# Astronaut shoots a bullet into the sun - gravity assists?

#### rcgldr

Homework Helper
This question came up at another forum. The delta-v of potential guns is limited to the range 3 km/s to 11 km/s (hydrogen gun). Assume the astronaut is either on the moon or orbiting the moon. I'm wondering if a single impulse followed by gravity assists (and ideal planet alignment) could result in the bullet's orbit intercepting the sun.

Would 3 km/s to 11 km/s be sufficient to achieve a gravity assist with Mars (assuming it's located in an ideal part of it's orbit)? If not, the next option would be a gravity assist from Earth and then one or more planets to achieve a Sun intercepting orbit?

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#### rootone

I am sure it's possible in principle to launch a small projectile that eventually collides with the Sun.
It could take a a while to get there though, and how could this be a useful project?

#### Tom.G

and how could this be a useful project?
As in "This is left as an exercise for the reader." Or to put a 'forbidden here', philosophical bent on it; "Is information that is not immediately used useful?" Or perhaps as a variation on a Hohmann transfer orbit. Or is it useful to have enough gasoline in your parked car to get to a refill station while you are in bed asleep?

#### rcgldr

Homework Helper
The escape velocity from earth at the average radius of the moon's orbit is about 1.438 km/s. The moons average orbital speed is about 1.022 km/s. Escape velocity from the surface of the moon is about 2.38 km/s, so a bullet speed of 3 km/s from the surface of the moon in the right direction should be enough to achieve escape velocity from earth and moon system. That in itself doesn't mean it would end up in an orbit that intercepts the sun, but perhaps enough to get a gravity assist from another planet that would do the trick.

The question was posted at stack exchange in reference to a comic strip.

https://space.stackexchange.com/questions/26276/could-an-astronaut-safely-shoot-the-sun-with-a-gun

I'm not sure of the point of the comic strip was, but the answers posted there didn't seem to consider gravity assist. A somewhat recent example of gravity assist was the 12 year path of Rosetta:

or the 5 man made objects that have left the Solar System.

https://en.wikipedia.org/wiki/List_of_artificial_objects_leaving_the_Solar_System

Voyager 1:

Staff Emeritus
Why not Venus?

Think of this as a problem of shedding velocity. It's presently going too fast to hit the sun.

#### rcgldr

Homework Helper
Why not Venus?

Think of this as a problem of shedding velocity. It's presently going too fast to hit the sun.
I forgot to include Venus as well as Mars as the two closest planets to use gravity assist to change the path of the orbit. I'm wondering if the thin atmosphere of Mars would allow for a closer flyby. My idea on this was to increase the eccentricity of the orbit more that slow down the speed relative to the Sun.

The Rosetta gravity assist was interesting in that it involved both the sun and the earth. The launch of the Rosetta put it into a solar orbit that intercepted the earth one year later, resulting in enough gravity assist for it to reach Mars.

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#### 256bits

Gold Member
The escape velocity from earth at the average radius of the moon's orbit is about 1.438 km/s. The moons average orbital speed is about 1.022 km/s. Escape velocity from the surface of the moon is about 2.38 km/s, so a bullet speed of 3 km/s from the surface of the moon in the right direction should be enough to achieve escape velocity from earth and moon system. That in itself doesn't mean it would end up in an orbit that intercepts the sun, but perhaps enough to get a gravity assist from another planet that would do the trick.
So one achieve a hyperbolic orbit of the bullet relative to the earth-moon system, but not all that much to deduct from the earth's speed revolving around the sun.
One thing to be looked at is the eccentricity of the bullet orbit wrt the sun and whether an intercept with a Martian or Venetian orbit is possible.

#### rcgldr

Homework Helper
So one achieve a hyperbolic orbit of the bullet relative to the earth-moon system, but not all that much to deduct from the earth's speed revolving around the sun. One thing to be looked at is the eccentricity of the bullet orbit wrt the sun and whether an intercept with a Martian or Venetian orbit is possible.
The goal here is more about increasing eccentricity to intercept the sun as opposed to reducing speed. Looks like setting up an orbit (relative to both earth and sun) that re-intercepts the earth about a year later can provide a fairly large gravity assist to increase eccentricity. Rosetta used this method to intercept the earth a year after launch which allowed it to reach and get a gravity assist from Mars (the timing with Mars orbit had to be setup for this to work).

#### 256bits

Gold Member
@rcgldr Oh. OK. I didn't read post #6 like a should have. You already took all that into account.

#### mfb

Mentor
In principle it is possible, but Venus or Mars won't help. You have to reduce the speed in the tangential direction to nearly zero to fall into the Sun, this corresponds to 35 km/s relative velocity for Venus, 30 km/s for Earth and 24 km/s for Mars. All these speeds are much larger than the escape velocity - to achieve them in a fly-by, you have to start with a high retrograde speed (as seen from the planets) already. A single fly-by won't make it and multiple fly-bys are impractical as well.

What you would need is a fly-by at Jupiter. A few kilometer per second are sufficient to reach Jupiter, and a single fly-by can reduce the tangential speed to nearly zero. Without course corrections your chance to hit the right spot at the Jupiter fly-by is small, but you can shoot many bullets to increase the chance...

"Astronaut shoots a bullet into the sun - gravity assists?"

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