Real gravity, not an animation
Most solar-system visualizations move planets along pre-drawn ellipses. This one doesn't. Every frame, the simulator computes the gravitational pull between every pair of bodies — Sun, eight planets, the Moon, the asteroid belt, and any comets you launch — and lets the orbits fall out of the physics. Nothing is on rails, and the Sun isn't nailed down: the planets tug it around the system's common center of mass just like the real one.
Because the orbits are computed rather than drawn, you can break them. Give Earth a shove with the thruster and it settles into a permanently different orbit. Crank a planet's mass and its neighbors feel it. Delete the Sun and everything sails away. The elapsed-time clock and the energy indicator beside playback track how faithfully the integration is conserving energy while you meddle.
What's real, what's illustrative
Real numbers
- Planet masses (in solar masses) and distances (in AU)
- Orbital eccentricities — Mercury's stretched path is genuine
- Orbital periods — not typed in, they emerge from gravity
- Axial tilts and rotation periods in the planet inspector and dock
- Satellite orbit altitudes in Earth view, up to the 35,786 km geostationary ring
Stretched to be visible
- Drawn sizes — at true scale Earth would be far thinner than a pixel
- Orbit spacing is gently compressed so all eight planets fit on screen — Display settings can switch to true-scale distances
- Everything orbits in one flat plane
- Satellite speeds in Earth view are sped up to stay watchable
- Time — a year passes in seconds, faster at higher rates
A dozen-plus experiments worth running
- Put a comet into Jupiter. Open Sandbox → Launch a comet, aim with the direction and aim sliders, and fire. A hit flashes on impact and lands in the event feed.
- Fire the comet storm. ~100 random specks at once — then watch the feed report captures, ejections, and impacts as gravity sorts them out.
- Invite a rogue star. A quarter-solar-mass intruder crosses the system: orbits bend as it passes, and anything it sweeps too close to is consumed — the event feed keeps score of the damage.
- Fly a probe to Mars, Jupiter, or Saturn. Missions → Probe to Mars: the spacecraft waits for the launch window (Mars has to lead Earth by about 44°), then coasts along a real Hohmann transfer orbit for roughly eight months while the planets keep moving — arriving where Mars will be, not where it is now. Jupiter and Saturn are the same idea at grand scale: bigger launch windows and multi-year cruises out past the asteroid belt. Or pick the Mars lesson in Explore and take the launch controls yourself — the rocket's push is fixed, and only the timing is yours.
- Fly the Voyagers from 1977 to today. Missions → Voyager 1 arranges the four giant planets in the rare arc they formed in 1977 — an alignment that comes around once every ~176 years — and launches on 5 September 1977. The probe coasts to Jupiter, swings a few planet radii past it in a slow-motion close-up (the real one passed 349,000 km out in March 1979), is bent onto Saturn for a second assist (November 1980), and comes out above the Sun's escape speed. No engine fires after launch: the speed it stole from the two planets' orbital motion is what carries it out. Voyager 2 flies Jupiter, Saturn and Uranus and, like the real one, needs small course-correction burns between planets to thread each flyby; the inlay logs every burn in metres per second. Its fourth target, Neptune, does not close from this approximate 1977 arrangement — the planets here sit on circular orbits at rounded 1977 longitudes, and a fourth flyby needs the true ephemerides — so the card says so rather than pretending. A small mission card keeps the clock, distance, speed against the local escape speed, light-time and a running log out of the way of the view, and at today's date it sets this flight beside the real spacecraft's. Every number is the simulator's own; the flyby distances and the escape fall out of gravity, not a script.
- Fly a probe to the Moon. Missions → Probe to the Moon opens a to-scale Earth–Moon view — Earth and the Moon at their true relative size and distance, with the geostationary-satellite ring drawn in for contrast. The probe rides a real translunar trajectory: injected at about 10.8 km/s (the actual Apollo departure speed) and coasting three days on Earth's and the Moon's gravity.
