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Had STS-51L been completed safely, the frequency with which Challenger herself flew into space in 1986 would have greatly eclipsed her three previous years of operations. With four more missions scheduled for May, July, September and December, she would have deployed the joint US/European Ulysses probe to explore the Sun's polar regions, followed by a third Tracking and Data Relay Satellite to replace the doddery TDRS-1, retrieval of the Long Duration Exposure Facility, finally, and a secrecy-enshrouded Department of Defense assignment just before Christmas. By far the most significant of these was the launch of Ulysses, which, uniquely for the Shuttle, would have been one of two deep space missions launched within only five days. By the beginning of May, it was expected that Challenger would be installed on Pad 39B and sister ship Atlantis, carrying the Jupiter-bound Galileo explorer, on Pad 39A to support launches on the 15th and 20th of that month. Both missions were fixed within a narrow launch window which could not be slipped.

John Young referred to the missions as the 'Death Star' flights.

Behind the dark humour, however, lay real concern for the then-chief of the astronaut corps. Even with an increasingly confident outlook on the Shuttle's capabilities as it entered its sixth year of flight operations, Young instinctively knew that Challenger's STS-6 IF mission and Atlantis' STS-61G voyage would be two of the riskiest ever attempted. Fellow astronauts Rick Hauck and Dave Walker, who would respectively command them, echoed his concern. "As with any flight," said Hauck, who also flew aboard STS-7 and would have been Challenger's 11th Commander, "if everything goes well, it's not risky. It's when things start to go wrong that you wonder how close you are to the edge of disaster."

The loss of STS-51L and, on February 1st 2003, of Columbia upon re-entry have illustrated how fine the line is between triumph and tragedy; a line - and risk - that every astronaut knows and accepts before clambering aboard. Yet Hauck and Walker's flights, scheduled to occur just five days apart in May 1986, would have carried additional danger. This was partly due to the importance of the Ulysses and

Had STS-51L survived, these four men would have been Challenger's next crew. Scheduled for launch on May 15th 1986, STS-61F would have deployed the joint US/European Ulysses probe on a five-year mission to explore the Sun's polar regions. Seated are Pilot Roy Bridges (left) and Commander Rick Hauck and standing are Mission Specialists Mike Lounge (left) and Dave Hilmers.

Had STS-51L survived, these four men would have been Challenger's next crew. Scheduled for launch on May 15th 1986, STS-61F would have deployed the joint US/European Ulysses probe on a five-year mission to explore the Sun's polar regions. Seated are Pilot Roy Bridges (left) and Commander Rick Hauck and standing are Mission Specialists Mike Lounge (left) and Dave Hilmers.

Galileo payloads, both of which were equipped with controversial Radioisotope Thermoelectric Generators (RTGs). The latter were nuclear power sources, fuelled by plutonium dioxide, and the implications of a launch accident and the consequences of depositing highly radioactive material across eastern Florida did not bearing thinking about.

This risk was compounded still further by the fact that, attached to the base of each nuclear hot potato in Challenger's and Atlantis' payload bays was a thin-skinned, liquid-fed rocket that many astronauts and managers had condemned as unsafe and unacceptable for use in conjunction with a manned spacecraft. Measuring nine metres long and four metres wide, it was called the 'Centaur-G Prime' and, for Rick Hauck, it was his baby.

Just like a baby, it was both temperamental and unpredictable.

"I was assigned to be the astronaut office's project officer for Centaur," Hauck recalled two decades later. "It's pressure stabilised, which means if it's not pressurised, it's going to collapse by its own weight. If it were not pressurised, but suspended, and you pushed on it with your finger, the tank walls would 'give' and you'd see that you're flexing the metal!" Nicknamed a 'balloon' because its rigidity thus depended on full pressurisation, the Centaur had long been viewed warily by NASA's human spaceflight people, whose safety rule of thumb on the Shuttle dictated that no single failure should be capable of endangering the vehicle or crew.

The Centaur-G Prime, however, did much more than that. Much of its pressure regulation hardware, disturbingly, was not redundant - it lacked a backup facility -and, worse, a failure of its internal bulkhead had the potential to rupture both its volatile liquid oxygen and hydrogen tanks. Additionally, it was recognised that the sheer mass of propellants - which totalled more than 16,500 kg - could cause 'sloshing' and a myriad of other controllability problems that could hinder Hauck or Walker if the need arose to execute an emergency landing shortly after lift-off.

