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"Now don't break our airplane," Judy Resnik joked.

Hank Hartsfield promised not to. It was October 1985 when the two astronauts shared their moment of cameraderie. Three months later, the STS-61A Commander would recall Resnik's light-hearted advice with sorrow; for it was on her flight, rather than his, that the 'airplane' - Space Shuttle Challenger - finally broke. Hartsfield had just returned from the week-long Spacelab-Dl mission, sponsored by West Germany, aboard Challenger and had immediately flown to Europe on his crew's public relations tour. He was aboard a commercial airliner, returning to the United States, when Resnik and the rest of Challenger's tenth crew blasted off.

The pair had good reason for their closeness: not only did they share the same career, but they had flown together on the maiden voyage of the Space Shuttle Discovery barely a year earlier. "You sort of become family," Hartsfield recalled two decades later. "We worked together for 13 months, partied together and you do get close." Whilst in Europe, he had followed with interest his former crewmate's seemingly fruitless efforts to return to space in January 1986.

Right from the start, launching Challenger's tenth mission had proved to be an exercise in frustration.

It was, furthermore, a frustration that NASA could ill afford. The planned six-day flight, designated 'STS-51L', would feature the first private citizen to fly aboard the Shuttle - a social studies high school teacher from Concord in New Hampshire, named Christa McAuliffe. Picked from over 11,000 applicants for the Teacher-In-Space initiative, she would teach two lessons from space, providing a much-needed public relations boon for the agency as it sought to demonstrate that its reusable fleet of orbiters were truly the spacegoing equivalents of commercial airliners. Indeed, years later, McAuliffe's mother, Grace Corrigan, would insist that the general atmosphere in the weeks leading up to Challenger's fateful launch was that the Shuttle was actually far safer than an airliner, simply due to the higher number of precautions taken by NASA. Even McAuliflfe herself had expressed jocular confidence that her only 'fear' was a failure of the orbiter's multi-million-dollar toilet.

When the STS-51L crew arrived at the Kennedy Space Center (KSC) in Florida in the third week of January, their launch was routinely postponed by delays in bringing Challenger's sister ship, Columbia, home from her own flight and weather concerns at a Transoceanic Abort Landing (TAL) site in Senegal. More trouble was afoot. Unacceptable weather in Florida put paid to a second attempt and, when Commander Dick Scobee and his six crewmates settled into their seats aboard Challenger on January 27th, they were again thwarted by high winds and a frozen handle on the hatch. That night, temperatures at the launch site plummeted to an unseasonal (for Florida) minus 13 degrees Celsius, forcing technicians to switch on safety showers and fire hoses at Pad 39B to prevent water pipes from freezing. This proved particularly worrisome for the ice inspection team, who began their final 'sweep-down' of the pad area in the early hours of January 28th, and they were obliged to knock huge, 30 cm long icicles away with broom handles as the countdown clock continued ticking towards launch.

As the Sun rose, temperatures climbed slightly to a few degrees above zero Celsius, producing the coldest conditions under which a Shuttle launch had ever been attempted, a fact that would be investigated in depth during the subsequent presidential inquiry into the cause of the tragic events later that day. The copious amounts of ice on Pad 39B then forced an additional two-hour delay to permit the Sun to thaw it. Nonetheless, many of the astronauts' families, including Scobee's wife, June, doubted that NASA would conceivably fly under such conditions. She was partially appeased by her husband's insistence, over the phone that morning, that he felt it was safe to do so. Hank Hartsfield, good-naturedly, had called the crew on a regular basis to jokingly ask what the hell they were up to. Would they ever launch, he wondered?

Tragically, as we now know, Resnik and her colleagues - Scobee, McAuliffe, Pilot Mike Smith, Mission Specialists Ellison Onizuka and Ron McNair and Payload Specialist Greg Jarvis - would indeed launch that frigid Tuesday, with catastrophic consequences. Two decades later, the world is familiar with the technical and human causes of Challenger's loss, but the disaster also put paid to plans for two important satellite deployments, a range of scientific and engineering experiments and a comprehensive survey of Halley's Comet.

