The article deals with the great challenges industries are facing with Cis Lunar Gateway and especially cleanliness on Esprit Refueling Module (ERM).

The first part of the article presents what is ERM on Gateway with its particular sensitive surfaces such as windows and propulsion fluid connectors

The second part of the article aims to show an overview of standard cleanliness issues on science or optical satellite orbiting around the Earth, which are mainly particulate contamination on ground and molecular contamination in flight.

Beyond industrial challenges involving many contractors to manage and tiny schedules, cleanliness challenges on ERM are described in the third part.

It is also described the contamination levels of usual optical spacecraft compared to ERM contamination levels expectations.

The goal of the article is to answer to :

• What are the contaminations?
• What are the contaminations levels?
• What are the sensitive surfaces?
• How to measure the contaminations before launch and their impacts?
• How to evaluate ground and flight contamination levels?
• How to mitigate or to clean up?