Minimize ISS RELL

ISS Utilization: RELL (Robotic External Leak Locator)

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NASA is sending humans forward to the Moon, this time to stay. Upcoming expeditions to the Moon will require making every moment of astronaut time outside the safety of the Gateway in orbit and lunar lander system on the surface count. Robotics will enable lunar crews to do more while minimizing their risk. 1) 2)

NASA’s Satellite Servicing Projects Division is teaming up with the International Space Station to develop the technologies for this kind of astronaut-robotic collaboration, and is launching a Robotic External Leak Locator (RELL) aboard Cygnus on Northrop Grumman’s 11th Commercial Resupply Services mission.

Spacecraft and habitats rely on extensive cooling systems. Just as coolant in a car is used to cool its engine, ammonia is circulated through a huge system of pumps, reservoirs and radiators on station to cool its complex life support systems, spacecraft equipment and science experiments. RELL is a “sniffer,” or a robotic, remote-controlled tool that helps mission operators detect the location of external ammonia leaks on space station and rapidly confirm a successful repair.


Figure 1: Photo of the RELL (Robotic External Leak Locator) flight unit before launch (image credit: NASA)

“RELL capabilities help mitigate the risk of the potentially severe impacts to the space station presented by an external ammonia leak,” said Christopher Craw, ISS Senior Systems Integration Lead at NASA’s Johnson Space Center in Houston.

When it arrives at ISS, this will be the second RELL on board and will serve as a spare.

Some background:

The first flight RELL-1 (also referred to a IRELL) is already on board the station (it was launched on 3 December 2015 on CRS-4 of Orbital ATK) where it successfully located a leak in one of these systems, significantly reducing astronaut time required outside of station to inspect and repair the leak. 3) 4) 5)

On-orbit operations with RELL-1 began in late November 2016 and following scanning activities to characterize the natural and induced environment of the ISS, RELL focused on the United States EATCS (External Active Thermal Control System). RELL successfully detected ammonia related to a known small ammonia leak in the port-side EATCS, with the highest pressure values around the inboard Radiator Beam Valve Module 1 (RBVM 1). An additional day of scanning was subsequently performed in December 2017 to focus on RBVM 1. RELL was approved for additional external operations in February 2017 with the goal of fine tuning the location of the leak. Using grid scanning patterns, RELL detected ammonia around RBVM 1 and located the approximate source of the leak. The potential leak site was inspected by a crew member during an Extravehicular Activity (EVA) in March 2017, and the suspected radiator-side lines were isolated from the port-side EATCS coolant loop in April 2017. Subsequent monitoring of the system pressures showed that the leak has stopped, indicating RELL accurately located the source of the EATCS leak. These activities verify that RELL enhances the ISS Program’s ability to not only locate small leaks, but isolate the source with minimal impact to the entire ISS system. 6)


Figure 2: Controlled by a team at NASA/JSC (Johnson Space Center), the Canadian Space Agency’s Dextre robot point IRELL toward the station’s cooling lines. A NASA ground team will monitor the signals from Earth (image credit: NASA/Goddard Spaceflight Center)

“The decision to build and fly another flight unit seemed like the obvious choice to ensure this capability was going to be available to the ISS Program through the rest of spacecraft’s life,” said Adam Naids, ISS Hardware Development Engineer at NASA’s Johnson Space Center.


Figure 3: Photo of astronaut Shane Kimbrough with RELL aboard the International Space Station (image credit: NASA)

After Cygnus delivers the second RELL to station, the plan is to store the unit 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 the 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 space station managers the information they need to understand and correct the problem.

Before RELL, astronauts manually searched for leaks on spacewalks, which always carry an element of risk. The Leak Locator that is currently stationed in-orbit has proven its worth, paving the way for the second unit.


Figure 4: The Robotic External Leak Locator on the end of the Dextre robot in February 2017 (image credit: NASA)

Both RELL units will eventually be stored in the RiTS (Robotics Tool Stowage), 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.