In spite of the hazards, the Centaur's key advantage was that its liquid propellants provided considerably more oomph to push large payloads out of Earth orbit and onto trajectories to other planets than solid-fuelled rockets could achieve. It was also well known that liquid-fed boosters produced a much 'gentler' thrust than the notoriously harsh impulse of solids. Still, the safety concerns rightly overshadowed and ultimately overwhelmed these benefits.

"The Shuttle was obligated to launch Ulysses and Galileo," explained Hauck. "[NASA] needed the most powerful rockets they could have [and] at some point the decision was made to use Centaur, which was never meant to be involved in human spaceflight. That's important because rockets that are associated with human spaceflight have certain levels of redundancy and certain design specifications that are supposed to make them more reliable. Centaur did not come from that heritage, so, Number One, that was going to be an issue in itself, but Number Two is [that] if you've got a Return to Launch Site abort or transatlantic abort and you've got to land - and you've got a rocket filled with liquid oxygen [and] liquid hydrogen in the cargo bay -you've got to get rid of [it], so that means you've got to dump it while you're flying through this contingency abort. To make sure that it can dump safely, you need to have redundant parallel dump valves, helium systems that control the dump valves [and] software that makes sure contingencies can be taken care of. Then, when you land, you're sitting with the Centaur in the bay that you haven't been able to dump all of it, so you're venting gaseous hydrogen out this side [and] gaseous oxygen out that side. This is just not a good idea!"

To support the new rocket on STS-61F and STS-61G, both Challenger and Atlantis underwent a series of extensive modifications, costing around five million dollars apiece, which included extra plumbing to load and drain the Centaur's propellants and control panels in their aft flight decks to monitor its performance. As NASA's newest orbiter, Atlantis had been made Centaur-capable during her initial construction and was destined to spend the first part of 1986 out at Pad 39B undergoing validation tests of the new hardware. Challenger, too, had received the Centaur upgrades, which also included the addition of an S-band transmitter to handle the booster's telemetred data. During typical, pre-launch loading operations, the Centaur's liquids would have been fed through plumbing 'tapped into' the Shuttle's main propulsion system feedlines. Emergency dumping vents - capable of draining all liquid oxygen and hydrogen from the booster within 250 seconds of an abort being declared - were situated on opposite sides of the aft fuselage, just beneath the Orbital Manoeuvring System pods, none of which filled Hauck or Walker with confidence due to the risk of leakages or explosions.

As part of her validation tests, Atlantis would have been rolled to Pad 39B

sometime in February 1986 - only weeks after STS-51L had vacated the same launch complex - with a 'real' Centaur-G Prime and a mock-up of Galileo in her payload bay. Whilst on the pad, the booster would have been fuelled with liquid oxygen and hydrogen and a series of tests carried out. Atlantis would then have removed from the pad, the 'real' Galileo installed and transferred to Pad 39A. By mid-April, she would have been joined on adjacent Pad 39B by Challenger, laden with Ulysses and its own Centaur.

Doubts over the reliability of the Centaur-G Prime riding the Shuttle had already, in the autumn of 1981, obliged NASA to cancel it and opt to install Ulysses and Galileo onto 'safer' - though less powerful - solid-fuelled Inertial Upper Stage boosters. For the exceptionally large Galileo, which comprised both a Jupiter orbiter and atmospheric entry probe, the swap from Centaur to IUS meant that its journey time to the giant planet would almost double to four and a half years and most likely would require the mission to be split into two 'halves'.

Predictably, its price tag soared, peaking at close to a billion dollars, until Congress pressed NASA in late 1982 to resume work on a Shuttle-borne Centaur and restore the Jupiter travel time to around two and a half years. Not only Galileo, but Challenger's Ulysses payload, required close encounters with the planet - the latter in order to alter its trajectory and rendezvous with the Sun's poles - and both missions were allocated the same, week-long launch window from May 15th to the 21st 1986. Hauck's crew would lift off from Pad 39B aboard Challenger at around 5:20 pm on the 15th, followed by Walker's team from adjacent Pad 39A aboard Atlantis five days later.

