# How difficult to gravity assist brake on entry to Alpha Centauri system? [duplicate]

Say a generation ship arriving at Alpha Centauri A system wants to get into orbit (or land) on an exoplanet there. After decelerating for the 2nd half of the journey, their approach velocity is still too high to land on the surface safely. They need to decelerate more than their ship thrusters can provide.

Can gravity assist slingshots provide enough braking? Say from 40+ kps to 8kps velocities.

I hope ~40kps should be slow enough entry to the system give ample time to calculate the orbits of other exoplanets needed for lining up gravity assists. I see only 1 unconfirmed planet for Alpha Centauri A that is 20-50 M⊕. But, there could be more. Hopefully that planet helps for the calculation though.

Let's also assume that the exoplanet the ship wants to visit is (fictionally) so similar to Earth that we have the same mass and the atmosphere of the Archeaen Earth (lacks the O2. I'm not sure if/how that effects atmos entry). Typical low earth orbit re-entry speeds are near 17,500 mph, or 7823.2 m/s. Let's assume the ship is at least 200,000kg. It is designed for this entry velocity in 1 piece, or breaking up into multiple entry cones with heat shields. Perhaps breaking up into modular smaller masses would help the gravity assist decel as well... I'm only guessing.

I'm curious about figure (g) in: https://upload.wikimedia.org/wikipedia/commons/7/7e/GravPoss.gif As it seems to reverse the vector... perhaps this is a good approach angle to max decel.

Looking on this site and in research, I mostly see gravity assists for accelerating, or minor adjustments for small probes into orbits, not larger deceleration. Thank you!

• BTW, 200 tons is pretty small for a generation ship. Also, if its top speed is 40 km/s, it'll take over 32,000 years to get there from here. – PM 2Ring Apr 4 at 19:56
• @OrganicMarble no it doesn't since the question and its answers only cover trajectories about a single body and the "Alpha Centauri system" offers n-body goodies that may allow at least temporary capture. This question should not be closed as duplicate just because a few words overlap with the other question. Look at the care with which this question is written and the thought put into n-body dynamics. There may be some duplicate but that's not one. – uhoh Apr 4 at 22:02
• Taking 32,000 years to arrive and not bringing enough delta-V to brake at the destination seems like one heck of a gamble to run with your generation ship. You'll probably be overtaken in transit. Maybe the people who got there thirty thousand years ago, terraformed the planet, and populated it, can send out tugs to help you.=) – notovny Apr 4 at 22:38
• @uhoh I disagree with your opinion here, this question has a lot of extraneous detail. The accepted answer to the suggested duplicate discusses multi-body cases. – Organic Marble Apr 4 at 22:44
• The question asks specifically "How difficult to gravity assist brake on entry to Alpha Centauri system?" It's a detailed question about a particular and well-studied system; HopDavid's answer offers no specific guidance here. I just don't think it is good to block all users from an opportunity to answer this question. Nobody is going to spontaneously add an answer about the Alpha Centauri system there. – uhoh Apr 4 at 23:07