above 800 km [500 mi] — air drag becomes less effective, and objects will generally remain in orbit for many decades.”

NASA and its counterparts draw sharp distinctions between the relative risks to people from controlled versus uncontrolled re-entries of spacecraft, including implications of the survival of debris long enough to reach aircraft altitudes. “Controlled entry normally occurs by [driving] the spacecraft to enter the atmosphere at a steeper flight path angle,” NASA said. “It will then enter at a more precise latitude, longitude and [human survivability] footprint in a nearly uninhabited impact region, generally located in the ocean. … A controlled re-entry is mostly necessary when it is known that a large satellite may not demise upon atmospheric re-entry. An example of this is the Mir space station.⁷

“An uncontrolled re-entry is defined as the atmospheric re-entry of a space structure in which the surviving debris impact cannot be guaranteed to avoid landmasses. … Usually, large objects that have impacted the ground are from uncontrolled entries or orbital decay, so the impact point cannot be calculated exactly.”⁸

These NASA documents also explain the current assumptions about demise/survival when a spacecraft or large fragments re-enter the atmosphere. “Generally, orbital re-entry is assumed to begin at the entry interface altitude of 122 km (400,000 ft),” NASA said. “Spacecraft that re-enter from either orbital decay or controlled entry usually break up at altitudes between 84 to 72 km [275,600 to 236,200 ft] due to aerodynamic forces that cause the allowable structural loads to be exceeded. The nominal break-up altitude for spacecraft is considered to be 78 km [255,900 ft]. … Individual components or fragments will continue to lose altitude and experience aeroheating until they either demise [largely depending on the material] or survive to impact the Earth.”⁹

In the future, if the voluntary international mitigations are postponed or fail, the constant disintegration caused by collisions among existing objects could worsen “space junk” effects — to perhaps include more uncontrolled re-entries of defunct spacecraft and other orbital debris, the U.N. and IADC concur. Ongoing research, such as the study of the few confirmed fragments found to have reached the Earth’s surface, meanwhile helps scientists to refine their theories and models of spacecraft demise and better predict the full range of consequences of debris survival.

Notes

1. IADC. “IADC Space Debris Mitigation Guidelines.” IADC-02-01, Revision 1. September 2007.
2. NASA. “Process for Limiting Orbital Debris.” NASA Technical Standard NASA-STD-8719.14A. Dec. 8, 2011.
3. For more details, see www.eurocontrol.int/news/safe-skies-satellites-return.
4. IADC.
5. NASA. NASA Handbook for Limiting Orbital Debris. NASA Handbook 8719.14, July 30, 2008. The handbook also notes in a discussion of the origins of orbital debris, “The intentional breakup of [China’s] Fengyun-1C spacecraft in January 2007 via hypervelocity collision with a ballistic object [missile] created the most severe artificial debris cloud in Earth orbit since the beginning of space exploration.”
6. ESA Space Debris Office. www.esa.int/esaMI/Space_Debris/SEMQQ8VPXPF_0.html, updated Feb. 20, 2009. ESA also said, “The first-ever, accidental in-orbit collision between two satellites occurred … Feb. 10, 2009, at 776 km [482 mi] altitude above Siberia [Russia]. … Both were destroyed, and a large amount of debris [in orbit was] generated.”
7. NASA. NASA Handbook for Limiting Orbital Debris.
8. NASA. “Process for Limiting Orbital Debris.”
9. NASA. NASA Handbook for Limiting Orbital Debris.