This fabled celestial wanderer, which frequents the inner Solar System only once every 75 years, was to be the focus of not only STS-51L, but also two other missions in the spring of 1986. Two weeks before Challenger lifted off, Columbia's STS-61C crew had been prevented from making significant observations, due to problems with their Comet Halley Active Monitoring Program (CHAMP) cameras. In March, another Columbia team on STS-61E was to have employed a battery of ultraviolet telescopes and a wide field camera to analyse the comet. Meanwhile, STS-51L would have utilised CHAMP and deployed a free-flying satellite to explore Halley's tail and the gaseous 'coma' around its peanut-shaped head.

Onizuka, a Hawaiian-born astronaut of Japanese and American parentage, was

Icicles on Pad 39B's launch tower on the morning of January 28th 1986. Note the 'patchwork' of black thermal protection tiles on Challenger's belly and the connecting propellant lines from the External Tank to the orbiter's aft compartment.

The STS-51L crew in the 'white room' on Pad 39B, during their Terminal Countdown Demonstration Test on January 8th 1986. From left to right are Christa McAuliffe, Greg Jarvis, Judy Resnik, Dick Scobee, Ron McNair, Mike Smith and Ellison Onizuka.

responsible for the CHAMP hardware and was to have buried himself on January 29th under a black shroud on the Shuttle's aft flight deck to ensure maximum darkness for his observations. "I will have about two minutes on four different orbits to photograph Halley's Comet in both the visible and ultraviolet spectrum," he told an interviewer. "The objective is to try to get this data as the comet approaches perihelion, which is just as it goes around behind the Sun and starts to head back out. It's a regime where we do not have any data at the present time, so I've been told we'll probably be the only human beings to see it at that time."

As Onizuka worked, his crewmates would have been involved in preparing the Shuttle Pointed Autonomous Research Tool for Astronomy - a mouthful that NASA's finest acronym-makers had somehow carved into the name 'Spartan' -for deployment on January 30th to commence its own series of Halley observations. Built by the agency's Goddard Space Flight Center (GSFC) of Greenbelt, Maryland, the small, boxy satellite had previously flown on Dan Brandenstein's STS-51G mission in June 1985 and was designed to be serviceable and capable of returning to orbit every six to nine months.

On STS-51L, it would have used a pair of ultraviolet spectrometers and two modified Nikon F-3 cameras to study the composition of the comet's dirty-snowballlike nucleus and million-kilometre-long shimmering tail. McNair, with Resnik, was responsible for deploying and later retrieving Spartan with the Canadian-built Remote Manipulator System (RMS). Interestingly, the spectrometers, produced

The official mission emblem for the Spartan-Halley project, showing the small, boxy satellite and highlighting its quest to better understand the celestial wanderer.

jointly by GSFC and the University of Colorado's Laboratory for Atmospheric and Space Physics, were derived from backups for an instrument aboard the Mariner-9 spacecraft, which had begun investigating the Mars atmosphere in 1971.

"Comets happen to be one of the remnants of the creation of the [Solar System]," McNair said before the mission, "and they're just a big mass of ice - of frozen gases -and the last time [Halley] came around, we weren't sophisticated enough to do the type of things that we're doing now. Scientists will be able to analyse the gases [and] emissions by looking at the Sun's reflection and the absorption of sunlight and give some credibility to some of the theories - or possibly tear them down - about the origin of the Universe. Who knows what we're going to find out of this? But these types of observations can change the way you think."

Several other missions, besides Spartan, were also watching Halley at the time: Europe's Giotto, the Soviet Union's twin Vega spacecraft and Japan's Suisei and Sakigake probes were heading for the comet, but NASA believed its Shuttle-borne studies had the 'edge' by conducting observations as it neared 'perihelion', at its closest point to the Sun.

