After last week’s explosion of a spent Russian rocket booster in the skies over the Cayman Islands, urgent questions are being asked about the threat to aviation posed by falling space debris.
The Cayman Islands Airports Authority last week minimised the danger, acknowledging the potential threat, but pointing out no damage had resulted when the booster, dubbed SL-14 R/B, having lifted communications satellite 39033 into orbit in December, decayed on Monday night over the coast of Honduras, tracing a fiery arc across Cayman, heading for Cuba and finally disintegrated in the night sky.
On Monday, former CIAA chairman Dick Arch echoed previous remarks, saying chances of a collision were so slim as to be negligible. “When you think of the speed and course [of debris], the chances are one in a billion,” he said, observing that lightning strikes to aircraft occur several times each year with little damage.
“I have never heard of an aircraft ever being hit” by falling debris, Mr. Arch said.
Nevertheless, the problem continues to loom, increasing with the proliferation of cheap air travel and the accumulation of space junk, which includes everything from spent rocket stages, old satellites, lost equipment, fragments from disintegration, erosion and collisions.
Estimates are that 19,000 pieces of debris larger than 2 inches orbit the earth, as well as 300,000 pieces smaller than 0.4 inches, including dust from rocket motors, surface-degradation products such as paint flakes and even coolant released by nuclear-powered satellites. The impacts of these particles are similar to sandblasting.
While most debris incinerates in the atmosphere, larger objects can reach the ground. The original Skylab station entered the earth’s atmosphere in 1979, trailing debris across the Indian Ocean and Southern Australia; a Navstar rocket crashed in Saudi Arabia in 2001 after a “catastrophic orbital decay”, while the Columbia Space Shuttle in 2003 shed debris across a swathe of the southern US, ending in Florida.
A US Federal Aviation Administration study after the shuttle accident determined the probability of an impact between Columbia debris and commercial aircraft in the vicinity was at least one in a thousand, and the chance of an impact with a general aviation aircraft was at least one in a hundred.
In March 2007, the pilot of a LAN Chile flight to New Zealand, carrying 270 passengers, notified Auckland air traffic controllers after seeing flaming wreckage from a Russian satellite five miles from his plane. He reported hearing the sonic boom as it passed. The Pacific Ocean is among the safest places in the world for a satellite to descend because of its vast uninhabited areas.
A pilot with Australian and International Pilots Association said the debris could have caused catastrophic consequences had it struck the aircraft.
“For [the pilot] to have heard it, one of two things – [the debris] was a lot closer than he thinks or it was bigger and going at quite a high speed,” Qantas Captain Steven Anderson said.
With more than 1,000 operating satellites and more than 13,000 objects in low earth orbit, the problem appears to be growing. More than 38 large debris objects have re-entered the atmosphere since the beginning of 2012.
Washington’s Federal Aviation Administration, the Office of Commercial Space Transportation and the US Department of Defence have developed aircraft vulnerability models, concluding that a fragment as small as 300 grams would prove “catastrophic” to aircraft, particularly if penetrating the fuselage or fuel tank.
Still Anthony Philbin, acting chief of the communications section of the Montreal-based International Civil Aviation Organisation, said the risk from space debris “is extremely low and, to date, we have no record of any incidents, adding that “no one has ever been killed by falling space junk, whether in an aircraft or not,” although five Japanese sailors and one Australian woman have been struck by falling fragments.
“Inbound meteorites are a more common occurrence,” Mr. Philbin said, pegging their numbers at 50 per day. “Space debris, in contrast, comes in at a rate of 150 pieces per year. “Larger pieces usually have functioning engines and are guided back by the [United] States who operate them in a controlled manner and with the required hazard warnings.”
In a May meeting in Montreal, the International Association for the Advancement of Space Safety assessed the risks to aviation of falling debris, saying 40 large space debris objects annually re-entered the Earth’s atmosphere, and that between 10 per cent and 40 per cent of their mass survived, “posing hazard to people and property”.
“The location of uncontrolled re-entries is unpredictable,” the society sad, “and the debris impacts in long, thin ground footprint[s]”.
Only four months earlier, in January, the group noted, the Russians issued a “Note to Airmen”, citing a decaying satellite and asking Europe to close its airspace for two hours. Authorities declined, however, estimating potential costs at 20 million euro.
The society said it hoped to complete the initial work this autumn, mapping air traffic density and calculating the probability of impacts and the vulnerability of aircraft, observing the impact point for surviving fragments was impossible to predict, the spread of fragments depended on where they are released, while wind speed and direction are critical.
Already, the outlines of a space-vehicle-mounted Reentry Direct Broadcasting Alert System are apparent helping ease concerns.
According to Space Safety Magazine, when a spacecraft carrying the system enters the atmosphere, it relays a message with the coordinates of the falling debris area to anyone with a receiver and a display.
Predictions remain uncertain, however, largely due to the skipping effect that an uncontrolled spacecraft will experience as its hits the upper atmosphere. When the skipping ends, trajectory and overall location of fragments can be calculated, but leaving little time for evasive manoeuvres.
The magazine quotes the system’s inventor, Tommaso Sgobba, saying an airplane would have between five minutes and seven minutes to get out of the way.
“With a typical hazard area being 1,000-2,000km (620-1,240 miles) long, but very narrow, 30-70km (18.5-43.5 miles), an escape manoeuvre from the risky area can be readily performed or the aircraft could hold its position until it is safe to cross the hazard area,” Mr. Sgobba said.
The system is unlikely, however, to address the random re-entry of decaying fragments, leaving wider worries outstanding.