Operation of RELL: The RELL design includes two sensors: a mass spectrometer and a total pressure gauge. The mass spectrometer measures the number of molecules present in any molecular mass to create a “mass spectrum” reading. Based on this data, analysts determine the composition of present gases. The mass spectrometer can distinguish between trace orbital gasses, which occur naturally, and chemicals potentially originating on station, such as ammonia. This tool can tell the difference from a football field length away.

The total pressure gauge measures the total pressure in space. After the general vicinity of a leak is known, the pressure gauge is able to pinpoint it within a few inches in real time.

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. At its 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.

The president’s direction from Space Policy Directive-1 galvanizes NASA’s return to the Moon and builds on progress on the Space Launch System rocket and Orion spacecraft, collaborations with U.S industry and international partners, and knowledge gained from current robotic assets at the Moon and Mars.

Whether reducing the risk to astronauts on station or one day “sniffing out” the environment of an extraterrestrial world, the human-robotics collaboration demonstrated by RELL will be a vital part of NASA’s exploration future.

Launch: The RELL-2 instrumentation was launched on 17 April 2019 as part of the Cygnus NG-11 (Northrop Grumman-11) CRS (Commercial Resupply Mission) to the ISS on the Antares 230 vehicle. The launch site was MARS (Mid-Atlantic Regional Spaceport) on Wallops Island in Virginia. 7)

Orbit: Near-circular orbit, altitude of ~ 400 km, inclination = 51.6º, period of ~ 92 minutes.

Events with RELL and RiTS on the ISS

• April 7, 2021: In this week’s Tech on Deck join us to learn about RELL and RiTS (Robotic Tool Stowage) and how robotic tools like these can help enable a sustained human presence in space. 8)

Figure 5: 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. - The Station has two remote-controlled tools, RELL (Robotic External Leak Locators) units, that can help detect the location of an external leak and quickly confirm a successful repair. To allow mission operators to more easily deploy these important tools using Station’s Dextre robot, the RELLs recently received a new home on the outside of Station—a protective housing unit called RiTS (Robotic Tool Stowage), video credit: NASA's Exploration and In-space Services

• July 21, 2020: Storage is just as important aboard the International Space Station as it is on Earth. While the space station is about the size of a football field, the living space inside is much smaller than that. Just as you wouldn’t store garden tools in a house when you could store them in a shed outside, astronauts now have a “housing unit” in which they can store tools for use on the exterior of the space station. 9)

Launch of RiTS: On Dec. 5, 2019, a protective storage unit for robotic tools called RiTS (Robotic Tool Stowage), was among the items launched to station as part of SpaceX’s 19th commercial resupply services mission for NASA. As part of a spacewalk on July 21, NASA astronauts Robert Behnken and Chris Cassidy installed the “robot hotel” where the tools are stored to the station’s MBS (Mobile Base System), where it will remain a permanent fixture. The MBS is a moveable platform that provides power to the external robots. This special location allows RiTS to traverse around the station alongside a robot that will use the tools it stores.


Figure 6: The Mobile Base System moves on the Mobile Transporter rail car along truss rails covering the length of the space station. It provides a movable platform for Canadarm2 and Dextre and can access any of eight worksites that feature power connections (image credits: NASA)

“RiTS provides thermal and physical protection for tools stored on the outside of station, not only freeing up room on board but also allowing the Canadian Space Agency’s Dextre robot to access them more quickly,” said RiTS Hardware Manager Mark Neuman.


Figure 7: RELL Engineering Development Unit (left) pictured alongside RiTS (image credit: NASA)

The first step in the RiTS installation process involved preparing the unit inside the space station. The astronauts unpacked RiTS’ occupants from storage – two units of a tool called the RELL (Robotic External Leak Locator) – and affixed them inside RiTS’ aluminum housing.


Figure 8: Astronauts Robert Behnken, Doug Hurley, and Chris Cassidy prepare RiTS for installation (image credit: NASA)

“RELL is a great example how robots with the right tools can simplify life for astronauts,” said Neuman. “Dextre can use RELL to detect ammonia leaks, eliminating the need for astronauts to perform the same task during a spacewalk.”

