The Problem: Exploring space is hard because you need a lot of fuel, which is heavy, which means you need more fuel, etc. Bad loop.
The Solution: Surf the gravity around planets, moving slower but at little to no energy cost.
In the 1400s the oceans were as huge and seemingly impassable as space is to us today. And early Chinese and European explorers were basically surfing, they had zero fuel to burn. But they did it. They used knowledge of the environment to their advantage and they found winds that could carry them across the world. It took timing, it took patience and it took skill, but that was the early exploration that shaped the world.
Today, we face similar resource constraints in space. We have fuel, but the weight of the fuel makes it a very scarce resource for interplanetary travel. Luckily, however, we also have something similar to the trade winds. All of the planets and objects in our solar system have gravity, and played carefully they can guide and land objects (and people) where we please. Just not when we please, but that, I assume, comes later.
The Long Way Round
Nowadays we freak out if we don’t have fuel, but our ancestors actually explored the world without it. They simply relied on the ocean currents and the wind. It was counter-intuitive sometimes (you’d sail away from your destination), but it worked.
In space we have a similar problem (paucity of fuel) and a similar solution. In 1990 a Japanese moon probe needed to get into orbit and it didn’t have enough fuel. The answer was to send it 1.5 million km in the opposite direction, allowing it to catch a gravitational ‘wind’ back.
Belbruno got Hiten to the Moon by spiralling it out 1.5 million km from the Earth, allowing our planet’s gravity to slow the probe. It manoeuvred into position and the engines were turned off. Hiten was then pulled back towards Earth and into a capture trajectory with the Moon.
Boom. Same thing. It’s slow like sailing (this procedure would take 2 years for what fuel could accomplish in a few days) but given the constraints of space, something like this seems necessary, for cargo at least.
interPlanetary Trade Winds
The amazing Medium article cited above calls the whole thing an ‘Interplanetary Superhighway’, but that’s perhaps not the right metaphor. This method of travel is really efficient but slow. A more apt comparison is to the trade winds of yore. Interplanetary Trade Winds would be my terminology.
This is apt, I think, because it’s similar applications. The first round of global exploration wasn’t about sending lots of people around very fast, it was about exploring first and then moving a lot of goods around very slow.
PLANETARY GAS STATIONS
The way this can work for modern space exploration is simple. You set up space ‘stations’ years in advance and stock them with fuel and whatever you need to reach the next station. These need not be full blown space stations like the ISS, they can just be orbiting caches of fuel and oxygen and food.
In the planetary subway map above you can see the ‘stations’. This map is based on straight rocket transfers, but you could make it more efficient by surfing the gravity winds. The stations (AKA orbits) would be the same. You could use rocket transfers for moving humans around, but that’s it. All the heavy stuff would be sent in advance.
It would still be quite a cost and effort getting supplies out of earth orbit (easier with a space elevator) but from there we could pinball little caches off supplies into orbits around planets throughout the solar system.
It requires another way of thinking about space travel, but it’s not a new way of thinking. It’s how humans explored our own planet centuries ago.