Editing Tidal acceleration (section)

== Tidal deceleration ==
[[File:tidal_acceleration_principle.svg|thumb|300px|In tidal acceleration (1), a satellite orbits in the same direction as (but slower than) its parent body's rotation. The nearer tidal bulge (red) attracts the satellite more than the farther bulge (blue), imparting a net positive force (dotted arrows showing forces resolved into their components) in the direction of orbit, lifting it into a higher orbit.<br />In tidal deceleration (2) with the rotation reversed, the net force opposes the direction of orbit, lowering it.]]
This comes in two varieties:
{{ordered list|1= ''Fast satellites'': Some inner moons of the [[giant planet]]s and [[Phobos (moon)|Phobos]] orbit within the [[synchronous orbit]] radius so that their orbital period is shorter than their planet's rotation. In other words, they orbit their planet faster than the planet rotates. In this case the tidal bulges raised by the moon on their planet lag behind the moon, and act to ''decelerate'' it in its orbit. The net effect is a decay of that moon's orbit as it gradually spirals towards the planet. The planet's rotation also speeds up slightly in the process. In the distant future these moons will strike the planet or cross within their [[Roche limit]] and be tidally disrupted into fragments. However, all such moons in the Solar System are very small bodies and the tidal bulges raised by them on the planet are also small, so the effect is usually weak and the orbit decays slowly. The moons affected are:
* ''Around [[Mars]]'': [[Phobos (moon)|Phobos]]
* ''Around [[Jupiter]]'': [[Metis (moon)|Metis]] and [[Adrastea (moon)|Adrastea]]
* ''Around [[Saturn]]'': none, except for the [[Saturn's rings|ring particles]] (like Jupiter, Saturn is a very rapid rotator but has no satellites close enough)
* ''Around [[Uranus]]'': [[Cordelia (moon)|Cordelia]], [[Ophelia (moon)|Ophelia]], [[Bianca (moon)|Bianca]], [[Cressida (moon)|Cressida]], [[Desdemona (moon)|Desdemona]], [[Juliet (moon)|Juliet]], [[Portia (moon)|Portia]], [[Rosalind (moon)|Rosalind]], [[Cupid (moon)|Cupid]], [[Belinda (moon)|Belinda]], and [[Perdita (moon)|Perdita]]
* ''Around [[Neptune]]'': [[Naiad (moon)|Naiad]], [[Thalassa (moon)|Thalassa]], [[Despina (moon)|Despina]], [[Galatea (moon)|Galatea]] and [[Larissa (moon)|Larissa]]

Some hypothesize that after the Sun becomes a red giant, its surface rotation will be much slower and it will cause tidal deceleration of any remaining planets.<ref name=Schroeder>
{{Cite journal|last1=Schröder|first1=K.-P.|last2=Smith|first2=R.C.|title=Distant future of the Sun and Earth revisited|journal=[[Monthly Notices of the Royal Astronomical Society]]|volume=386|issue=1|pages=155–163|date=2008|doi=10.1111/j.1365-2966.2008.13022.x|doi-access=free |arxiv=0801.4031|bibcode=2008MNRAS.386..155S|s2cid=10073988 }} See also {{Cite news|last=Palmer|first=J.|title=Hope dims that Earth will survive Sun's death|url=https://www.newscientist.com/article/dn13369-hope-dims-that-earth-will-survive-suns-death.html|work=[[New Scientist]]|date=2008|access-date=2008-03-24
}}</ref>|2= ''Retrograde satellites'': All retrograde satellites experience tidal deceleration to some degree because their orbital motion and their planet's rotation are in opposite directions, causing restoring forces from their tidal bulges. A difference to the previous "fast satellite" case here is that the planet's rotation is also slowed down rather than sped up (angular momentum is still conserved because in such a case the values for the planet's rotation and the moon's revolution have opposite signs). The only satellite in the Solar System for which this effect is non-negligible is Neptune's moon [[Triton (moon)|Triton]]. All the other retrograde satellites are on distant orbits and tidal forces between them and the planet are negligible.
}}
[[Mercury (planet)|Mercury]] and [[Venus]] are believed to have no satellites chiefly because any hypothetical satellite would have suffered deceleration long ago and crashed into the planets due to the very slow rotation speeds of both planets; in addition, Venus also has retrograde rotation.