It was during a narrow, five-week 'window', from January 20th until February 22nd 1986, that the agency hoped Halley - then some 225 million km from Earth and only 97 million km from the Sun - would be chemically at its most active and yield the most desirable scientific data. The Spartan mission to explore the comet, codenamed '203', would have got underway on the second day of STS-51L, when Scobee and Smith were scheduled to fire Challenger's Orbital Manoeuvring System (OMS) engines to nudge it to a slightly higher altitude, about 245 km above Earth. The assignment of Spartan-203 had already resulted in some changes to the Shuttle's own launch period; originally targeted for a morning lift-off, it was moved to the afternoon, in order to provide the best lighting conditions for the satellite's observations. However, an afternoon start would delete the option of touching down in Casablanca in the event a TAL abort to Morocco became necessary. Ultimately, as Challenger's launch was pushed into the final week of January, conditions for optimum viewing of Halley, based on an afternoon window, could no longer be met and the lift-off time was shifted back to the morning hours. In a sense, therefore, the delay was actually beneficial.

By January 30th, after Spartan-203's software had been uploaded from NASA's Johnson Space Center (JSC) in Houston, Texas, and voltage and current checks carried out, it would have been 'hung' over the payload bay wall and released into free flight by the mechanical arm. The satellite would then have executed a slow, minute-and-a-half-long pirouette to prove that it was working properly, after which Scobee would have pulsed the Reaction Control System (RCS) thrusters to achieve a maximum separation distance of around 145 km. This would have ensured that sunlight reflected by Challenger's pristine white surfaces did not 'confuse' Spartan-203's sensors.

Following two orbits of further tests, the aperture doors covering the satellite's two ultraviolet spectrometers would have automatically retracted to initiate an aggressive, 40-hour-long phase of free flight, of which more than half would have been dedicated to studies of the photodissociation of water in Halley and analysis of its various nitrogen-, carbon- and sulphur-containing molecules. Meanwhile, its cameras would have offered an ongoing record of the 'large-scale' activity of the comet itself, including outbursts in its nucleus and asymmetries in its coma. Retrieval would have followed on February 1st and Spartan-203 would have been repositioned on its Mission Peculiar Equipment Support Structure (MPESS) carrier in the forward section of the payload bay.

Had Onizuka and his six crewmates survived their violent climb to orbit on January 28th, however, the delicate and tricky Spartan-203 deployment, two days of station-keeping and retrieval would have actually been the secondary task of their mission. By far the largest, most expensive and most powerful payload aboard STS-51L was NASA's second SI00 million Tracking and Data Relay Satellite - known as

'TDRS-B' - which, it was expected, would enable future Shuttle astronauts to communicate directly with Mission Control for most of each 90-minute circuit of the globe.

"That's going to be a big improvement," Smith told an interviewer in the weeks leading up to the launch, "not only for the Shuttle, but also for the space station when it gets up later on." Until the early 1980s, US missions had relied on a network of ground stations to relay communications between orbiting crews and Houston-based controllers. The TDRS network of at least two large satellites, positioned in orbits 35,600 km high, would gradually bring this era to a close.

Onizuka, though, was simply thrilled at having the chance to help deploy "one of the largest communications satellites ever!" His words were, to say the very least, an understatement. The 2,540 kg TDRS-B would, when fully functional in its operational orbital 'slot' and numerically renamed 'TDRS-2', resemble a colossal windmill with four 'paddles' extending from beryllium booms affixed to a hexagonal 'bus'. Two of these paddles held electricity generating solar panels, while the others carried umbrella-like S-band and Ku-band antennas. Between the tips of its solar panels, TDRS-2 would have spanned an impressive 12 m when full unfurled in orbit, making it virtually identical to the satellite launched by Paul Weitz' STS-6 crew almost three years before.

In order to achieve its high orbit, it was attached to a Boeing-built Inertial Upper Stage (IUS), whose two solid-fuelled sections would have delivered the satellite, over a period of about seven hours, into its operational location. Deployment of the 14-m-long combo would have consumed most of Challenger's first day in space and, although all five 'career' members of the crew would have been involved, the lengthy procedure would have been conducted under the direction of Onizuka and McNair.