The two flights had scarcely an hour apiece available to them in which to launch and, in order to minimise weight, both would carry just four astronauts. Hauck would have been joined by Pilot Roy Bridges - a veteran of the STS-51F flight - and Mission Specialists Mike Lounge and Dave Hilmers, while Walker's crewmates were Pilot Ron Grabe and Mission Specialists Norm Thagard and James 'Ox' van Hoften. There would be no secondary experiments and their payload bays would be empty, save for the probes and their attached Centaur boosters and support structures. Even some elements of crew equipment in the middeck, including the galley, would have been eliminated to save weight. In January 1986, NASA accepted a recommendation to fly Atlantis with her main engines running at a never-before-tried 109 per cent rated thrust: launching at the standard 104 per cent, it was argued, would have meant the heavy 2,270 kg Galileo spacecraft's Centaur would have been forced to carry less propellant and limited its launch window. Ulysses, on the other hand, was considerably lighter than Galileo (at just 370 kg) and Challenger's engines for STS-61F were manifested to run at the 'standard' thrust rating.

Additionally, the two Death Star flights - scheduled to last between two and four days apiece, according to various sources - were headed for lower than normal, 168 km orbits because, said Hauck, "you need the performance to get the Centaur up because it was so heavy". Moreover, assuming an on-time lift-off of STS-6IF, the astronauts would have had no more than about nine hours to get Ulysses out of Challenger's payload bay and on its way to Jupiter, since the Centaur was required to periodically dump its 'boiled-oflf' gaseous hydrogen to keep tank pressures within their mandated limits. After too much time, it would have 'bled' so much hydrogen that the remainder would not be sufficient to perform its trans-Jovian engine burn.

Consequently, three deployment opportunities were manifested for both missions: in the case of STS-61F, the first chance for Hauck, Bridges, Lounge and Hilmers came at 11:20 pm, some six hours after launch. Two additional options followed at 12:50 am and 2:20 am on May 16th. The Centaur-G Prime's twin Pratt and Whitney-built RL-10A-3A engines - each generating a thrust of 7,300 kg - would then have ignited about 45 minutes later and Ulysses would have been on course, first for a Jupiter rendezvous in July 1987 and ultimately for passage over the Sun's polar regions in 1989-1991.

The idea of sending a spacecraft out of the 'ecliptic' - the plane on which most of the planets circle the Sun - to investigate the mechanics of our parent star can be traced back almost half a century, although it was not until the mid-1970s that the newly-formed European Space Agency (ESA) and NASA began designing possible scenarios. One of the earliest plans was for a dual-spacecraft project, both directed initially towards Jupiter to acquire a gravity-assisted boost, after which one would head for the Sun's south pole and the other for its north pole to make simultaneous observations. This 'Out of the Ecliptic' (OOE) venture was approved in 1976, its scientific research payload defined by the following year and launch provisionally targeted aboard the Shuttle in February 1983. However, financial cutbacks obliged the cancellation of one of the two spacecraft and ESA decided to go ahead with a single probe, built in Europe, with half of its instrumentation and the RTG supplied by the United States. Ongoing development problems with the Centaur-G Prime served to push the launch back still further to the early summer of 1986.

When finally built, the boxy spacecraft was renamed 'Ulysses' and, only days before the STS-51L disaster, was shipped to Florida to commence pre-flight processing and integration with its Centaur-G Prime. Its main feature was a 1.65 m diameter high-gain antenna, through which it would communicate with ground controllers via NASA's Deep Space Network of worldwide tracking stations. Affixed to the side of Ulysses was a 5.6 m radial boom, to keep its three sets of scientific instruments well away from the main spacecraft and, in particular, from potential interference with the plutonium-fed RTG nuclear power source. These instruments included magnetometers to explore the extent of the solar field, together with plasma and ion investigations and gamma ray and X-ray detectors. Ulysses' primary scientific objective was to characterise the 'heliosphere' - a vast region of interplanetary space occupied by the Sun's atmosphere and dominated by the outflow of the solar wind - at latitudes higher than 70 degrees at both the north and south poles. Of particular interest was the behaviour of the solar wind itself, in addition to the physical properties of solar radio bursts and plasma waves, X-rays and solar and galactic cosmic rays. Near the ecliptic, the wind was known to be very turbulent, but at higher latitudes was expected to be a radial flow and to be much faster.

