How would an interstellar probe navigate the pull of gravity from stars and other large objects on the way?

Question

Refer to the ambitious probe sent by Yuri Milner, Stephen Hawking and Mark Zuckerberg to Alpha Centauri:

Yuri Milner is spending $100 million on a probe that could travel to Alpha Centauri within a generation—and he's recruited Mark Zuckerberg and Stephen Hawking to help. In an interview with ​The Atlantic, Milner makes his case for star travel.

There are so many unknown gravitational sources along the way, and human cannot hope to steer the direction of the space probe in real time in order to avoid them all. Assuming that you want to change the course of the probe when it's halfway through ( about 2 light years from the earth), your signal will have to travel at least 2 years before reaching the probe, but by then the probe might already have collided with a planet. It's too late. The remote control cannot be done in real time.

So how can scientists and engineers ensure that this probe actually reaches Alpha Centauri-- a galaxy that is a few light years away from earth-- instead of colliding with or being sucked into stars, planets, comets and other unknown gravity sources ( like dark matter perhaps)?

Answer 1

There are a number of reasons why the probe does not stray off course on its journey.

1. There are no known large objects between the solar system and Alpha Centauri. Alpha Centauri is the closest star system to our own which means that the only objects in the probe's path will be in the interstellar medium.
2. Interstellar space is extremely rarefied. There is hardly any matter in the space between the large centres of gravity (stars). The probability of the probe passing a stellar object at a distance that affects its path is highly unlikely.

So it is simply very unlikely that that the probe will encounter any significant gravitational influence until it reaches the Centauri system

Answer 2

Others have already clarified that the chance of encountering another massive body in interstellar space is astronomically small. However, there is still something to consider for course corrections, which is that a very small error in velocity early in your transfer orbit to Alpha Centauri can result in a huge error in your intercept with the system.

Think of it like somebody firing a single atom while in orbit around Earth, trying to set up the trajectory so that hundreds of years later, it precisely hits a microscopic target that was traveling in a different orbit. For extra fun, also try to take into consideration miniscule perturbing effects that cause the orbits to deviate from their idealized model over that period of time.

In practice, the above means that you do need some way to perform course corrections if you're going to intercept your target. As you say, 'real-time' remote control is not feasible, so what this means is that the probe would need some automated control systems to gather information and autonomously correct course along the way.

There's a trade-off in when you make the corrections. The earlier you make them, the less delta-V (and hence fuel) required. But, due to the mindbogglingly huge distances involved, the more precise/accurate you need to be. In practice, you probably schedule a number of short burns along the way, so that you keep the delta-V required for your 'late' adjustments to a minimum. How you schedule those adjustments is likely a matter of careful optimization based on estimated error magnitudes.

Answer 3

The effects of gravity falls off very quickly (following the inverse square law) at large distances, so the pull of other stars would be negligible on the probe. This is even more so for planets in other systems, or comets/asteroids. As they are so far away, stars and planets would not be an issue.

Encountering an ejected planet, asteroid, comet, etc. along the route between the stars is always possible. However, the probe would have to come very close to the object or even impact the object to be disturbed from its trajectory. There's a very low chance of this happening, as interstellar space has extremely low densities.