The ability to locate and repair ammonia leaks efficiently is important since ammonia is used to operate station’s cooling system (Figure 4).

“By locating and monitoring this ammonia leak, RELL verified station’s safety status and avoided a risky spacewalk to find the leak,” said Adam Naids, RELL project manager at NASA’s Johnson Space Center in Houston. “RELL performed spectacularly and successfully carried out the job it was built to do.”

RELL-1 was launched to the space station aboard a Orbital ATK resupply mission in 2015. It was stowed aboard the space station for several months before NASA astronaut Kate Rubins loaded it onto the Japanese Experiment Module slide table. The slide table was extended into space where robotic operators from NASA Johnson controlled the Dextre robot to grab RELL and run it through initial testing. Engineers on the ground verified RELL was functioning properly and effectively communicating back to Johnson. 10)

RELL’s instruments provide directionally sensitive measurements of the amount and kind of gases present. By taking samples, RELL established baseline readings for amounts and kinds of gases which are normally present outside the station. These baseline readings matched what engineers expected, but RELL detected an ammonia signature during the final portion of testing.

Operators were able to identify the ammonia source, an isolation valve for the ammonia cooling loop on the station. Robot operators maneuvered Dextre to position RELL within 12 inches of the leak to collect measurements for several days. With the data gathered, personnel on the ground were able to determine the leak posed no risk to the station or astronauts on board.


Figure 9: Astronaut Kate Rubins loading the Robotic External Leak Locator for deployment into space (image credit: NASA)

Later, ground controllers returned RELL to take high resolution scans from a variety of angles to identify the specific location of the leak. Assisted by RELL’s precise data, station managers choreographed a spacewalk which saw astronauts perform tests to gather additional data that would inform a plan to solve the leak issue. Afterwards, station operators successfully vented and isolated the leaking hose connection from the rest of the cooling system. The station team was able to confirm the leak had stopped and the ammonia coolant loop was intact.

To date, RELL has logged nearly 190 total hours in space, completed thousands of scans, and continuously demonstrated its value aboard the space station. RELL is currently inside the station, ready to return outside when needed.

“Robotic tools apply to many areas of space exploration,” said Benjamin Reed, deputy division director for the Satellite Servicing Projects Division (SSPD) at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “RELL is at the forefront of the great work being done with robotic tools on space station, and we couldn’t be more pleased with its successes.”

RELL is a collaboration between the Engineering Directorate at Johnson and SSPD at Goddard.

1) Kathryn Cawdrey, Rob Garner, ”NASA ‘Nose’ Importance of Humans, Robots Exploring Together,” NASA, 16 April 2019, URL:

2) ”NASA 'Nose' importance of humans, robots exploring together,” Space Daily, 17 April 2019, URL:


4) ”International Space Station Robotic External Leak Locator,” NASA Facts, URL:

5) ”Space Station Receives New Space Tool to Help Locate Ammonia Leaks,” NASA, 22 December 2015, URL:

6) Alexandra M. Deal, Katie L. Fox, Alvin Y. Huang, Michael J. Heiser, William A. Hartman, Ronald R. Mikatarian, Matthew J. Davis, Adam Naids, Timothy A. Bond, Brien Johnson, and Dino J. Rossetti ,"Robotic External Leak Locator (RELL) leak plume field detection on the International Space Station (ISS)", Proceedings of SPIE 10748, 'Systems Contamination: Prediction, Control, and Performance 2018, 1074807,' 19 September 2018, doi: 10.1117/12.2324738,

7) ”Northrop Grumman Heads to Space Station with New NASA Science, Cargo,” NASA Release 19-031, 18 April 2019, URL:

8) ”Tech on Deck, Episode 6” NASA, 7 April 2021, URL:

9) Vanessa Lloyd, ”NASA’s 'Robot Hotel' Gets Its Occupants,” NASA Feature, 21 July 2020, URL:

10) Peter Sooy, ”NASA’s Robotic ‘Sniffer’ Confirms Space Station Leak, Repair,” NASA Feature, 14 September 2017, URL:

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (

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