# Good year for start of an interstellar mission, due to gravity-assist

What is the next excellent year for a interstellar mission (beyond Neptune). The main acceleration for those missions are gravity-assists with Jupiter, Saturn, Uranus and Neptune.

Voyager 2 managed the gravity-assist for each of the 4 gas-giants, Voyager 1 used only Jupiter and Saturn. New Horizons, even through it started at a significantly higher speed, will never reach Voyager 2 because it only visited Jupiter (see the related question here).

I wonder what is the next year with an ideal constellation of (at least) the four gas giants to speed up a space-craft, for reaching interstellar space faster?

A safe upper limit should be ($OP$=Orbital Period in years) $$t =OP_{\text{Jupiter}} \times OP_{\text{Saturn}} \times OP_{\text{Uranus}} \times OP_{\text{Neptune}} ~= 4.8 \text{ million years}$$. That's the maximum time needed for reaching the exact same configuration. But of course space-crafts are flexible, thus the least common multiple should be much smaller.

I'm interested in the next good year (and also on how to calculate that).

Added later: Part of the question has already been answered here, and also on Wikipedia on gravity assist. I would like to understand how to calculate the year (teach a man how to fish...), and what the years for three-planet line-up is.

• Related When is the next Outer Planet lineup (Voyager). Also, to clarify, did you mean "extrasolar travel"? Interstellar travel that would require gravity assists by the four outer giants would take so much time that waiting for the next planetary alignment would seem "just around the corner" in comparison. ;) – TildalWave Jan 5 '15 at 17:36
• The answer to TildalWave's related question says the next 4 planet lineup is not till 2151-2154. Would you be interested (as I would) in possible 3-out-of-4 lineups? – Jerard Puckett Jan 5 '15 at 17:42
• @TildalWave perhaps for the Neptunian equivalent of the maneuver on slide 10 here: niac.usra.edu/files/library/meetings/misc/trieste_may02_mtg/… – Jerard Puckett Jan 5 '15 at 18:16
• @JerardPuckett That one seems to employ the close perihelion burn (at 3-4 solar radii according to this PDF) just like the von Ehricke trajectory that Deer Hunter mentions in his answer. It's not a "grand tour" trajectory. Jupiter seems to be used only to increase orbital eccentricity to allow the probe to make a perihelion dive for propulsive solar gravity assist. ;) – TildalWave Jan 5 '15 at 18:54
• Props to Deer Hunter for reminding me what the maneuver was called/who came up with it. See comment under his answer. – Jerard Puckett Jan 5 '15 at 18:57

An interstellar mission does not need to visit more than two planets, since your interest by definition lies in objects beyond the Solar System. The Voyagers were first and foremost planetary exploration craft, albeit with symbolic messages to alien civilizations.

As I wrote in an answer to another question, the best trajectory (a.k.a. the Krafft Arnold Ehricke trajectory) to get out of Solar System with the highest velocity $V_\infty$ will have to use:

• A gravity assist from Saturn
• A gravity assist from Jupiter
• A perihelion propulsive maneuver (aka firing engines) near the Sun (as close as allowed by the spacecraft's thermal control system).

Claudio Maccone proposed a following trajectory yielding an exit velocity of 51 km/s:

• A gravity assist at Jupiter
• A powered maneuver near the Sun
• Another Jovian gravity assist.

The opportunities for a multiple-assist + propulsive maneuver mission are much more frequent due to the flexibility that powered swingbys give.

References:

Gregory L. Matloff. Deep Space Probes: To the Outer Solar System and Beyond. 2nd ed., 2005. P.46-48.

Claudio Maccone. Solar Foci Missions, 2nd Int'l Conf. on Low-Cost Planetary Missions, Laurel, MD, 1996. IAA-L-0604.

Krafft Arnold von Ehricke, "Saturn-Jupiter Rebound. A method of high-speed spacecraft ejection from the Solar System". Journal of the British Interplanetary Society, Vol.25, 1972. Pp.561-571.

• Any utility in going much further out before plunging back towards the sun, as in i.stack.imgur.com/R1gLH.png? – Jerard Puckett Jan 5 '15 at 18:53
• @JerardPuckett - too long, and the $GM$ is too low :) – Deer Hunter Jan 5 '15 at 18:57
• Oh that is highly interesting, thanks for the intro and the references. So my idea that gravity-assist of four planets is required is too naive, and several nice alternatives exist. Now I have some research to do on modern space travel ideas :) – NicoDean Jan 5 '15 at 19:59