LEO to Sun-Earth L1 and L2: Corridor Detail

Corridor record

The Fields Every Corridor Carries

One record covers both points: the Δv, transit and light time are the same.

From: LEO
To: SEL-1, 1.5 million km sunward; SEL-2, 1.5 million km anti-sunward
Δv: 3.40 km/s to either
Transit: About 30 days; JWST took 29
Gear ratio: 15.1× on LOX/methane, from Earth's surface
Window: Daily departures
Light time: 5 s one way
Carries: Solar and space-weather monitors at SEL-1; telescopes at SEL-2
Status: Flown
Retired: No
Rated: Observed

Two points

SEL-1 Watches the Sun; SEL-2 Hides From It

Both points sit about 1.5 million km from Earth on the Sun-Earth line, where a spacecraft keeps pace with Earth around the Sun. Neither is an Earth-Moon libration point; EML-1 and EML-2 are 100 times closer.

SEL-1 sits between Earth and the Sun with an unbroken view of the Sun. Solar wind passes it about an hour before reaching Earth, which is the warning time DSCOVR and SWFO-L1 give power-grid and satellite operators.

SEL-2 sits beyond Earth, so the Sun, Earth and Moon all lie in one direction. A single sunshield blocks all three, which is why JWST, Euclid and Roman are there: infrared telescopes have to stay cold.

Δv budget

3.4 km/s and Then a Few Meters per Second a Year

Almost all of it is the departure burn from LEO.

Δv by leg
Leg	Δv	Flown by	Rated
LEO → transfer injection	3.2 km/s	Ariane 5 upper stage on JWST; Falcon 9 on DSCOVR	Observed
Corrections and halo-orbit insertion	0.1-0.2 km/s	JWST's own thrusters in its first month	Observed
Total	3.40 km/s	The same to either point	Observed
Station-keeping	2-4 m/s a year	Every halo-orbit mission; the orbits are unstable	Observed

Halo orbits around both points are unstable, so every mission carries station-keeping propellant; running out of it, or of coolant, is what ends most missions there.

Who has flown it

Flown Since ISEE-3 in 1978

A selection by first and by instrument; the US, Europe, China, India, Russia and Japan have all used these points.

Missions at SEL-1 and SEL-2
Mission	Operator	Point	Years	Role	Rated
ISEE-3	NASA	SEL-1	1978	First libration-point mission	Observed
SOHO	ESA and NASA	SEL-1	1996-	Solar observatory; three decades on station	Observed
ACE and Wind	NASA	SEL-1	1997-	Solar wind upstream of Earth	Observed
DSCOVR	NOAA and NASA	SEL-1	2015-	Space weather; EPIC camera images the sunlit Earth	Observed
Chang'e-5 orbiter	CNSA	SEL-1	2021	Extended mission after the lunar sample return	Observed
Aditya-L1	ISRO	SEL-1	2024-	India's first solar observatory	Observed
IMAP	NASA, with NOAA SWFO-L1	SEL-1	2025-	Launched September 2025 with the Carruthers Geocorona Observatory	Observed
WMAP	NASA	SEL-2	2001-2010	First mission stationed at SEL-2	Observed
Herschel and Planck	ESA	SEL-2	2009-2013	Infrared and microwave; ended when coolant ran out	Observed
Chang'e-2	CNSA	SEL-2	2011-2012	First Chinese libration-point mission, then on to asteroid Toutatis	Observed
Gaia	ESA	SEL-2	2014-2025	Star catalog; retired in 2025	Observed
Spektr-RG	Roscosmos and DLR	SEL-2	2019-	X-ray survey	Observed
JWST	NASA, ESA and CSA	SEL-2	2022-	Infrared telescope; 6.5 m mirror behind a five-layer sunshield	Observed
Euclid	ESA	SEL-2	2023-	Dark-energy survey	Observed

Who flies it next

Three European and US Telescopes Bound for SEL-2

Every telescope in development for SEL-2 is an infrared or survey instrument.

Missions in development
Mission	Operator	Point	Role	Rated
Nancy Grace Roman Space Telescope	NASA	SEL-2	Wide-field infrared survey	Building
PLATO	ESA	SEL-2	Exoplanet transit survey	Building
ARIEL	ESA	SEL-2	Exoplanet atmospheres; launch 2029	Building
LiteBIRD	JAXA	SEL-2	Cosmic microwave background polarization	Projected

Gear ratio

Every Ton at SEL-1 or SEL-2 Costs About 15.1 Tons Launched

Rocket equation for 3.40 km/s from LEO, dry mass fraction 0.10, times the 5.2× Earth-to-LEO launch factor.

Gear ratio by propulsion class
Propulsion	Isp	Mass ratio	Gear ratio	Rated
Storable, MMH/NTO	320 s	2.95	17.1×	Observed
Methane, LOX/CH₄	360 s	2.62	15.1×	Observed
Hydrogen, LOX/LH₂	450 s	2.16	12.5×	Observed

Nothing is made at either point; the gear ratio prices the observatories and any future servicing visit, not local production.

Time and windows

A Month Out, Any Day

Direct transfers take about 30 days; slower ones save propellant.

JWST reached SEL-2 29 days after launch in December 2021. ISRO's Aditya-L1 took about four months in 2023-2024 because its PSLV launcher left it in Earth orbit and it raised itself in steps.

Light time is about 5 seconds each way, so both points are run from the ground like a GEO satellite with lag.

China

China Has Used Both Points With Lunar Spacecraft

Both Chinese visits were extended missions after a lunar job was done.

Chinese programs at the Sun-Earth points
Program	Data point	Rated
Chang'e-2 · CNSA	Reached SEL-2 in 2011 after its lunar mission; China's first libration-point stay	Observed
Chang'e-5 orbiter · CNSA	Reached SEL-1 in 2021 after delivering the lunar samples	Observed
Queqiao-1 · CNSA	Earth-Moon L2 relay, not Sun-Earth; see the Node Index	Observed

Contested

Deliberately Not Claimed

The load-bearing uncertainties, tagged with the confidence they actually earn.

Modeled

No spacecraft has been serviced at either point. JWST was built without servicing fittings; proposals to refuel or repair observatories at SEL-2 remain studies.