Explore the Sun-Earth L2 Lagrange point where space telescopes like JWST
reside. Adjust the halo orbit size, observe temperature equilibrium
panels, and compare famous telescope configurations.
This simulation visualizes a space telescope orbiting the Sun-Earth L2 Lagrange point, the gravitationally balanced spot ~1.5 million km beyond Earth where the James Webb Space Telescope resides. At L2 a sunshield can permanently block the Sun, Earth and Moon, letting infrared instruments cool to cryogenic temperatures. It is fascinating because this single orbital choice unlocks views of the earliest galaxies and exoplanet atmospheres that ground-based and Earth-orbit telescopes simply cannot see.
How it works
The Sun, Earth, Moon and the L2 point are plotted to scale along the Sun-Earth line.
The telescope traces a halo orbit (an ellipse around L2) whose size you control.
A layered sunshield, when enabled, splits the hot Sun-facing side from the cold instrument side.
Thermal equilibrium, angular resolution and collecting area are recomputed live from your settings.
Key equations
T_eq = T_sun · sqrt(R_sun / (2·d)) — T_sun is the Sun's surface temperature (5778 K), R_sun its radius, and d the Sun-telescope distance. With a sunshield the cold side settles near 40 K. Angular resolution follows θ ≈ 1.22 · λ / D, where λ is wavelength and D the mirror diameter.
Controls
Telescope presets — load JWST, Hubble, Herschel or a custom L2 configuration.
L2 halo orbit radius — sets how large the halo orbit around L2 is drawn.
Sun shield — toggles the 5-layer shield that cools the cold side.
Mirror diameter — scales the mirror, changing collecting area and resolution.
Did you know?
JWST's sunshield is roughly the size of a tennis court, yet its five ultra-thin layers drop the temperature by over 250 °C — from a sun-baked +85 °C on the hot side to around −235 °C on the cold side.
Telescope Presets
🌌 JWST
🔵 Hubble (LEO)
🟡 Herschel
🔮 Custom L2
Orbit & Telescope
Thermal & Performance
Distance from Earth—
Orbital period—
Hot side temp—
Cold side temp—
Angular resolution—
Collecting area—
L2 — 5th Lagrange point:
~1.5 M km beyond Earth,
always in Earth's shadow.
Halo orbit ≈ 6-month period.
Teq = T☉·(R☉/2d)^(1/2)
Shield: Tcold ~ 40 K
Why L2?
The Sun-Earth L2 point is located ~1.5 million km beyond Earth away
from the Sun. It is a quasi-stable equilibrium where the gravitational
attraction of the Sun and Earth cancel with the centrifugal force of
the co-rotating frame, allowing objects to orbit there with minimal
station-keeping. The key advantage for infrared telescopes: a
sunshield can simultaneously block light from the Sun, Earth, and
Moon, allowing the telescope's mirror to cool to extremely low
temperatures (~40 K for JWST's cold side). JWST observes in infrared
wavelengths requiring temperatures below 50 K — impossible in Earth
orbit where the telescope would be alternately heated and cooled as it
enters/exits Earth's shadow. At L2, a single 5-layer sunshield
provides a stable, cold thermal environment, enabling observations of
the earliest galaxies and exoplanet atmospheres.
About this simulation
This simulation places a space telescope at the Sun–Earth L2 point, fixed at one astronomical unit from the Sun plus 1.5 million km beyond Earth, and recomputes its physics live from three sliders. Thermal equilibrium uses T_eq = T_sun·√(R_sun / 2d), the true radiative-balance temperature at that solar distance. Enabling the sunshield doesn't move the telescope — it splits it thermally into a Sun-facing side near 1.6×T_eq and a shielded instrument side clamped to about 40 K, matching JWST's real operating temperature. The drawn halo orbit is an ellipse around L2, scaled by the radius slider and stretched 1.5× vertically like an actual 3-axis halo trajectory. Angular resolution follows the diffraction limit θ ≈ 1.22λ/D at λ = 2 µm, while collecting area scales as π(D/2)², so the mirror-diameter slider sharpens resolution and boosts light-gathering power together.
What it shows
The Sun, Earth, Moon and L2 point drawn to a schematic scale along the Sun–Earth line, with the telescope tracing a halo-orbit ellipse around L2, a hexagonally segmented mirror, and a five-layer sunshield that appears whenever shielding is enabled.
How to use it
Click a telescope preset (JWST, Hubble, Herschel, Custom L2) to load its orbit radius, shield state, and mirror size in one step, or drag the three sliders yourself and watch the Thermal & Performance panel update instantly.
Did you know?
JWST's real sunshield is roughly the size of a tennis court. Its five ultra-thin layers drop the temperature by over 250 °C — from a sun-baked +85 °C on the hot side to around −235 °C (about 40 K) on the cold instrument side.
Frequently Asked Questions
What is the Sun–Earth L2 Lagrange point and why do telescopes orbit there?
L2 is a point roughly 1.5 million km beyond Earth (away from the Sun) where the combined gravity of the Sun and Earth, plus the centrifugal effect of orbiting alongside Earth, lets a spacecraft stay in near-constant alignment with both bodies. From L2, a single sunshield can block the Sun, Earth, and Moon at the same time, so an infrared observatory like JWST can passively radiate its heat away and cool to cryogenic temperatures without any onboard refrigeration.
Why does the telescope orbit L2 in a halo orbit instead of sitting exactly at the point?
L2 itself is an unstable equilibrium along the Sun–Earth line — a telescope parked exactly there would drift away and require constant fuel to correct. Instead, real telescopes (and this simulation) fly a large halo orbit around L2, which keeps the Sun, Earth, and Moon reliably behind the sunshield while needing only small course-correction burns every few weeks, and gives the ~6-month orbital period this simulator reports.
How does the simulator calculate the hot-side and cold-side temperatures?
It first computes the equilibrium blackbody temperature at the telescope's fixed Sun distance, T_eq = T_sun·√(R_sun/2d), using the Sun's surface temperature (5778 K) and radius. If the sunshield is off, both sides sit near this T_eq. Turning the shield on splits the telescope thermally: the Sun-facing side heats to roughly 1.6×T_eq (close to JWST's real +85 °C sunshield-facing layer), while the shielded instrument side is clamped near 40 K — matching JWST's actual mid-infrared operating temperature.
What determines the telescope's angular resolution in the simulation?
Angular resolution is computed from the diffraction limit, θ ≈ 1.22λ/D, evaluated at an infrared wavelength of 2 µm and reported in arcseconds. Because D is the mirror-diameter slider, a bigger mirror shrinks θ (sharper images) while simultaneously increasing the collecting area π(D/2)² shown in the Performance panel — the same trade-off that drives real observatories toward larger primary mirrors.
Are the preset telescope configurations (JWST, Hubble, Herschel) realistic?
They are simplified illustrations, not exact mission specs. The JWST preset (800,000 km halo radius, shield on, 6.5 m mirror) is close to the real observatory. Hubble is included without a shield and with its actual 2.4 m mirror purely for comparison — in reality Hubble orbits low Earth orbit, not L2. Herschel's preset approximates a passively cooled L2 infrared telescope at reduced mirror scale rather than reproducing its full 3.5 m aperture.