National Aeronautics and Space Administration

NASAfacts

International Space Station 

Robotic External Leak Locator 

All You Need to Know about RELL

Ops plan: Controlled by a team at NASA’s Johnson Space 

Center (JSC), the Canadian Space Agency’s Dextre robot points 

RELL toward the station’s cooling lines. A NASA ground team 

monitors from Earth.

That’s when the game of “Hot and Cold” begins. When the 

tool is pointed at a leak, the tool’s signal goes up. The closer 

the tool comes to the leak source, the higher the reading gets. 

Houston, we’ve found a leak.

Dextre robot

Cooling lines

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.

RELL launched to the International Space Station in December 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. The first RELL unit’s success paved the way for a second, spare unit. 

Double the Instruments, Double the Detection

How the tool works: Two instruments working in sync 

give RELL its ammonia-detecting superpowers.

Mass spectrometer: Designed for use in a vacuum, 

this device measures the number of molecules present 

in any molecular mass to create a “mass spectrum” 

reading. From this spectrum, analysts can distinguish 

between gasses that are naturally present in the orbital 

environment, versus ammonia – which could only be 

coming from the space station itself. Far more sensitive 

than a human nose, the instrument can detect a leak 

from a football field’s length away.

Ion vacuum pressure gauge: True to its name, this 

device measures total pressure in space. It cannot 

distinguish between different gas molecules, but it 

can sniff for a large leak up close – and locate a leak’s 

position to within a few inches.
The Robotic External Leak Locator on the end of the Dextre robot 

in February 2017.

Astronaut Kate Rubins loading the Robotic External Leak Locator 

for deployment into space.

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.

National Aeronautics and Space Administration

Goddard Space Flight Center

8800 Greenbelt Road

Greenbelt, MD 20771

www.nasa.gov

NASA Facts

NASA astronauts during a May 2013 leak repair spacewalk.

FS-2015-11-357-GSFC (Rev. 04/19)

For more information, visit:

sspd.gsfc.nasa.gov/rell.html

sspd.gsfc.nasa.gov

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.

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 JSC 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.

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 its core, RELL is a roboticscontrolled 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.