On this day, 21 years ago, the Space Shuttle Challenger and her crew of seven were lost when the vehicle broke up after 73 seconds of ascent, sending shockwaves through manned space flight that still remain today.

The failure of right Solid Rocket Booster’s (SRB) O-ring seal that ultimately destroyed Challenger changed not only the Space Shuttle program forever, but also highlighted the decision-making process, that at the time proved to be a death sentence.

It was to be the ‘Year of the Shuttle’ in 1986, with a launch on average every month, but the pressure was already raising grave concerns in the summer of 1985, as engineers from Morton-Thiokol – the company that supplied the SRBs to NASA – started to feel cut off from the decision-making process of launch parameters for the boosters.

Thiokol’s Bob Ebeling wrote the infamous ‘Help!’ memo in October 1985, informing everyone within his capabilities of the growing concerns about low temperature launches relating to the seals on the SRBs.

‘We didn’t think we were getting anyone’s attention and we thought by using the subject ‘Help!’ would get them (Thiokol top level management) to read at least the first sentence of the memo,’ said Ebeling.

Such concerns might not of been an issue if Challenger had launched within known parameters of the SRBs, but fate would come about in the strangest of circumstances. First a decision not to fill the External Tank with liquid oxygen and liquid hydrogen on the 26th of January came from an incorrect forecast from the Weather Station of bad weather.

‘We had been told on Saturday evening by the weather folks that we had a lot of wind and rain moving through,’ said Gene Thomas, Kennedy Space Center (KSC) launch director for STS-51L. ‘We decided we would not launch on Sunday – and Sunday was a beautiful day. We missed an opportunity to launch.’

Monday was the next opportunity, but a screw failed to release from the crew hatch of the orbiter. A drill was ordered to be driven to the pad, but the battery was flat. Nine more batteries were sent to the pad, but all, for no apparent reason, became flat by the time they arrived at the launch pad. Time ran out and the launch attempt was scrubbed.

NASA decided on a 24 hour turnaround, but there was a new problem at the Weather Station, with exceptionally cold temperatures forecast for Florida.

‘We prepared forecast for the next day,’ said John Weems of the NASA Weather Station. ‘We knew the winds would be decreasing, but the real concern was the very cold temperatures that were due in the area. We put together a 12 hour forecast and presented it to the Mission Management people, presenting temperatures of 24F (minus 5 Celsius) at the pad for the next morning.’

NASA remembered that Morton-Thiokol had been concerned about low temperature launches and made a call to their Utah headquarters.

‘A manager came by my room and asked me if I was concerned about an 18 degree launch,’ recalled Ebeling. ‘I said ‘What?’ – because we’re only qualified to 40 degrees. I said ‘what business does anyone even have thinking about 18 degrees, we’re in no man’s land, we’re in a big grey area.’

Ebeling called his O-ring task force team to assemble in his office, given the O-rings had never been tested below freezing, but now the estimated temperatures the exposed SRBs would experience were some 18 degrees colder.

‘We discussed what might happen below our 40-degree qualification temperature and practically to a man we decided it would be catastrophic,’ added Ebeling.

A call was immediately made to the NASA Marshall Space Flight Center (MSFC) in Alabama to raise concerns.

‘Thiokol recommended that we could not launch until the weather warmed up in the afternoon,’ said NASA senior manager Jud Lovingood. ‘Well I told them they couldn’t make that recommendation. They had to give us a temperature that we could launch with.’

A formal presentation would have to be made, two hours after speaking with Lovingood and just 15 hours before launch, via a teleconference at which Thiokol would need to given their reasoning for a no launch decision – a power contractors held, but were scared to make given the effects on the Shuttle schedule.

Thiokol engineer Roger Boisjoly – one of two specialists (the other being Arnie Thompson) on the SRB joint seals – grabbed anything he could from his office to show how the temperature would lead to a failure of the SRB’s O-ring and the destruction of the Shuttle.

‘Unfortunately in our rush we didn’t have time for a dry run at what we’d present to NASA,’ noted Boisjoly. ‘I had no idea what my colleagues would present and I had no idea what I’d bring to the meeting.’

Thiokol engineers still managed to give what they believed to be compelling evidence that the low temperature would slow down the sealing of the O-ring primary and secondary seal, leading to hot gas leaking out of the joints and an explosion on the launch pad as soon as the SRBs ignited.

‘The entire Thiokol group recommended no launch,’ remembered Ebeling, as they recommended a minimum launch temperature of 53F (11C). The expected rubber stamping of that recommendation was expected from NASA on the other end of the teleconference. However, they would be proven wrong.

‘I thought it was a very poor briefing,’ said Lovingood. ‘To make such a huge statement on flight safety, it was an extremely poor briefing.’ NASA engineers at MSFC started to pull apart Thiokol’s data.

‘You don’t do data by emotion,’ added Lovingood. ‘You can’t go up there and say ‘hey, I’ve got a gut feeling this thing is going to blow up.’ They’d take you to the funny farm.’

‘We’re always probed on rational, but that night I was hammered (by NASA engineers) way more than I had experienced as an engineer in the aerospace industry,’ said Boisjoly.

‘We always do that with our contractors, ‘ countered Lovingood. ‘What we did that night was mild, compared to what we normally do.’

The problem escalated during the meeting. NASA could not go against a contractor’s ‘no launch’ recommendation. However, such a recommendation of a minimum launch temperature would destroy the ambitious launch schedule of the Shuttle.

