STS-37 (Atlantis / Compton GRO)

Background

By the late 1980s, NASA's Great Observatories program represented one of the most ambitious astrophysical undertakings in the agency's history. The concept was straightforward in ambition if not in execution: place a suite of large, complementary space telescopes into Earth orbit, each tuned to a different portion of the electromagnetic spectrum, so that the universe could be studied with unprecedented coverage and sensitivity. The Hubble Space Telescope had launched in April 1990 as the first of these platforms. Next in line was an observatory dedicated to the most energetic phenomena in the cosmos — gamma-ray sources including pulsars, blazars, and gamma-ray bursts — a class of objects that ground-based astronomy could not directly observe because Earth's atmosphere absorbs high-energy radiation before it reaches the surface.

The instrument built for that purpose was the Compton Gamma Ray Observatory (CGRO), named in honor of physicist Arthur Holly Compton, whose work on the scattering of X-rays and gamma rays earned him the Nobel Prize in Physics in 1927. At roughly 17 metric tons, Compton was one of the heaviest scientific payloads ever flown aboard the Space Shuttle. It carried four complementary instruments covering an enormous range of gamma-ray energies, giving astronomers the ability to survey the entire sky and to study individual sources in detail.

Crew and Preparations

Space Shuttle Atlantis was assigned to carry Compton to orbit on mission STS-37. Commander Steven Nagel and Pilot Kenneth Cameron led a crew that included mission specialists Jerry Ross, Jay Apt, and Linda Godwin. Ross and Apt were designated as the primary spacewalkers, assigned to conduct two planned extravehicular activities during the mission. The crew trained extensively on deployment procedures and contingency operations for the observatory, which was stowed in Atlantis's payload bay. Launch took place on April 5, 1991, and the vehicle reached a stable orbit approximately eight and a half minutes after liftoff.

Deployment and an Unplanned Spacewalk

The mission's central objective, releasing Compton into orbit, was scheduled for approximately twenty-five hours into the flight. On cue, the observatory was lifted from the payload bay by the Shuttle's robotic arm and released. What followed, however, was not in the flight plan.

Shortly after deployment, ground controllers and the crew discovered that Compton's high-gain antenna had failed to deploy. The antenna was critical: without it, the observatory's ability to transmit scientific data to the ground at full capacity would be severely compromised, and the mission's scientific return could be substantially diminished. Engineers on the ground worked through options while the crew assessed the situation.

The decision was made to conduct an unplanned EVA — only the second time in the Shuttle program's history to that point that an unscheduled spacewalk had been called to resolve an emergency. Ross and Apt suited up and ventured outside. Working at the end of the payload bay, the two astronauts manually shook and manipulated the antenna mechanism. The intervention worked: the antenna snapped free and locked into its deployed position. The unplanned EVA, logged at approximately thirty hours and thirty-three minutes into the mission, lasted long enough to resolve the problem and confirm that the antenna was secure. The intervention saved what might otherwise have been a severely degraded scientific mission.

The two astronauts also completed their originally scheduled EVAs during the flight, making STS-37 notable for the volume of extravehicular activity conducted across the mission. The planned spacewalks tested hardware and techniques relevant to future assembly operations, adding operational knowledge to the mission's already significant accomplishments.

Landing and Legacy

After a mission lasting nearly six days, Atlantis executed its deorbit burn at approximately 142 hours and 47 minutes after launch. The orbiter touched down at Edwards Air Force Base in California at roughly 143 hours and 33 minutes into the flight, completing a mission that had exceeded expectations despite its mid-flight complications.

The Compton Gamma Ray Observatory went on to become one of the most productive astronomical instruments ever placed in orbit. Over the course of its operational life, Compton surveyed the entire sky in gamma rays and made foundational discoveries about the nature of gamma-ray bursts, definitively establishing that these transient events were distributed isotropically across the sky — a finding that confirmed they were cosmological in origin, occurring throughout the universe rather than within the Milky Way. Compton also detected and catalogued thousands of gamma-ray sources, mapped the distribution of radioactive elements synthesized in stellar explosions, and studied the high-energy environments around black holes and neutron stars.

The improvised antenna repair conducted by Ross and Apt is remembered as one of the more striking examples of human problem-solving in spaceflight history. The ability to send trained astronauts to physically correct a mechanical failure that no amount of remote commanding could resolve demonstrated precisely the kind of value that human presence in orbit provides, and it reinforced arguments for crewed servicing as a design consideration for major space observatories.

STS-37 thus occupies a significant place in the record of both operational spaceflight and astrophysics. It delivered a transformative scientific instrument to orbit, preserved that instrument's full capability through timely human intervention, and advanced EVA techniques that would prove relevant to subsequent missions. The Compton Gamma Ray Observatory remained in operation until 2000, when it was safely deorbited after a gyroscope failure raised concerns about controlled reentry. Its scientific legacy, built in part on the quick thinking of an Atlantis crew in April 1991, continues to shape high-energy astrophysics.