# Could the sun be used as a gravity assist outside the solar system (with current tech)?

I think I might have understood what this XKCD comic is trying to do. The diagram is proposing to use the Oberth effect at closest approach to the sun to escape the solar system!

With current technology (something like the New Horizons or Voyager engines), could this kind of assist actually get it outside the solar system?

Additional note: I thought the bottommost planet in the comic was meant to indicate Jupiter. This is apparently a minority opinion on other forums - their logic is that "Kuiper" could refer to a maneuver that ultimately crosses the Kuiper Belt.

• The idea could be to perform a gravity-assist around a gas-giant to get onto an highly-eccentric sun-orbit and then use the oberth-effect during a burn at perihelion. Apr 8 '14 at 14:15
• But once in interstellar space one should be able to use an extremely tight binary white dwarf/neutron star to get a very good gravity assist. Apr 8 '14 at 14:44
• @LocalFluff Using realistic technology, there's a fairly hard limit on the Delta v our engine can impart. Even if we could get something crazy like 50 km/s, it'll be millennia before we can use that binary pair boost! Apr 8 '14 at 15:05
• Basic confusion with the answers is you bundling "gravity assist" in question title and "Oberth maneuver" in question body. In short, "gravity assist: no, Oberth maneuver, yes." Not that it would be very practical (due to mission times involved), but definitely possible.
– SF.
Jul 16 '17 at 0:14
• The spacecraft would be destroyed anyway when passing the sun in a small distance due to the extreme concentration of sunlight.
– Uwe
Jul 16 '17 at 10:09

No. What a gravity assist does is change the velocity with respect to other objects, but not the one you were approaching. Nasa provided a nice diagram to assist with understanding this.

In fact, in more ordinary terms, it could be though of as the below diagram shows. The baseball is thrown at the train at 30 miles per hour. From the view point of the train, the ball is first coming at it at 80 miles per hour, then leaving at up to 80 miles per hour. From the train's perspective, the relative velocity is the same. However, from the perspective of the person, the ball is moving much faster after hitting the train.

So, enter the idea of using the sun as a gravity slingshot to leave the solar system. It won't work, because the relative speed of flying by the sun won't be changed by flying close it it. One could fly by the sun, use a rocket to accelerate it's movement and leave the solar system behind (Due to the previously mentioned Oberth effect), but that's about the best that could be accomplished by a flyby of the sun. In fact, this was proposed in a paper, which states that if a large amount of acceleration occurs near the Sun quickly, the spacecraft could achieve a speed of 20 AU/year. That would allow for exploring the interstellar medium, but still make it difficult to explore another star.

• @AlanSE - of course, solar sails would benefit from passing close to the Sun. Getting out of the ecliptic plane may also be what you need. Jul 29 '13 at 21:17
• Is the latter what the orbital diagram on slide 10 of this NASA Institute for Advanced Concepts presentation shows?
– gerrit
Jul 30 '13 at 13:37
• @gerrit That trajectory looks similar enough that I suspect it was part of Randal's inspiration. Jul 30 '13 at 16:06
• Actually, 1 ly = 63,239.7263 AU, so 20 AU is only 0.000316256903 ly, or 0.000316256903 c, or 1/3162 c. or 527 times less than 1/6 c. ;) Apr 7 '14 at 21:26
• -1. The answer would be okay if Alan were asking about a solar gravity assist. But the question was asking about the Oberth effect, not a gravity assist. Nov 29 '14 at 18:19

Hefty gravity wells can give a healthy Oberth benefit. Doing a burn deep in Neptune's well makes sense. Suggesting an Oberth maneuver near a Pluto sized object is pretty silly. Don't know if Randall Munroe knows this. Maybe that's part of his joke.

Heading all the way back from the Kuiper belt to the inner solar system take 30 years. Then back out another few decades. Seeing as a mission to the outer system already takes decades, I don't think mission planners would go for this. They like to see their probe return data within their lifetimes. And it's doable to send a probe out of the system with existing rockets and assists from Jupiter. This is already demonstrated.

