Where to go in LEO
With the retirement of the ISS coming in the next few years, NASA has begun funding projects to build a commercial successor. Other private ventures plan to do it on their own.
The International Space Station has allowed a continuous presence of humanity in space for almost 24 years now. Despite its very high cost relative to the science it has produced, it will likely be remembered well for this alone, and for providing a platform which allowed for the birth of commercial space companies, notably SpaceX.
There is already another space station in Earth orbit, operated by the China National Space Agency, and it has already got to the stage of doing crew handovers. This means that the chain of continuous human occupation begun by the ISS is unlikely to be broken even it retires before a successor is ready. Humanity is out there, albeit tentatively, and is not ever going to retreat.
NASA for its part are supporting the development of successor stations through the Commercial LEO Destinations (CLD) contract, although at a fairly low level for now. Under this contract, NASA would purchase an “end-to-end service” from the supplier: space station, crew and cargo transport all included.
(There is also, in the next few years, the plan to construct Gateway in orbit of the Moon for the Artemis program. This is a small space station, which doesn’t serve a lot of the functions planned for LEO stations, so I won’t cover it here in much depth.)
The CLD development program budget request for FY 2025 is $170 million, Compare this to the ISS budget of $1.3 billion, and the deep space exploration budget of $7.6 billion. The amount spent on exploring commercial space stations is closer to the $140 million allocated for STEM engagement. This isn’t to say NASA isn’t keen on having a successor for the ISS - but budgets are constrained and other priorities have squeezed it out for now. The elephant in the room being the Artemis program of course, which is unlikely to ease up its own budgetary demands this decade. The ISS will also continue to cost a great deal until it is retired as well.
In short, NASA wants commercial space stations to happen, but is unlikely for the foreseeable future to be able to be the kind of deep pocketed anchor tenant that can nurture commercial providers. So can the companies involved make it work without them?
Commercial Destinations
There were a number of CLD applicants but at time of writing, there are only two projects receiving development funding through the program.
Blue Origin and partners including Sierra Space and Redwire are offering Orbital Reef, a large multimodule station based around 7m diameter core modules to be launched on the upcoming New Glenn launch vehicle.
Starlab is designed to rapidly provide a usable space station through a single Starship launch - named perhaps as a spiritual successor to Skylab, which exploited the lifting power of the Saturn V to similarly get a large station in place quickly.
Not a recipient of the current round of CLD funding, but pursuing a commercial space station anyway, is Axiom Space. The company was founded by a former ISS managed, and in 2020 received a contract under the NextSTEP program to build out modules on to the ISS, which they plan to detach at a later time to form an independent space station, and have recently received another contract to adapt their lunar spacesuits for the ISS. They presently conduct their own Crew Dragon flights to ISS for private customers and non-US space agencies. Overall the company seems, to me, to be a commercial spin out of NASA space station operations.
Finally there is Vast who are racing to get a small, single module station to orbit next year (2025) before moving on to more complex projects. They have not received CLD funding either but have an unfunded collaboration agreement with NASA to work on space station technology.
The Market
These companies hope to be able to make a profit from their space stations. Aside from NASA and other space agencies, what is the market for these space stations?
Varda Space have shown that on-orbit manufacture does not necessarily need humans with the recent succesful conclusion of their first automated on-orbit manufacturing mission, although the argument could be made that it might be easier on a station.
For applications that don’t require very long periods of microgravity, a Starship might be cheaper. Orbital tourism is likely viable, at the right price, but again his the problem that most people would be satisfied with a Starship ride - these have been booked by 3 groups already, and its not clear what added value a longer duration mission would offer for the additional cost.
During the Space Shuttle program, the European Space Agency provided the Spacelab module, which was a temporary microgravity lab that travelled in the Shuttle’s cargo bay. When it first flew, the Soviets were operating the later variants of the Salyut stations as long term labs, and the US had no permanent station. Up to the duration of a Shuttle mission, there wasn’t any experiment a Salyut station could do that the Shuttle could not. Later on the US moved the focus of its microgravity experiments to the ISS, with the last Spacelab science mission flying in 1998, the year ISS was started. But this decision was made in the context of politics not in the context necessarily of trying to best serve scientific customers, especially because the Shuttle was significantly overpriced for what it could do. Starship will be cheap, have a cavernous internal volume and a large payload capacity - the assumption that microgravity experiments and commercial projects will choose a space station over this has not yet been properly tested in the marketplace.
These stations may also have an issue with scale. Not only can Starship provide a good temporary microgravity lab, but it can also operate as a passenger transport on a truly disruptive scale - but that is contingent on having somewhere to actually offload the passengers. For any activity that a space station is interested in, it is surely better the more people are able to do it, and Starship can make it incredibly affordable if the correct destination is there.
Above is the relation between passenger count in a Starship travelling to LEO versus the cost per passenger. The plot ends at 200 which is a conservative maximum seat count for Starship, based on my previous estimates, and Starship will likely be stretched considerably before it carries humans anyway. The price per launch is based on Elon Musk’s aspirational target, a conservative estimate, and a central value (which I think is close to where the long term price will be).
For any price of a Starship launch, it is clear that such small stations would not come close to exploiting its ability to get crew costs down, even if every crew member is regularly rotated out. Such crew counts make sense if the station is resupplied by Dragon, Starliner or Dreamchaser - but NASA expects the transition from CLD development to operations to occur around the end of this decade. These designs are aiming for where the ball is and not where it will be.
Future Space Stations
So what would a space station look like, if we assume Starship disrupts the launch market?
As indicated above, it would be a lot bigger than either ISS or any funded concept to potentially replace it. It would have to also provide services that cannot be found on Earth or easily replicated with single Starship launches - and one that stands out is hypogravity research on humans.
A rotating space station that can produce Lunar or Martian gravity would significantly advance our knowledge - such a station would need to be large, to avoid the impacts of high rotation speeds on the crew, but given that stations need to be built larger anyway that should not be a problem. Such a station would also have the advantage of allowing longer human stays. Normal ISS stays are 6 months, in part due to the health impacts of weightlessness over that period. A habitat with enough gravity to offset the impacts of this would allow the same flight to allow more person-days on orbit, further improving the economics of human activity in space.
In other words, now is the time to move beyond the ISS mode, think big, and look to the space colony proposals of the 1970s again.
There aren’t many companies taking this seriously though. Above Space have a prospective design for a ring shaped space station, and Vast is working towards a small (8 person) artificial gravity station. Interestingly their website did also used to include an image of a larger wheel type station to be developed in the 2040s, but that is no longer there.
The proposal that comes closest to what I see as being the necessary direction for space stations is Al Globus’ concept for Kalpana Two: a hundred metre diameter drum that would rotate at 4rpm and house 500 people. It would be located in an equatorial LEO in order to minimise the radiation shielding required and thus the mass could be as low as 8,500 tonnes.
This is only a study though. To my knowledge, nobody is seriously investing in it. Currently the serious money from both public space agencies and private investors is going towards just replicating the ISS, or similar, which is disappointing. When will we see a well backed effort to produce a space station worthy of the 2030s?
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