ScienceDaily
Tidal locking
When a body's spin has been dragged into matching its orbit, so one face never turns away.
Tides raised by a larger body bulge the smaller one, and friction in that bulge bleeds off rotational energy until the spin period equals the orbital one. The Moon reached that state long ago, which is why we see one hemisphere from here and why the far side was unknown until 1959. Most large moons in the solar system are locked, as are many exoplanets close to their stars — and a locked planet has a permanent day side and night side, which is a much harder place to imagine weather on.
Where it has come up lately
Related terms
Gravity assist
Stealing a fraction of a planet's orbital momentum to go faster for free.
Regolith
The layer of broken rock and dust covering an airless world — and a serious engineering problem.
Sample return
Bringing a piece of somewhere else back to a laboratory on Earth.
Opposition and conjunction
The two alignments that decide when a planet is worth looking at and when it is unreachable.
Questions
What is Tidal locking?
When a body's spin has been dragged into matching its orbit, so one face never turns away.
Why does it matter?
The Moon reached that state long ago, which is why we see one hemisphere from here and why the far side was unknown until 1959. Most large moons in the solar system are locked, as are many exoplanets close to their stars — and a locked planet has a permanent day side and night side, which is a much harder place to imagine weather on.