# Is a solar free-return trajectory possible?

I'm coining a new phrase: "Dyson circuit". Is it possible, with current solar orbital capabilities (see Parker Solar Probe), to drive artificial satellites in a free-return trajectory around the Sun?

As a second part to this question, would that make it possible to harvest excess kinetic/radiation energy from this "circuit"?

• Interesting idea and Welcome to Space! The orbital mechanics part of your question is really interesting and I think it can stand on its own. The last part and harvest excess kinetic/radiation energy from this "circuit"? is a whole separate question. Why not delete that from here and post a second question about that separately? You can ask as many good questions as you like, and the narrower and more focused each question is, the better the answers may be.
– uhoh
Mar 28, 2019 at 15:12
• I do want to clarify the second part. I was wondering if a mass-driver/thruster added could serve as a focused exhaust that transfers kinetic energy (or possibly solar), and if that could in turn decrease the viable perihelion that a satellite achieve (a surplus trajectory, if you will). Mar 28, 2019 at 16:38
• Where do you think you can gain energy on one circuit? If we exclude drag of any kind, and effects from all bodies other than the sun, the satellite will return to where it started with zero gain in energy. There will be no excess. In reality, it will need energy input to it, because of all those things we just excluded... Definitely worth getting rid of that second question! Mar 28, 2019 at 19:34
• I suppose what I'm asking is if the baseline trajectory is viable (assuming, with a suspension of disbelief, that the solar craft has a means of compensating for factors such as drag). The second part, and this is my guess, is that there is an application for a mass driver in free fall that can aim payloads around a gravity well, especially if the source of propulsion can found in the ambient environment. Mar 28, 2019 at 20:07
• Any orbit whose period is some fraction of Earth's and crosses Earth's orbit will eventually coincide again if we ignore perturbations. For example, a 6-month orbit with aphelion at 1AU will return to Earth exactly a year later
– Jack
Mar 28, 2019 at 20:34

Is it possible to drive artificial satellites in a free-return trajectory around the Sun?

No. Getting a satellite close in to the Sun takes a lot of energy.

Parker Solar Probe used the biggest rocket available to get a really high-speed launch, and then:

Parker Solar Probe will use seven Venus flybys over nearly seven years to gradually shrink its orbit around the sun, coming as close as 3.83 million miles (and 6.16 million kilometers) to the sun, well within the orbit of Mercury and about seven times closer than any spacecraft has come before.

So a high-speed launch plus a bunch of gravity assists to get to a highly elliptical orbit with an aphelion near Venus:

Source

Second, a 'free return trajectory' does not really apply to the Sun. Apollo 13 went into a free return trajectory, using the Moon to bend its trajectory back toward Earth and capture into Earth's gravity well.

The Parker Solar probe is in an elliptical orbit around the Sun.

tl;dr: To the narrow question:

Is a solar free-return trajectory possible?

Yes, a trajectory that starts near the Earth, goes around the Sun, and returns to a place near Earth is possible.

I'm not an expert but let me try to clear up some points though:

1. Traditionally, a free return trajectory has been envisioned starting from the large close-to-fixed body (e.g. Earth), swinging past a small body that's orbiting around it (e.g. Moon), then returning to the large body.
2. As @Hobbes points out you need a big impulse to go from low orbit around the large body to the free-return orbit.
3. A free-return orbit is not necessarily the same thing as a cycler orbit; if you didn't put on the breaks after the first complete cycle and swung back out a second time, the small body would not be there the next time in most practical cases, and then you'd be in a pickle.

I'm coining a new phrase: "Dyson circuit".

So I think you are looking for periodic solutions, and there are certainly quasi-periodic 3-body orbits that can pass alternately near the Sun and near the Earth. In reality perturbations would not allow these to be quasi-periodic for very long, but at least they could be station-kept.

From this answer to the question What sort of orbital elements are used to describe halo orbits?:

The extremely cool and colorful paper E. J. Doedel et al, (2007) Elemental periodic orbits associated with the libration points in the circular restricted 3-body problem International Journal of Bifurcation and Chaos 17, 2625 (2007). https://doi.org/10.1142/S0218127407018671 builds a system of illustrations that show all of the known, periodic, orbits in the CR3BP (Circular Restricted Three-Body Problem). This includes many kinds or classes of orbits but excludes Lissajous Orbits because they are not in general periodic. (note: ignore the drawing in the Wikipedia article!)

You can and probably should also download the paper from its non-paywalled ResearchGate site, make some coffee, then spend six months enjoying it.

Here are some 3-body orbits that might do the job you are looking for. As mentioned, in the real world you would have to include some station-keeping to maintain the orbit over time. The images are in a rotating frame.

These images are drawn using the Earth-Moon system, but you can also imagine the Sun-Earth system where the Moon in the picture is the Earth, and the Earth in the picture is the Sun.

These are schematically drawn, when you change to the much larger mass ratio of the Sun-Earth system, the shapes will change.

It's not a coincidence that these two resemble the two cis-lunar co-planar free-return orbits in the last plot below!

## Back to 1963!

Below are four free-return trajectories starting from Earth orbit and swinging past the Moon. These are drawn in a rotating frame of reference so that both the Earth and Moon are pinned to the x-axis of these plots. These are from NASA Technical Note D-1833; Trajectories in the Earth-Moon Space with Symmetrical Free Return Properties written in 1963 by Arthur J. Schwaniger at the George C. Marshall Space Flight Center.

In the paper, a free-return is defined:

For manned exploration the first flights will very likely be such "fly-by" trips with no plan for landing on the moon. Indeed, when the manned mission is to land on the moon's surface, a "free return" trajectory may be used so that if unforeseen difficulties arise which would make landing undesirable or impossible (particularly a failure of the propulsion system to brake the vehicle speed so as to make landing possible) the astronauts will return safely to the earth.

They are all free-return trajectories starting and ending near the Earth, and except in special cases they work one-time only. You need a big propulsive maneuver to enter into one from LEO in the beginning, and you need a 2nd big propulsive maneuver to get back into LEO (or reenter the atmosphere) when you get back to Earth.

They're called symmetrical because the 2nd half is nearly a mirror-image of the first half.

There are certain combinations of injection altitudes and velocities that can produce periodic or cycler-like behavior but in general these will not approach the Moon very closely.

In a non-rotating frame I think that the