system
Given the following post, respond as if you're commenting on Reddit:
human
If I wanted to travel to the Sun, at which direction do I point my thrusters?

**In a ship that are in earths orbit around the sun.**
I figure we already have momentum, so if I pointed the thrusters in the opposite direction of my momentum. Then I would decrease my speed around the sun and gravity should pull me in.
^(I think)

Or is it just as simple as pointing my engines away from the sun and head inwards? Like walking on one of the arms of a carousel towards the center.
gpt
Too many wrong answers here.

If you accelerate against your motion, you'll need about 30 km/s relative to the Earth (as this is Earth's orbital velocity). You are in Earth's orbit, you need sqrt(30^(2)+11^(2))=32 km/s there to escape from Earth where 11 km/s is the escape velocity of Earth, but you also have ~7 km/s from the orbit, assuming a low Earth orbit (LEO) with suitable orientation. Total: ~25 km/s. With an efficient chemical rocket (400s I_sp) that needs a mass fraction of about 500. To get 100 kg towards the Sun you need 50 tonnes in LEO. That is the fastest (2-3 months) somewhat feasible way to reach the Sun but also the most inefficient one. It is similar to a Hohmann transfer.

If you accelerate with the motion of Earth until your orbit goes very far away from the Sun, you need about 21 km/s relative to the Earth. Close to Earth this needs 23.7 km/s, you have 7 km/s already => 16.7 km/s needed. Once you are very far away from the Sun, you can make a tiny burn to reduce your tangential velocity to zero, and from there you fall down to the Sun directly. Can take decades, but it is much more fuel-efficient. To get 100 kg to the Sun you only need 6.5 tonnes in LEO.

What a spacecraft would actually do is a gravity assist at Jupiter. Accelerate by 6.5 km/s to escape Earth at a speed of sqrt((7+6.5)^(2)-11^(2))=8 km/s, enough to reach Jupiter when timed properly. There, make a fly-by that reduces your tangential velocity to zero and gives you a bit of velocity towards the Sun already, then fall down towards the Sun. Takes a few years, but only 6.5 km/s and a few course correction maneuvers. To get 100 kg to the Sun you only need 500 kg in LEO. This is both more fuel-efficient and faster than the previous method. You can reduce the fuel requirements further if you add a gravity assist at Mars or Venus, but then you get quite limited in the possible launch times. With Jupiter alone such a mission is possible roughly once per year.

If you have impossibly strong magical thrusters, you can accelerate directly towards the Sun and hit it. But then you have to be so fast that Earth's orbital velocity simply does not matter. You would need speeds in excess of 1000 km/s to do that properly. With a ridiculously strong but not completely magical thruster you can also accelerate a bit backwards and then inwards to speed this up.