Shortly after reaching space and opening the payload bay doors - thus exposing the folded-up satellite and its booster to the harsh environment of low-Earth orbit for the first time - the two men, located at instrument panels on the aft flight deck, would have run through a series of checks and eventually hoisted the 'stack' to a pre-deployment angle of 29 degrees using the ring-doughnut-shaped 'tilt table'. As Scobee and Smith manoeuvred Challenger into the correct attitude, Onizuka and McNair would have switched TDRS-B over from the Shuttle's electricity supply to the IUS' internal batteries.

Next, they would have commanded the tilt table to raise the combination to an angle of 59 degrees and, precisely ten hours after leaving Earth, spring-ejected it, such that it swept smoothly over Challenger's cabin roof. Nineteen minutes later, Scobee would have fired the OMS engines to create a safe separation distance in anticipation of the IUS' first stage ignition. After computing the stack's correct attitude by taking star sightings, the IUS would have fired its engine an hour after deployment and run for two and a half minutes. An additional burn by the second stage, lasting just under two minutes, would then have inserted TDRS-B into near-geosynchronous orbit.

Whilst still attached to the now-exhausted second stage, the satellite's solar arrays would have opened - "like an insect coming out of a cocoon," astronaut Mike Lounge, who deployed TDRS-C in September 1988, would later remark - and, eventually, so too would its communications payload. Over a period of several months, during a series of extensive tests, it would have gradually drifted westwards to its final orbital position over the Pacific Ocean, directly above the equator south of Hawaii, at 171 degrees West longitude.

It was a complicated task and one for which the IUS itself had made a rather inauspicious start. When TDRS-1 was launched in April 1983, the second stage of its booster had malfunctioned during the circularisation manoeuvre and delivered it into a lower-than-planned orbit; this forced controllers to use three-quarters of the satellite's precious hydrazine fuel to limp into the correct slot, reducing its operational lifespan. The investigation into the embarrassing failure led to the postponement of several other IUS-dependent Shuttle missions and the TDRS-B launch, originally targeted for August 1983, was repeatedly delayed. Additional problems with a timing circuit aboard TDRS-1 pushed it back yet further from March 1985 until the spring of the following year.

However, following his first mission aboard Space Shuttle Discovery in January 1985, which featured the successful deployment of a top-secret Department of Defense satellite affixed to an IUS, Onizuka expressed confidence in the weeks leading up to STS-51L in the training and procedures involved with releasing both the enormous TDRS and its problem-prone booster. "The basic training was the same," he said of the similarities between his first and second flights. "Once we enter the area of payload and mission operations, there were some differences, [but] I'm very familiar with the IUS; very comfortable with it."

Capable of handling up to 300 million bits of information per second - roughly equivalent to processing a couple of hundred 14-volume encyclopaedias every minute - TDRS-B would technically bring the system up to fully operational status. Nevertheless, a third satellite was scheduled to be ferried into orbit by Challenger's STS-61M crew in July 1986 to replace the degraded TDRS-1. Until the arrival of this third member of the network, TDRS-B would operate from an initial 'spare' orbital slot of 136 degrees West longitude, providing much-needed backup services for its prematurely ageing sibling. After the launch of the third satellite, however, TDRS-B was scheduled to be moved to its final position at 171 degrees West longitude.

Despite its important contributions to astronomy and communications, the STS-51L mission naturally attracted media attention, as NASA had intended, thanks to the presence of teacher observer McAuliffe. Explorers, journalists and entertainers were considered in the early 1980s as the agency weighed up options for which profession would yield 'the best' private citizen to send aloft on the pioneering mission. Ultimately, in August 1984, President Ronald Reagan announced that a teacher would fly first. Dick Scobee agreed that it was the right decision.

"Teachers teach the lives of every kid in this country through the school system and if you can enthuse the teachers about doing this, then you enthuse the students and impress on them that's something to expect in their lifetime," he explained in the weeks leading up to Challenger's launch. "Man needs to explore and that's part of the thing we have to do to ensure our future. So as far as I'm concerned, it's a good insurance policy for the human race."