‘I turned to my fellow managers and said if these guys persist in this decision not to launch, then we can’t launch, and they agreed with me, we were at their mercy,’ added Lovingood.

‘As soon as the button was pressed to mute NASA from our meeting, the managers said ‘we have to make a management decision,’ said Boisjoly. ‘It was obvious they were going to change their decision to launch decision to accommodate their major customer.

‘But surely the photographs I had showed that the more black that you see between the seals, the lower the temperature, the closer you are to a disaster. I was told I was literally screaming at the managers to look at the photos, but they wouldn’t look at them.

The general manager of Thiokol turned to his three senior managers and asked what they wanted to do. Two agreed to go to a launch decision, one refused.

‘So he (the general manager) turns to him and said ‘take off your engineering hat and put on your management hat’ – and that’s exactly what happened,’ said Boisjoly. ‘He changed his hat and changed his vote, just 30 minutes after he was the one to give the recommendation not to launch. I didn’t agree with one single statement made on the recommendations given by the managers.’

The teleconference resumed and NASA heard that Thiokol had changed their mind and gave a recommendation to launch. NASA did not ask why.

‘That was stupid on our part, that was dumb,’ said Lovingood. ‘We should have said ‘give us your rational for changing your mind’ but a guy sits in a meeting, that is a good for launch meeting and he doesn’t stand up in front of the train to stop it, he’s go. No one stood up, so everyone was go for launch.

‘But I remember going home and telling my wife that I sure hoped we made the right decision, as I had misgivings about it.’

‘I went home, opened the door and didn’t say a word to my wife, ‘ added Boisjoly. ‘She asked me what was wrong and I told her ‘oh nothing hunny, it was a great day, we just had a meeting to go launch tomorrow and kill the astronauts, but outside of that it was a great day.”

The next morning ice was cleared from the pad and the countdown continued smoothly.

‘Bob Ebeling got hold of my arm and asked me to watch the launch,’ recalled Boisjoly. ‘At first I said no, I didn’t want to see the launch, I didn’t want to see the failure. He spoke with me for a while and convinced me to watch the launch.

‘When Challenger’s SRBs ignited and the vehicle cleared the tower, I turned and whispered to Bob that we’d just dodged a bullet, as it was my expectation – and that of my colleagues – that it would blow up right on the pad.’

‘I made a prayer, and that prayer was a negative one,’ added Ebeling. ‘Lord, make me and all these other engineers wrong, let it go. I thought we were home free too.’

But Ebeling and Boisjoly were correct. As soon as the SRBs ignited, a plume of black smoke was seen to come out of the aft of the SRB, as the O-rings failed. The only reason the Shuttle didn’t explode on the pad was due to SRB grain and debris of the joint temporarily sealing the gap.

As Challenger went through the maximum area of aerodynamic pressure, Challenger was hit by the largest wind sheer ever experienced by a Shuttle. That broke the temporary seal created by the debris and a flame was seen breaching through the SRB joint. Burning through an attach joint between the SRB and External Tank (ET), the booster swivelled into the ET.

While Challengers computers tried desperately to counter the forces with extreme steering commands, the booster swung into the ET, causing it to fail, with the collapse of the ET vaporising the liquid oxygen and liquid hydrogen in seconds, forcing the orbiter to break up into segments due to massive aerodynamic overloading.

The crew compartment survived intact, with the crew surviving the initial break-up. The events that followed for the crew are known to some of engineers close to the mission and need not be published – but ultimately the seven astronauts died when their cabin hit the ocean a few minutes later at over 200 mph.

NASA administrator Mike Griffin placed an important positive surrounding the loss of Challenger, noting their loss was anything but in vain, as the lessons learnt from the failure of STS-51L will ultimately take man back to the Moon.

The Solid Rocket Booster failure that destroyed Challenger led to many hundreds of millions of dollars being spent on making the booster safer, making it one of the safest forms of the first stage of ascent, one which will be used on NASA’s next generation manned space vehicle.

The CEV (Crew Exploration Vehicle) will come into service in around six years time, replacing then retired Space Shuttle fleet which hasn’t suffered a SRB failure since Challenger. It will be the same SRB technology and concept that will be utilised in the first stage of the new vehicle.

‘The Challenger crew that died, because of a mistake, helped make a better SRB,’ said Griffin during the New Horizons press conference. ‘The SRB has the best record of any spaceflight component we use. It will be a key part of the next steps and we got that from the Challenger crew.

The losses NASA has suffered, from the three astronauts on Apollo 1, to the seven on Columbia in 2003, will all leave their legacy in the expansion of safety and procedures that will ultimately make it that little bit safer for future crews.

‘When I think about the crews, there’s a book called Fate is the Hunter – and in that book when you open it up the writer has a small salute to those that didn’t make it, as we learned as flawed human beings we tried to make flight safe,’ added Griffin.

‘The knowledge we’ve gained has been made through many, many losses, trying to do something very difficult.’

‘Space Flight is the most complex thing we do. It is difficult, it is dangerous. We’re learning to take this technology into new arena. The losses we’ve sustained so far reflect the difficult involved in inventing this new technology. It’s part of a learning process.

‘We will learn in the same way we learned how to do aviation flight. It will be difficult.’

Thanks to Blast! Productions and JWM Films for content/quote additions and photography from the BBC documentary ‘Challenger: Go for launch.’ This article is a combination of previous Challenger articles published on this site.