SpaceX’s Starship Human Touchdown System (HLS) stands tall on the lunar floor on this artist’s rendering. HLS is certainly one of two industrial landers NASA will take a look at on the company’s upcoming Artemis 3 low Earth orbit mission. The HLS is at the moment underneath contract to land crew on the Moon for the Artemis 4 mission in 2028. Credit score: SpaceX
When the Artemis 2 crew splashed down within the Pacific Ocean on April 10, the joy was palpable — Reid Wiseman, Victor Glover, Christina Koch, and Jeremy Hansen had swung across the Moon, damaged a human spaceflight distance document, and are available residence safely. However for the Artemis program, it was only the start.
Between April’s historic splashdown and boots on the lunar south pole lies a sequence of technical challenges extra advanced than something Artemis 2 was requested to resolve, together with newly revised mission structure and quite a few so-far unproven applied sciences. However maybe probably the most essential merchandise on the to-do listing is to perform one thing that has by no means been completed within the historical past of spaceflight: refueling a rocket in orbit. And the clock is ticking.
The rocket equation
When Neil Armstrong and Buzz Aldrin landed within the Sea of Tranquility in 1969, they arrived aboard a spacecraft that had launched as a single stack from Kennedy House Middle. The Saturn V carried the whole lot — the Command Module, the Service Module, and the Lunar Module — unexpectedly, on one rocket. The Saturn V may ship roughly 47 tons to translunar injection, price the equal of billions of {dollars} per flight, and was completely expendable. That very same method received’t work for the cadence and payloads required for constructing a everlasting base.
The Artemis program is constructed across the House Launch System (SLS) — a core stage supplemented with strong rocket boosters that mix to ship probably the most liftoff thrust of any rocket in historical past. However regardless of its lifting energy, SLS is definitely much less succesful than the Saturn V with regards to sending mass to the Moon: In its present configuration, referred to as Block 1, SLS can ship round 27 tons to lunar orbit. A variant with extra highly effective higher levels — which might have delivered the Lunar Gateway — was cancelled as a part of the Artemis restructure led by NASA Administrator Jared Isaacman.
Extra critically, SLS was by no means designed to hold a lunar lander. All the Artemis structure is constructed round a individually launched lander assembly the crew in house and delivering them to the lunar floor.
NASA has contracted two corporations to construct these landers: SpaceX, with its Starship Human Touchdown System (HLS), and Blue Origin, with its Blue Moon. And getting both car to the Moon requires one thing Apollo by no means wanted: refueling in orbit.
The necessity for orbital refueling comes all the way down to a tough constraint of physics. A Starship or Blue Moon loaded with sufficient propellant to fly to the Moon and land can be too heavy to launch. The gasoline required to elevate that gasoline requires but extra gasoline, a compounding downside that engineers name the tyranny of the rocket equation. The answer is to launch the lander with its tank of propellant almost empty and fill it as soon as it reaches low Earth orbit (LEO), earlier than it units off for the Moon.
Plane have been refueling in flight for the reason that Nineteen Twenties. A tanker, flying in shut formation with a receiver, connects the 2 via a versatile hose or a inflexible increase, transfers gasoline, and the plane flies on. Rockets are completely different.
SpaceX’s Starship runs on liquid oxygen (LOX) and liquid methane; Blue Origin’s New Glenn, which can launch Blue Moon, runs on LOX and liquid hydrogen. These are cryogenic propellants — fuels that exist as liquids solely at extraordinarily low temperatures. Transferring them between two autos the scale of skyscrapers, within the microgravity of LEO, introduces a set of issues that aerial refueling has by no means needed to remedy: preserving the gasoline chilly sufficient that it doesn’t boil off, and managing the truth that in microgravity, liquid and fuel don’t behave the way in which they do on Earth.
A historical past of makes an attempt
Transferring cryogenic propellants between two unbiased spacecraft has by no means been completed earlier than. However engineers have been chipping away at items of this downside for many years.
The primary precedents date to as early as 1986, from the Soviet and Russian house packages. Each the Mir house station after which later the Worldwide House Station (ISS) had been serviced by Progress cargo autos that transferred propellant to maintain the stations in orbit. However these transfers used propellants that stay liquid at room temperature.
