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Explore Neptune

Sail through Neptune’s sapphire gales, where supersonic winds, drifting methane clouds, and the shifting Great Dark Spot swirl above a hidden ocean of mysteries.

4.5 B km
Distance from Sun
49,244 km
Diameter
−201 °C
Temperature
59,800 days
Year Length

Physical Characteristics

Bulk Composition

Neptune is an ice giant with no solid surface. Its visible “surface” is a deep envelope of hydrogen, helium and methane that merges downward into a hot, dense fluid layer rich in water, ammonia and methane ices.

Hydrogen ~80 %
Helium ~19 %
Methane ~1 %

Atmosphere

Neptune’s atmosphere—dominated by hydrogen and helium, tinted deep blue by trace methane—hosts the fastest winds in the Solar System (up to 2,100 km h-1) and ever-shifting storms like the Great Dark Spot.

Hydrogen (H₂) ~80 %
Helium (He) ~19 %
Methane (CH₄) ~1 %

Gravity

Neptune’s greater mass gives it a surface (cloud-top) gravity about 14 % stronger than Earth’s. A 100 kg person on Earth would feel roughly 114 kg on Neptune.

Earth
100 kg
Neptune
114 kg
Gravity is 1.14 × Earth’s (11.15 m/s²)

Day Length

Neptune spins once every 16 hours 6 minutes, giving it one of the shortest days among the planets.

24 h
16 h 6 m
Neptune’s day is about two-thirds of an Earth day.

Atmospheric Phenomena

Neptune’s Great Dark Spot
Feature:
Great Dark Spot
Neptune, has no solid surface—every feature floats within its deep atmosphere.

Notable Atmospheric Features

Great Dark Spot

A massive anticyclonic storm about the size of Earth, first seen by Voyager 2 in 1989. It drifts westward and can appear or disappear within just a few years.

Winds exceed 1 ,900 km /h along its edge.

“Scooter” Bright Cloud

A brilliant, high-altitude methane-ice cloud that races around the planet every 16 hours, often seen alongside dark spots and casting thin shadows on the deeper haze layers.

Travels at ~300 m/s—nearly supersonic in Neptune’s atmosphere.

Supersonic Jet Streams

Neptune boasts the fastest sustained winds in the Solar System, driven by internal heat and rapid rotation. Equatorial jets and mid-latitude bands shear past each other, powering frequent storm formation.

Peak measured speed: ≈ 2 ,100 km /h.

Neptune’s Moons

Neptune hosts 14 known moons, yet a handful—led by Triton—stand out for their unique size, orbits and geology.

Triton

Triton

Diameter
2 710 km (1 680 mi)
Orbital Period
5.88 days (141 h)
Distance from Neptune
354 760 km (220 380 mi)

Triton orbits retrograde, has nitrogen geysers, and may harbour a subsurface ocean—making it a prime target for future life-detection missions.

Proteus

Proteus

Diameter
420 km (262 mi)
Orbital Period
1.12 days (26.9 h)
Distance from Neptune
117 650 km (73 100 mi)

Irregularly-shaped and heavily cratered, Proteus is almost large enough to be spherical—yet just shy of that threshold.

Nereid

Nereid

Diameter
340 km (211 mi)
Orbital Period
360 days (8 640 h)
Distance from Neptune
5.51 million km (3.42 million mi)

Nereid follows one of the most eccentric moon orbits known—its distance from Neptune swings nearly 15-fold each revolution.

Larissa

Larissa

Diameter
194 km (121 mi)
Orbital Period
0.55 days (13.3 h)
Distance from Neptune
73 550 km (45 700 mi)

Discovered twice—first in 1981 by stellar occultation, then re-found by Voyager 2—Larissa is likely a rubble-pile remnant of a shattered larger moon.

Orbital Period Comparison

Origin Theory

Triton’s retrograde capture likely scattered Neptune’s original moon system; debris from those interactions may have coalesced into the current inner moons.

Future Exploration

NASA’s proposed Neptune Odyssey flagship (launch late-2030s) would conduct multiple Triton fly-bys and deploy atmospheric and ring probes.

Neptune Exploration History

1980s
First Flyby
Voyager 2 at Neptune

Voyager 2

August 25, 1989 Flyby

Humanity’s only close encounter with Neptune. Voyager 2 discovered six new moons, mapped the faint ring system, measured supersonic winds and imaged the transient Great Dark Spot.

Flyby NASA
No missions available for this category.

