Space Radar Lab
Tiny space simulators
105 small pages, each teaching one idea about space. Every number is quoted from its source, and every page says what is and is not to scale. Open one and move the slider.
Orbits and gravity
Binary Dance
Two stars really orbit a point between them, closer to the heavier one.
Comet Tail Compass
A comet's tail always points away from the Sun, even when the comet is leaving.
Escape Velocity Ladder
Fire straight up faster and you go higher, but only at about 11.2 km/s does the ball never come back.
Five Parking Spots (Lagrange Points)
In the rotating frame of two bodies there are five spots where a third thing can stay still.
Gravity Slingshot Steal
A spacecraft can steal a little of a planet's speed by swinging past it.
Gravity Well Rubber Sheet
Mass bends the pattern of space-time like a ball in a stretched sheet, and light or marbles follow the dips.
Hohmann Transfer Hop
To climb from a low orbit to a high one you do not fly straight up: you fire twice, once to stretch the orbit and once to round it off.
Janus and Epimetheus
Two Saturn moons share nearly one orbit and swap places every four years without colliding.
Kepler Equal Areas
A planet sweeps out equal areas in equal times, so it speeds up as it nears the Sun.
Kepler's Third Law Race
The farther a planet is from the Sun, the longer its year, and the rule is exact: the square of the period matches the cube of the distance.
Mercury Rosette
Mercury's orbit slowly turns, and Newton alone could not explain all of it.
Moons in Step
Io, Europa and Ganymede orbit Jupiter in a 4:2:1 beat, and their tugs keep the pattern locked.
One Launch, Four Orbit Shapes
Circle, ellipse, parabola and hyperbola are all the same orbit with different speeds.
Orbit Painter
Each throw gets one push, then gravity bends the path into an orbit, a plunge into the star or an escape.
Orbit Zoo
The higher a satellite flies, the slower it goes and the longer its lap.
The ISS Always Falls
The Space Station is dragged down a little every day and must be pushed back up.
The Moon Spins to Match
Over time tides slow a moon's spin until it faces its planet with one side forever.
Three Bodies on a Figure Eight
Three equal stars can chase each other along one figure-eight path forever.
Throw It Faster
Throw a ball faster and it lands farther away; at about 7.9 km/s the ground curves away as fast as the ball falls, and it never lands.
Why GEO Stays Put
A satellite 35,786 km above the equator goes round in exactly one day, so it hangs over the same spot.
Planets and moons
Greenhouse Dial: Why Venus Is Hot
A thick CO2 blanket can heat a planet far above what its sunlight alone would do.
Habitable Zone Slider
The zone where liquid water can sit on a planet moves in for a dim star and out for a bright one.
Io: The Squeezed Moon
Jupiter's tug squeezes Io like a stress ball, heating it enough to power hundreds of volcanoes.
Mercury Double Sunrise
On Mercury the Sun can rise, stop, retreat and rise again because of its spin and orbit.
Olympus Mons vs Everest
The biggest volcano on Mars, Olympus Mons, stands about 2.4 times as tall as Mount Everest.
Orbit and Escape on Other Worlds
The smaller the world, the slower you need to go to circle it or to leave it for good: on Phobos a hard throw would never come back.
Roche Limit Shredder
A moon that gets too close to its planet is torn apart by tides, which is how rings can form.
Saturn Rings Edge-On
Saturn's rings are 280,000 km wide yet in places only about ten metres thick.
Tilt Makes the Seasons
Earth's tilt, not its distance, makes summer and winter.
Two Tides, Not One
The Moon pulls the oceans into two bulges, so tides come twice a day.
Venus Day Race
Venus spins once in 243 Earth days and circles the Sun in 225, so its day is longer than its year.
Scale and distance
Asteroid Belt Is Not Crowded
Despite pictures, the asteroid belt is almost empty between rocks.
Earth From the Sun, Shrinking
From farther out Earth shrinks from a ball to a single pixel.
Earths in Jupiter
More than 1,300 Earths would fit inside Jupiter's volume.
From Ground to Space in 100 km
Space is only about 100 km up: roughly the drive between two cities.
Milky Way Address
Earth is about 27,000 light-years from the centre of our galaxy, in a quiet outer arm.
Nearest Star by Voyager
At Voyager's speed the nearest star would take tens of thousands of years.
