# What are the advantages and disadvantages of assembling a space station in LEO before sending it to its intended Lunar orbit?

Government space agencies and some of the largest space companies are talking about building a space station somewhere in cis-Lunar space, the so called "Gateway". As far as I've heard, the (still very preliminary) plans are to assemble its modules in the intended final orbit.

Wouldn't it be safer and cheaper to assemble it in Low Earth Orbit instead?

Safer given the experiences from the ISS and with much better radiation protection for the crew and the option for quick evacuation to Earth.

Cheaper given that chemical rockets would only need to launch the modules of the space station to LEO instead of to a higher delta-v orbit at the Moon. The space station, when ready to commence operations, could then be towed outwards to its intended orbit by a Solar electric propulsion spacecraft. Since the now canceled Asteroid Retrieval Mission was formulated for over half a decade ago, it has been a priority for NASA to find some kind of mission for a powerful SEP-tow to prove the abilities of this propulsion technology.

Here's a recent Boeing presentation of a Gateway station concept: http://spirit.as.utexas.edu/~fiso/telecon/Duggan_8-9-17/

• Some advantages that you already mentioned are that it is cheaper, lower radiation contamination and it is closer to Earth. But what is our actual goal? Is it to stay in the Earth and not care about our surroundings? If so, then release it in LEO. But if our objective is to explore then assemble it in the farthest place that we can. – Matthew Aug 21 '17 at 11:30
• @Matthew But why, really? Rocket engines are not assembled in space "because it is hard", but on the safe practical ground (which is hard enough). The assembly of entire 10+ton modules for space stations and interplanetary spacecrafts benefit from microgravity, so why not use the safest and cheapest orbit to do it at? When everything is proven to work in relative safely, then push it out to its destination. Just common sense, but I've noticed that common sense isn't always applicable in space flight, so I wonder what the Devil in the details is here. – LocalFluff Aug 21 '17 at 12:08

Making this a community wiki answer so others may add to it as they see fit:

• LEO assembly allows less mission risk if assembly does not go completely as planned. It's much easier to schedule and launch replacements or corrective hardware.
• LEO assembly allows for checkout and stress testing of hardware in a location where failure allows a safe return of crew in a matter of hours, rather than days.
• "Once you're in orbit, you're halfway to anywhere." -- Keeping the upmass of individual launches small allows the use of existing, proven launch technology to get components up for assembly, lowering the program risk posture and reducing the consequences of a single launch failure. As the question implies, transport to higher orbits can then be accomplished over a longer period of time with less mechanical stress to the vehicle using a low-thrust, high-$I_{sp}$ electric thrust system.