Question: This is part of a series of questions.

*Context*: a friend of mine is writing a novel about a rogue planet around the mass of Mars passing by the solar system before continuing its journey in interstellar space (it must not be captured by the Sun). Given sufficient heads-ups, Earth sends a research mission to land on it, study it for as long as possible, and return.

We would like Earthians to have as much time as possible on the rogue planet.

**How slow and close could a Mars-like planet pass near the Solar System to be plausible and not cause catastrophic Earth orbit perturbation?**

I am guessing that there is no theoretical lower speed limit, as it can barely have the escape velocity of its birth system, then be slowed down by more stars behind it that in front of it. (right?) Or maybe a theoretical limit is the solar system escape velocity, which is around 700km/s, as even an almost stale object relative to the solar system will fall at that speed?

Answer: 
**Numbers**

I concur with Avun Jahei's answer, but feel that it's worth quantifying the minimum non-catastrophic close approach distance.

This object has a mass comparable to Mars - 6.9 x 1023 kg

Earth's moon has a mass of 7.35 x 1022 kg and orbits at an average distance of 3.8 x 108 m.

For the purposes of calculating effects, let's assume that the rogue planet is exactly 10 times more massive than Earth's moon.

If the rogue planet passed by travelling on a trajectory perpendicular to the ecliptic and just outside lunar orbital distance then it would not be catastrophic for Earth's *orbit* - one never-repeated pass would be insufficient to have much effect - but it would be disastrous for Earth *civilisation* - massive tidal waves and energetic disruption to weather patterns, possible perturbation of the moon's orbit and so on. Quite apart from being freakishly unlikely that an approach would be that close - as Douglas Adams famously said, "Space is big" - this is probably not what you are looking for.

Fortunately, gravity works on an inverse square relationship. If we say that the rogue passed by with closest approach being 10 x lunar orbit away then it's tidal effects experienced on Earth will only be one-tenth that of the moon. The object is 10 times more massive, but the gravitational attraction is 100 times weaker at that distance (about 12-13 light-seconds). These effects would be somewhat noticeable, for example unusually high/low tides, but not catastrophic.

Increase the distance of closest approach by another order of magnitude and make closest approach about two light minutes away - still freakishly close for an interstellar object - and the effects from a single close encounter will be 0.1% as strong as lunar tidal effects. Scientists will be able to measure the effect and amateur telescopes will get a good look at it, but there will be no perceptible difference for people or other lifeforms on Earth.
Considering that the solar system is a multi-body system, we are sure it is a chaotic system in the long run. For the solar system, its [Lyapunov time](https://en.wikipedia.org/wiki/Lyapunov_time) is 5 million years.

This means that the effect of any perturbation on the system cannot really be reliably predicted past 5 million years. Plus what are you going to do with the Oort cloud bodies disturbed by the passage of the planet and that will likely end up showering the inner solar system?

For sure any body which can shoot through the solar system will need to have at least the sun escape velocity.

The additional problem is that landing on something moving that fast is going to be tricky: first you need to reach that velocity to be able to land, and then slow down to get back on Earth. Considering that the Apollo mission had to struggle being in the range of 11 km/s, you can see the challenge you are up to.