Planets Fit Moon Gap
The other seven planets laid side by side almost fit in the gap between Earth and the Moon: not quite at the average distance, yes at the farthest.
Powers of Ten Zoom Out
Zoom out by factors of ten and Earth, solar system, stars and galaxy fall away in seconds.
The Solar System Is Mostly Empty
To scale, planets are specks separated by huge gaps.
The Sun as a Marble
Shrink the Sun to a marble and Earth is a grain of sand several metres away.
Voyager Odometer
Voyager 1 has been travelling since 1977 and is still the farthest thing humans made, at 17 km/s.
Walk the Solar System
If the Sun were at your door, Neptune would be a long walk away.
Your Closest Stars
The nearest stars are enormous distances away and mostly dim red dwarfs.
Light and time
Crossing the Galaxy at Light Speed
A beam of light needs about 87,000 years to cross the Milky Way's disc, longer than human history.
Doppler Siren Waves
Waves bunch up in front of a moving source and stretch behind it, changing pitch or colour.
Inverse Square Flashlight
Light spreads as it travels, so at twice the distance it is four times dimmer.
Light Speed Road Trip
Sunlight takes about 8 minutes to reach Earth and over four hours to reach Neptune.
Radio Ping-Pong With Mars
Radio signals take 3 to 22 minutes to reach Mars and just as long to return, turning every message and reply into a slow game of ping-pong.
Redshift Stripes
The faster a galaxy flies away, the more its light shifts toward red.
Spacetime Diagram
On a spacetime diagram a moving observer's time line and line of now both tilt toward the path of light, so flashes that happen together for one observer happen one after another for the other.
Starlight Bends Past the Sun
The Sun's gravity bends the light of a star that passes close to it, twice as much as Newton's gravity would, and the 1919 eclipse measured it.
Stars Bunch Up Ahead
At near light speed the stars crowd into a bright tunnel in front of you.
Sunshine by Planet
Sunlight gets weaker with the square of the distance, so Earth gets about 900 times more of it than Neptune.
The Sun Vanishes
If the Sun vanished, we would keep seeing it and keep orbiting for 8 minutes 19 seconds.
Twin Clock Time Dilation
A clock moving close to the speed of light ticks slower than one standing still.
Rockets and spaceflight
Aerobraking Dip
A probe can shrink its orbit by dipping into the thin upper air, saving fuel.
Delta-v Subway Map
Moving around the solar system is like a metro map where each line costs a number of km/s.
Free-Return Loop to the Moon
A spacecraft can swing around the Moon and come back to Earth without extra fuel.
Ion Drive vs Chemical Rocket
With the same fuel, an ion engine pushes weakly for years and ends up faster than a single chemical burst.
Mars Launch Window
You can only launch to Mars cheaply when the planets line up, once every 26 months.
Reentry Heat: Speed Squared
Slowing from orbit turns a huge amount of energy into heat, because it scales with speed squared.
Rocket Equation
Each extra km/s of speed costs a fifth of what is left, so a rocket bound for orbit is almost all fuel.
Solar Sail Surf
Sunlight pushes a thin sail a tiny bit all the time, and that adds up to high speed.
Staging: Drop the Dead Weight
Dropping empty tanks lets the rest of a rocket go much faster than one big tank could.
To Catch Up Slow Down
In orbit, to catch a craft ahead you drop to a lower orbit, which is faster.
Stars and their lives
An HR Diagram of Our Neighbours
Sort the stars near the Sun by colour and by true brightness, and most of them turn out to be small, cool, faint red dwarfs, not the bright stars of the night sky.
Cepheid Cosmic Yardstick
A pulsing star's period tells you its true brightness, so you can tell how far away it is.
Element Factory
Heavier elements are made in stars in layers, ending at iron.
Find a Planet by Its Wobble
A planet tugs its star to and fro, and real measurements of the star's speed, folded at the right period, reveal the planet and its mass.
Find a Planet in Kepler Data
Twelve days of real Kepler measurements of the star TrES-2 dip every few days, and folding them at the right period stacks the dips into one clean planet transit.
Planet Shadow: Transit Light Curve
When a planet crosses its star the star dims a little, and that is how we found thousands of worlds.
