Use this page as both a player guide and a technical reference. The first half explains the program loop and controls. The second separates published inputs, calculated results, and explicit planning assumptions so none of the important physics is a black box.
Defend Earth closes a threat clock. Moon, Mars & beyond plans one expedition. Campaign keeps infrastructure, vehicles, and flight history.
Choose the destination or orbit, payloads, transportation, and recovery method. The verdict explains missing capability instead of letting an impossible stack launch.
Roll out each vehicle, watch staging and recovery, then advance to the next dated event. A RUD pauses the program for repair, retry, or replanning.
Research an uncertain threat, choose a deflection method, secure permission, and launch before impact. If the first push falls short and time remains, its measured orbit change carries into the next turn.
Select a destination and assemble relays, scouts, rovers, power, cargo, habitats, or crew. The manifest orders launches and arrivals around capacity, launch windows, and prerequisites.
Infrastructure and hardware survive between missions. Delivered assets remain available to fly again, and the inventory counts each delivery at its destination or Earth orbit. A LEO propellant depot stores fuel for later departures and can be replenished while other spacecraft are already traveling.
The launch vehicle gets mass off Earth. Engines, tanks, diameter, stages, and boosters determine thrust-to-weight, delta-v, payload, and recovery compatibility.
The engine laboratory develops propulsion for future custom rockets. Solid, liquid, and hybrid engines have different shutdown, restart, pressure, throttle, and schedule limits.
The spacecraft does the job after launch. Orbiters, landers, rovers, and atmospheric flyers trade science payload against bus, power, propulsion, antenna, and structure.
Recovered cores receive B-series tail numbers; fairing halves receive F-series numbers. The game retains flights, outcomes, losses, soot, and museum retirement.
Reading level, trip and campaign programs, custom mission vehicles, booster inventory, and depot propellant use versioned browser storage. Reloading the page keeps them.
Rocket share codes use the hosted Earth Guard service and contain only the bounded public rocket design schema. Political verdicts are optional narrative calls and never decide the physics.
Earth Guard has three kinds of numbers: published vehicle or planetary data, values calculated from equations, and planning assumptions used where no final vehicle or mission exists. This page labels all three. Scientist mode also prints the live equation with your mission's numbers on the screen where it is used.
The destination radius and the Sun's gravitational parameter are published inputs. The transfer time and C3 are derived. The Moon uses a separate 5.9 km/s surface-delivery budget from low Earth orbit.
Stage wet mass, dry mass, thrust, specific impulse, diameter, and published low-orbit payload are inputs. The ascent is a 2D point-mass integration at 0.1 second steps with drag, falling gravity, centrifugal relief, propellant use, and a closed-loop gravity turn.
Falcon Heavy's side cores separate first and can return to the launch site while the throttled center core continues alone to a higher-energy separation. The game therefore allows different recovery choices for side boosters and center core and prevents a high-energy center core from pretending it can make the same return-to-launch-site maneuver.
The nozzle solver is derived from ideal-gas, choked-flow equations. Propellant temperature, ratio of specific heats, gas constant, and cycle efficiency are representative approximations. Cycle pressure and single-chamber thrust caps prevent obviously impossible combinations. Conventional solids burn until the grain is exhausted. Liquid engines can shut down, but restart hardware must be designed and qualified rather than assumed.
A weather or mapping satellite can use a roughly 705 km sun-synchronous orbit for repeatable lighting. Communications relays use geostationary orbit at 35,786 km for a fixed view. Hubble works in low Earth orbit; Webb uses a halo orbit near Sun–Earth L2; NEO Surveyor uses L1 to search near the Sun's direction.
A random threat begins as a point of light. Brightness cannot separate size from reflectivity, and a short observation arc does not fix the orbit. The game therefore blocks mission design until another instrument is chosen. Uncertainty in size, density, and momentum enhancement is rolled at intercept.
When a first mission falls short but the window remains open, its measured orbital change carries into the next turn. The game shows impact only after the clock closes.
Build capability in order. A crew cannot arrive before power, communications, and a place to live. Robotic missions can fly alone.
Choose a propellant, feed cycle, chamber pressure, thrust class, and nozzle. The nozzle equations are ideal-gas physics; development time follows NASA technology-readiness steps. A new engine is a years-long campaign asset, not a last-minute defense trick.
Start with the job, then pay the mass for structure, power, propulsion, communications, and science instruments. The result becomes a persistent mission element you can launch in expeditions and campaigns.
