The slow road and the fast road to Mars
Our company is named after a spot in space, so here's some orbital mechanics. Space travel has a trade-off you'll recognize from money: you can save fuel or you can save time, but it's hard to do both.
Every rocket burn changes a spacecraft's speed. Engineers call that change delta-v (Δv), measured in kilometres per second. Δv is a budget: each km/s costs fuel, and fuel is weight you have to launch. So the big question for a trip is how much Δv it needs.
Hohmann transfer
In 1925 Walter Hohmann worked out the cheapest common way to move between two circular orbits. Burn once to stretch your orbit into an ellipse that just touches the outer orbit. Coast halfway around. Burn again to round the orbit out.
- Lowest Δv of any two-burn route between circular orbits (until the outer orbit is about 12 times wider; then other tricks win).
- Fixed trip time: exactly half an orbit of the ellipse. For Earth to Mars, about 259 days.
- Fixed departure dates: Mars has to arrive at the meeting point when you do, so windows open only about every 26 months.
Earth to Mars, counting only the Sun's pull: about 2.9 km/s to leave and 2.6 km/s to arrive.
Lambert transfer
Lambert's problem, named after the 18th-century mathematician Johann Lambert, asks a different question: "I'm here now. I want to be there at exactly this time. Which orbit connects the two?" Solve it and you get a transfer for any start, any finish and any trip time you choose.
- You pick the time. Leave now and arrive when you need to, which is how missions plan intercepts and rendezvous.
- Speed costs Δv. A faster path cuts across the orbits instead of gliding along them, so both burns get bigger.
- The cheapest Lambert path is the Hohmann one. When the timing lines up, the two answers meet.
Mission planners chart thousands of these on a "porkchop plot", a map of Δv for every launch date and arrival date.
Trade time for fuel
Slide the trip time and watch the Δv change. The lowest point is the Hohmann transfer. Rush the trip and the cost climbs fast; dawdle and it climbs too. The shape is illustrative, not a mission plan.
Hohmann sweet spot: the lowest Δv.
The same trade-off, closer to home
Long-range plans work the same way. Steady saving over decades is the Hohmann route: slow, predictable and cheap. Trying to catch up in a few years is the Lambert express: possible, but every year you cut costs more. Our apps are built to help you take the efficient road early.