I was playing around with Universe simulator 2 and they have simulated NASA missions. It seems to actually take care of n-body simulations surprisingly well- in terms of a video game that is. So I was rather surprised when I let the Parker Solar Probe simulation run all the way to 2075 and suddenly a close pass with either Mercury or Venus circularized Parker's orbit right below Mercury's. I was wondering if this is actually going to happen- or if the game isn't as accurate as I had hoped (I checked the orbital elements of the big planets, and they were correct).

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    $\begingroup$ Gravity assists from Venus or Mercury couldn't circularize Parker's orbit below Mercury, only at it or above, but potentially other effects could... $\endgroup$
    – Jack
    Commented Jan 13, 2019 at 17:37
  • $\begingroup$ @Jack I've mentioned your comment here but that could really be expanded as a separate answer if you are interested. $\endgroup$
    – uhoh
    Commented Jan 13, 2019 at 22:27

1 Answer 1


As I discuss in this answer Universe Sandbox 2 does not include General Relativity according to it's FAQ.

In that answer I say:

As noted by @MikeG in this comment, the Universe Sandbox 2 FAQ addresses the question, but not in a satisfying or helpful way:

Does it account for relativity?

No, the physics in Universe Sandbox² is currently only [Newtonian][4].

Why? The short answer is that you need a supercomputer to accurately simulate general relativity. (emphasis added)

Jenn, astrophysicist and Universe Sandbox² developer, [explains more in a blog][3] post: "General relativity requires simulating the spacetime itself. That is, taking your simulation space, discretizing it to a hi-res 3-D grid and checking the effect that each and every point in that grid has on all neighboring points at every timestep. Instead of simulating N number of bodies, you are simulating a huge number of points. You start with some initial data of the shape of your spacetime and then see how it evolves according to the Einstein equations, which are 10 highly non-linear partial differential equations."

In that answer I also point out that this is somewhat misleading. To add GR effects to orbital propagation you only need to make a relatively small change to the equations.

I won't copy/paste the whole answer here, please read there for more details.

So both Mercury and Parker will be precessing due to GR in a way not calculated Sandbox 2. Further, they have set it up with some educated guesswork about Parker's trajectory. With all the upcoming flybys in the future there are two more problems

  • Specific maneuvers to make fine adjustments to the flybys that will happen years from now are impossible to predict
  • The problem is extremely non-linear, a few km shift in a flyby could result in thousands of km difference in trajectory, and that could cause a hundred-thousand km shift following the next flyby in a cascade fashion.

So nobody knows where Parker will be in 2075!

In this answer I plot an example of a planned trajectory from the Horizons database. There is no guarantee it will actually be like this, small changes are likely, large changes are possible, but 2075 is anybody's guess!

Circularization: It's very unlikely that a flyby in the future, or even a sequence of them, could make the orbit circular. It would have to add quite a bit of energy to raise Parker's periapsis to match that of Mercury's again, much less that of Venus. As @Jack points out in his comment, the flyby of a planet ensures that the the apoapsis must be at least as far away from the Sun as that planet.

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    $\begingroup$ Ahh... I had just gotten the game because I heard it had some cool scenarios. I figured it was too good to be true for anything other than maybe simple modeling. $\endgroup$ Commented Jan 15, 2019 at 8:22

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