After deployment from Challenger, and following the nine-minute-long Centaur firing, Ulysses would also have snared another record by becoming the fastest ever man-made machine, hurtling to Jupiter at 15.9 km/sec! One can imagine that, despite their joy at getting this important international mission underway, Hauck and his crew would have been glad to see the back of both it and the Centaur. Throughout the second half of 1985 and into the spring of 1986, in addition to their rigorous training regimes, both Hauck and Walker found themselves routinely questioning their own judgement over how many potential failure modes and problems they could live with.

"In early January 1986," Hauck recalled, "we were working an issue to do with redundancy in the helium actuation system for the liquid oxygen [and] liquid hydrogen dump valves and it was clear that the [senior Shuttle management] was willing to compromise on the margins in the propulsive force being provided by the pressurised helium. We were very concerned about it. We had discussions with the technical people, but we went to a [review] board to argue why this was not a good idea to compromise on this feature. The board turned down the request. I went back to the office and said to my crew, in essence, 'NASA is doing business differently from the way it has in the past. Safety is being compromised and, if any of you want to take yourself off this flight, I will support you'. Two or three weeks later, Challenger blew up. Now, there is no direct correlation between my experience and Challenger, but it seemed to me that there was a willingness to compromise on some of the things that we shouldn't compromise on."

Years later, Hauck remained undecided as to whether he would have refused, personally, to fly STS-61F, but admitted that Shuttle programme managers were taking unacceptable risks in the months preceding Challenger's fateful launch. Only days after the tragedy, any lingering doubts were resolved for him. The Kennedy Space Center's safety office refused to approve advanced processing of the first Centaur-G Prime, citing "insufficient verification of hazard controls" from both NASA and the booster's manufacturer, General Dynamics. Additional safety concerns, and cost overruns to the tune of $100 million, ultimately led to the project's cancellation in June 1986. Fortunately, a few years later, the Galileo and Ulysses missions went ahead, reverting to the less powerful IUS to get them successfully -though not without incident and requiring longer journey times - to their celestial targets. History has shown us that both achieved considerably more than expected and truly revolutionised humanity's understanding of both our parent star and our planetary big brother.

Ulysses was finally launched by Space Shuttle Discovery's STS-41 crew in October 1990, reaching Jupiter 16 months later, thanks to the combined thrust of its IUS and a Payload Assist Module (PAM)-S booster. It reached its maximum latitude of 80.2 degrees at the Sun's south pole on September 13th 1994, then crossed the ecliptic and travelled through high northern latitudes between June and September of the following year. Both series of observations were conducted during 'quiet', or 'minimum', periods of the Sun's 11-year cycle of activity; a further set of studies at 'maximum' solar conditions were completed between November 2000 and December 2001. Additionally, Ulysses undertook serendipitous analysis of Comet Hyakutake, passing through its billion-kilometre-long tail in May 1996, and observed Comet Hale-Bopp the following year. In August 1998, employing its gamma ray experiment, it also recorded a magnetic burst from the star SGR1900+ 14 in the constellation Aquila, some 20,000 light years from Earth.

The success story which Ulysses later became - and, indeed, still is, for its operational mission has since been extended until at least March 2008 - could scarcely have been further from NASA or ESA's collective mind on January 28th 1986, as Challenger's wreckage tumbled into the Atlantic from the STS-51L fireball. All Shuttle missions, predictably, were indefinitely suspended until the Rogers Commission, whose staff panel included former astronaut Neil Armstrong and STS-7 and STS-41G veteran Sally Ride, had completed its inquiry and made recommendations.

Among its conclusions were that NASA and Thiokol's operation of the Shuttle was seriously flawed: concerns from individual engineers were not reaching appropriate managers, 'critical' items were not being given the attention they demanded and the need to stick to a 'schedule', partly in a bid to please customers, was overriding 'safety'. Not only was NASA attempting to accommodate its major customers but, evidenced in a teleconference with Marshall Space Flight Center and Kennedy Space Center managers on the evening of January 27th 1986, Thiokol showed that it was prepared to ignore the safety concerns of several of its engineers to accommodate NASA, its own major customer. Worries of potential O-ring failure under the near-freezing weather conditions predicted for the following morning, expressed by Roger Boisjoly and others, were ignored, downplayed and Thiokol collectively voted that Challenger was fit to fly, unwittingly signing the STS-51L crew's death warrants in the process.