McAulifle's selection as the primary candidate for the mission, with Idaho teacher Barbara Morgan backing her up, was revealed by Vice-President George Bush in July

1985 and a few weeks later both women arrived in Houston to begin training in earnest. Her tasks included performing two, 15-minute-long lessons: the first, entitled 'The Ultimate Field Trip', was a guided tour of the Shuttle to familiarise students with onboard living and working conditions, while the second, called 'Where We've Been, Where We're Going', focused on NASA's fledgling plans for a permanent space station. Both were to have been aired by the Public Broadcasting System sometime on February 2nd and McAuliffe would have explained the roles of her six crewmates, identified and summarised the experiments aboard Challenger and enthused 'her' students with a vision of the future.

"I think it's going to be very exciting for kids to be able to turn on the TV and see the teacher teaching from space," she said. "I'm hoping that this is going to elevate the teaching profession in the eyes of the public and of those potential teachers out there. Hopefully, one of the secondary objectives of this is students are going to be looking at me and perhaps thinking of going into teaching as professions."

McAuliffe and Jarvis were both 'Payload Specialists' - candidates chosen by their respective companies, agencies or organisations to operate specific experiments, but not 'career' astronauts like their five crewmates - and both joined the STS-51L line-up relatively late in the training flow. Yet both were quickly accepted and grew to become highly respected members of the team.

"It's refreshing to have somebody on board that's really dedicated and enjoys doing what they're doing," Scobee remarked, "but also she goes into the training with a positive attitude and stays out of the way when she needs to stay out of the way, she gets involved when she needs to get involved and does basically all the right things, and so does Greg Jarvis. Both of them, from our standpoint, are good Payload Specialists. They came onboard with a good, open mind, they're accommodating to our system, we try to be accommodating to theirs and it's a nice trade-off."

The level of respect was, of course, mutual and Jarvis, a Hughes aircraft engineer, recalled one particular session as an example of the astronauts' ability to operate seamlessly together. "When you watch them work through the malfunctions they work through, you get very comfortable that they know what they're doing," he said. "One time when we were in the Motion Base Simulator, the lights went out for the visual for the landing. The Commander called down and said 'Aren't the lights out?' And they [Mission Control] said, 'I think so, we'll get back to you on that'. The conversation went on for about two or three minutes and it turns out they had mistakenly turned the lights out on the visuals. The thing you didn't realise was that he made a perfect landing without any lights!"

The arrival of Jarvis in October 1985 had come particularly late in the crew's training period. During the mission, he was assigned to conduct a battery of investigations using spinning, fluid-filled plastic models on Challenger's middeck to evaluate 'optimum' shapes for future satellite fuel tanks. The reason for his late assignment was primarily linked to the fact that payloads for Shuttle missions were in constant flux prior to STS-51L; indeed, the cargo for Dick Scobee's flight had changed several times, as had the identities of 'his' Payload Specialists.

Ironically, one of the main reasons for flying Greg Jarvis was to allow a representative of the Hughes company - which had built several Shuttle-ferried communications satellites - to observe and analyse the physics of an actual deployment in depth. Originally assigned to fly aboard Columbia on STS-61C, he was transferred to STS-51L, ostensibly because the Hughes-built Westar-6S satellite scheduled to ride aboard the former mission had experienced technical problems and been delayed. This sounded perfectly reasonable, but for one thing: STS-51L, also, had no Hughes satellite aboard! The more likely reason for reassigning Jarvis, wrote Mike Mullane, was that Congressman Bill Nelson had requested a Shuttle flight and the space agency had hurriedly complied. "NASA bumped the oft-abused Jarvis one mission to the right," Mullane recalled in his 2006 memoir. "The next time he would pose for a crew photo would be for STS-51L, the mission that would kill him. He would die on a mission that had no Hughes satellite to deploy, the singular event that had been the original justification for his assignment to a Shuttle flight."

