Inflatable habitat modules like Bigelow Aerospace's BEAM may point the way to future space station architecture, but with so many micro-meteoroids and bits of space debris shooting around in low Earth orbit, are they safe? To help minimize the risk, researchers from Embry-Riddle Aeronautical University and LUNA Innovations Inc. are developing ultrasensitive sensors made from carbon nanotubes that can detect impacts by near-microscopic objects.
Inflatable modules, which can be sent up in a single rocket launch and then expand to a volume larger than the International Space Station (ISS), may seem fragile, but are designed to provide better radiation shielding and impact protection than conventional metal structures. This is achieved by a hull made of layers of a proprietary Kevlar-like fabric and closed-cell vinyl polymer foam.
The problem is the one faced by all spacecraft, but especially those in the more crowded orbits around Earth. According to NASA, there's over 170 million pieces of space debris above our heads, ranging from discarded boosters to flecks of paint. In all, there are over 5,500 tonnes of debris circling the Earth with 98 percent made up of 1,500 objects in low Earth orbit. Added to this are micrometeoroids of various sizes. The impact of any of these can cause anything from a scratched lens to a punctured habitat.
To better deal with micrometeoroids and orbital debris (MMOD), the Embry-Riddle team led by Aerospace Engineering Professors Daewon Kim and Sirish Namilae are developing a new class of smart material sensors made out of buckypaper based on carbon nanotubes, which are 50,000 times thinner than a human hair, yet are 500 times stronger than steel.
These sensors consist mainly of carbon nanotubes loosely bonded together with epoxy to form thin sheets, and thousands of these would be coated on the large flexible membrane of an inflatable space habitat or other structures like aircraft wings or motor car engines. These are enhanced with micrometer-sized graphite platelets and should be capable of detecting the impact of extremely small micrometeoroids.
The Embry-Riddle project is based on a NASA request for sensors that can detect and locate impact of objects up to 3 mm in diameter and traveling at up to six miles per second (9.6 km per second). It was funded in part by a 2016 US$125,000 Phase I grant to produce two sensor prototypes for monitoring the structural health of inflatable habitats as well as detecting impacts.
The new sensors that are designed to cover multiple outer layers of a habitat are being developed in conjunction with the LUNA project to modify high-definition fiber optic strain sensors to monitor the interior layer. According to Embry-Riddle, the team is now working on highly sensitive strain sensors that have piezoresistive properties when subjected to mechanical deformations.
Kim says that the sensors have already demonstrated dynamic impact detection and are currently undergoing Phase II testing to improve their sensitivity and make them into a flexible and compliant material that can expand as the modules are inflated. Meanwhile, Namilae is developing a computational modeling algorithm that can determine an impact's severity and exact location.
The next step will be at the University of Dayton Research Institute's Hypervelocity Impact Facility where hypersonic 3-mm projectiles will be fired at a sensor array embedded in multiple impact-resistant layers separated by vinyl polymer foam. This will be to demonstrate the space-worthiness of the sensors as well as determining the power needed to operate the system.
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