Pulsar Lighthouse
A pulsar is a spinning dead star that sweeps a beam past Earth like a lighthouse.
Sunspot Cycle Counter
Sunspots rise and fall every 11 years, with storms at the peak.
Supernova Shockwave
A massive star's core collapses and the outer layers blast away within seconds.
What Happens to Our Sun
In about five billion years the Sun swells into a red giant and swallows the inner planets.
Wobbling Star
A planet pulls its star in a little circle, shifting its light blue and red.
Sky from Earth
Mars' Loop in the Sky
Planets seem to loop backward as faster Earth overtakes them.
Meteor Shower Radiant
Meteors in a shower look like they come from one point because the streams are parallel.
Midnight Sun
Above the Arctic Circle the Sun doesn't set for weeks in summer.
Moon Phases From Above
The Moon is always half lit; the phase is how much of the lit half faces us.
Star Trails by Latitude
The sky spins around different points depending on where you stand, from the pole to the equator.
Two Eclipses One Geometry
Solar eclipses happen when the Moon is between us and the Sun; lunar eclipses when Earth is between.
Venus Phases (Galileo's Proof)
Venus shows phases like the Moon, proving it circles the Sun, not us.
Cosmology and black holes
Black Hole Lens in 2D
A black hole bends light around itself, so a background star can show up twice or as a ring.
Dark Matter Rotation Curve
Stars at the edge of galaxies orbit nearly as fast as inner ones, which suggests unseen mass holds them.
Every Galaxy Moves Away
Space stretches evenly, so from any galaxy all the others move away, and one twice as far recedes twice as fast.
Frame Dragging by a Spinning Black Hole
A spinning black hole drags space round with it, so a stone dropped straight in is swept sideways, and the closest safe orbit moves in with the spin and out against it.
Galaxy Merger Dance
Galaxies can collide and pass through each other, with their stars rarely touching.
Gravitational Wave Chirp
Two black holes spiralling together shake space itself, and the wave they send out rises in pitch and strength until they merge.
Hubble Diagram from Supernovae
Faraway exploding stars look fainter than they would if gravity were slowing the universe down, so its expansion is speeding up.
Orbits Near a Black Hole
Close to a black hole an orbit no longer closes: each lap swings round further than the last, and near the hole the turn grows far beyond the weak-field formula.
Sound Waves in the Baby Universe
Sound rang through the hot early universe until light and matter parted, and the distance it had travelled is why the microwave sky has spots about a degree across.
Spaghettification
Falling feet-first into a black hole, your feet are pulled harder than your head and you are stretched.
The Afterglow Is a Perfect Blackbody
The microwave glow left from the hot young universe has exactly the colours of a glowing body at 2.725 degrees above absolute zero.
The Pulsar Pair That Spirals In
Two neutron stars that lose energy to gravitational waves fall closer and orbit faster, so each lap comes round a little early, by just as much as Einstein's theory predicts.
The Shadow of M87's Black Hole
A black hole's shadow is about 2.6 times as wide as its horizon, so its size in a telescope weighs the hole, and the 2019 picture of M87* matched 6.5 billion Suns.
Time Freezes at the Horizon
Seen from afar, a clock falling into a black hole ticks slower and slower and never quite crosses.
Universe Timeline Scrub
The universe is 13.8 billion years old, and humans arrived in the last sliver.
Stars and galaxies
Big Stars Burn Fast
The heavier a star, the shorter its life: a star 25 times the Sun's mass lasts a few million years, the Sun about ten billion.
Parallax Thumb Trick
A nearby star seems to shift against distant ones as Earth goes round the Sun, and the size of the shift gives its distance.
Squeeze Earth Into a Marble
Squeeze the whole Earth until its radius is under 9 millimetres and it would be a black hole.
Star Colour Is Temperature
A hotter star glows bluer and a cooler one redder, and the colour tells you the temperature.
Earth, Moon and Sun
Sun and Moon Look the Same Size
The Sun is about 400 times wider than the Moon and about 400 times farther away, so they look almost exactly the same size in the sky.
The Moon Drifts Away
The Moon moves about 3.8 centimetres farther from Earth every year, so one day total solar eclipses will stop.
Add your own simulator
One HTML file, no libraries, no network, about ten minutes. Copy the starter, change the numbers, quote your source, open a pull request.