Skip to main content
Astronomy

You are not logged in. Your edit will be placed in a queue until it is peer reviewed.

We welcome edits that make the post easier to understand and more valuable for readers. Because community members review edits, please try to make the post substantially better than how you found it, for example, by fixing grammar or adding additional resources and hyperlinks.

16
  • 5
    $\begingroup$ Note that 70,000 km also happens to be Jupiter's radius, so the planets would need to be touching. (And there's not a "the" Roche limit; it depends on the density of the secondary object, that is, Earth in this example). $\endgroup$
    – hmakholm left over Monica
    Commented May 27, 2019 at 11:03
  • 5
    $\begingroup$ People aren't held together by gravitational forces, so being inside the Roche limit won't pull them apart like it would for a planet. In that regard, people on the surface do behave differently than rocks - a pile of gravel will separate into individual pebbles as it gets sucked into Jupiter's gravitational well, but a person will remain intact. $\endgroup$
    – Nuclear Hoagie
    Commented May 27, 2019 at 13:44
  • 17
    $\begingroup$ @NuclearWang But... people are held to the earth purely by gravitational forces. Being within the roche limit would not pull humans apart but we would still act like those pebbles in the sense that individual humans would not be stuck to the earth by gravity any longer. $\endgroup$
    – J...
    Commented May 27, 2019 at 13:59
  • 7
    $\begingroup$ At just inside the Roche limit, the math in @uhoh's answer shows that loose objects aren't literally ripped off the surface. I think the mechanism is more gradual even for a totally non-rigid aggregate of gravel: with its own gravity being unopposed in the other directions, it would elongate in the tidal-force direction. This puts the ends farther and farther from the centre of mass, and increases the distance for the gravity gradient. This eventually leads to it being torn apart, but one quick pass wouldn't rip loose objects off the surface (esp for a stiff / viscous object like Earth) $\endgroup$
    – Peter Cordes
    Commented May 28, 2019 at 4:10
  • 5
    $\begingroup$ @HenningMakholm: I think this answer accidentally took Jupiter's actual radius as its Roche limit!, because the Wikipedia page has a table of density and radius for objects in our solar system at the top of the section for Roche limits for pairs of bodies. The actual rigid-body Roche limit (where objects are pulled off the surface by tidal forces) is $R_m * (2 \rho_M / \rho_m) ^ {1/3}$ = 71493000 * (2 * 1326/5513)^(1/3) = 56 018 km using the "fully rigid-satellite" formula from Wikipedia. $\endgroup$
    – Peter Cordes
    Commented May 28, 2019 at 4:35