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To Follow After Knowledge: STS-41 and the Odyssey of Ulysses

Pictured in its horizontal position in Discovery’s payload bay, the circular dish antenna and Radioisotope Thermoelectric Generator (RTG) of Ulysses are clearly visible in this image. Photo Credit: NASA

Human hands have stretched far into the cosmos during our half-century of exploring the final frontier. Men and women have circled hundreds of miles above the protective gaseous veil of Earth’s atmosphere and a handful of men have ventured further and left their footprints on the flat plains and undulating hills of our closest celestial neighbour, the Moon. Many machines crafted by human hands have been sent into the most inaccessible reaches of the Solar System…and several of those were delivered, personally, by humans. In June 2009, one such machine fell silent after two decades exploring the poles of the Sun. The joint US-European Ulysses mission, now defunct, continues to orbit our parent star, completing a full circuit every six years or so, and its legacy stands testament to the ingenuity of the scientists, engineers, visionaries and thinkers who laboured to put it there. From its launch in October 1990 to the end of its life, Ulysses pushed the boundaries of knowledge about our Sun and fundamentally altered our understanding of how it works.

The Sun is, quite literally, the reason that life exists on Earth. Most scientists accept that the Solar System formed from a vast cloud of gas and dust, around 4.5 billion years ago, with immense temperatures and pressures serving to form a proto-star and an enormous disk which eventually coagulated into the primordial versions of the planetary attendants that exist today. On the third of those attendants, the largest of the innermost, ‘rocky’ planets, life eventually arose; life which would someday build and despatch Ulysses to learn more about the star which had given it life. Ulysses was the first spacecraft to venture outside the ‘ecliptic plane’ – the plane of Earth’s orbit – to directly explore the Sun’s northern and southern polar regions. In doing so, it enabled physicists to study the star in three dimensions and provide an accurate assessment of the total solar environment, across a full range of heliographic latitudes. Since the ecliptic plane differs from the solar equatorial plane by only 7.25 degrees, it was previously only possible to observe the Sun from low solar latitudes. To explore from higher inclinations demanded a prohibitively large launch vehicle, but by utilising the enormous gravity of the planet Jupiter a significant ‘plane change’ could be effected, enabling travel outside of the ecliptic.

More than four decades ago, consideration was given to launching a Pioneer spacecraft in 1974 for precisely this purpose, but it failed to gain approval. A seed of interest had been sown, however, and ultimately bore fruit as the International Solar Polar Mission (ISPM). In its original incarnation, this was a truly ‘international’ endeavour, employing two separate spacecraft – one built by NASA, the other by the European Space Agency (ESA) – to travel towards Jupiter. One would hurtle ‘beneath’ the giant planet’s south pole, using its gravity to direct it northwards, out of the ecliptic, towards northern solar latitudes. Meanwhile, the other craft would do the reverse, travelling ‘above’ Jupiter’s north pole to bend its trajectory southwards to explore southern solar latitudes. The result would be a pair of in-situ instruments to provide simultaneous measurements of both solar hemispheres for mapping, measurements of magnetic fields and observations of the anamolous ‘solar wind’, a stream of charged particles known to emanate from the Sun at hundreds of thousands of miles per hour.

Mounted atop its IUS and PAM-S boosters, Ulysses departs Earth at the start of its odyssey to explore the Sun. Image Credit: NASA

The ISPM was formally approved in 1976, its scientific instruments were agreed the following year and work on the spacecraft began in October 1978. Both would be launched on a single Space Shuttle mission in February 1983 and were to be boosted towards Jupiter by a solid-fuelled rocket known as the Inertial Upper Stage (IUS). However, the limited capabilities of this rocket was already raising eyebrows in scientific and political circles and there was doubt that it was powerful enough to deliver the twins as far as Jupiter. As a result, in April 1980 the ISPM was split into two halves and rescheduled for separate launches in 1985. The IUS woes continued, however, and the infant Shuttle drew voraciously on NASA’s funds. In February 1981, the space agency slowed the development of its ISPM craft and the IUS was dropped in favour of a more powerful, liquid-fuelled booster, built by General Dynamics. It was called the Centaur-G Prime and its implementation pushed the launch back still further to May 1986. It also opened an entirely new can of worms.