A extra direct precursor got here in 2007, when DARPA’s Orbital Specific mission efficiently demonstrated autonomous rendezvous, docking, and fluid switch between two spacecraft. However once more, the propellant was storable hydrazine gasoline, not cryogenic.
NASA’s Robotic Refueling Mission 3 (RRM3) got here near the cryogenic problem. Launched to the ISS in December 2018, RRM3 was designed to show each long-duration cryogenic storage and an precise tank-to-tank switch of liquid methane — the primary anybody had tried in orbit. It achieved the primary objective: Particular cryocoolers saved the methane secure for 4 months. Then on April 8, 2019, the coolers stopped responding and couldn’t be revived. The methane was safely vented, and the switch demonstration by no means ran.
A number of years later, SpaceX was capable of succeed the place RRM3 had failed. In March 2024, SpaceX’s Starship Flight Take a look at 3 grew to become the primary car in historical past to switch cryogenic propellant in house, shifting a minimum of 10 metric tons of LOX between two inside tanks. This demo came about inside one car, not two, with no docking interface to barter. But it surely was a vital first step, and an vital proof that cryogenic switch in house is bodily doable.
The best way to fill a rocket in house
Docking
The micro-gravity downside
Ullage, boil-off, and stress
The stress delta
The boil-off downside
One of many greatest challenges is preserving cryogenic gasoline in its liquid state. LOX boils at –297.3 levels Fahrenheit (–183 levels Celsius); liquid methane at –258.7 F (–162 C). Liquid hydrogen is much more excessive, boiling at –423.2 F (–253 C) — simply 20 levels above absolute zero. Regardless of how insulated a gasoline tank’s partitions are, warmth from daylight and radiation will always leak via. This causes liquid propellant to boil into vapor, which collects within the house across the liquid — a area referred to as the ullage. As extra vapor accumulates, stress within the sealed tank climbs. For a gasoline depot sitting in orbit for months, it’s a basic downside. “Probably the most underappreciated problem shouldn’t be a single cryogenic switch occasion, however long-duration cryogenic storage in orbit,” says Qiyun Cheng, an engineer and postdoctoral affiliate at MIT. “Over weeks or months, even very small warmth leaks can accumulate.” The passive resolution — venting the vapor via a reduction valve — throws away propellant the mission wants. A NASA evaluation for a hypothetical Mars mission discovered that passive boil-off losses for a big liquid hydrogen tank carrying 38 tons of gasoline would run roughly 16 tons per 12 months, that means the propellant would boil-off lengthy earlier than the mission arrived at Mars.
Holding propellants chilly requires lively cooling via what engineers name zero boil-off (ZBO) storage (like what was used throughout RRM3). Two important mechanisms have been developed. The primary is jet mixing: A stream of chilly liquid is pumped via the tank, cooling and condensing vapor earlier than stress can construct. The second is droplet injection: Chilly droplets are sprayed immediately into the ullage, the place they take in warmth from the vapor and cut back stress with out venting.
NASA has been testing ZBO storage in microgravity aboard the ISS since 2017 via a challenge referred to as the Zero Boil-Off Tank experiments. The outcomes confirmed the fundamental method works. On that very same hypothetical Mars mission, the evaluation discovered ZBO storage may lower boil-off losses by 42 %, making the mission viable. Nevertheless, questions on how the expertise performs at operational scale stay unresolved.
For one, lively cooling methods for cryogenic gasoline demand important energy. “It takes a variety of electrical work to maneuver a small quantity of warmth out of the tank,” Álvaro Romero-Calvo, an aerospace engineer at Georgia Tech who directs the college’s Low-Gravity Science and Expertise Laboratory, tells Astronomy. “On Earth, we now have just about infinite power, however in house, energy is proscribed by what your photo voltaic panels and batteries can present, and mass prices cash.” Extra energy means extra mass at launch — extra insulation may cut back the wanted energy however carries its personal mass penalty.
SpaceX might want to discover a candy spot in a tangle of trade-offs: Each kilogram dedicated to insulation and cooling is a kilogram of propellant that doesn’t make it to orbit. What number of tanker flights it takes to prime off the lander relies on how effectively the corporate navigates that stability.
Blue Origin faces the identical problem, compounded by hydrogen’s extra excessive boiling level. In July 2025, the corporate introduced that floor testing of its ZBO expertise had met all NASA aims, demonstrating efficiency at what the corporate referred to as “two occasions the efficiency of the present cutting-edge.”