- Send a lightspeed message. Missions → Lightspeed message: a radio pulse leaves Earth for Mars (3 to 22 light-minutes away, depending on where the orbits stand) or Jupiter (33 to 54). The planets hold still while it crawls — because at any speed where you can see planets move, light is effectively instantaneous.
- Visit Jupiter's moons. Click Jupiter and press Visit Jupiter's moons — or follow it and keep zooming in. Io, Europa, Ganymede and Callisto orbit at their real distances in a simulation of their own, pulling on each other as well as on Jupiter. Count along with the lap counter: Io laps four times for every two of Europa and one of Ganymede, a rhythm their gravity holds in place. Moons go dark as they cross Jupiter's shadow and vanish behind the planet; drag up or down to tilt the view. More on Jupiter's moons →
- Visit another date. The sky opens on today. Sandbox → Go to a date (or tap the date under the play button) puts the planets and the Moon where they were on any day from 1800 to 2200 — try Mars's close approach of 5 September 1877, when Schiaparelli drew his "canals" — then play or rewind and the date counts along.
- Find the next eclipse. Sandbox → Eclipses lists the coming solar and lunar eclipses from the sky's date — real dates and kinds — and jumps to the moment of each. A side view shows the Moon's real height above or below Earth's orbit: at an eclipse its shadow lands on Earth, or it turns red inside Earth's shadow; most months it passes above or below and nothing happens.
- Watch the Moon's phases. Sandbox → Moon phases shows Earth and the Moon from above, lit from the Sun's real direction and starting at tonight's phase, with the Moon as seen from Earth beside it. Two stopwatches time one lap round Earth and one cycle of phases; the second comes out about two days longer, because Earth moves on round the Sun. Tonight's Moon →
- Find the Lagrange points. Sandbox → Lagrange points turns the view with Jupiter so its five Lagrange points stand still. Tap each one to drop a probe: three lose it, two keep it. The green dots crowding the stable two are Jupiter's Trojan asteroids, which also ride along in the normal view. More on Lagrange points →
- Zoom into Saturn's rings. Click Saturn and press Visit Saturn's rings. The C, B and A rings, the Cassini Division, the Encke Gap and the F ring are drawn from their real edges, with Saturn's shadow falling across them. Scroll toward any part of the rings to zoom in. The small blue particle laps twice for every lap of Mimas: that 2:1 rhythm is why the B ring ends where the Cassini Division begins. Zoom out to find Titan.
- Delete the Sun. Click it, open Adjust, delete. The planets fly off along near-straight lines — the leash made visible.
- Make Jupiter heavier. Drag its mass slider upward and watch the rest of the system start to wobble.
- Drag Earth out to Mars's distance. Grab any planet and pull: time pauses, the orbit guide previews the circle it will settle onto, and the inspector's year, perihelion and escape speed change under your hand. Let go and it orbits there — the Moon comes along. Switch on the velocity & gravity arrows in the view options first and watch both arrows shrink as you move it outward.
- Add a planet where you click. Sandbox → Add a planet: pick a mass anywhere from a dwarf planet to a dozen Jupiters, then click a spot in the system. A ghost shows the orbit it would get as you move; the placed world starts on a circular orbit there and joins the physics for real — it pulls, gets pulled, and can be flung out like anything else. Drop a second Jupiter beside the first and see how long the neighbours last. Reset clears whatever you added.
- Stretch Earth's orbit into an ellipse. Click Earth, open Adjust, and slide the eccentricity: the orbit guide stretches through Earth's current position, the perihelion and aphelion tiles follow, and the readout warns if the new orbit dives into the Sun. Let time run and watch Earth race through perihelion and crawl at aphelion — Kepler's second law, live. The toggle underneath decides whether the point you're at becomes the closest or the farthest end.
- Nudge Earth. A small thruster burn permanently changes its orbit — there's no script pulling it back.
- Launch satellites. Press E for Earth view, then launch at 200 km and again at 35,786 km and compare the orbits.
- Run time backwards. The « button reverses the integration — orbits retrace themselves.