The Centaur carried an enormous load of cryogenic hydrogen and oxygen – totalling more than 36,000 pounds – and came to be nicknamed a ‘balloon tank’, since it required total pressurisation in order to become fully rigid. In fact, if it was not fully pressurised, a single push from a finger could literally flex its metal walls. Right from the star, the Centaur was viewed warily by NASA’s safety officials, whose rule of thumb dictated that no single failure should ever be capable of endangering the Shuttle or her crew. Disturbingly, the Centaur’s pressure regulation hardware lacked a backup facility and, worse, a failure of its internal bulkhead had the potential to rupture the walls of both of its propellant tanks. Moreover, the dangers of ‘sloshing’ of these propellants risked a whole range of controllability problems for the Shuttle itself…but, balanced against these enormous risks was the promise that the Centaur was powerful enough to boost the ISPM and other deep space probes, including the Galileo mission to Jupiter. In the end, it was not enough and the Centaur was removed from consideration in favour of the less powerful, but safer IUS.

Potential disaster hit the ISPM in September 1981, when NASA was forced by the House Appropriations Committee to terminate the production of its spacecraft. However, ESA pressed on with its own craft, which, at 815 pounds, was small and light enough to be reassigned back onto an IUS in January 1982. (In fact, it was so small, said astronaut Dick Richards, that it could quite easily be fitted onto the back of a pickup truck.) By this time, the absence of the Centaur was creating massive financial consequences: Galileo was a hugely important voyage and to be launched by an IUS meant that its journey time to Jupiter would double, its mission duration would effectively be halved and its overall scientific harvest would be seriously compromised. Within a matter of months, plans changed yet again. A groundswell of support for the Centaur, spearheaded by New Mexico Senator and former Moonwalker Harrison ‘Jack’ Schmitt, led to its reinstatement. In spite of the cost of changing boosters again and the lingering safety fears, the reduced journey times and increased scientific bounty which the Centaur could offer Galileo and the ISPM were deemed worthy of the risk. In July 1982, President Ronald Reagan himself approved the change.


The Centaur-G Prime – originally destined to loft Ulysses out of Earth orbit in May 1986 – is pictured during pre-flight processing. In the aftermath of the Challenger disaster, the human-rated Centaur was considered to be unacceptably hazardous and the programme was cancelled. Photo Credit: NASA

The ISPM, therefore, reverted back to the Centaur and, since both it and Galileo needed to travel to Jupiter, both were scheduled for two separate Shuttle missions during the same ‘launch window’ in May 1986. In the meantime, by 1983, the Europeans had completed the fabrication of their spacecraft – a small, boxy machine, with an attached dish antenna and a NASA-provided Radioisotope Thermoelectric Generator power unit. It would be spin-stabilised at five revolutions per minute and its attitude would be managed by four pairs of hydrazine thrusters. Ten scientific instruments were manifested, half of them provided by ESA and half by NASA, to explore radio wave emissions from solar plasmas, together with measurements of magnetic fluxes, observations of electrons, ions, neutral gas, dust and cosmic rays and analysis of the solar wind.

As the ISPM changed, so too did its name. One leading contender was ‘Odysseus’, to honour the mythical Greek hero of the Trojan War, whose ten-year journey back home to reclaim his kingdom of Ithaca and his suitor-pestered wife, Penelope, has made his name a synonym for a voyage with many changes of fortune. The name was entirely fitting. In the same way that the ISPM would follow an indirect path to explore an uncharted destination, so the mythical Odysseus had taken many unexpected twists and turns before reaching the end of his journey. At length, the Latinised version of Odysseus’ name – Ulysses – was picked instead. It had been proposed by an Italian physicist, Bruno Bertotti of the University of Pavia, whose gravitational wave experiment was aboard the mission. In Bertotti’s mind, the name drew upon not only the rich cultural heritage of Troy, but also offered a nod toward other more recent writings: Alfred, Lord Tennyson’s epic poem, James Joyce’s novel and Dante Alighieri’s Inferno. In the latter, Ulysses famously guided his crew westwards into the unexplored waters beyond the Strait of Gibraltar. His terrified men mutinied, but Ulysses calmed them and encouraged them “to follow after knowledge and excellence”.

As with all voyages of exploration, Ulysses and its Centaur booster required massive preparation on the ground. Challenger, the vehicle assigned to deliver the spacecraft into low-Earth orbit, underwent extensive modifications for the purpose. Extra plumbing and emergency dumping vents were installed into the Shuttle to load and drain the Centaur’s propellants, control panels were fitted in the flight deck, an S-band telemetry antenna was added and a huge Centaur Integrated Support Structure in the payload bay served to position the ‘stack’ for deployment.