The NASA Workplace of Inspector Basic (OIG), nonetheless, present in a report launched March 10, 2026, that cryogenic fluid administration stays certainly one of each Blue Origin and SpaceX’s most vital technical challenges.
No gravity, no movement
On Earth, buoyancy does the work of separating liquid from fuel — much less dense vapor rises, denser liquid settles. “In house, they only float round, creating froth layers or advanced liquid/fuel mixtures which might be very onerous to separate,” says Romero-Calvo.
SpaceX’s method to the gravity downside combines two methods. First, small thrusters fireplace briefly to push the spacecraft ahead, making a faint synthetic gravity that settles propellant towards the again of the tank. The approach has been used for many years inside single rockets earlier than engine ignition, however making use of it throughout a cryogenic switch between two docked autos is unprecedented. As soon as the propellant is settled, movement is pushed by stress moderately than pumps — managed boil-off raises stress within the donor tank, and that differential pushes propellant throughout the umbilical gasoline line into the receiving car. “The elemental movement mechanism is due to the stress delta throughout the umbilical,” Amit Kshatriya, NASA’s affiliate administrator, advised the NASA Advisory Council’s Human Exploration and Operations Committee in April 2024, in response to reporting on the time from Ars Technica.
The total particulars of how SpaceX will handle pressures, sequencing, and switch stay in growth. Kshatriya advised the committee that SpaceX nonetheless had work forward, as reported by SpaceNews. “The purpose of their flight take a look at program earlier than we do that is to verify they totally perceive the slosh dynamics, totally perceive how the ullage is being maintained, what the settling thrust must be,” he mentioned. “We’ve gone via it with them when it comes to their plan for this. It’s a great plan.”
It’s price noting that even on the bottom, with gravity doing its job and skilled crews managing established infrastructure, cryogenic propellant tanking prompted repeated delays throughout Artemis 2 launch preparations. In orbit, these benefits disappear completely.
A brand new roadmap
On Feb. 27, 2026 — a month earlier than Artemis 2 even left the pad — NASA Administrator Jared Isaacman introduced a sweeping overhaul, restructuring the Artemis program right into a extra incremental sequence of missions modeled on Apollo’s method.
Underneath the brand new structure, Artemis 3, at the moment scheduled for mid-2027, is not going to land on the Moon as initially deliberate. As a substitute, the four-person crew — commander Randy Bresnik, pilot Luca Parmitano of the European House Company, and mission specialists Frank Rubio and Andre Douglas — will launch aboard an Orion crew capsule and rendezvous in low Earth orbit with one or each of NASA’s contracted industrial landers — SpaceX’s Starship Human Touchdown System (HLS) and Blue Origin’s Blue Moon. Whereas there, they can even take a look at this system’s new spacesuits and life-support methods. The mission has drawn comparisons to Apollo 9 — the 1969 Earth orbit shakedown of the Lunar Module that cleared the way in which for Apollo 11 4 months later.
Artemis 4, concentrating on early 2028, is now the primary touchdown mission. NASA has mentioned they may select whichever industrial lander is prepared and licensed. If SpaceX is chosen, a four-person crew will launch aboard Orion, journey to Earth orbit, and dock with Starship HLS for the journey to lunar orbit. If Blue Origin is chosen, Blue Moon’s path to lunar orbit is much less clear. As soon as in lunar orbit, two astronauts will descend to the lunar south pole, the place they may develop into the primary people to face on the Moon since Apollo 17 in December 1972. Over roughly per week on the floor, they may conduct moonwalks, gathering samples and performing experiments, earlier than ascending again to lunar orbit and rejoining their crewmates aboard Orion for the journey residence. The mission is anticipated to final round 30 days in complete.
Artemis 5, at the moment concentrating on late 2028, will see two astronauts descending to the south pole, and the start of NASA’s building on the floor infrastructure for a everlasting base.
Past Artemis, NASA’s formal Moon base plan requires $20 billion to $30 billion in funding over the subsequent decade, unfolding in three phases towards a nuclear-powered everlasting outpost by 2032. A deliberate house station in lunar orbit referred to as the Lunar Gateway has been scrapped. Within the phrases of Carlos Garcia-Galan, the previous Gateway program deputy supervisor who now heads the house company’s Moon base effort, each useful resource is now “centered on one factor, and that’s to construct the Moon base.”