- Zoom all the way out. Find the asteroid belt, push the speed to ×32, and watch Mercury lap the outer planets.
What if? — guided experiments
Explore → What if? turns the sandbox into a lab. Each experiment introduces a what-if, runs a live demonstration, and explains what happened. No answers or guesses are required. Six are live so far.
What if the Sun vanished? Remove the Sun and watch the planets coast away. Earth carries on at roughly 30 km/s along a nearly straight path, with the planets’ mutual gravity still acting; the overlay draws a straight reference line, and the coach reports its speed before and after. That is Newton's first law with the training wheels off — the circle was the Sun's doing all along. The one thing the simulator can't show is that gravity travels at light speed, so the real Earth would keep circling a vanished Sun for another 8 minutes 20 seconds. Read the answer →
How long does a message to Mars take? Send a real radio pulse across whatever gap the planets are at right now. The planets freeze on purpose: at any speed where you could see them move, light would be instantaneous. Today's answer lands somewhere between 3 and 22 minutes depending on where Mars is, a reply takes just as long coming back, and that round trip is the reason Mars rovers drive themselves. Then try Jupiter, where the delay runs past half an hour. Read the answer →
Why can't we fly straight to Mars? Watch an automatic launch, or explore the timing yourself. The rocket is fixed — it can leave Earth about 3.7 km/s faster than Earth is already moving, a realistic figure for a Mars launcher — and the only thing you choose is when. Leave with Mars in the wrong place and the probe coasts out to Mars's distance from the Sun and finds empty space: over an eight-month transfer Mars travels well over a hundred degrees further around its orbit, so it has to be tens of degrees ahead of Earth at the moment you go. From whatever moment you pick, the simulator prices the cheapest real transfer (one row of the "porkchop plot" mission planners draw) and flies it if the rocket can afford it. The brief stretch when it can is the launch window — and because Earth laps Mars only every 26 months, that's how often it comes around. Read the answer →
How far is the Moon, really? See how a basketball Earth puts the Moon across a room — then fly there in a view drawn to true relative size and distance. The answer is 30 Earth-diameters, about 7 m on the basketball scale, with the geostationary satellite ring not even a tenth of the way out. The probe rides the real three-day Apollo-speed trajectory, and the debrief notes that light makes the same trip in 1.3 seconds. Read the answer →
Does the Sun move? Watch from close in with the system's centre of mass marked. The Sun loops around it at about 12 m/s — a cyclist's pace — once every 12 years, dragged mostly by Jupiter, with the loop a little larger than the Sun itself. That wobble, read as a rhythm in a star's light, is one way astronomers find exoplanets; the lesson lets you make Jupiter ten times heavier and watch the Sun's speed jump.
Where does Voyager's speed come from? Watch a simulated 1977 launch and compare the speed before and after Jupiter’s gravity assist. Relative to Jupiter the speed is unchanged; relative to the Sun the probe gains several km/s, paid for by Jupiter's own orbital motion, which slows by an amount far too small to ever measure. The same flight then carries on: past Saturn for a second assist, above the Sun's escape speed — the point past which gravity can slow it but never stop it — and out to today, nearly half a century and well over a hundred AU later. Read the answer → · The full 1977 → today flight →
Using it in a classroom
The simulator runs free in any browser with no account, no install, and nothing to set up — it projects well and works on student laptops, tablets, and phones. A few demonstrations it makes concrete:
- Closer means faster. Click any planet and the inspector shows its orbital period. Mercury's fraction of a year against Neptune's 165 isn't a table entry here — you can watch it happen.
- Eccentric orbits change speed. Launch a slow comet and watch it crawl at the far end of its arc, then whip around the Sun — Kepler's second law, live.
- Axial tilt. The dock's mini-planets spin at their real tilts and relative rates: Uranus rolls on its side, Venus turns backwards, slowly.
- The Sun moves too. Zoom in tight on the Sun and watch it circle the system's center of mass — the same wobble astronomers use to find planets around other stars.
