Aerocapture is an orbital transfer maneuver used to reduce the velocity of a spacecraft from a hyperbolic trajectory to an elliptical orbit about the targeted celestial body.
Aerocapture uses a planet’s or moon’s atmosphere to accomplish a quick, near-propellantless orbit capture to place a space vehicle in its science orbit (most science orbits require a near circular orbit about the celestial body). The aerocapture maneuver starts as the spacecraft enters the atmosphere of the target body from an approach trajectory. The aerodynamic drag generated by the dense atmosphere slows the craft. After the spacecraft slows enough to capture into orbit, it exits the atmosphere and executes a small motor firing to circularize the orbit. This nearly fuel-free method of deceleration could significantly reduce the mass of an interplanetary spacecraft. Less spacecraft mass allows for more science instrumentation to be added to the mission or allows for a smaller and less-expensive spacecraft, and potentially a smaller, less-expensive launch vehicle. However, this approach requires significant thermal protection and precision closed-loop guidance during the maneuver. This level of control authority requires either the production of significant lift, or relatively large attitude control thrusters. 
Benefits of aerocapture
NASA technologists are developing ways to place robotic space vehicles into long-duration, scientific orbits around distant Solar System destinations without the need for the heavy fuel loads that have historically limited vehicle performance, mission duration, and mass available for science payloads.
A study published in the Journal of Spacecraft and Rockets called “Cost-Benefit Analysis of the Aerocapture Mission Set” showed that using aerocapture over the next best method (propellant burn and aerobraking) would allow for a significant increase in scientific payload for missions ranging from Venus (79% increase) to Titan (280% increase) and Neptune (832% increase). Additionally the study showed that using aerocapture technology could enable scientifically useful missions to Jupiter and Saturn.
Aerocapture technology has also been evaluated for use in manned Mars missions and found to offer significant mass benefits. For this application, however, the trajectory must be constrained to avoid excessive deceleration loads on the crew. While there are similar constraints on trajectories for robotic missions, the human limits are typically more stringent, especially in light of the effects of prolonged microgravity on acceleration tolerances.
Aerocapture spacecraft designs
The aerocapture maneuver can be accomplished with three basic types of systems. The spacecraft can be enclosed by a structure covered with thermal protection material also known as the rigid aeroshell design. Similarly another option is for the vehicle to deploy an aerocapture device, such as an inflatable heat shield, known as the inflatable aeroshell design. The third major design option is of an inflatable, trailing ballute—a combination balloon and parachute made of thin, durable material towed behind the vehicle after deployment in the vacuum of space.
Blunt body, rigid aeroshell design
The blunt body, rigid aeroshell system encases a spacecraft in a protective shell. This shell acts as an aerodynamic surface, providing lift and drag, and provides protection from the intense heating experienced during high-speed atmospheric flight. Once the spacecraft is captured into orbit, the aeroshell is jettisoned.
NASA has used blunt aeroshell systems in the past for atmospheric entry missions. The most recent example is the Mars Exploration Rovers, Spirit and Opportunity, which launched in June and July 2003, and landed on the Martian surface in January 2004. Another example is the Apollo Command Module. The module was used for six unmanned space flights from February 1966 to April 1968 and eleven manned missions from Apollo 7 in October 1968 through the final manned Apollo 17 lunar mission in December 1972. Because of its extensive heritage, the aeroshell system design is well understood. Adaptation of the aeroshell from atmospheric entry to aerocapture requires mission-specific customization of the thermal protection material to accommodate the different heating environments of aerocapture. Also, higher-temperature adhesives and lightweight, high temperature structures are desired to minimize the mass of the aerocapture system.
Inflatable aeroshell design
The inflatable aeroshell design looks much like the aeroshell or blunt body design. The inflatable aeroshell is often referred to as a hybrid system, with a rigid nosepiece and an inflated, attached decelerator to increase the drag area. Just prior to entering the atmosphere, the inflatable aeroshell extends from a rigid nose-cap and provides a larger surface area to slow the spacecraft down. Made of thin-film material and reinforced with a ceramic cloth, the inflatable aeroshell design could offer many of the same advantages and functionality as trailing ballute designs. While not as large as the trailing ballute, the inflatable aeroshell is roughly three times larger than the rigid aeroshell system and performs the aerocapture maneuver higher in the atmosphere, reducing heating loads. Because the system is inflatable, the spacecraft is not enclosed during launch and cruise, which allows more flexibility during spacecraft design and operations.
