Solar System Simulator

A real one. Each frame numerically integrates the gravitational attraction between every pair of bodies — an N-body simulation. That's why you can permanently alter, destroy, or rearrange the system; there is no loop to snap back to. Press R to rebuild the solar system from scratch.

The physics uses real AU distances, solar-mass ratios, and eccentricities. The default view gently compresses the space between orbits so all eight planets fit on screen — switch on true-scale distances in ☰ › Settings to see the honest spacing (mostly emptiness; zoom in to find the inner planets). Drawn sizes are never to scale: at true scale Earth would be invisible, far less than a pixel wide, so bodies are enlarged enough to see and click.

Because the planets pull back. The Sun and planets all orbit their common center of mass, and Jupiter is heavy enough to drag the Sun in a small circle around it — about 12 m/s, once every 12 years, in a loop slightly larger than the Sun itself. The real Sun does the same, and detecting that wobble in other stars is one of the main ways exoplanets are discovered: 51 Pegasi b, the first planet found around a Sun-like star, showed up in 1995 as a 56 m/s swing repeating every 4.2 days. Explore → The Sun's wobble zooms in to show the loop.

About 384,000 km — 30 Earth-diameters. Shrink Earth to a basketball and the Moon is a tennis ball about 7 metres away, much farther than most pictures suggest. Light takes 1.3 seconds to cross the gap; Apollo took about three days. The simulator's Earth–Moon view draws both at true relative size and distance, and Missions → Probe to the Moon flies a probe across it. How far is the Moon, to scale →

Relative to the planet, it doesn't: a spacecraft leaves a flyby at the same speed it arrived, because gravity gives back on the way out what it took on the way in. But the planet is moving around the Sun — Jupiter at about 13 km/s — and the flyby bends the spacecraft's path to travel more along the planet's direction, so relative to the Sun it gains speed. Momentum is conserved: the planet slows by an immeasurably tiny amount. Voyager used this at Jupiter, Saturn, Uranus and Neptune; Missions → Voyager 1 flies the whole 1977 mission to today — watch the speed readout through the Jupiter flyby. How a gravity assist works →

Because nothing needs to push it — a force is needed to stop it, and the Sun's gravity is the only thing trying. Gravity weakens with the square of distance, so the total slowing the Sun can ever apply from any point outward is finite; that total is the escape speed there (42 km/s at Earth's distance, 18.5 at Jupiter's, about 3 where Voyager 1 is now). Below it the Sun eventually turns you around: an ellipse, an ordinary orbit. Above it the Sun runs out of pull before you run out of speed, and you coast away forever. Voyager 1 crossed that line at Jupiter and Saturn, has not fired an engine since 1977, and is still slowing — toward about 16.6 km/s, never zero. The simulator's Voyager does the same, and the inspector's "escape speed here" tile makes the comparison for any body you click.

The planets would fly off in straight lines at whatever speed and heading they had at that instant — about 30 km/s for Earth — because nothing would be pulling them into a curve any more. That is Newton's first law. The Moon would stay with Earth, since Earth's own gravity is unaffected. One subtlety the simulator can't show: gravity travels at the speed of light, so the real Earth would keep orbiting the vanished Sun for 8 minutes 20 seconds before it noticed. Click the Sun, open Adjust and delete it to run the experiment for real. What if the Sun disappeared? →

Between about 3 and 22 minutes one way, depending on where Earth and Mars are in their orbits — the gap between them swings from roughly 55 to 400 million km, and radio travels at the speed of light, 300,000 km per second. A reply takes just as long coming back, so a round trip is 6 to 44 minutes, which is why rovers are driven by pre-planned commands rather than live. Explore → Lightspeed message sends a real pulse across today's gap and times it. How long a message to Mars takes →

Because the rocket is the fixed part and the timing is the free part. A launcher can add only so much speed beyond Earth's own orbital motion, and the cheapest path to Mars — a transfer orbit that just reaches Mars's distance — only works if Mars will be at the far end of it when the probe arrives. Mars keeps moving during the roughly eight-month trip, so it has to be a specific angle ahead of Earth at launch. Earth catches up to that geometry once every synodic period, about 780 days. Missions → Probe to Mars shows it: the probe waits for the window, then flies the transfer and arrives where Mars will be. The next Mars launch windows →

Their own gravity locks them. Io laps Jupiter four times for every two laps of Europa and one of Ganymede, and each time Io and Europa pass on the same side of Jupiter the tug lands in the same place, so small drifts get pulled back rather than adding up. One result: the three can never all line up on the same side of Jupiter. In the simulator, open Explore → Jupiter's moons (or click Jupiter and choose Visit Jupiter's moons): the four large moons orbit to scale in their own N-body simulation, a counter tallies the laps as they happen, and nothing is scripted — the lock is the moons' pulls on each other. Jupiter's moons →

The Cassini Division, the dark gap between the bright B ring and the A ring, sits where a ring particle would lap Saturn exactly twice for every lap of the moon Mimas. Repeated tugs from Mimas at the same point in every second orbit clear particles out of that zone. In the simulator, open Explore → Saturn's rings (or click Saturn and choose Visit Saturn's rings): the rings are drawn from their real edges, a blue marker particle laps at that 2:1 radius beside Mimas, and you can zoom into the ring edges, the Encke Gap and the thin F ring.

