Airliner’s mid-air impact prompts space-debris speculation - though a weather balloon may be the more likely culprit

A Boeing 737 Max 8 airliner was struck mid-flight by a suspected re-entry object, cracking its laminated windscreen while flying over Utah at an altitude of 36,000 feet.  

The United Airlines flight UA1093, enroute to Los Angeles from Denver on 16 October, diverted to Salt Lake City. Although inner windscreen laminations helped maintain pressurisation, the impact of the strike left windowpanes crazed with cracks. The captain of the flight was slightly injured by lacerations to his arm. The rest of those onboard, six crew members and 134 passengers, were unhurt. 

The incident is being investigated by the National Transportation Safety Board (NTSB). The captain reportedly said that the cause of the impact appeared to be re-entering space debris.  

Orbital analyst Jonathan McDowell has noted that no major spacecraft or orbiting rocket body re-entered at this time. However, smaller undetectable pieces may have made it through the atmosphere.

A more likely scenario is that the damage was caused by a weather balloon. Windbourne Systems, a company which operates weather balloons, has contacted the NTSB with its suspicion that one of its balloons may have been responsible for the strike.  

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Whether a weather balloon was involved or not (pun intended), space debris strikes on airliners have occurred before. On 25 December 1996, the cockpit windscreen of a Boeing 757, operated by China Southern Airlines and travelling from Beijing to Wuhan, was cracked by suspected falling space debris. Thankfully, given that the altitude at the time was 31,500 feet, pressurisation was maintained. The chance of an airliner being struck is likely to increase due to the exponentially growing number of spacecraft that are being launched, driven mainly by the growth of satellite constellations.  

When a spacecraft re-entry is expected, NOTAM warnings are issued to aircraft to avoid a projected impact area of any surviving parts. For example, this is usually the case for the controlled re-entries of Progress spacecraft in the South Pacific.  

However, more often space debris returns unannounced – prediction of times and landing areas of naturally re-entering hardware remains an inexact science. The satellite parts most likely to survive re-entry are low radius spherical objects, especially small propellant tanks made from stainless steel (SpaceX uses it on Starship for this reason).

Nonetheless, airliner flights remain a very safe way to travel. Commercial passenger aircraft have an annual fatal accident rate of 1 in 1 million, according to International Air Transport Association (IATA).  

In comparison, the fatal accident rate for human spaceflight is about 1 in 100, according to the Slingshot Seradata database. Most space-related strikes on the ground have usually involved falling or exploding stages during the launch. There has only been one incident involving a re-entering part directly hitting a human. A woman called Lottie Williams, 48, was exercising in Tulsa Park, Arizona, on 22 January 1997, when she was gently hit by a blackened six-inch piece of a Delta II rocket that had re-entered. Thankfully, she walked away from the incident unharmed.

The chance of an injury to an individual in an aircraft as the result a re-entering piece of space debris is less than one in a trillion, according to the FAA. Whether this will be re-assessed remains to be seen.