Daily Orbit — today's sky, one probe, six launches
Every day the simulator places the planets where they really are on that date and picks one as the target. Press Daily Orbit in Explore and you get two dials — how hard the probe is pushed away from Earth, and in which direction — and six launches to get captured there. Everyone in the world gets the same board, so the result you share is a fair comparison.
Nothing is on rails. Each launch is flown under the same N-body gravity as the rest of the page, so Jupiter tugs on your probe, a near pass of Venus bends it, and the target keeps moving while you're in flight. The dotted preview shows the probe's path around the Sun alone; what it can't show is where the planet will be when you get there — that's the puzzle. After each launch the card says how far ahead or behind the target was when you crossed its orbit. Planet ahead? You were late: more speed. Planet behind? You were early: ease off.
Par is the cheapest launch the physics finds for the day, and fuel is counted the way mission planners count it: the launch burn, any mid-course correction, and the braking burn at the planet. Your probe carries a small correction tank (1.5 km/s): a launch that would miss by a little fires one Voyager-style burn on the way, and that fuel counts against you. Three stars is within 5% of par, two within 25%, one star for any arrival. When the day is over you see par's own path as a gold ghost line and the porkchop strip — fuel against flight time for every route the scan found.
Frequently asked questions
Is this a real simulation or a looping animation?
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.
Are the sizes and distances to scale?
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 the Display 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.
Why does the Sun wobble?
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. The Explore → What if? lesson zooms in to show the loop.
How far away is the Moon?
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 the Explore → What if? lesson flies a probe across it.
How does a gravity assist make a spacecraft go faster?
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; the Explore → What if? lesson measures the gain in the simulator, and Missions → Voyager 1 flies the whole 1977 mission to today.
Why does Voyager keep getting farther away instead of orbiting the Sun?
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.
What would happen if the Sun suddenly disappeared?
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. The Explore → What if? lesson runs the experiment for real.
How long does it take a radio signal to reach Mars?
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. The Explore → What if? lesson sends a real pulse across today's gap and times it.
Why do Mars missions only launch every 26 months?
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. The Explore → What if? lesson lets you test it: launch whenever you like and see where Mars actually is when you get there.
Why are Jupiter's moons Io, Europa and Ganymede in resonance?
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, 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.
What makes the gap in Saturn's rings?
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, 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.
Can planets collide or be destroyed?
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.
Can I see where the planets were on a particular date?
Yes. The simulator opens on today's sky, and Sandbox → Go to a date moves every planet and the Moon to where they really were, or will be, on any day from 1800 to 2200 — the date under the play button opens the same picker. 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.
What are the controls?
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 panel on the left (a sheet at the bottom on phones) opens from Explore, Missions or Sandbox; Display settings has scale, guides and the data table. Keyboard: Space pause, ↑↓ time rate, E Earth orbit, C comet, D data table, M mute, 0 whole system, R reset the system, Esc leave the current view.
What is Daily Orbit?
A daily launch puzzle inside the simulator. Each day the planets are placed where they really are on that UTC date and one of them is the target. You set a launch speed and heading, and the probe flies under the simulator's full gravity — six launches to get captured, scored on fuel against a par the physics finds for that day, with a shareable emoji result and a streak. Everyone in the world gets the same board.
What do the Daily Orbit dials mean?
Speed is how fast the probe leaves Earth, in kilometres per second over Earth's own orbital motion. Aim is the heading of that push: 0° is straight along Earth's direction of travel, positive angles swing it outward from the Sun, and 180° is a braking burn against Earth's motion, which is how you fall inward to Venus or Mercury. Reaching the target's orbit is the easy half; arriving when the planet is there is the puzzle.
What is par in Daily Orbit?
The cheapest launch the physics can find for that day: hundreds of candidate transfers are computed and the best are flown for real in the simulation until one arrives and survives a small aiming error. Fuel counts the launch burn, any mid-course correction, and the braking burn at the planet. Three stars is within 5% of par, two within 25%, one star for any arrival.
Is it free?
Yes — free, in the browser, no download and no signup, like everything on this site.