During that fateful teleconference, Thiokol's vice-president for engineering, Bob Lund, argued that his team's 'comfort level' was not to fly SRBs at temperatures below 12 degrees Celsius - some 53 degrees Fahrenheit - for fear of catastrophic 'blow-by' of the O-rings and field joints, but he could present no evidence to Marshall that 'proved' it was unsafe to do so. In a lengthy debate, Lawrence Mulloy - based in Florida as Marshall's KSC representative manager at the time - and other NASA officials challenged Thiokol's data and questioned its logic. At one stage, the MSFC director of science and engineering, George Hardy, remarked that he was "appalled" at the company's decision. So was Mulloy, who scornfully exploded with "For God's sake, Thiokol, when do you expect me to launch? Next April?" Neither man, however, was prepared to ignore the recommendation of their major contractor. Lund stood firm and, had he continued to do so, NASA would have had little choice but to postpone the STS-51L launch. Shortly thereafter, Thiokol requested a five-minute recess from the teleconference to consider the situation. Five minutes ultimately became half an hour.

Throughout this recess, Boisjoly and fellow engineer Arnie Thompson continued to argue persuasively that it was unsafe to fly outside of their proven field joint temperature range, but the Thiokol senior executives in attendance felt the O-rings should still seat and function properly, despite the cold weather. "Arnie actually got up from his position and walked up the table, put a quarter pad down in front of the management folks and tried to sketch out once again what his concern was with the joint," Boisjoly told the Rogers Commission, "and when he realised he wasn't getting through, he stopped. I grabbed the photos and tried to make the point that it was my opinion from actual observations that temperature was indeed a discriminator and we

Members of the Rogers Commission, including chairman William Rogers (centre) arrive at the Kennedy Space Center on March 7th 1986, during the course of their inquiry into the Challenger disaster.

should not ignore the physical evidence that we had observed. I also stopped when it was apparent that I couldn't get anybody to listen."

Then, executive Jerry Mason - presumably aware of the need not to upset NASA - explicitly asked Lund to "take off your engineering hat and put on your management hat". When the teleconference resumed, Lund indeed changed his vote and Thiokol changed its position on the issue. The company's new recommendation was that, although frigid weather conditions remained a problem, their data was indeed inconclusive and the launch of STS-51L should go ahead the following morning. None of the engineers wrote out the new recommendation - "I was not even asked to participate in giving any input to the final decision charts," Boisjoly told the Rogers hearing - and none but the executive managers signed it.

However, when MSFC and KSC managers asked for any additional comments from around the Thiokol table before closing the teleconference, none of them voiced their concerns. Boisjoly, in particular, remained silent; a fact which would later lead some observers to brand him a witness who turned 'state's evidence', rather than a noble 'whistleblower'. When questioned by a Rogers panel member, he emphasised that "I never [would] take [away] any management right to take the input of an engineer and then make a decision based upon that input, and I truly believe that. There was no point in me doing anything any further than I had already attempted to do ... [but] I left the room feeling badly defeated. I personally felt that management was under a lot of pressure to launch and that they made a very tough decision, but I didn't agree with it."

Having analysed the results of the teleconference, and interviewed the participants, the Rogers report concluded that "there was a serious flaw in the decisionmaking process leading up to the launch ... A well-structured and managed system, emphasising safety, would have flagged the rising doubts about the Solid Rocket Booster joint seal." In fact, when brought to testify before the panel, key officials intimately involved with the decision-making process, including STS-51L Launch Director Gene Thomas, Shuttle programme manager Arnie Aldrich and NASA's Associate Administrator for Spaceflight Jesse Moore admitted that they had not been privy to the issues raised at the January 27th teleconference.

