Autonomous Inspection of Spacecraft
How much astronaut time would be saved by using robots to inspect the exterior of a spacecraft in flight?
Spacecraft carrying human crews may need periodic inspections while in flight to determine whether they have sustained damage from launch, meteoroids or other causes. When astronauts conduct the inspections themselves, current NASA safety protocol requires two astronauts to perform an EVA (extra-vehicular activity) while one stays inside the vehicle and monitors them. All EVAs are hazardous, and the need for three people to be involved makes the task very costly in terms of astronaut-hours.
This study examined the savings in astronaut time of using robots to perform external vehicle inspections on crewed missions. Robotic inspectors would reduce or eliminate the need for human EVAs. Only one astronaut would be required to monitor the robot at current and near-future levels of robot autonomy, and could do so from inside the vehicle. (Future autonomy improvements may make even one human monitor unnecessary, but that scenario was not considered in this study.) So even if robots took as long or somewhat longer than humans to conduct a given inspection, they would yield a savings in crew-hours, freeing the astronauts to perform other work. As it happens, the figures used in our study indicate that because of the constraints attendant to performing EVAs, a robot could conduct a vehicle inspection considerably more quickly than a human counterpart, producing even greater savings in crew time. Note, however, that evaluating the effectiveness of a robotic inspection vs. a human inspection was beyond the scope of this effort.
At the time of this study, the design of the Crew Exploration Vehicle (CEV) -- which NASA plans to deploy by 2012 for missions to the International Space Station, the Moon, and ultimately Mars -- was unknown. So as a proxy, we used the dimensions and inspection requirements of the current shuttle spacecraft.
The robotic inspector is represented in our study by the Autonomous Extravehicular Robotic Camera (AERCam), a free-flying robotic inspection vehicle. A version of the AERCam was demonstrated as a remotely piloted flight experiment during a shuttle mission in 1997, and it has been improved since then.
The Mini AERCam, developed at NASA's Johnson Space Center
To determine the savings in astronaut time that a robotic inspector would offer, we needed to calculate how long it would take a robot to conduct an inspection -- during which time a human astronaut would operate or monitor the robot -- and subtract that amount from the total crew hours required for an equivalent human inspection.
At the time of this study, no shuttle mission had included an external vehicle inspection. However, NASA's return-to-flight plans after the fatal Challenger mission included a human vehicle inspection which would take six hours. Since three astronauts would be involved during that entire time (two EVA and one inside the vehicle), the inspection would take 18 crew-hours.
To calculate how much time the robot would need, we multiplied the robot's flying speed by the distances it would traverse to reach three inspection sites (the shuttle's nose and the leading edge of each wing), and added the hours which, according to a robotics expert at JPL, the robot would need at the sites to conduct the inspection. This gave us the state-of-the-art (SOA) inspection time for a robot.
We looked at three phases of human space flight: low-Earth orbit (LEO) within the next 10 years (e.g., to and from the International Space Station), and then flights to the Moon and later to Mars.
We assumed that each subsequent phase would see improvements in robotic autonomy and/or performance which would reduce the time required for a human to monitor each robotic inspection. As a proxy for this improvement, we used faster flight times for the robot in calculating the inspection time for each successive phase. The AERCam's SOA flight speed is 0.033 meters per second (mps). We used figures of 0.05 mps for LEO missions, 0.1 mps for lunar missions, and 0.2 mps for Mars missions.
Total time for a robotic inspection, and the savings in crew time compared to human inspection. "IVR" stands for "Intra-Vehicular Robotic activity," i.e., the activity performed inside a spacecraft by an astronaut who would either monitor the robot or control the robot via teleoperation. "Translation" refers to the robot's travel time.
We calculated a total robotic inspection time of 3.1 hours at the SOA, meaning that the robot and human monitor would each spend 3.1 hours on the task. To determine the savings in crew time, we subtracted the human monitor's 3.1 hours from the 18 hours of crew time that would be required for a human inspection (2 human EVAs and 1 human monitor each working for 6 hours) to yield a resulting savings of 14.9 hours. With anticipated technology improvements, the savings jumps to 15.3 hours for near-future LEO, 15.7 hours for lunar missions, and 15.9 hours for Mars.
The total savings in crew time increases greatly if we multiply these per-inspection results by the number of inspections that each kind of mission is likely to require.
As an illustration, we calculated the total savings in crew time for each kind of mission in each of three scenarios for the rate at which the number of inspections might grow.
This table shows how many astronaut hours would be saved by robotic inspections under three different scenarios for each of the three types of missions. Fixed growth means that the number of inspections per mission (100) would remain constant across the three kinds of missions. Slow growth means the number of inspections would increase to 150 for the lunar phase and to 200 for the Mars phase. Rapid growth means the number of inspections would increase to 200 for the lunar phase and to 300 for the Mars phase. Note that these numbers are merely parametric, and are not based on actual projections by domain experts.
If provided with the necessary information regarding the cost of technology development for autonomous robotic inspection, we are equipped to extend this study to determine which robotic technology development would provide the best return on investment (ROI), and also whether the best ROI would be achieved by targeting development of robotic inspection technology for LEO, the Moon, Mars, or combinations of them. (A study of this type would look very much like the portfolio analyses in many of the other case studies described in this section of the web site.)
We anticipate that the value of robotic inspection would increase exponentially with the addition of more-complex missions represented by the Moon and Mars in NASA's Vision for Space Exploration, and that the additional cost of each new mission would decrease incrementally since each one would build on the foundation laid by its predecessor. Therefore, the rate of return on investment, measured in terms of benefit divided by cost, should increase markedly as we progress to more complex missions in the Vision.