First-time astronaut Scott “Doc” Horowitz was convinced he would not be launching into space on 22 February 1996. Fully suited in their orange pressure suit and buckled into their seats aboard Columbia—queen of NASA’s shuttle fleet and about to embark on the 19th mission of her career—the seven STS-75 astronauts had been training hard for more than a year.
But 25 years ago today, misfortune almost visited Columbia right on the cusp of liftoff, triggering a possible emergency which might have heralded an on-the-pad launch abort or even worse.
To understand STS-75, it is important to understand its purpose. The TSS was one of the most unusual payloads ever carried aboard the shuttle: an egg-shaped satellite extended into the upper atmosphere at the end of a 12.6-mile (20.5 km) conducting cable. It was meant to demonstrate the “electrodynamics” of a conducting tether in the electrically-charged ionosphere, which extends from roughly 37 miles (60 km) to 620 miles (1,000 km) above Earth.
Scientists were hopeful that such demonstrations could eventually lead to systems which could use tethers to generate electricity, effectively using our planet’s magnetic field as a power source. And by reversing the direction of the current in the tether, the force thus created by its interaction with the magnetic field could conceivably put objects into motion, boosting a spacecraft’s velocity without the need for precious propellant.
Such technologies might, it was theorized, lead to new instruments to trail scientific platforms far below orbital altitudes in difficult-to-study regions, such as the fragile ozone layer above the South Pole. Other applications for tethers included service as extremely low-frequency antennas, capable of penetrating both land and seawater, as well as the generation of artificial gravity and the raising of payloads to higher orbital altitudes.
The TSS first flew aboard shuttle Atlantis in the summer of 1992, but achieved only partial success when the tether snagged on a bolt in the deployment reel mechanism and refused to unroll more than about 850 feet (260 meters). Still, it was successful enough for a reflight to be authorized. Most of the original crew was kept intact for the second mission. Commanding STS-75 was Andy Allen, who had piloted the first flight, and he was accompanied by former TSS-1 crewmates Jeff Hoffman, payload commander Franklin Chang-Diaz and Swiss astronaut Claude Nicollier.
Joining them were Horowitz as pilot and Italy’s Maurizio Cheli as the flight engineer. TSS-1’s lead flight director, Chuck Shaw, reprised the same role in Mission Control for STS-75, whilst Italian astronaut Umberto Guidoni—who served as a backup crewman on the first flight—was assigned as the sole payload specialist.
Unusually on STS-75, the crew was split into no fewer than three shifts for the TSS deployment activities, after which it was expected that they would revert to a standard, Spacelab-style dual-shift system thereafter. The “red” team comprised Horowitz, Cheli and Guidoni, with Chang-Diaz and Nicollier on the “blue” team and Allen and Hoffman forming a unique “white” team.
This final, staggered shift was added to enable the astronauts to operate a suite of instruments aboard the TSS to gather real-world data about how conducting tethers could generate electricity. After tether operations concluded, it was planned that Allen would join the blue team and Hoffman the reds.
“The complexity of the experiment,” Allen wryly noted before launch, “is extreme.” The launch itself would be nothing less.
Early on 22 February 1996, the seven astronauts donned their pressure suits in the Operations & Checkout (O&C) Building at the Kennedy Space Center (KSC) in Florida, before being bussed out to Pad 39B where Columbia awaited them. As they awaited their turns to board the shuttle, the seven men hammed for the White Room cameras.
Allen displayed a placard sending love to his daughters, Jessica and Meredith, whilst Guidoni did likewise—in English and Italian—for his wife, Mariarita, and son, Luca, as did Nicollier for his wife, Susana, and daughters, Maya and Marina. But perhaps the best “gotcha” of the day came courtesy of Maurizio Cheli, serving as Mission Specialist No. 2, who proudly displayed a card with the inevitable legend: “MC2”.
The countdown proceeded normally towards an on-time liftoff at 3:18 p.m. EST. “Arriverderci, au revoir, auf wiedersehen and adios,” Allen radioed cheerily from his seat on Columbia’s flight deck shortly before launch. “We’ll see you in a couple of weeks.”
Unfortunately, the first few seconds of ascent caused hearts to leap into throats.
As the pilot, it was Horowitz’s job to monitor the performance of the three Space Shuttle Main Engines (SSMEs), which together punched out more than 1.2 million pounds (540,000 kg) of thrust at liftoff. As the final phase of the countdown ticked away, Horowitz’s attention was focused with laser-like precision upon the SSME cockpit indicators, whose data-tapes were meant to climb smoothly to full power. “I was a rookie pilot on-board,” he said later, “so my eyes were real big, staring at those main engine gauges about this point.”
Ordinarily, Horowitz would issue the standard call “Three at a hundred” to verify that STS-75 had three good engines at full throttle. But the data on the Chamber Pressure (PC) Tape Meter did not tell him that. Rather than showing all engines at 104-percent of their rated performance, it indicated that the left-hand engine—Serial Number 2034, a veteran of six prior shuttle launches—was firing at only 40-45 percent of its intended output.
Horowitz and Allen were certain there would be no launch for Columbia that day; they would soon hear alarms start blaring and would succumb to a Redundant Set Launch Sequencer (RSLS) abort, as had happened five times previously between June 1984 and August 1994. “We were convinced we were not going anywhere,” Horowitz said later. “We were Pad Abort today.”
To everyone’s surprise, the countdown continued and at T-0 the astronauts felt the staccato crackle of the twin Solid Rocket Boosters (SRBs) and a definitive kick in the back as the shuttle stack left Earth and roared into the clear afternoon sky.
“Houston, Columbia, we’re in the roll,” radioed a somewhat surprised Allen, as the shuttle executed her computer-controlled Roll Program maneuver at T+10 seconds.
“Roger Roll, Columbia,” radioed Capcom Blaine Hammond, seated at his console in Mission Control.
A few seconds passed. Then Allen’s voice was heard over the communications loop again. “Gauge is showing 45 percent on the left.”
“Roger,” replied Hammond. “Stand by.”
It soon became clear that the data was erroneous. “We’ve got good data, Flight,” came the call. “Showing left engine at 104.”
Hammond radioed Allen. “Columbia, we’re showing good engines, good commands on all three.”
“Okay, thank you very much,” replied Allen.
After reaching orbit safely, the astronauts stated that the meter tracked the other PC Meters during ascent, but exhibited a bias of approximately 60 percent. “The meter went to zero during the throttle-down for the throttle bucket and then returned to 40 percent at throttle-up after the period of maximum dynamic pressure,” NASA noted in its STS-75 Flight Problem Summary. “Downlink showed no discrepant engine parameters and the engine correctly responded to all throttle commands throughout ascent.”
Post-mission analysis revealed that the condition was caused by “an anomalous output” from a Multiplexer-Demultiplexer (MDM) and that all Launch Commit Criteria (LCC) were met in a timely fashion, all three SSMEs declared themselves as “Engine Ready” at the proper time and both the High Pressure Oxidizer Turbopump (HPOTP) and High Pressure Fuel Turbopump (HPFTP) performed within mandated specifications.
It was a momentary scare at the start of what would go on to become a highly successful mission of 16 days. But if the erroneous data had been a real emergency brewing in the engines, it would have required Allen and Horowitz to perform a hairy Return to Launch Site (RTLS) abort at the Shuttle Landing Facility (SLF). “We had a couple of moments there that we got a little adrenaline rush,” Allen said later, with a slight smile. “It looked like a bad run in the simulator.”