In addition to mandated changes in communication channels, such that individual engineers could express concerns more openly, the most important requirement which had to be met before the Shuttle could fly again was the redesign of the Solid Rocket Booster's field joint and O-ring seal to prevent future combustion gas leakages. In its July 1986 response to President Ronald Reagan and the Rogers Commission, NASA announced its plans: to redesign the joint's metal components, insulation and seals, thereby providing "improved structural capability, seal redundancy and thermal protection". New capture latches would reduce joint movements caused by motor pressure or structural loads and the O-rings were redesigned to not leak under structural deflection at twice the expected level. Internal insulation was modified to be sealed with a deflection relief flap, rather than putty, and new bolts, strengtheners and a third O-ring were added. External heaters with integrated weather seals would maintain future SRB joint temperatures at 24 degrees Celsius or above and prevent water from entering the seals. "The strength of the improved joint design," read NASA's reply to Reagan, "is expected to approach that of the [SRB] case walls."

Another key result of Challenger was that the Shuttle would henceforth only be employed for missions which explicitly required its unique capabilities and those of its crews. Particular focus would be granted to scientific research. More than two dozen commercial and military satellites, previously booked to fly aboard the orbiters in 1986 and beyond, were transferred to expendable rockets. Ironically, in pre-Challenger days - and in line with the 'Shuttle-only' policy inherent in the designation of the orbiter as the National Space Transportation System - these rockets were in the process of being phased out. Apart from a handful of contracts signed prior to STS-51L, including several top-secret Department of Defense payloads that had been configured to fly only aboard the Shuttle and the Italian Space Agency's second Laser Geodynamics Satellite (LAGEOS-2), no further commercial 'primary' cargoes would be trucked aloft by future crews. A deviation from that policy came in May 1992, when, on her maiden voyage, the Challenger-replacement orbiter, Endeavour, conducted a breathtaking retrieval, repair and redeployment of the stranded Intel-sat-6-F3 communications satellite. Although successful and once more demonstrative of the Shuttle's unique capabilities, the STS-49 retrieval was an initial, worrying hint that the lessons from STS-51L were fading from NASA's mind.

Former astronaut Tom Henricks, who flew four missions between November 1991 and July 1996, noted that, as SRB design changes appeared to 'work' successfully, safety was once more being compromised. "The pendulum [after Challenger] had swung to as conservative as they could make it," he said, "but then that pendulum started swinging back almost immediately and it was very prevalent by the time we were going to [the Russian space station] Mir. We were still sending Americans to Mir after a fire and a collision. Near the post-Challenger timeframe, that wouldn't have happened." Henricks actually turned down the chance to command a mission to Mir in June 1998 due to these safety concerns. Ultimately, added Mike Mullane, these fears and the one-off decision to fly former astronaut and US senator John Glenn on STS-95 in October 1998 contributed to a sense of over-confidence in the Shuttle which culminated in the loss of Columbia.

Among the safety improvements made to increase the survivability of future crews in the wake of STS-51L were upgraded brakes and tyres, the development of a drag chute to support the Shuttle's high-speed touchdowns and the incorporation of an escape pole which could be used to bail out of the vehicle's middeck side hatch in the event of serious problems. It was recognised that, without a pole to provide sufficient clearance, astronauts evacuating a crippled orbiter in flight would quickly impact the left wing. Unfortunately, the pole - which was attached to the middeck ceiling during a mission - could only be used when the Shuttle was in controlled, gliding flight, and not much higher than the altitude of Challenger when she disintegrated. The seven astronauts aboard Columbia for STS-107, which broke up 61 km above Earth on February 1st 2003, stood no chance.

In the wake of Challenger, each astronaut was provided with a partial-pressure suit - later upgraded, in 1994, to a fully pressurised ensemble - which would provide hyperbaric protection during ascent and cold-water immersion protection in the event of an emergency ditching in the ocean, together with parachute and life raft. However, Mullane, who flew the second Shuttle mission after STS-51L in December 1988, commented "I was strapped into a fortress that would keep me alive long enough to watch Death's approach. If fire was to kill me, I would have time to watch the flames. If a multi-mile fall was to kill me, I would watch the Earth rushing into my face. Even a cockpit depressurisation would no longer mercifully grant us unconsciousness, as it might have spared the Challenger crew. We now wore pressure suits that would keep us alive and conscious through any cockpit rupture ..."

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