Even when the five NASA crew members were assigned in January 1985, with a projected lift-off in November, the mission's payload was changing every few months: first they would deploy an Australian communications satellite and operate a pharmaceutical processing factory, then for a short time 'their' orbiter was switched from Challenger to Atlantis and, finally, back to Challenger again. In fact, one of the issues raised by the Rogers Commission - headed by former Secretary of State William Rogers - was this practice of constantly juggling payloads between missions. In STS-51L's case, this had led to no fewer than six postponements of the critical Cargo Integration Review, an essential meeting at which payload requirements are assessed and the development of final flight products can begin. Although the commission admitted that most payload adjustments were complete by the time the review finally took place in June 1985, it was particularly critical of the late assignment of Jarvis and his experiments, just three months before launch.

"The launch minus five months Flight Planning and Stowage Review was conducted on August 20th 1985," continued the Rogers report, "to address any unresolved issues and any changes to the plan that had been developed to that point. Ideally, the mission events are firmly determined before the review takes place. For 51L, however, Mr Jarvis was not added until October 25th 1985 and his activities could not be incorporated into mission planning until that time. There were changes to middeck payloads, resulting from the addition of Mr Jarvis, that occurred less than three months before launch. The most negative result of the changes was a delay in publishing the crew activity plan. [This] specifies the in-flight schedule for all crew members, which in turn affects other aspects of flight preparation."

Furthermore, Rogers investigators expressed concerns that changes were being made to flights at very short notice - not only Payload Specialist adjustments, but also satellite swaps and experiments being added, delayed or dropped entirely - which, of course, would directly impact the training time available for crews. "Had we not had the accident," said Hank Hartsfield in his testimony to the commission, "we were going to be up against a wall; STS-61H [a Columbia mission, scheduled for June 1986] ... would have had to average 31 hours in the simulator to accomplish their required training and STS-61K [an Atlantis flight in October] would have to average 33 hours. That is ridiculous. For the first time, somebody was going to have to stand up and say [that] we have got to slip the launch because we are not going to have the crew trained." Training was also affected by the presence of only two Shuttle simulators at JSC, capable of supporting crews for no more than 12-15 missions per annum. "The flight rate at the time of the accident," read the Rogers report, "was about to saturate the system's capability to provide trained astronauts for those flights."

At length, with everything (TDRS-B and Spartan-203) and everybody in place, the STS-51L mission-specific training commenced in the late autumn of 1985, with the astronauts averaging 49-hour work weeks to ensure proficiency in robot arm operations, Spartan deployment and retrieval activities, IUS systems, ascent and re-entry procedures and each of the experiments crammed into Challenger's middeck. The mission itself was deemed "moderately complex" in view of the Spartan commitment, although both it and a TDRS deployment had already been 'baselined' on previous flights.

Still, despite a hectic six days in space, all seven astronauts intended to spend some moments appreciating the uniqueness of where they were. "We have a fairly busy timeline and it's nice to have time to go look out the windows," Scobee, who had flown once before on Challenger in April 1984, said during one of his last interviews. "I guess one of the things that pleasures me most is to have a quiet time where you can go look out the windows, turn out the lights and look at the stars and Earth and thunderstorms. Just the sheer joy of doing it is probably the most fun part because it's hard to single out one thing, but even the hard work of it is generally fun. I enjoy the flying. I enjoy the excitement and thrill of the ascent, because it is really dramatic. Entry is fiery -just an amazing light show - and the fires of hell are burning outside your window and you're sitting there nice and comfortable watching all this go on and it's just a neat feeling."

Nonetheless, Scobee had already announced before setting off that STS-51L would be his last space mission; doubtless, he intended this one to count even more so than his previous flight. His last comments of encouragement to his crewmates over Challenger's intercom in the final seconds of the countdown were words that conveyed enthusiasm, dedication, professionalism, childlike wonder - and an uncanny, though unwitting, preview of what would happen.

"Everybody strap in tight," he told them cheerily. "We're about to go for the ride of our lives."

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