Current Neptune Missions

No Active or En-Route Missions

As of 2025, no spacecraft are operating at Neptune or on their way there. The planet’s only close visitor to date was Voyager 2, which performed a single fly-by on 25 August 1989.

Concepts such as NASA’s proposed Neptune Odyssey flagship and the earlier Trident fly-by mission remain under study, but none have yet been funded for launch. For now, Neptune’s realm awaits its next explorer.

Future Neptune Exploration

Upcoming Missions

Neptune Odyssey

Target Launch late-2030s → Arrival ~2050

NASA flagship concept featuring a nuclear-powered orbiter plus atmospheric and ring probes to study Neptune, its rings and Triton during a 10-year tour.

In pre-Phase A study (Decadal Survey “priority”)

Trident Fly-by

Proposed 2031 launch*

A New Frontiers–class mission aiming for a single close pass of Triton to sample its active plumes, map the surface at 50 m resolution and probe a suspected subsurface ocean.

*Not yet selected; could re-compete next NF call

Ice-Giant Atmospheric Probe

2030s Proposal

Joint NASA-ESA study for a one-hour descent through Neptune’s clouds to directly measure noble gases and isotopic ratios, delivered by a carrier-relay spacecraft.

ESA Voyage 2050 candidate

Long-Term Visions

Looking toward the 2040s – 2050s, researchers imagine ambitious projects:

  • Triton Lander / Cryovolcano Hopper to sample nitrogen geysers and search for biosignatures.
  • Ring-Skimmer & Polar Orbiter mapping Neptune’s dusty arcs and high-latitude storms.
  • Neptune Upper-Atmosphere Balloon drifting for months to log temperature, winds and aerosol chemistry.
  • CubeSat & Nano-probe Swarm released from an orbiter for multipoint magnetosphere studies.

Challenges & Solutions

Cryogenic Temperatures

Triton’s surface and Neptune’s upper clouds sit near –220 °C, taxing batteries, lubricants and sealants.

Potential Solutions:
  • MMRTGs & RHUs for constant heat
  • Low-temperature electronics & aerogels
  • Active thermal cycling during cruise

Long Cruise & 4-h Light-Time

A direct trajectory takes 12–15 years; two-way radio signals lag ~8 hours, complicating navigation and real-time commanding.

Potential Solutions:
  • Autonomous fault-protection & science scripts
  • Efficient Jupiter gravity-assists
  • Optical comms for higher data rates

Low Solar Flux (30 AU)

The Sun is only 0.1 % as bright as at Earth—solar arrays become impractically huge.

Potential Solutions:
  • Next-generation MMRTGs providing 500+ W
  • Hybrid systems with advanced Li-S batteries
  • Fold-out 100 m2 ultra-thin solar sails (for secondary power)

Neptune Image Gallery

Voyager 2 Global Portrait

Voyager 2 Global Portrait

Only close-range, true-colour view of Neptune, taken 4.4 × 104 km away on 25 Aug 1989; Great Dark Spot and bright methane clouds visible.

JWST 2023 Rings & Triton

JWST 2023 Rings & Triton

James Webb’s NIRCam image resolves faint rings and shows Triton gleaming above the planet in reflected sunlight.

Hubble 2020 New Dark Spot

Hubble 2020 New Dark Spot

HST captured a fresh mid-latitude vortex and a smaller “companion” dark spot forming to its south.

Hubble 2021 Dark-Spot Portrait

Hubble 2021 Dark-Spot Portrait

OPAL programme image (7 Sep 2021) shows a persistent dark vortex, dim northern clouds and a dusk-lit south-polar band.

Voyager 2 Forward-Scattered Rings

Voyager 2 Forward-Scattered Rings

Back-lit view highlights the Adams, Le Verrier and Galle rings—proof Neptune has a complex dusty ring system.

Triton Global Mosaic

Triton Global Mosaic

High-resolution Voyager 2 composite shows Triton’s nitrogen-ice cap, cantaloupe terrain and young fractures.

Keck 2023 “Clouds Gone” View

Keck 2023 “Clouds Gone” View

Near-IR Keck image (21 Jun 2023) shows Neptune almost cloud-free—part of a study linking cloud cover to the solar cycle.

Voyager 2 Ring Arcs

Voyager 2 Ring Arcs

High-contrast processing exposed Neptune’s mysterious ring “arcs,” later explained as dust corralled by Galatea’s gravity.