The original Ulysses deployment crew of Mission 61F were scheduled to launch on 15 May 1986 in what would have been Challenger’s next flight after the ill-fated 51L. Three members of this crew would go on to fly STS-26. Photo Credit: NASA

According to the crew activity plan for Challenger’s mission, released by NASA in mid-January 1986, the Shuttle and its crew of four – astronauts Rick Hauck, Roy Bridges, Mike Lounge and Dave Hilmers – would launch at 4:10 pm EST on 15 May. Assuming an on-time liftoff, the Shuttle carried provisions for a four-day flight and the sheer weight of the Centaur was such that a number of crew provisions, including the galley, had to be removed. Hauck’s crew would have entered a relatively low orbit of just 105 miles and had just nine hours to get Ulysses out of the payload bay. The Centaur was required to periodically dump its boiled-off gaseous hydrogen to keep tank pressures within their mandated limits and, beyond nine hours, it would have ‘bled’ so much propellant that the remainder would have been insufficient to perform the engine burn for Jupiter. After deployment, the Centaur’s twin engines would have ignited to carry Ulysses towards a rendezvous with the giant planet in July 1987 and from thence an encounter with the Sun’s polar regions in 1989-1991.

All of these plans ground brutally to a halt on 28 January 1986, when Challenger exploded during liftoff, killing her entire crew. The resulting investigation uncovered many safety flaws in the reusable Shuttle, several of which related directly to the Centaur, and in June the tempestuous booster was formally cancelled by NASA Administrator Jim Fletcher. With both Ulysses and Galileo now forced to wait out lengthy delay before the Shuttle flew again, other options to deliver them to Jupiter had to be worked out. At this point, the IUS returned to the fore and in April 1987 a firm launch target of October 1990 was established for Ulysses. It would be a narrow ‘window’, just two weeks long, and achieving it would be critical if the spacecraft was to properly rendezvous with Jupiter in February 1992 and go on to explore the solar poles in 1994-95.

Veteran astronaut Dick Richards commanded the Ulysses deployment flight. He had previously flown as pilot of STS-28 in August 1989. Six weeks after his return, at the end of September, he was named to lead the Ulysses flight, STS-41. It was an incredibly rapid turnaround and a ‘plum’, of sorts, for Richards had waited an unenviable nine years for his first flight…longer than any of his contemporaries in his astronaut class. In fact, when he did his debriefing after STS-28, he described himself as “the plank-holder”, for having waited the longest time for a mission, and expressed his fervent hope that no one else would be subjected to the same. “I guess management felt like they owed it to me to make it up to me,” Richards told the NASA oral historian, “and so they had turned me around and got me ready for my first command on STS-41, right away.” He was joined by pilot Bob Cabana – today’s Director of the Kennedy Space Center – and mission specialists Bruce Melnick, Bill Shepherd and Tom Akers.

Richards saw it as his duty to ensure that his crew was ready; “I had the luxury of nine years getting ready to go fly,” he said, but “they didn’t have that much time”. To make them as confident with the Shuttle as possible, he decided to put together a crash course in systems knowledge – they ended up giving each other weekly lectures from their perspective – and although Richards admitted that the move was both popular and unpopular, the end fulfilled the means. “I spent a lot of time worrying about their systems knowledge,” he said, “and ship basics, because of the lack of their shelf life. By the time we got done on that crew, we knew the vehicle backwards and forwards.” It helped, of course, that all five men were incredibly smart, focused and self-motivated individuals and that all were active military personnel. They knew the chain of command, they understood the importance of duty and single-minded devotion to accomplishing The Mission, and performed admirably.

The five men of STS-41 – Tom Akers, Bob Cabana, Dick Richards, Bruce Melnick and Bill Shepherd – pose next to Discovery after triumphantly returning to Earth on 10 October 1990. Photo Credit: NASA

On STS-41, Akers was primarily responsible for overseeing the deployment of Ulysses, which was mounted, uniquely, atop an IUS and a PAM-S booster. The latter was a special variant of McDonnell Douglas’ Payload Assist Module, whose primary objective was to deliver the spacecraft out of Earth orbit and onto a trajectory towards Jupiter. Equipped with a Star-48B solid rocket motor, the PAM-S was designed to be spin-stabilised after separation from the final stage of the IUS.

After Discovery reached orbit on 6 October 1990, the payload bay doors were opened, allowing unfiltered sunlight to flood across Ulysses for the first time, and Tom Akers took the lead in preparing the solar explorer for its voyage. Deployment was scheduled for six hours and one minute into the mission. Watching his movements, Dick Richards could not hide his admiration. “There was a time-critical bunch of steps,” he recalled, none more so that the purging of coolant from Ulysses’ plutonium-powered Radioisotope Thermoelectric Generator (RTG). “Tom had to get down on this switch panel, which was, for some reason, located in this obscure corner of the flight deck.”

