# Braking using Escape Velocity

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I'm wondering if it's possible to use the gravitational field of a massive object to decelerate a spacecraft moving at high velocity. An example is this: if a spacecraft moving purely on inertia at one percent light-speed performed a flyby of a one sol mass neutron star (or black hole) at a given altitude, would the gravity field cause the spacecraft to swing round the star, slow it up or else deflect its trajectory in a meaningful way? According to one online calculator the escape velocity of a solar mass neutron star at a distance of 25,000 km equals 3,000 km/s or 0.01 c. At this distance the tidal pull exerted by the star's gravity would be minimal, therefore represent no hazard to the ship or its crew. That said, I'm curious to know what would be the outcome.

PS. It's the escape velocity I'm interested in: not the star's proper motion as a means of affecting a change in delta-v.

Many thanks.

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An object moving at 0.01c while still very far from the Neutron star, will be accelerated by the star's gravity as it approaches, by the time is is 25,000 km from the star, it will be moving at ~0.0148c relative to the star ( nearly 50% faster than escape velocity for that distance).  As it climbs back away from the star, it will lose that gained speed and once it gets back to being very far from the star is will again be moving at 0.01c with respect to the star.

It's path will be deflected by some amount.  Now you could use this deflection to alter the object's speed with respect to its target, but this would involve the Neutron star itself as having a significant velocity with respect to the target.    You would being doing the reverse of what happens when we use a planet for a gravitational assist, where we transfer some the planet's momentum with respect to the Sun to the probe.   This can also be used to go the other way, transferring momentum from probe to planet.

In this case we would need to transfer momentum from our object to the neutron star.  This will allow use to reduce the object's speed relative to some reference system, but only if the Neutron star has the proper velocity with respect to that reference frame.

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Yes, I understand now. In other words the only 'practical' way for an object to decelerate while passing through an intense gravitational field would depend on powered means, not inertial. Moreover, the fact that the incoming object is moving at 0.01c relative to a given mass would (I should imagine) rule out all variations of power assist - this insofar that the proper velocities of nearly all stars aren't remotely relativistic, that's to say within local frames.

Many thanks for clearing up this matter for me.

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• 4 weeks later...
On 12/17/2017 at 8:34 AM, GeeKay said:

Yes, I understand now. In other words the only 'practical' way for an object to decelerate while passing through an intense gravitational field would depend on powered means, not inertial. Moreover, the fact that the incoming object is moving at 0.01c relative to a given mass would (I should imagine) rule out all variations of power assist - this insofar that the proper velocities of nearly all stars aren't remotely relativistic, that's to say within local frames.

Many thanks for clearing up this matter for me.

Actually, Aerobraking is another way instead of using powered means.

It basically means you use the atmosphere of the planet to slow you down, potentially putting you in orbit around the planet.

Then you just wait until you reach the apogee and then increase your velocity a little bit and you'll gain height at the periapsis, preventing you from slowing down further.

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On 12/17/2017 at 9:34 PM, GeeKay said:

Yes, I understand now. In other words the only 'practical' way for an object to decelerate while passing through an intense gravitational field would depend on powered means, not inertial. Moreover, the fact that the incoming object is moving at 0.01c relative to a given mass would (I should imagine) rule out all variations of power assist - this insofar that the proper velocities of nearly all stars aren't remotely relativistic, that's to say within local frames.

Many thanks for clearing up this matter for me.

Not specifically. You can use gravity of one object to alter your velocity with respect to the third object. For example, you have a space ship coming back from Mars towards Earth. You can use lunar gravity assist to slow it down a little with respect to Earth rest frame. But in Moon FoR craft will have the same velocity at same distances as it approaches and recedes.

It's the same as you can use Jupiter gravity assist, this time to increase your velocity wrt Sun rest frame, but then again, velocity of your craft (unless powered assist is used) will be the same at same distances from Jupiter as you approach and leave.

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