Whereas supposed to be a less complicated structure, the timeline stays formidable. From Isaacman’s announcement to the primary lunar touchdown is lower than two years. And earlier than both lander flies, each corporations should remedy the refueling downside.
From demo to depot
If the engineering challenges will be solved, the Starship structure works like this: SpaceX launches a devoted propellant depot — a modified Starship, basically a big, insulated tank with docking ports — into LEO. Then, over a interval starting greater than 200 days earlier than the crew launches, a sequence of Starship tanker flights elevate off from the bottom, rendezvous with the depot, and switch their propellant. NASA is concentrating on greater than 10 such tanker flights, at a cadence of roughly one each six days. Whether or not it takes 10 or nearer to twenty relies on boil-off losses and switch effectivity. As soon as full, the Starship HLS then launches individually, docks with the now full depot, tops off its tanks, and departs for the Moon.
Earlier than that occurs, SpaceX will show the propellant switch with a take a look at flight. Two Starships will launch from Boca Chica, Texas, a number of weeks aside. The primary, the goal car, will attain orbit and wait. The second, the tanker, will chase down the primary, and dock autonomously. Subsequent, the tanker will switch its cryogenic propellant to the goal. Each autos will then deorbit and return to Earth. If it really works, it will likely be the primary time cryogenic propellant has been transferred between two unbiased spacecraft.
The demonstration has slipped twice. It was initially deliberate for March 2025, then delayed 12 months to March 2026, in response to a NASA OIG report. SpaceX then missed the March 2026 date as effectively, with the demo now anticipated in some unspecified time in the future in 2026 however with no confirmed launch date introduced. The OIG report explicitly flagged the delay as a danger to the general Artemis schedule.
A profitable Starship propellant switch demonstration will clear the trail for an uncrewed Starship HLS lunar touchdown demo. Success there clears the trail for Artemis 4’s crewed lunar touchdown in 2028.
Blue Origin’s steeper climb
Blue Origin faces a much less sure path. In contrast to SpaceX, which has an outlined structure and a pending demo flight, Blue Origin doesn’t have a printed plan for the way refueling will work. Its unique structure trusted docking with the Lunar Gateway, which has since been canceled. How that modifications Blue Origin’s refueling method has not been introduced.
Blue Origin should additionally fly an uncrewed demonstration mission earlier than NASA will certify its lander to hold crew. That demo will probably be flown by Endurance, the Mark 1 variant of Blue Moon — a smaller, uncrewed cargo and science lander designed to show precision touchdown, cryogenic propulsion, and nil boil-off storage on the lunar south pole. Solely after Endurance succeeds can the crewed Mark 2 fly on Artemis 5.
Endurance accomplished thermal vacuum testing at NASA’s Johnson House Middle in early Could — however that milestone has been overshadowed. On Could 28, the New Glenn rocket Blue Origin was making ready for an Endurance launch exploded on its pad throughout a static fireplace take a look at, destroying the car and severely damaging the corporate’s solely New Glenn pad. At a June 9 press convention, Blue Origin’s vice chairman of lunar permanence John Couluris mentioned the corporate’s factories are working around-the-clock and that Mark 1 will probably be prepared for launch this 12 months, including: “We’ll measure ourselves not solely by our successes, however how we reply to setbacks.” Whether or not Blue Origin can return New Glenn to flight, resolve its refueling structure, and land Endurance on the Moon in time to certify Blue Moon for Artemis 5 stays to be seen.
What comes subsequent
“Crack large-scale orbital propellant switch, and propellant turns into a commodity you launch cheaply and mixture in LEO, which decouples deep-space functionality from single-launch mass,” Romero-Calvo says. Briefly, the Moon, Mars, the outer planets — none are reachable with out propellant switch.
“It is a relay race,” Artemis 2’s Christina Koch mentioned from house the day earlier than splashdown. Artemis 2 ran its leg. When humanity will lastly land on the Moon to remain, and Mars after that, hinges on the subsequent legs — legs which might be longer, extra advanced, and depend upon as-of-yet unproven expertise.
Brooks Mendenhall is a workers author at Astronomy, based mostly in Chattanooga, Tennessee, fueled by an never-ending curiosity concerning the universe. A former classroom trainer, he has a knack for breaking down advanced ideas for a large viewers.