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Truth behind Challenger disaster, how weather was to blame

Scott Sutherland Meteorologist/Science Writer

Monday, January 28, 2019, 10:30 AM - Thirty-three 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.

Originally published on Jan 28, 2016, this article has been updated.

On the chilly Tuesday morning of January 28, 1986, seven U.S. astronauts climbed on board the Space Shuttle Challenger, ready for an 11:37 a.m. ET launch into Earth orbit for a six day stay in space.

Just 73 seconds after liftoff, at 11:38 a.m. EST, their mission tragically ended, when the shuttle broke apart and crashed back down to Earth. Killed 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 Challengerdisaster 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 - a rubber O-ring near the base of one of the two solid rocket boosters. These O-rings function to seal the field joints between the sections of the booster, and they are meant to expand and contract quickly, as the sections of the booster flex and shift under the stresses of launch, to prevent hot exhaust gases from escaping through these joints.



A schematic of the solid rocket booster used by Challenger. The 'field joint' closest to the nozzle is the location of the O-ring that failed during launch. Credit: NASA

Although these O-rings had functioned without incident for 24 shuttle launches prior to this one, there was an inherent flaw in their design that contributed heavily to the loss of Challenger: they were not rated for operations below 4°C.

RECORD COLD LAUNCH MORNING

During the launch before Challenger's fateful liftoff, just two weeks earlier, when the Space Shuttle Columbiaflew into space early in the morning on January 12, temperatures at Cape Canaveral were roughly 13°C.

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 -14°C, while Montgomery, AL, had a low of -9°C.

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:



Long icicles hang off Challenger's launch tower.

Credit: NASA

This component was nearly completely iced over.

Credit: NASA

Ice on the launch tower is not necessarily a problem for a launch, though. With the launch delayed to allow temperatures to warm with sunrise, they managed to climb above freezing by the 11:37 a.m. launch time, although only around 2°C above, and ground crews were able to address any direct problems from ice build up.

However, it was specifically the effect the cold had on 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 direct sunlight to warm up 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." For reference, 75°F = 23.9°C, while 30°F = -1.1°C.

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 Challengerlifted 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 will actually cause 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 through the joint, for a longer period of time, which vapourized much of the O-ring in the process.



This recovered piece of the booster shows the melted rubber from the O-ring. Credit: NASA

This view shows the hole the rocket exhaust burned through the booster. Credit: NASA

If that had been the only problem encountered by the shuttle, it's very likely that it would have still made it to space safely and even completed its mission. This is because, according to the NASA report, the very by-products of the burning rocket fuel and vapourizing O-ring combined to form a reasonably strong seal between the two solid booster sections, thus stopping the leak.

So, if Challengerhad encountered no other complications, shuttle mission STS-51-L would probably have gone down in history as simply STS-51, and it would have been yet another successful trip to space and back for NASA. The only indication they would have had that anything had gone wrong was when they retrieved the booster from its splashdown, and saw the damage done to the O-ring.

NOT JUST COLD, BUT TURBULENT

Unfortunately, the cold O-ring was not the only complication the weather visited upon this particular shuttle mission.

At 37 seconds after liftoff, Challenger began passing through a series of wind shear events - different layers of the atmosphere along its launch trajectory where the winds changed direction and/or speed, very suddenly and dramatically, between those layers.

A weather balloon launched a few hours prior to liftoff revealed strong winds aloft, but did not indicate any particularly strong shear or turbulence. Reports from the time say that pilots conducting test flights across the area did experience some wind shear, however it was said to be within acceptable limits. Thus, the upper-air weather and pilot reports cleared the controllers to go ahead with the launch.

By the time of launch, however, conditions had apparently become significantly worse.

In a study published in the Bulletin of the American Meteorological Society in October 1986, led by scientists at NASA's Goddard Space Flight Center, the researchers performed a detailed examination of the weather conditions above the southeastern United States for the morning of January 28. They found that two different 'jet streaks' - strong flows of wind, high up in the air, which are embedded within the flow of the jet stream - were overlapped on top of one another above north-central Florida, at the time of the Challengerlaunch. One of these jet streaks, the 'polar front jet' or PFJ, was blowing from a northwesterly direction, while the other, the 'sub-tropical jet' or STJ, was accelerating over the region, above the PFJ, from the west-southwest. Simulations performed at the time showed that Challengerwould have passed through several layers of moderate to strong wind shear, which "had the potential for clear-air turbulence in north-central Florida at the time of launch."

For a full 27 seconds, the shuttle plunged through this turbulence, with the flight computer reacting exactly as it should have for the situation, making corrections as necessary to keep Challengeron course. As the NASA report noted, however, "[t]he wind shear caused the steering system to be more active than on any previous flight."

This unfortunate situation put even greater stresses on the already compromised right solid rocket booster. Towards the end of the shuttle's sequence of maneuvers, a plume of flame became noticeable from the booster by those observing on the ground, as those added stresses broke the seal on the right booster rocket, and allowed the exhaust gases to escape through the joint, once again.



Views from the launch site tracker camera show the emergence of the exhaust plume from Challenger's right solid rocket booster. Credit: NASA

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 seam, and then began to burn a hole in the exterior fuel tank. Once breached, the exterior fuel tank began 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. The damaged solid rocket booster and breached fuel tank failed, igniting the hydrogen fuel in the tank.

Without the proper thrust, Challengerveered 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.



Space Shuttle Challenger breaks apart, at 11:38 a.m. EST, January 28, 1986. Credit: NASA

While safety measures for space flights have improved since, in the aftermath of this tragic accident, and even now, 33 years later, weather still remains a major influence on the launch schedule of NASA and other space agencies, both public and private.

Challengerstands as the example of exactly how badly things can go with the wrong combination of conditions, and for why launch control crews are extremely careful when it comes to avoiding adverse weather conditions.

Sources: NASA| NOAA | Weather Underground| BAMS (pdf)

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