However, the sun offers potentially huge Oberth benefits should we want to leave the solar system at a good clip.

At .1 A.U. from the sun, solar escape velocity is about 133.2 km/s. Falling from Neptune, an object would be moving 133 km/s by the time it reaches a .1 A.U. perihelion. From there it would only need .2 km/s to achieve solar escape. Doing a 4.2 km/s burn at this perihelion would give a solar V infinity of 33 km/s.

To go from earth to Neptune would take a 8.3 km/s LEO burn. Once at a 30 A.U. aphelion, a gravity swing by from Neptune could be more than sufficient to throw the ship back to a .1 AU perihelion.

To get a solar Vinfinity of 33 km/s, an 18 km/s LEO burn would be needed.

Edit: A 33 km/s solar Vinfnity might be good for doing an inner Oort Cloud object fly by within a reasonable time. But 33 km/s is only a little more than .0001 c. It'd take nearly 40,000 years to reach Alpha Centauri.

• Munroe used to work for NASA, I expect it's part of the joke. Aug 5 '18 at 16:17

OK, we've got three problems here: First, in the title of your question, you are asking about "a gravity assist outside the solar system" but in the body of the question you ask about passing close to the Sun from within the Solar system. I think the title of this question should be edited to be "Could the Sun be used as a gravity assist to acheive an escape from the Solar system (with current tech)?"

The second problem is with PearsonArtPhoto's accepted answer wherein the Sun is assumed to be a stationary object but in fact it is in orbit around the center of the galaxy. Therefore, swinging around the Sun in the direction of its orbit will produce exactly the same sort of gravity assist as going past Jupiter (or any other relatively massive object).

The third problem is that a gravity assist from the Sun is not required to attain escape velocity from the Solar system - we already have FOUR probes that are leaving, never to return - two Pioneers (10 and 11, launched in 1972 and 1973), and two Voyagers (launched in 1977).

Pioneer 10's last, very weak signal was received on 23 January 2003. NASA engineers calculated its radioisotope power source has decayed to where it does not have enough power to send additional transmissions to Earth.

Pioneer 10 will continue to coast silently as a ghost ship through deep space into interstellar space, heading generally for the red star Aldebaran, which forms the eye of Taurus (The Bull). Aldebaran is about 68 light years away and it will take Pioneer over 2 million years to reach it.

Pioneer 11 studied energetic particles in the outer heliosphere.

The Pioneer 11 Mission ended on 30 September 1995, when the last transmission from the spacecraft was received. There have been no communications with Pioneer 11 since. The Earth's motion has carried it out of the view of the spacecraft antenna. The spacecraft cannot be maneuvered to point back at the Earth. It is not known whether the spacecraft is still transmitting a signal. No further tracks of Pioneer 11 are scheduled. The spacecraft is headed toward the constellation of Aquila (The Eagle), Northwest of the constellation of Sagittarius. Pioneer 11 will pass near one of the stars in the constellation in about 4 million years.

Voyager 1 is already in interstellar space, and Voyager 2 is in the heliosheath, and both spacecraft are still sending scientific information about their surroundings through the Deep Space Network.

The answer is no, because the Sun and planets are all moving though the galaxy at the same speed. However, you could do an Oberth maneuver (sometimes called a powered gravity assist) but which is different from gravity assist. If you want to climb out to say Neptune and then fall back you could get there free (not counting leaving Earth's gravity) by using several gravity assists, Venus/Earth/Jupiter/Saturn for example, (unpowered) then use Neptune to send you back and do an Oberth maneuver using the Sun. The gravity assist at Jupiter would put you past breakeven for leaving the solar system but if you want more speed you could do the Oberth maneuver using the Sun.

• The first phrase obviously is false. If Mercury moves at 47 km/s wrt Sun, then how can one seriously claim that they “are moving though the galaxy at the same speed”? 47 km/s is a noticeable speed for stellar dynamics even in the Milky Way (let alone dwarf galaxies). Jul 31 at 16:47