Reliability and maintainability are key concepts in spacecraft. Reliability is critical for spacecraft, due to their inaccessibility for service after launch and great cost of replacement, and—especially for crewed spacecraft—the potentially catastrophic consequences of failures. The economics of space are rapidly changing, but customers still generally need their spacecraft to work the first time, every time, and to be highly fault-tolerant and long-lasting.
NASA has an entire website dedicated to reliability and maintainability, and several extensive documents detailing their standards for spacecraft. In one, NASA RELIABILITY AND MAINTAINABILITY (R&M) STANDARD FOR SPACEFLIGHT AND SUPPORT SYSTEMS, they spell out the importance of the subject in their objectives: "The top-level objective of R&M activities in NASA support systems programs and projects is to ensure that systems perform as required over their lifecycles to satisfy mission objectives including safety, reliability, maintainability, and quality assurance requirements"Maintainability is an interesting consideration in space. With few exceptions to date, spacecraft cannot be serviced once launched, so they must be designed to survive their design lifetime without needing any repair. What can be maintained, however, is a particular orbit; most spacecraft operate in low Earth orbit (LEO), where a small amount of atmosphere still exerts drag that gradually pulls the trajectory back towards Earth; gravity variations have a similar effect on low lunar orbits. Spacecraft generally counter this through periodic station-keeping, by firing thrusters or rocket engines, and that propellant must be accounted for in the design. The flip side of atmospheric drag is that it helps de-orbit spacecraft from LEO at the end of their life. But atmospheric drag is also highly variable--the density of the atmosphere at any given tie depends on temperature and many other factore.
Maintenance and upgrades are not wholly out of the question, either. Software upgrades, in fact, are common, as code can be transmitted through the communication and data system. Hardware upgrades (outside of the ISS) are virtually unheard of for the reasons above, but as automation and crewed spaceflight capabilities increase, this will become a more common consideration. Other lifecycle services are becoming a reality as well—Northrop Grumman has demonstrated the ability to extend the lives of geostationary satellites through an autonomous “helper” spacecraft , and NASA recently awarded several research grants for on-orbit refeueling technology demonstrations.
Maintenance and upgrades are not wholly out of the question, either. Software upgrades, in fact, are common, as code can be transmitted through the communication and data system. Hardware upgrades (outside of the ISS) are virtually unheard of for the reasons above, but as automation and crewed spaceflight capabilities increase, this will become a more common consideration. Other lifecycle services are becoming a reality as well—Northrop Grumman has demonstrated the ability to extend the lives of geostationary satellites through an autonomous “helper” spacecraft , and NASA recently awarded several research grants for on-orbit refeueling technology demonstrations.
