Is this a viable theoretical method of fast interplanetary space travel?

Question

Let's say in the future humanity has traveled to all parts of the solar system, but they want to be able to travel between planets faster. In pursuit of this, they create massive space probes with solar sails attached. These satellites then go into "orbit" between two planets and due to the constant boost from the solar sails and the lack of resistance in space they travel between the two planets faster and faster. Whenever a spaceship wants to travel quickly between planets they simply grab on to the space probe and ride it to their destination planet.

1. Would this be possible? Can space probes running on solar sails navigate between two planets without getting caught in another planets gravitational field? Could the probes actually "slingshot" to reverse a la The Martian?

2. Would this be fast? Could the space probes actually gain and maintain enough speed to travel between planets faster than a spaceship using the conventional means?

3. Would this be practical? Could spaceships actually attach themselves to the space probe without either accelerating to a fast enough speed to make the probe redundant or getting pulled to pieces? Could the ships slow down enough to avoid hitting or overshooting the planet?

Answer 1

Combine solar sails with planetary slingshotting and ion thrusters

You can't use a solar sail to accelerate towards the Sun. At best, you can slant the sail to accelerate at nearly a right angle to the sunlight, basically increasing or decreasing orbital speed (which will also work to increase or decrease the orbital radius).

Hence, you could increase orbital speed this way when moving from the inner planet to the outer planet and decrease orbital speed when moving in. You could feasibly adjust the acceleration to adjust for the planet's varying positions along their orbits (sometimes, they will be on the same side of the Sun, sometimes on opposite sides, and anywhere in between).

When arriving at the outer planet, you can reduce your velocity with a close pass to the planet (slingshotting), which theoretically can reduce velocity by up to twice the planet's orbital velocity, though lower in practice if you want to avoid catastrophic atmospheric friction and tidal strain. Disembark after this passage to reduce the amount of deceleration your shuttle has to do. When arriving back at the inner planet, use a similar maneuver to increase orbital speed by up to twice the orbital velocity of that planet. Here, disembark before this maneuver, since your orbital velocity is likely greater than that of the planet.

There are some constraints to this method. The fastest way to move inwards is to totally kill your orbital velocity, basically free-falling inwards. This is limited by the gravitational pull of the Sun at the distance you are, which is likely to be a very small fraction of one gravity. At Earth's orbit, the Sun's gravitational pull is 0.006 m/s/s (meters per second squared), and this pull decreases as the square of the distance from the Sun. Moving from Mars to Earth, the average pull will be around 0.0036 m/s/s. Moving from the (average) orbit of Mars to that of Earth will then take a minimum of 76 days. Going in from Saturn to Jupiter will take far longer; on the order of 3.5 years.

There are no similar constraints going outwards. Giving a large enough solar sail, your acceleration is only limited by the mass per square meter of the sail. Given a light (and strong) enough material (unobtanium), you could feasibly maintain an acceleration of 1 g. Accelerating at 1g all the way, you could move out from the orbit of Jupiter to the orbit of Saturn in just 4-5 days! (arriving at very high speeds, though). Or you could if there was no solar wind. The solar wind, consisting of charged particles emanating from the Sun, has a much higher energy density than sunlight and will begin to brake your solar sail once you reach outbound velocities exceeding around 400 km/s (the average speed of main component of the solar wind). This increases the minimum travel time out from Jupiter to Saturn to around 20 days. Your ship will need to decelerate from 380 km/s (400 km/s minus twice Saturn's orbital velocity of 10 km/s).

In other words, moving out can be done in a matter of days, but going in will take months or years, depending on how far out you travel from. To make your system feasible, you probably need some kind of engine to provide thrust going in (with solar sails furled or slanted perpendicular to sunlight and solar wind). A fusion-powered ion thruster is probably your best bet, but your ship would have to tank up on He3 before going in. Such a thruster can also decelerate your spaceship going out, arriving at your destination at near-zero speed.

To sum up: A solar sail will help you accelerate going and to adjust the orbital speed of your solar spaceship to match that of the planet you arrive at. However, you will need a thruster (ion or otherwise) to kill your final speed going out and to give you speed going in (where braking can be handled by solar sails). The good news is that you don't have to worry about matching the speed of your payload / passenger module to your solar spaceship, since it will have to kill its speed anyway before reversing direction.

Answer 2

Frame challenge:

Orbits in space do not work as roads back on Earth.

On Earth if you need to go from A to B, you can do it a 50 km/h, 100 km/h, 200 km/h or even faster, at most depending on the cops.

In space, if you want a closed orbit (like you seem to suggest) you can't simply accelerate over and over, because very quickly you will no longer be orbiting, but rather drifting away.

That said, also solar sails do not work as Earth sails. On earth one can use sails to go against the wind and gain speed, in space that is not the case: light can push away radially from the star, so you can't use a sail to "accelerate back and forth between two planets continuously increasing velocity".

