Getting to the Moon is not simply a matter of pointing a rocket at it and firing. The 384,400 kilometers between Earth and the Moon is filled with competing gravitational forces, complex orbital geometry, and a ruthless energy budget measured in kilometers per second. Every kilogram of propellant a spacecraft carries to accomplish a maneuver is a kilogram that didn't go toward science instruments, life support, or payload. Cislunar space, the volume of space between Earth and the Moon and extending slightly beyond, is governed by some of the most complex orbital mechanics in practical spaceflight. Understanding that complexity is essential for anyone trying to make sense of mission design choices, fuel costs, or why a spacecraft that travels to L2 might take four months instead of three days. The delta-v stages of a lunar mission. Each number represents the velocity change required at that phase. Credit: AI-generated illustration Delta-V: The Currency of Spaceflight Before getting into orbital geometry, there is one concept that explains almost every trade-off in mission design: delta-v , written as Δv and pronounced "delta-vee." It is the total change in velocity a spacecraft must perform across all its maneuvers, measured in kilometers per second. Think of it as the budget for a road trip, except instead of dollars, you spend propellant, and running out is fatal. The tyranny of the rocket equation, first formalized by Konstantin Tsiolkovsky in 1903, means that propellant requirements grow exponentially with delta-v. A mission that needs twice the delta-v doesn't need twice the fuel. It might need four or eight times as much, depending on the specific impulse of the engine. This relationship is why orbital mechanics is not just an academic exercise. It directly determines whether a mission is feasible at all. ~9.4 km/s Earth surface to LEO ~3.1 km/s LEO to Trans-Lunar Injection ~0.9 km/s Lunar Orbit Insertion ~1.9 km/s Descent to lunar surface ~5.9 km/s Total: LEO to lunar surface ~2.4 km/s Lunar surface to orbit These numbers explain why lunar landers are small and why returning large amounts of mass from the Moon is so difficult. Every kilogram lifted from the lunar surface requires propellant, which adds mass, which requires more propellant. The Moon's surface gravity is about 1.62 m/s² (roughly 17% of Earth's), but its lack of atmosphere means there is no aerobraking. Every bit of deceleration on the way down must come from rocket thrust. Getting There: Hohmann Transfers and Low-Energy Trajectories The most fuel-efficient way to move between two circular orbits in a two-body gravitational system is the Hohmann transfer , named for German engineer Walter Hohmann who described it in 1925. The concept is elegant: burn the engine once to raise the spacecraft into an elliptical transfer orbit, coast along that ellipse until reaching the destination orbit, then burn again to circularize. For a lunar mission, a Hohmann-style transfer looks like this: the spacecraft fires its engine at perigee (lowest point near Earth) to enter a highly elliptical orbit whose apogee just reaches the Moon's distance of roughly 384,400 km. This is called trans-lunar injection (TLI). Three days later, after coasting most of the way to the Moon, it fires again to slow down and enter lunar orbit. That second burn is lunar orbit insertion (LOI). AI-generated image A schematic of the main propulsive stages in a lunar mission. TLI, coast phase, LOI, and surface descent each have distinct delta-v requirements. The classic Hohmann transfer to the Moon takes roughly 72 hours. It is fast and relatively simple to execute, but it is not the only option. A class of trajectories called low-energy transfers (also called ballistic lunar transfers or weak stability boundary trajectories) can reach the Moon using about 25% less fuel on the final insertion burn. The catch: the trip takes three to four months instead of three days. Low-energy transfers exploit the gravitational influence of the Sun to help steer the spacecraft. By flying out beyond the Moon's orbit and curving around the Sun-Earth L1 or L2 Lagrange points before falling back toward the Moon from the far side, the spacecraft arrives at lunar orbit with a much smaller relative velocity. Smaller relative velocity means a smaller braking burn, which means less propellant. Japan's SELENE-2 precursor mission, the lunar orbiter RSAT, used this approach. NASA's GRAIL mission in 2011 flew a similar low-energy trajectory to save fuel, reaching the Moon in about four months. For robotic science missions that aren't in a hurry and have tight mass constraints, low-energy transfers can be decisive. For crewed missions where time is critical, the fast direct route is usually preferred. The Three-Body Problem: Why Cislunar Math is Hard Keplerian orbital mechanics, the math most people learn first, assumes a spacecraft is governed by exactly one large gravitational body. A satellite in orbit around Earth follows an ellipse defined by Earth's gravity alone. The equations are clean and have exact analytical solutions. Cislunar space breaks that assumption. A spacecraft between Earth and the Moon is pulled simultaneously by Earth's gravity, the Moon's gravity, and, to a lesser but non-negligible degree, the Sun's gravity. This is the three-body problem, and it has no general closed-form solution. Mathematicians have known this since Henri Poincaré proved it in 1887. Mission designers work around it using numerical integration, running orbital simulations forward in small time steps, recalculating forces at each step. 2026 Reality Check: Artemis II, Gateway's Pause, and Why Trajectory Choice Suddenly Matters More Since this guide first ran, cislunar mechanics has moved from classroom topic to front-page operational issue. NASA's Artemis II launched on April 1, 2026 at 6:35 p.m. EDT and spent roughly 10 days proving Orion on a crewed lunar flyby. The mission did not enter lunar orbit or NRHO. Instead it used a free-return style trajectory , which is exactly the kind of path mission designers choose when crew safety and abort options outrank loiter time at the Moon. That one mission was a live demonstration of the trade this article has described all along: different cislunar routes buy different combinations of risk, energy, flexibility, and mission duration. Then NASA changed the architecture around it. On March 24, the agency said it was halting work on the lunar Gateway and shifting focus toward a multi-phase lunar base effort. That decision does not make NRHO or Lagrange-point dynamics less important. It changes who needs those orbits, and when . Without Gateway as the near-term anchor tenant, the industry's immediate cislunar traffic mix leans more toward direct flybys, south-pole landing campaigns, relay and navigation infrastructure, and cargo routes optimized around surface delivery rather than orbital habitation. What the last three weeks taught us about cislunar mechanics Development Why it matters mechanically Practical takeaway Artemis II lunar flyby Validated a crewed free-return style mission profile instead of a capture into lunar orbit. For early human missions, simplicity and abort geometry can beat long loiter capability. Gateway work paused Removes the near-term need to optimize every architecture around sustained NRHO operations. NRHO remains useful, but it is no longer the automatic center of gravity for every lunar plan. Surface-first lunar planning Pushes attention toward landing windows, relay coverage, polar lighting, and cargo mass fraction. Trajectory design is becoming more mission-specific, not less. NRHO Is Still Important, Even If Gateway Is Not Driving the Schedule A lot of commentary treated Gateway's pause as proof that NRHO was a dead-end concept. That is too simplistic. NRHO is still attractive because it offers long visibility of the lunar south pole, relatively modest stationkeeping, and repeating geometry