In the early evening of October 23, 2001, Pacific Daylight Time, the Odyssey spacecraft, now cruising through interplanetary space, will reach the planet Mars. At that time it will activate its main propulsion engines and perform a Mars Orbit Insertion (MOI) maneuver that will place the spacecraft in orbit around the red planet.

The MOI maneuver is the most critical single event in the Odyssey mission profile. If the propulsion system does not perform its function within several minutes of its designated time, the spacecraft may race past Mars and become a permanent interplanetary traveler. In that case the mission, carrying a scientific payload designed to observe Mars from orbit, will be a failure.

In space engineering, aerobraking is the term applied to the practice of using a planetary atmosphere to change the orbit of a spacecraft. When a spacecraft encounters an atmosphere, even a tenuous one, some of its orbital energy is converted to thermal energy by the aerodynamic drag on the vehicle induced by the particles of the atmosphere. The net impact of such an encounter, called a drag pass in engineering parlance, is to impart a velocity change to the spacecraft that reduces its energy with respect to the planet that it is orbiting. The spacecraft is also heated by part of the thermal energy generated by the drag pass.

The amount of energy converted during each aerobraking drag pass, and therefore the size of the velocity change, depends upon the density of the atmosphere through which the spacecraft passes. This makes logical sense. If the atmosphere is denser, then more particles are encountered, more energy is converted, and the velocity change is larger. In general, atmospheric density is highest near a planet’s surface, and decreases monotonically with altitude. Hence, for a planetary mission, the deeper a spacecraft dips into the atmosphere during its orbit, the larger the velocity change that is imparted and the higher the heating on the spacecraft.

For several decades visionary engineers and science fiction writers have predicted the eventual use of aerobraking for those scientific space missions that require close orbits around planets with atmospheres. Science instruments mounted on an orbiting spacecraft generally want to be as near as possible to the planet that is being observed, for their resolution is a direct function of their distance from the target. However, using standard chemical propulsion techniques, an enormous amount of fuel is required to transfer a spacecraft directly from an interplanetary trajectory, which is hyperbolic with respect to the target planet, to a tight orbit around the target. Often that fuel requirement, plus the mass of the tanks and ancillary equipment necessary to support the larger propulsion system, makes the total mass of the spacecraft to be launched from the Earth prohibitively large.

Aerobraking is a particularly effective way to mitigate that total mass problem. If aerobraking is used, the chemical propulsion system on the spacecraft can be much smaller. However, nothing comes without a price. The addition of aerobraking to a planetary mission design significantly increases the operational complexity and places an additional major risk element in the overall reliability equation.

Still a comparatively new technology for actual space missions, aerobraking was first demonstrated at Venus in an experiment at the end of the flight of the Magellan spacecraft in 1994. After Magellan showed that the basic concept of aerobraking was sound, the technique was incorporated into the baseline design for the Mars Global Surveyor (MGS) mission, which reached Mars in September 1997. Like Odyssey, MGS entered a loose orbit around Mars after performing its MOI with a chemical propulsion system. The preflight mission design then called for an aerobraking phase of a few months duration. During the aerobraking phase, repeated passes by the spacecraft through the upper regions of the Martian atmosphere were supposed to reduce its orbital period and result in the tight observational orbit required by the science payload.

Flight controllers on MGS were unable to implement aerobraking on the predetermined timeline. Part of the difficulty was that one of the MGS solar panels was structurally impaired, forcing the MGS team to be especially conservative in their approach to aerobraking. But other problems were encountered as well. For example, major short term fluctuations in the density of the upper Martian atmosphere, which mapped into significant uncertainties in the amount of velocity change imparted with each successive drag pass, made it almost impossible for the MGS flight team to predict future orbit parameters with any precision. The recurring atmospheric uncertainty plus other operational complexities made managing the spacecraft during aerobraking much more labor intensive than originally envisioned. As a result, the flight team eventually decided to break the aerobraking phase for MGS into two separate time periods, with a several month no aerobraking interval in between. Although the aerobraking implementation ultimately turned out to be successful, and MGS eventually way more than fulfilled all its preflight objectives, the desired science orbit for the MGS spacecraft was not attained until well over a year after MOI.