STS-85 (Discovery)

Background

By the mid-1990s, scientific understanding of the middle atmosphere — the stratosphere and mesosphere lying roughly between 15 and 100 kilometres above Earth's surface — remained incomplete in critical ways. Trace gases present in minute concentrations play outsized roles in ozone chemistry and energy balance, yet their global distributions were difficult to characterize from ground stations alone. The solution was a free-flying orbital observatory that could observe Earth's limb without the contamination introduced by an attached spacecraft. Germany's space agency, DARA (later folded into DLR), developed the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere – Shuttle Pallet Satellite, known as CRISTA-SPAS, to meet precisely that need. A first flight of the instrument in 1994 aboard STS-66 had demonstrated the concept; STS-85 would give it a second, more ambitious opportunity.

Alongside the atmospheric science objective, NASA had a forward-looking motive for the mission. The International Space Station was moving from drawing board toward construction, and robotic systems intended for station assembly and maintenance needed on-orbit validation. STS-85 would carry the Technology Experiment for Robotic Exchange of Replaceable Units, known as EFFORTS' companion experiment — the Japanese-built MACE-II and, most significantly, the MSX satellite tracking effort — but its headline robotics test was the demonstration of Canada's evolving arm concepts and operational procedures that would directly inform station work. The mission therefore combined pure atmospheric science with practical engineering pathfinding.

Crew and Orbiter

Space Shuttle *Discovery* had accumulated a distinguished record before its STS-85 assignment and remained one of NASA's most reliable orbiters. Commander Curtis Brown brought experienced leadership to the flight deck, with pilot Kent Rominger alongside him. Mission specialists Jan Davis, Robert Curbeam, and Stephen Robinson rounded out the NASA contingent; Robinson, flying his first shuttle mission, would go on to a celebrated career including a dramatic spacewalk repair on STS-114. Representing Canada, payload specialist Bjarni Tryggvason was a fluid physicist and pilot whose own research in microgravity fluid dynamics was among the secondary investigations carried in *Discovery*'s payload bay. The crew of six reflected the international character that had become standard for shuttle science missions by the late 1990s.

The Flight

*Discovery* lifted off on 7 August 1997, reaching a stable orbit approximately eight and a half minutes after launch. The crew moved quickly through activation of onboard systems, and early in the mission the CRISTA-SPAS observatory was released from the payload bay. Rather than remaining berthed throughout the flight, CRISTA-SPAS flew independently — separated from *Discovery* by a substantial distance so that thruster firings and outgassing from the orbiter could not corrupt its sensitive infrared measurements. Over the following days the instrument scanned Earth's atmospheric limb, building up a high-resolution, three-dimensional map of trace species including ozone, water vapor, nitrogen oxides, and a range of other minor but chemically significant constituents. The geometry of limb-sounding allowed CRISTA-SPAS to sample a far wider swath of the middle atmosphere in a single orbit than any ground-based network could achieve in months.

After the free-flight observation period, *Discovery* maneuvered to rendezvous with the satellite and the robotic arm was used to retrieve it, returning CRISTA-SPAS to the payload bay for the journey home. This retrieve-and-return architecture was fundamental to the experiment's design: the raw instrument data were supplemented by in-orbit calibration checks and the hardware itself was recovered for post-flight analysis, allowing scientists to verify the instrument's performance in ways impossible with an expendable satellite.

Parallel to the CRISTA-SPAS operations, the crew conducted the robotic demonstrations central to the mission's secondary objectives. Procedures for exchanging hardware components in orbit, tracking moving objects with camera systems, and assessing arm dynamics under realistic loading conditions were all evaluated. The results fed directly into planning for the station assembly flights beginning the following year.

Science and Legacy

The data returned by CRISTA-SPAS on its second flight proved highly productive for the atmospheric science community. By correlating the instrument's trace-gas maps with concurrent measurements from other satellite platforms and balloons, researchers could probe the dynamics of the stratosphere with unprecedented spatial resolution. The CRISTA datasets contributed to studies of planetary-wave activity, the transport of ozone-depleting substances across the tropics, and the validation of general circulation models — work that carried direct relevance to international assessments of stratospheric ozone recovery then ongoing under the Montreal Protocol framework.

*Discovery* completed the mission after nearly twelve days in orbit, executing its deorbit burn and landing at Kennedy Space Center. The clean touchdown closed out a mission that had, with little drama and considerable efficiency, advanced two distinct NASA priorities simultaneously. STS-85 represents an underappreciated model of how the shuttle program packaged scientific return alongside infrastructure investment: the same flight that yielded peer-reviewed atmospheric chemistry had also quietly strengthened the engineering foundation for one of the most complex construction projects in spaceflight history.

For Stephen Robinson and Robert Curbeam, STS-85 was the beginning of long and consequential shuttle careers. For Bjarni Tryggvason, it was a culmination of years of preparation within the Canadian astronaut program. For the CRISTA-SPAS instrument, now retired after two successful flights, the mission stands as confirmation that reusable, retrievable free-flyers could deliver world-class atmospheric measurements from low Earth orbit — a legacy that continues to inform the design of Earth-observing platforms to this day.