Traveling to the moon is expensive. Every kilogram of fuel a spacecraft carries has to leave Earth, and the Earth itself burns more fuel. That’s why aerospace engineers and orbital mechanics researchers spend a lot of time looking for minimum efficiencies in the way spacecraft fly. Saving a few meters per second can translate into millions of dollars in savings per mission. Now, an international team of researchers say they have found a more efficient route between Earth and the moon, calculated using advanced computer models, and that it has been hiding in plain sight.
How Gravity Powers Spacecraft for Free Through an Interstellar Transport Network
To understand the significance of this discovery, it helps to understand how the spacecraft actually moves. The engine only fires certain times. For much of its journey, the spacecraft relies on the gravitational pull of planets, moons, and the sun to follow natural paths through the solar system. These gravity-determined routes, often called interstellar transport networks, run like invisible highways through the solar system. Using them is essentially free propulsion.So finding a cheap route to the Moon has a lot to do with gravity, specifically the gravitational pull of the Earth and Moon. Researchers who study these paths look for so-called “variables,” natural trajectories that guide a spacecraft toward a desired trajectory without the need for constant engine thrust. The question is always from which direction to enter which part of the variable.
This counterintuitive discovery upends traditional lunar navigation
That’s where the new study takes an unexpected turn. Conventional thinking suggests that the most logical approach is to enter the lunar orbit variable at its closest point to Earth, which is the obvious, intuitive choice. But the researchers found that it was actually better to enter the variable from the other side. “Rather than assuming that it is easier to choose the variable part closest to Earth, we can use systematic analysis and faster methods to try to find important solutions,” said study co-author Vitor Martins de Oliveira, a postdoctoral researcher at the University of São Paulo in Brazil. In other words, going around is more efficient than going straight.
How 30 million simulations led researchers to a more fuel-efficient route to the moon
The approach behind this discovery is rooted in functional connectivity theory, a mathematical framework that reduces the computational load required to run complex orbital simulations. Using this approach, the team simulated 30 million different routes to the moon and cited 280,000 simulations in its published research. An analysis of this scale is much more difficult using older methods, which is part of the reason why this particular route has never been identified before. The newly reported route consumes 58.80 meters less fuel per second than the previous cheapest known route. This may not sound impressive, but in orbital mechanics, delta-v (the measure of change in velocity required by a spacecraft engine) is a common means of mission planning. Less delta-v means less fuel, less launch mass and lower cost per stage of the mission.
Why does this new lunar orbit also eliminate communication blackouts in space?
Increased efficiency isn’t the only advantage. The orbit proposed by the researchers would also allow for uninterrupted communication with Earth, something that existing routes cannot always guarantee. “For example, the Artemis 2 mission lost contact with Earth for a time because it was directly behind the moon,” Oliveira noted. The new orbit avoids this problem entirely, allowing the spacecraft to maintain constant contact with ground stations throughout its journey.
What the discovery of the Moon’s path means for the future of lunar space missions
The researchers are careful to view this as a start rather than a final answer. Their model only takes into account the gravitational pull of the Earth and Moon. Future studies could include other variables, such as the sun’s gravitational influence, potentially producing more cost-effective trajectories.“The systems analysis we applied in our work could be adopted more widely in the future,” said the study’s lead author Allan Kardec de Almeida Júnior, a researcher at the University of Coimbra in Portugal.The research was published in the journal Astrodynamics in April. As agencies and private companies plan increasingly ambitious lunar missions over the next decade, the value of tools that make these journeys cheaper and more reliable will only continue to grow. It turns out that this thing was always there waiting to be discovered from the other side.