Step by critical step, the instant of deployment drew closer: forward payload restraints were released, the aft frame of the IUS’ support structure tilted the stack to an angle of 29 degrees, Richards and Cabana manoeuvred to the correct attitude and electrical power was switched from the orbiter to the IUS. Finally, the three-minute purge of RTG coolant occurred, minutes before deployment. As Akers worked, his crewmates anxiously eyed the clock, keenly aware that a few minutes hence Ulysses would have to be gone. It brought back memories from a couple of their pre-flight simulations, in which Akers had been momentarily late with switch throws, but Richards trusted him implicitly to complete the job and for a few minutes left him alone. At length,
however, the anxiety was pressing.

“Tom?” he asked. “How you doing?”

Akers looked up from his work and gave a broad grin. “Never had so much time!”

The tension in Discovery’s cabin was thus broken and, precisely on time, the ordnance to separate the IUS umbilical cables was activated and the stack was tilted to its deployment position of 58 degrees above the payload bay. Seemingly in slow motion, the spacecraft drifted smoothly and serenely away. Nineteen minutes later, Richards and Cabana fired the orbiter’s thrusters to manoeuvre to a safe distance in anticipation of the firing of the IUS’ first stage engine. That occurred three-quarters of an hour after deployment, unseen by the crew because Discovery had been oriented with her belly facing the direction of Ulysses to protect the orbiter’s windows from the exhaust plume. The first stage burned out, as planned, after a 150-second firing and was jettisoned; whereupon the second stage ignited for almost two minutes, before separating itself.

Ulysses drifts serenely above Earth, shortly after release from Discovery’s payload bay on 6 October 1990. Shortly after this image was taken, the IUS and PAM-S boosters would propel the craft faster than any previous man-made object out of Earth’s gravitational clutches. Photo Credit: NASA

Next came the turn of the PAM-S. Firstly, it ‘spun-up’ Ulysses to 70 revolutions per minute for stability, then executed an 88-second burn to provide the final velocity increment and set the spacecraft on its way to Jupiter. After the burnout of the PAM-S, the spacecraft was ‘yo-yo-despun’ – with weights deployed at the end of cables – to less than eight revolutions per minute. By now, departing Earth’s gravitational well at an escape velocity of 34,510 mph, Ulysses became the fastest man-made object yet to leave the vicinity of the Home Planet; a record which would remain unbroken until the New Horizons spacecraft was boosted towards Pluto in January 2006.

For the astronauts, their involvement with Ulysses was now effectively at an end and responsibility passed to an army of flight controllers and trajectory specialists who would guide it towards a rendezvous with the Solar System’s largest planet in February 1992 and, later, for its exploration of the Sun. Although the role of the STS-41 crew had been exclusively to launch Ulysses, they had undertaken several trips to Europe, and particularly Holland and Germany, where much of the contracting and project management was undertaken. On one occasion, Richards recalled, it gave him a slightly unsettling introduction to European culture. At the end of each afternoon, at 4:30 pm, the German team would reach the end of their day, open up their cooler and pull out several kegs of beer. “We’d all sit around there, next to Ulysses,” he recalled, “toasting Ulysses and having beer. We didn’t do that here in the United States, so that was different. I kinda liked it.”

STS-41 ended, as planned, after just four days – a woefully short period of time, according to Richards, who felt the monumental effort to simply get there could have been monopolised by more time aloft and more experiments  – and the orbiter returned to a desert landing at Edwards Air Force Base in California on 10 October. By the time Discovery landed, Ulysses had already traversed almost a million miles in its journey toward Jupiter. The spacecraft reached the giant planet on 8 February 1992, utilising its gravitational influence to increase its inclination to the ecliptic plane by 80.2 degrees and bend its trajectory southwards to encounter the solar south pole in June-October 1994. From then until the end of operations, its mission would profoundly alter our knowledge of the Sun, demonstrating the dynamic nature of solar magnetism and highlighting the strength of the solar wind. Its northward journey carried it for the first time over the solar north pole in June-September 1995. From its unique vantage point, Ulysses was also employed to observe Jupiter and Comet Hale-Bopp from afar, as well as examining highly energetic gamma ray bursts and interstellar dust from beyond the Solar System.

For a mission which came so close to cancellation, Ulysses transformed itself into one of the greatest success stories and one of the grandest adventures of scientific exploration ever undertaken in the annals of human history.


This is part of a series of History articles, which will appear each weekend, barring any major news stories. Next week’s articles will focus upon two important Shuttle science flights performed 20 years ago, during International Space Year.

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