On the cheapest path, a transfer orbit that just reaches the planet's distance, about 2 years 9 months to Jupiter and 6 years to Saturn. As with Mars, the launch has to wait until the planet is the right angle ahead of Earth, which comes round a little over once a year for either. Real missions often go faster by stealing speed from a planet on the way: Voyager 1 reached Jupiter in 18 months and Saturn in just over three years. Missions → Probe to Jupiter or Saturn waits for the window and flies the transfer under the simulator's full gravity.

Five places where a small object can keep pace with a planet, held there by the planet's and the Sun's gravity together. Three sit on the line through the Sun and the planet, and they are unstable: anything parked there slowly drifts off. The other two, 60° ahead of and behind the planet, are stable, and Jupiter's are crowded with more than ten thousand Trojan asteroids. Spacecraft use Earth's unstable points with small engine burns to stay put; the James Webb Space Telescope sits at L2, 1.5 million km beyond Earth. Explore → Lagrange points turns the view with Jupiter so the points stand still, and you can drop a probe on each. Lagrange points →

Half of the Moon is always lit by the Sun, just like half of Earth. As the Moon goes round Earth we see different amounts of that lit half: none of it at new moon, when the Moon is between us and the Sun, and all of it at full moon, when it is on the far side of Earth. A full cycle takes 29.5 days, about two days longer than one lap round Earth, because Earth moves on round the Sun in the meantime. Earth's shadow plays no part; when it does fall on the Moon, that is a lunar eclipse. Explore → Moon phases shows it from above, lit from the Sun's real direction. Moon phase today →

Because the Moon's orbit is tilted about 5° to Earth's. At most new moons the Moon passes above or below the Sun as seen from Earth and its shadow misses us; at most full moons it passes above or below Earth's shadow. An eclipse needs a new or full moon just as the Moon crosses the plane of Earth's orbit, and that lines up in two windows a year, called eclipse seasons. Each season brings two or three eclipses, so every year has at least four and at most seven. Explore → Eclipses lists the real coming ones and shows the Moon's height in a side view. The next eclipses →

Because a fast-moving clock runs slow, and everything aboard runs slow with it, the traveller included. Picture a clock made of light bouncing between two mirrors. On a fast ship, seen from Earth, the mirrors move sideways, so the light has to follow a longer zigzag to reach each one. Light travels at the same speed for everyone, so every tick takes longer. At 80% of light speed the ship's clock runs at 0.6 of Earth's rate: a 10.6-year round trip to Proxima Centauri takes 6.4 years aboard. It isn't symmetrical, because only the traveller turns round. It has been measured: atomic clocks flown round the world on airliners in 1971 came back out of step with clocks that stayed home, by the amount relativity predicts. Explore → Time dilation flies the trip with a light clock for each twin.

Several planets gathered on the same side of the Sun in our sky, so they share the evening sky (after sunset) or the morning sky (before dawn). It is never a straight line in space: the planets sit at very different distances in very different directions, and only from Earth do they look bunched. Five or more at once comes round every few months; six or seven is rarer, as on 28 February 2025, when all seven were in the evening sky. The tightest line-ups are often invisible, because the planets are bunched around the Sun and lost in its glare, as on 5 May 2000. Explore → Planet alignments draws a line of sight from Earth to every planet, lists the next parades from any date, and jumps to famous ones. The next planetary alignments →

Yes. Mass curves space, and light follows the curve. Starlight grazing the Sun turns by 1.75 arcseconds, about the width of a coin seen from 3 km. Einstein predicted that figure in 1915, twice what Newton's gravity gives if light falls like a stone. The Sun normally drowns out the stars beside it, so the test waited for the total eclipse of 29 May 1919, when Eddington's expeditions photographed stars next to the darkened Sun and found them shifted outward by Einstein's amount. Today the same effect, gravitational lensing, is used to map dark matter around clusters of galaxies. Explore → Bending light draws the bend and the 1919 sky.

On 14 April 1970, 56 hours into the flight and about 330,000 km from Earth, an oxygen tank exploded in the service module and the Moon landing was off. The crew used the lunar module as a lifeboat and fired its engine to get back onto a free-return path, one that swings around the Moon and falls home on its own. They passed 254 km above the far side, farther from Earth than anyone had been, a record that stood until Artemis II in 2026, and splashed down in the Pacific on 17 April. Missions → Apollo 13 replays the loop in a view that turns with the Moon.

