Author: CislunarSpace
Source: https://cislunarspace.cn

Earth-Moon Transfer Orbit

Definition and Classification

An Earth-Moon transfer orbit is the orbital segment connecting a Low Earth Orbit (LEO) to a target orbit in cislunar space, such as an NRHO, DRO, or the lunar surface. Depending on the transfer strategy, these orbits can be classified as follows:

Impulsive Transfer

Uses one or a few high-thrust maneuvers to complete the transfer, typically a Hohmann Transfer or a direct trans-lunar injection. The typical $\Delta V$ from LEO to TLI (Trans-Lunar Injection) is approximately 3.1–3.3 km/s.

Low-Energy Transfer

Exploits the dynamical characteristics of the Earth-Moon system — such as the Weak Stability Boundary (WSB) and stable manifolds of halo orbits — to achieve transfer at a lower energy cost. The trade-off is a significantly longer transfer time, ranging from several weeks to months. The Weak Stability Boundary transfer is a representative example.

Ballistic Capture Transfer

Uses lunar gravity assist so that the spacecraft is "captured" by the Moon's gravity upon approach without requiring propulsion. A small maneuver is then performed to enter the target orbit. This method can substantially save propellant but demands precise launch timing.

Energy Budget

The energy requirement for an Earth-Moon transfer is commonly characterized by the hyperbolic excess velocity ( $C_3$ ):

C_3 = v^2 - \frac{2\mu}{r}

where $v$ is the spacecraft's velocity in the Earth-centered inertial frame and $\mu$ is Earth's gravitational parameter.

From LEO (185 km circular orbit) to TLI, the typical $C_3 \approx -0.5$ km $^2$ /s $^2$ (corresponding to $v \approx 10.9$ km/s), with a required $\Delta V \approx 3.1-3.3$ km/s.

Transfer Time vs. Energy Tradeoff

Earth-Moon transfer is a classic bi-objective optimization problem balancing time and fuel:

Transfer Type	Typical Duration	Typical ΔV	Suitable For
Direct transfer	3–5 days	~3.2 km/s	Urgent missions, cargo
Low-energy transfer	2–4 weeks	~3.0 km/s	Crewed missions, probes
Ballistic capture	1–3 months	~2.9 km/s	Small probes, CubeSats

Rendezvous with NRHO and DRO

After arriving in cislunar space, the spacecraft must perform rendezvous and docking or orbit insertion with the target orbit:

NRHO rendezvous: Orbit insertion maneuver near the L1 point ( $\Delta V \approx 200-400$ m/s) to enter the NRHO.
DRO rendezvous: Insertion maneuver when approaching the target DRO ( $\Delta V \approx 100-300$ m/s).
Lunar surface rendezvous: Descent maneuver from NRHO or transfer orbit to the lunar surface ( $\Delta V \approx 1.5-2.0$ km/s).

Simulation

You can design Earth-Moon transfer orbits in the Satellite Orbit Simulation Lab and observe transfer trajectories under different $C_3$ values and launch windows.