Answer 3

This will never work in a realistic, respectful of physics, setting for 4 reasons.

1. orbital trajectories vary with speed. Unlike with a car on a road, accelerating/decelerating in space necessarily modifies your trajectory. The reason is that a spaceship is in freefall, with no force acting on it other than gravity, while a car uses friction with the road to keep on track even if its speed changes (somewhat, if the car goes too fast it will drift off). The faster a space ship goes around the sun, the more time the sun gravity takes to bring it back, the wider its orbit becomes, to the point that a Mars bound probe would spend more time beyond the Mars orbit than between the Earth and Mars, where you want it to be.

2. Slingshot maneuvers don't change a spaceship speed relative to the planet it slingshots by. It can accelerate or decelerate relatively to the other planets or the sun, but relative to the slingshot planet it will gain just as much speed while falling towards it than it will lose while getting away from it. It means your passenger ship is on its own when it comes to decelerate and land on the planet, and will probably have to burn much more fuel than it would have by just going with a usual trajectory as it goes much more fast.

3. A good analogy to your system is someone trying to travel from one side of a soccer field to the other by grabbing a bullet fired from a pistol. The first result is it would shatter their hand. The reason is the bullet is much faster than they are, like your probe is faster than the ship, and upon contact delivers a huge amount of cinetic energy. In order to make the grab possible you would have to run almost as fast as the bullet flies, so that your relative speed is somewhat equivalent to something you can effectively grab, like a baseball tossed at you by a human. But then, you did most of the work yourself, so there is not much point.

4. Even if you had superpowers that let you grab the bullet without hurting your hand, it won't take you to the other side of the field. The reason is the bullet, while much faster, is also much, much less heavy than you. The product of mass and velocity has to remain constant, so if you are immobile and 100 times heavier than the bullet as soon as you grab it your common speed will be divided by about 100. In the same way, if your cargo ship contains any amount of payload it's reasonnable that it would be orders of magnitude heavier than the probe, and reduce its speed by orders of magnitude when they make contact. Similarly baseballs, when pitched by a pro, can reach an average speed of 150km/h. The catcher who is much heavier catches it, yet is not projected into the public. It's the much faster but much much lighter ball who stops.

Now this is if you want to stay realistic. If your setting is more lax with physics and works with the rule of cool you don't have to bother. You do you.

Answer 4

A small refinement of the approach:

Instead of probes, put solar sails on a large number of asteroids, accelerating them to a very fast, very long solar orbit. When you want to go somewhere, grab the next asteroid that flies by.

The mass of the asteroids works as kinetic energy storage: the momentum harvested by the solar sail on the way out is usable when the asteroid eventually returns on its natural orbit.

There are two main problems with this approach:

1. Setting up the orbits so that they are useful, but do not hit the planets. Because the thrust from a solar sail is very low, you'll have to calculate the orbits for centuries ahead of time.

2. Catching a ride without having to accelerate with rockets. A lightweight probe that attaches a strong tether to the asteroid could work to pull the payload up to speed. It's going to have to be very long to provide reasonable rate of acceleration. While there is no gravity, accelerating the weight of the tether itself will prove to be problematic in similar ways as space elevators are.

Answer 5

The most you are going to reasonably get from this sort of maneuver is going to be a few 10's of km/s. You can't go arbitrarily fast because the planet only has so much gravitational pull. If you are going too fast you just bounce and don't get much benefit.

The Earth's orbital velocity around the sun is roughly 30 km/s. By being pretty tricky with slingshot stuff you might get 60 km/s. If you were to use the planet Mercury, it has an orbital speed of 47 km/s. Double and round that up to 100 km/s. Probably the best you could reasonably hope for. The farther out from the sun the slower the orbital speed.

This is to compare to the Pioneer probes that achieved around about 37 km/s.

The speed of light is 300,000 km/s. So you have now got one part in 3,000. The nearest star is therefore more than 12,000 years away using this method. You would need to be mighty patient.

Answer 6

I think one question mentioned here was not addressed: "If the ship can accelerate to safely catch the sail, why not just travel?"

I suppose one answer could be the resistance of interstellar medium (all those loose atoms and dust particles add up), presumably the sail (maybe boosted by laser from Earth/Moon) would allow the bundle to just move on without wasting fuel as otherwise a ship would have to keep doing -- and the huge sail would not have to be part (and weight) of the ship.

Also note that the ship anyway would also need to have the fuel to decelerate at its target planet. Maybe its own (smaller? lighter?) sails/parachute could be used for some of that (brake against the incoming star, e.g. detach from buoy at the Solar system border, open the parachute and let time pass).

Another rationale for your design could be that if this buoy with sails is massive enough to grab whole ships and not care much, it could be more protected from the space radiation etc. Effectively a sailing hotel between planets, park your thin-foil ship nearby and come inside for safe travel (even if hibernation). Maybe a repurposed asteroid or some such...