Trailing ballute design
One of the primary inflatable deceleration technologies is a trailing ballute configuration. The design features a toroidal, or donut-shaped, decelerator, made of a lightweight, thin-film material. The ballute is much larger than the spacecraft and is towed behind the craft, much like a parachute, to slow the vehicle down. The “trailing” design also allows for easy detachment after the aerocapture maneuver is complete. The trailing ballute design has performance advantages over the rigid aeroshell design, such as not constraining the spacecraft size and shape, and subjecting the vehicle to much lower aerodynamic and thermal loads. Because the trailing ballute is much larger than the spacecraft, aerocapture occurs high in the atmosphere where much less heat is generated. The ballute incurs most of the aerodynamic forces and heat, allowing the use of minimal thermal protection around the spacecraft. One of the primary advantages of the ballute configuration is mass. Where the rigid aeroshell may account for 30-40% of the mass of a spacecraft, the ballute mass fraction could be as little as 8-12%, saving mass for more science payload.
Aerocapture has not yet been tried on a planetary mission, but the re-entry skip by Zond 6 and Zond 7 upon lunar return were aerocapture maneuvers, since they turned a hyperbolic orbit into an elliptical orbit. On these missions, since there was no attempt to raise the perigee after the aerocapture, the resulting orbit still intersected the atmosphere, and re-entry occurred at the next perigee.
Aerocapture within fiction can be read in Arthur C. Clarke's novel 2010: Odyssey Two, in which two spacecraft (one Russian, one Chinese) both use aerocapture in Jupiter's atmosphere to shed their excess velocity and position themselves for exploring Jupiter's satellites. This can be seen as a special effect in the movie version in which only a Russian spacecraft undergoes aerocapture (in the film incorrectly called aerobraking).
Players of the video game Kerbal Space Program often employ aerocapture when exploring the satellites of Jool (a gas giant that serves as the game's analogue to Jupiter). The game's simplified model of aerodynamics (prior to the game's official release) allows for aerocapturing to occur without heat shielding.
Aerocapture is part of a unique family of "aeroassist" technologies being developed by NASA for science missions to any planetary body with an appreciable atmosphere. These destinations could include Mars, Venus and Saturn's moon Titan, along with the outer planets.
Aerobraking is another aeroassist maneuver that shares some similarities but also some key marked differences with Aerocapture. Aerobraking also uses the celestial body’s atmosphere to slow the aircraft down into the desired orbit however unlike aerobraking aerocapture only uses one pass through the atmosphere to reduce its velocity while aerobraking takes on the order of 100 to 400 passes to achieve the desired velocity reduction.
• Rapid process (hours to days)
• Descent into a relatively dense mid-atmosphere
• Requires a heavy heat shield due to rapid deceleration resulting in high g-forces
• Gradual process (weeks to months)
• Descent into sparse outer atmosphere
• Small reductions in spacecraft velocity per pass thus no additional mass for a heat shield is necessary
One of the main advantages of using an aerocapture technique over that of an aerobraking technique is that it enables mission concepts for human spaceflight due to the rapid process of transitioning to the desired orbit.
- NASAfacts, “Aerocapture Technology.” https://spaceflightsystems.grc.nasa.gov/SSPO/FactSheets/ACAP%20Fact%20Sheet.pdf. 12 September 2007
- Cruz, MI (May 8–10, 1979). "The aerocapture vehicle mission design concept". Technical Papers.(A79-34701 14-12). Conference on Advanced Technology for Future Space Systems, Hampton, Va. 1. New York: American Institute of Aeronautics and Astronautics. pp. 195–201.
- Hall, J. L., Noca, M. A., and Bailey, R. W. “Cost-Benefit Analysis of the Aerocapture Mission Set,” Journal of Spacecraft and Rockets, Vol. 42, No. 2, March–April 2005
- Physiologically constrained aerocapture for manned Mars missions, JE Lyne, NASA STI/Recon Technical Report N 93, 12720
- Physiological constraints on deceleration during the aerocapture of manned vehicles, JE Lyne, Journal of Spacecraft and Rockets 31 (3), 443-446
- "SCIENCE TEAM AND INSTRUMENTS SELECTED FOR MARS SURVEYOR 2001 MISSIONS". 6 November 1997.
- Percy, T.K. and Bright, E. and Torres, A.O. (2005). "Assessing the Relative Risk of Aerocapture Using Probabilistic Risk Assessment" (PDF).
- Way, D. and Powell, R.W. and Edquist, K. and Masciarelli, J.P. and Starr, B. (2003). "Aerocapture simulation and performance for the Titan Explorer mission" (PDF). AIAA Paper 495 (1).