Gravity-assist by Sun?

  1. Jorrie

    Jorrie 853
    Gold Member

    The use of gravity-assist flybys of planets are well known for interplanetary missions. My question: can a Sun flyby be used to gain orbital energy relative to the Galactic center for interstellar missions?

    I assume one must first attain solar escape velocity by other means, since bound orbits around the Sun cannot gain flyby energy. I also assume one can arrange the open orbit such that the spacecraft passes just behind the Sun in its galactic orbit.

    If this has already been discussed, my apologies - just point me to the thread.

  2. jcsd
  3. D H

    Staff: Mentor

    No, for two reasons. (1) The trajectory needs to take the vehicle fairly close to the planet to make the gravity assist effective. (2) The Sun's orbital velocity around the Sun-Jupiter center of mass is about 74 meters/second. There's not much velocity to steal.
  4. Jorrie

    Jorrie 853
    Gold Member

    But the Sun's orbital velocity around the Galactic center is some 220 km/s, which is more relevant to the question.
  5. sylas

    sylas 1,745
    Science Advisor

    We already have that velocity... The Sun might be useful to a traveler coming in from another galaxy. But I presume that was not what you had in mind.
  6. Jorrie

    Jorrie 853
    Gold Member

    Isn't arriving at the Sun at or above solar escape velocity (as per OP) equivalent to coming in from another star? With planetary gravity assist, all that is required is coming in correctly at or above the planet's escape velocity.
  7. Hi there,

    Not at all. When leaving, heading toward the Sun, you already have that 220km/s velocity. We are on Earth, which is following the Sun (hopefully for us), therefore, we live in roughly the same reference as the sun, therefore having the same velocity.
  8. Jorrie

    Jorrie 853
    Gold Member

    In the same way, the craft doing a flyby at Earth already has Earth's 30 km/s relative to the Sun. Yet, it can get a delta-V in heliocentric coordinates if it comes in at a relative speed above Earth's escape velocity and at the right angle, passing 'behind' Earth. Why can't this principle work in Galactic coordinates with the Sun as well?
  9. Jorrie

    Jorrie 853
    Gold Member

    Hi All. OK, it looks like since I've asked a 'half-baked' question, I've got a few 'half-baked' answers! :smile: Let me try and state the question more clearly.

    I think it is accepted that if a spacecraft were to come from in another star's vicinity, naturally arriving in the vicinity of the Sun at a speed exceeding solar escape speed, it could get a gravity-assist energy boost from the Sun. With this is understood that the spacecraft gained some mechanical orbital energy in Galactic coordinates.

    Now let a spacecraft from Earth be 'directly' boosted towards Jupiter for a gravity-assist that boosts its energy further relative to the Sun and takes it to Saturn (Cassini-like). At Saturn its orbit is arranged for a flyby that does two things: slingshot it to back to near the Sun at somewhat above solar escape velocity, having robbed some of Saturn's orbital energy. (If it had the means of propulsion, an advanced technology could just as well have shot the spacecraft 'directly' from Earth to the vicinity of the Sun, ensuring that it exceeds escape speed of the Sun).

    The spacecraft passes as close as possible 'behind' the Sun (relative to the Sun's galactic orbit) and naturally exceeds local solar escape velocity. Is there any reason why this spacecraft cannot receive a gravity-assist delta-V from the Sun? In other words, why would this scenario not be equivalent to a spacecraft coming in from another star?
    Last edited: May 21, 2009
  10. Janus

    Janus 2,446
    Staff Emeritus
    Science Advisor
    Gold Member

    A gravity assist is basically a transfer of momentum from one object (in this case the Sun) to another(the spacecraft), where gravity provides the mechanism of the momentum transfer. It is essentially an elastic collision without the physical contact.

    So let's use an elastic collision to explain what's going on.

    Let's say you have a massive truck moving down the road at 100 kph. We are standing at the side of the road holding a small ball. We toss the ball forward at 10 kph, so that passes in front of the truck and the truck hits it. At the moment of impact, the ball is moving forward at 10 kph and the truck 90 kph, so the relative velocity velocity of the two is 90 kph.

    Upon impact, the ball rebounds from the truck with the same 90 kph relative speed, but its relative velocity is in the opposite direction, so that relative to you it is moving 90 kph + 100 Kph = 190 kph. It has picked up 180 kph relative to the road in the collision.

    This is more or less how a standard gravity assist works.

    Now imagine that you are traveling along the road just ahead of the truck at 100 kph. You want to duplicate the impact of the above experiment. This means that you have to throw the ball backwards at the truck at 90 kph. The ball hits the truck, rebounds and then passes you with a relative velocity of 90 kph in the forward direction (again moving 190 kph relative to the road.) But that is just as fast as you threw it; you would have done just as well if you had just thrown the ball 90 kph forward in the first place. You gained nothing by bouncing the ball off of the truck.

    This is what you are trying to do by using the Sun in a gravity assist for a spacecraft launched from within the Solar system. You wouldn't gain anything by matching the velocity of an incoming object in order to cause it to whip around the Sun over just launching your spaceship in the direction of the Sun's galactic motion to begin with.
    (Besides, it is a lot easier to use an outer planet to give a probe a gravity boost that takes it directly out of the Solar system than it is to try give it velocity boost while at the same time kicking it in toward the Sun.)
  11. Jorrie

    Jorrie 853
    Gold Member

    Thanks for the very clear explanation Janus. I understand now.
  12. Of course you can still get a decent gravity well boost from a solar "fry-by" and doing a perihelion burn. Even more of kick can be gotten from doing likewise around a white-dwarf, but naturely much closer than you can get to the Sun. And then there's the super-boost you can extract from a Dyson gravity machine - twin white-dwarfs in very close orbits, which can boost you to ~3,000 km/s or so.
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