Tap a rocket to see it, then launch it. All five are drawn to the same scale, so you can see how much bigger some of them really are.
Two stages. Pick the engines, decide how much propellant each stage carries, and how fat the body is. Everything else — how tall it ends up, whether the engines physically fit, how much it can lift — falls out of the physics. It will tell you when a design cannot work, and why.
Drawn to scale. A Falcon 9 is shown beside it for comparison.
Looking up at the first-stage engines. This is the layout that decides how wide the rocket has to be.
Right now we have a blurry survey detection and not much else. We do not know how big it is, what it is made of, or exactly where it will be. Observing costs time off the clock, and time is the thing that makes deflection easy.
These all point at the same rock at the same time, so they run together. What you pay is the slowest one you pick, not the total — which means adding a fast one next to a slow one costs you nothing.
Deflection depends on the target's mass (M = ⁴⁄₃πr³ρ) and its momentum enhancement factor β. Both come from size and composition. If you skip characterization, the game rolls your real values out of the uncertainty band you left yourself, so a mission that works on paper can still miss. This is a real problem: before DART flew, β for Dimorphos was known only to within a factor of several.
Different tools win in different situations. The clock usually decides.
Everything below is normal-times rules: range safety, launch queues, months of pad turnaround. If the object is big enough, none of that survives contact with the news.
A rocket's famous payload number is to low Earth orbit. Escaping Earth costs another 3.2 km/s, and every bit of extra speed eats into the mass you can deliver.
Earth spins eastward at 465 m/s at the equator. Launch east from near the equator and you get that speed for free. Launch from high latitude, or south over open ocean, and you buy it out of your own tanks.
One impulsive burn from a 200 km circular parking orbit (vcirc 7.784 km/s, vesc 11.007 km/s) using the vehicle's real upper stage, subtracting its dry mass. Assumption: ignores gravity losses on the departure burn, staging above the second stage, and launch-window geometry. Lands within roughly 20% of published payload-vs-C₃ curves.
Two dials. Departure energy buys speed but costs mass. Lead time is how far ahead of the collision you hit it — the earlier you push, the more the miss distance grows on the way in.
Speed left over after escaping Earth: v∞ = √C₃ = —
Cruise needed: — · Time you have: —
The orange line is Earth's radius plus a safety margin. The bar has to get past it.
Everything else on this page is physics, and physics does not have an opinion. Getting permission is the other half of the problem, and it is the half that is actually hard. This asks a real AI to play the National Security Council, the UN, and the public, and to decide whether your plan is allowed to fly — and how many days the arguing costs you.
All stations are go for launch.
A 2D point-mass gravity turn integrated at 0.1 s steps. Thrust and mass flow come from each stage's real propellant load, thrust and specific impulse; drag uses an exponential atmosphere (ρ = 1.225·e^(−h/8500)) with Cd 0.3 over the vehicle's frontal area; gravity falls off as 1/r² and the centrifugal term is included. Assumption: no steering losses beyond the pitch program, no throttling, no aerodynamic lift. It produces realistic-shaped ascents, not flight-certified ones.
Coasting to intercept.
Short answer: not very. But the honest version is more interesting than either "we're all doomed" or "it'll never happen", so here it is properly.
Estimates differ between sources, which is why several of these are ranges rather than numbers. The small end comes from counting fireballs detected by satellites; the big end comes from counting craters and from how many objects the telescopes have found.
In 1859 the Sun threw a cloud of plasma at Earth and set telegraph offices on fire. Estimates of how often that happens vary a lot — from roughly 1 in 200 per decade up to 1 in 8 per decade, depending on which statistical method you trust — so it is far more likely than a dangerous impact. It would also do far less harm: transformers, satellites and power grids, not craters. More probable, much less lethal. That is usually how risk works, and it is why "how likely" and "how bad" have to be held in your head at the same time.
In any given year the chance that something big enough to matter hits Earth is very small — smaller than almost anything else you already do not worry about. Over a hundred thousand years it becomes close to certain. Both of those are true at once, and neither is a reason to lie awake. It is a reason for a few hundred people to keep pointing telescopes at the sky, which is exactly what they do.
Sources: NASA CNEOS and the Planetary Defense Coordination Office for the survey numbers, the National Academies planetary defense report for impact frequencies, and the space-weather literature for solar storm rates, where the disagreement between studies is genuine and worth knowing about.