Oddly enough, the use of a planetary atmosphere as a brake, bleeding off excess kinetic energy from interplanetary travel, began with human exploration. Because it was never used, it is not well known that aerocapture (see Chapter 10) was the backup plan for the Apollo astronauts as they returned from the Moon. Should their primary chemical propulsion system have failed, they would have achieved Earth orbit using aerocapture. Fortunately for the Apollo Program, unfortunately for advocates of aerocapture technology, the maneuver was never executed and it remains undemonstrated to this day.
How might it be used? In the near term, it would be very useful for spacecraft returning to Earth from the Moon or Mars. Recall that most of the mass of an interplanetary spacecraft, human or robotic, is devoted to its chemical propulsion system. Trade studies show that replacing the chemical propulsion system with the structure required for aerocapture saves anywhere from 20 to 70% of the total propulsive mass. Its use on lunar missions might not be widespread, at least initially. The Moon is close and few operational scenarios call for astronauts to remain in Earth orbit on their return. More likely, they will aeroentry into the atmosphere, like Apollo, for an immediate landing. Mars, however, is a different situation.
There is vigorous scientific debate on the topic of planetary biological protection. If life still exists on Mars, might it be a risk to Earth if it was accidentally brought back here by one of our visiting expeditions? Might we be facing a scenario similar to that depicted in the movie, The Andromeda Strain? Bluntly, we do not know and we might not know until we actually send a crew there to explore and search for signs of life—past or present. Many have suggested that we isolate a retuning crew in Earth orbit for an extended period of time to make sure that no unwanted biological passengers accompanied them on their trip home. There might even be some sort of decontamination facility in low Earth orbit to examine any materials returning from Mars without endangering the Earth's biosphere. If this were to be the case, then the rationale for Earth orbit aerocapture becomes quite strong. Why carry propellant at the expense of other needed cargo or crew for a maneuver that can be accomplished using the resources that nature has already provided?
The other leg of Martian exploration that would benefit from aerocapture is at Mars itself.Various studies by NASA and others have shown the mission-level benefits of using a non-propulsive capture at Mars. The mass savings from its use ripple through the mission design and result in fewer initial rocket launches to get the Mars vehicle into space and lower costs per mission.
A gradual infusion of this technology into robotic exploration will be required before human mission planners seriously consider its use, primarily due to the perceived risk. After all, braking a 1,000-kilogram robotic spacecraft into Mars orbit is far different from a human-piloted spacecraft that weighs 600 times as much. The technological barriers are basically the same: thermal protection, precision terminal guidance (to make sure the spacecraft is where it needs to be), and knowledge of the target atmosphere. All of these potential barriers are actively being broken down for the smaller, robotic systems.
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