Truth behind Challenger disaster, how weather was to blame
Sunday, January 28, 2018, 12:13 - Thirty-two years ago, the coldest and windiest weather ever recorded during a launch in central Florida contributed to one of the worst space disasters in history. Here's how.
On a chilly Tuesday morning, on January 28, 1986, seven US astronauts climbed on board the Space Shuttle Challenger, ready for a six day trip into orbit around the Earth. Just 73 seconds after liftoff, at 11:38 a.m. EST, their mission ended when the shuttle broke apart and crashed back down to Earth.
Lost in the accident were Francis Scobee, Michael Smith, Ellison Onizuka, Judith Resnik, Ronald McNair, Gregory Jarvis, and Christa McAuliffe.
Many things can go wrong during a space launch. Thousands of parts go into the construction of a space vehicle, and even a minor failure can turn very bad given the stresses put on these vehicles as they climb towards space or return back down to Earth. A common refrain heard when something goes wrong is "space is hard."
Challenger sitting on the foggy launch pad for its very first flight into space, in April of 1983. Credit: NASA
So, what happened with the Challenger disaster that took the lives of seven people and grounded the shuttle program for over two and a half years?
The technical problem uncovered during the investigation of the accident was found to be with one specific part of the shuttle - an O-ring near the base of the solid rocket boosters, which seals the gap between two sections of the booster, to prevent hot exhaust gases from escaping through that gap. Although these O-rings had functioned for 24 shuttle launches prior to this one, there was a flaw in their design that contributed heavily to the destruction of the Challenger. They were not rated for operation below around 4oC (39 F).
Record cold launch morning
During the launch before Challenger's fateful liftoff, just two weeks earlier, when the Space Shuttle Columbia flew into space early in the morning on January 12, temperatures at Cape Canaveral were roughly 13oC (55 F).
On the morning of January 28, however, the mercury had plunged below freezing in central Florida, as part of a cold snap that had enveloped parts of US Southeast that day (to the north, Atlanta, GA had an overnight low of near -14oC (7 F), while Montgomery, AL, had a low of -9oC (16 F)).
For several communities in the area of Cape Canaveral, they experienced their coldest January 28 morning on record, and the day retains that record even now.
Temperature records from NOAA, for the morning of January 28, 1986. Credit: NOAA NCDC
These record cold temperatures caused significant icing of Challenger's launch tower, as seen in the images below:
Ice on the launch tower is not necessarily a problem for a launch, though. Temperatures managed to climb above freezing by the 11:37 a.m. launch time, although only around 35 F, and ground crews were able to address any direct problems from ice build up.
However, it was specifically how the cold affected the O-ring on the right solid rocket booster - the one that was still in shadow as the Sun rose, and thus did not benefit from as much heating prior to launch - that was the problem.
According to the investigation of the accident, "[a] warm O-ring that has been compressed will return to its original shape much quicker than will a cold O-ring when compression is relieved," and "[a] compressed O-ring at 75 degrees Fahrenheit is five times more responsive in returning to its uncompressed shape than a cold O-ring at 30 degrees Fahrenheit."
Thus, the cold that seeped into the O-rings during that chilly overnight caused them to stiffen and become less responsive, and thus less capable of doing the job they were designed for.
When Challenger lifted off the pad and rose into the sky, the cold O-ring was not able to respond fast enough to the stresses being exerted on the right solid rocket booster. This opened up gaps between the two parts, allowing hot exhaust gases to escape. Normally, these hot gases would have actually caused the O-rings to expand, forming a tighter seal and thus limiting any danger. In this case, though, the cold weather slowed this process, allowing more gases to escape for a longer period of time, which vapourized much of the O-ring in the process.
If that had been the only problem encountered by the shuttle, Challenger likely would have still made it to space, intact and safe, and could have even completed its mission. This is because the aluminum oxide by-products of the burning rocket fuel formed a strong seal between the two solid booster parts, stopping the leak. Thus, the right solid rocket booster could have held together long enough to reach the height where those boosters were jetisoned. In that case, rather than having STS-51-L on NASA's record as a failure, it would have gone down in history as yet another successful space shuttle mission.
Not just cold, but windy
At 37 seconds after liftoff, however, Challenger passed through a number of wind shear events - where the direction and speed of the wind changes very suddenly (and often dramatically). For a full 27 seconds, the shuttle plunged through these sudden changes in wind speed and direction, with the flight computer reacting exactly as it should for the situation. As the NASA report noted, however, "[t]he wind shear caused the steering system to be more active than on any previous flight."
According to a study that appeared in the Bulletin of the American Meteorological Society shortly after the accident, although there was some indication that there could be wind shear and clear air turbulence over north-central Florida that morning, there were no direct measurements of it, so there was no way to determine, ahead of time, how strong the wind shear would be. Shuttles are rated to handle certain levels of wind shear, but without knowing the conditions, it would not have been possible to tell if they exceeded what the shuttle could handle.
As the shuttle was subjected to the wind shear conditions and the flight computer compensated for them, this put even greater stresses on the already compromised solid rocket booster, and towards the end of the sequence of maneuvers, the seal created by the aluminium oxide particles broke, and a plume of flame became noticeable from the booster by those observing on the ground.
By the time the shuttle cleared the wind shear, at just 64 seconds after launch, the plume had grown larger, as it burned through the joint and began to burn a hole in the exterior fuel tank. Once breached, the tank started leaking hydrogen fuel, causing more smoke to stream away from the shuttle.
With all of this going unnoticed by the on-board crew and the flight controllers, when the order was given to throttle up for the rest of the journey into orbit, the stresses on the spacecraft - as a result of both the cold and the wind shear - proved to be too much. Both the damaged solid rocket booster and the breached fuel tank failed, igniting the remaining fuel.
Without the proper thrust, Challenger veered off course, encountering wind stresses from the air flow roughly four times what it was designed to withstand. The vehicle was subsequently torn apart and crashed back down to Earth.
In the years since this tragic accident, there have been many treatices on the ultimate cause - from the official NASA investigation, to far less official efforts.
From examining everything that was presented about the incident, while there was a design flaw in the O-rings, preventing them from performing properly at low temperatures, and it was human decisions that put Challenger in the sky on the morning of January 28, 1986, there is still no reason to believe that the shuttle was doomed the moment it lifted off from the launch pad. Had there been calm weather conditions along the flight path, even the compromised solid rocket booster likely could have continued to do its job, delivering the shuttle safely to orbit.
It was the strong wind shear that Challenger encountered that ultimately caused the booster to completely fail, cutting the shuttle's mission short and claiming the lives of the seven people on board.
Safety measures for space flights have improved since, in the aftermath of this tragic accident. Even so, now 32 years later, weather still remains a major influence on the launch schedule of NASA and other space agencies, both public and private.
Challenger stands as the example of exactly how badly things can go with the wrong combination of conditions, and why launch control crews are extremely careful when it comes to avoiding adverse weather.