On 28 July 2061. Halley goes round the Sun every 75 to 76 years on a long, tilted orbit that runs the opposite way to the planets. In 1986 it rounded the Sun on the far side from Earth and was a poor sight; Europe's Giotto probe flew 596 km from its nucleus that March. In December 2023 it turned around 35 AU out, beyond Neptune's orbit, and it is falling back now. In 2061 it rounds the Sun on Earth's side and passes about half an AU from us. Missions → Halley's Comet replays both passes on its real orbit.

A comet that Jupiter captured and then destroyed. On 7 July 1992 it passed 25,000 km above Jupiter's clouds, close enough for Jupiter's tides to pull it into 21 pieces. Carolyn and Eugene Shoemaker and David Levy found the string of fragments in March 1993. Two years after the breakup, between 16 and 22 July 1994, the pieces hit Jupiter one after another at 60 km/s, leaving dark scars as wide as Earth: the first collision between two Solar System bodies anyone had watched. Missions → Shoemaker–Levy 9 replays the breakup, the two-year loop and the impacts.

The first object ever seen passing through the Solar System from another star. Pan-STARRS in Hawaii found it on 19 October 2017, five days after it passed Earth at 24 million km and six weeks after it swung round the Sun inside Mercury's orbit. It was moving too fast for the Sun to hold, 26 km/s even far away, so its path is an open curve rather than an orbit. It was small, elongated and showed no tail, and it has left for good. Missions → 'Oumuamua replays its path through the planets.

The third known visitor from another star, found on 1 July 2025 by the ATLAS survey telescope in Chile. It is the fastest of the three, arriving at 58 km/s, and it came in almost flat along the planets' plane but going round the opposite way. It passed 29 million km from Mars on 3 October 2025, where spacecraft orbiting Mars photographed it, rounded the Sun on the far side from Earth, and passed Jupiter in March 2026 on its way out. Missions → 3I/ATLAS replays its path through the planets.

About 32,000 km above the ground on 13 April 2029, closer than the geostationary satellites. Apophis is a rock about 340 m across. Soon after it was found in 2004 it briefly looked like it might hit Earth in 2029; better tracking ruled that out, and in 2021 ruled out any impact for at least a hundred years. Earth's gravity will bend its path and stretch its year around the Sun from 324 days to about 423. It should be visible without a telescope from Europe and Africa. Missions → Apophis replays the pass at true scale, then the old and new orbits.

A small one, with enough warning, yes: NASA's DART spacecraft rammed the asteroid moonlet Dimorphos in 2022 and changed its orbit. Warning time is what matters, because a small change in speed grows into a big miss over years. Ten years out, a rock the size of Texas, like the film's, would need a push of about 1 cm/s, less energy than one large bomb. With the film's 18 days it would need about 6 m/s, thousands of times the energy of every nuclear weapon on Earth. Sandbox → Armageddon lets you pick the warning time and the push, then plays it out in the real Solar System.

Yes. Bodies that meet merge — the larger absorbs the smaller — and comets can strike planets or the Sun. You can also delete any body from the inspector, including the Sun. The event feed narrates every impact, capture, and escape.

Yes. The simulator opens on today's sky, and the date under the play button moves every planet and the Moon to where they really were, or will be, on any day from 1800 to 2200. From there gravity takes over: play to move forward, rewind to go back, and the date counts along. The positions come from NASA JPL's published orbital elements; Earth and Mars line up within a third of a degree at every opposition checked from 1877 to 2035.

Scroll or pinch to zoom, drag empty space to pan, click or tap any body to inspect it, drag a planet to move it to a new orbit. The bar above the sky holds the time controls, the date, zoom, the compressed/true-scale switch and the whole-system button. The panel on the left (a sheet at the bottom on phones) opens from Sandbox, Explore, Now or Games; the ☰ menu (top right) has Settings (scale, guides, labels, trails, miles or kilometres and the data table), Share, and Help with the controls; ⟲ Reset all (top right) puts the real Solar System back. Keyboard: P pause, hold Space for turbo, ↑↓ time rate, + − zoom, W A S D or the arrow keys pan, T tilt, 0 whole system, X data table, F fullscreen, M mute, Esc back out of the current menu or view.

Open Games and pick Mercury, Venus, Mars, Jupiter or Saturn. The real Solar System keeps moving the whole time. Your rocket waits just off Earth: aim it, set the launch speed and launch, then steer it there: turn, and fire the engine. Nothing shows you the route; reach the planet and you have arrived.

On a computer, before launch ← and → aim, ↑ and ↓ set the launch speed, and Space launches; in flight ← and → turn the rocket and holding ↑ or Space fires the engine. On a phone, aim with ↺ and ↻, set the speed with the slider and tap Launch; in flight, ↺ ↻ turn and holding Thrust fires the engine. The time controls still work, and changing the speed of time doesn't change your score.

Your time is the days from launch to reaching the planet: the fewer the better. The second record is how much of your fuel tank you used, launch and engine together: the less the better. Both are kept for each planet and go on the site's leaderboards.

Yes — free, in the browser, no download and no signup, like everything on this site.