Astronaut Shane Kimbrough with RELL on station RELL attached to ISS's Japanese Experiment Module slide table for testing in late fall 2016 Introducing the Robotic External Leak Locator, a device that could help sniff out and locate potential leaks on the International Space Station.  NASA engineers at NASA's Goddard Space Flight Center prepare the RELL for flight.

Robotic External Leak Locator

Batten down the hatches! Just like a ship, the International Space Station is carrying precious lives and cargo through an unforgiving environment - and its operators want to make sure that its critical resources, like the ammonia that helps keep the station's cooling system working properly, do not escape into space. NASA's Robotic External Leak Locator (RELL) is a robotic, remote-controlled tool that helps mission operators detect the location of an external leak and rapidly confirm a successful repair. With RELL added to their tool chest, the space station team has another helper in their quest to keep the orbiting research center and all its onboard experiments operating at the right temperatures.

Helping Space Station Operate Optimally

Just as coolant in your car is used to cool its engine, ammonia is circulated through a huge system of pumps, reservoirs and radiators on the space station to cool all of its complex life support systems, spacecraft equipment and science experiments. This coolant system contains thousands of feet of tubing and hundreds of joints. Throughout its lifetime, this system has experienced tens of thousands of thermal cycles orbiting through night and day and the normal wear and tear that comes with over a decade in service. The space station also has to contend with micrometeoroids: tiny objects whizzing through space at speeds that can easily exceed 20,000 miles per hour - and that can cause unwanted, microscopic holes in spacecraft equipment.

Over time, there have been intermittent component failures and leaks on the ammonia cooling loop. Astronauts have undertaken spacewalks to help diagnose, troubleshoot and replace components within the complex active thermal control system. Without a way to robotically locate leaks with high accuracy, astronauts have used spacewalk time to inspect and isolate a potential leak site before addressing the problem at hand.

How It Works

RELL is stored until an ammonia leak is detected. Then, a game of “hot and cold” would begin. Affixed to the Canadian Space Agency’s Dextre robot arm, RELL would be moved around the outside of Station using its mass spectrometer “sniffer” to locate ammonia leaks. When RELL is directed toward a leak, it returns a higher signal. The higher the signal, the closer the leak. This process allows RELL to pinpoint the source of any given ammonia leak, giving Station managers the information they need to understand and correct the problem.

RELL IN ORBIT The Robotic External Leak Locator on the end of the Dextre robot in February 2017

Before RELL, astronauts manually searched for leaks on spacewalks, which always carry an element of risk due to the nature of spacewalks.

Two instruments working in sync give the RELL its ammonia-detecting superpowers.

The RELL So Nice, They Got It Twice

RELL launched to the International Space Station on December 3, 2015. Since then, it has proven its worth, successfully locating a leak and significantly reducing astronaut time required outside of Station to inspect and repair the leak.

Because RELL was built inexpensively, the electronics are commercial off-the-shelf and could be affected by the radiation environment in space, so Space Station always planned to request a spare. The first RELL unit’s success paved the way for a second, spare unit.

The second RELL unit is scheduled to launch to station on April 17, 2019 aboard Cygnus NG-11.

Both RELL units will eventually be stored in the Robotics Tool Stowage, or RiTS, which is still in development. Once installed to the outside of Station, RiTS will store the instruments so they are available when needed to track down a leak.

Beyond ISS

The benefits of leak detection have already been proven on Station, and this ability could be similarly helpful for long-term human habitation on the lunar Gateway, a lunar habitat, and perhaps one day a crewed voyage to Mars. Furthermore, at their core, RELL is a robotics-controlled characterizer of the local environment. This same ability could be used to determine the composition of nearby environments for exploration on the lunar surface, and for scientific and resource utilization purposes.

A Tale of Two Centers

Working together, the Engineering Directorate at JSC and the Satellite Servicing Projects Division (SSPD) at NASA's Goddard Space Flight Center (GSFC) developed RELL for the ISS Program so that astronauts could dedicate their resources to other duties.

The JSC Engineering Directorate houses many of the space station subsystem managers that are responsible for keeping the immense, orbiting research asset operating safely and reliably for global use. With a wide array of technology development experts, designers, analysts, project engineers and project managers in house that have developed ISS systems and supporting hardware, the Engineering Directorate seeks to support the ISS program by providing solutions like RELL that can enhance the ISS mission and systems reliability.

The SSPD team is driving the cutting edge in new NASA servicing technologies. In creating RELL, SSPD leveraged the experience they gained building and executing the Robotic Refueling Mission (RRM), an experiment on space station that is successfully demonstrating tools, technologies and techniques to service spacecraft that were not designed for in-flight repair.


  • Engineering Directorate, NASA's Johnson Space Center
  • Satellite Servicing Projects Division, NASA's Goddard Space Flight Center

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