Asteroid

Asteroids are tiny-large floating Stone deposits in the sky, which house multiple Hardmode ores (mainly Adamantite/Titanium), and very rarely, a Pre Hardmode ore may be in the mix.

These giant rocks are helpful for advancing in Hardmode (once Wings are obtained, or you can just build up in hopes of finding one). However, they only spawn after one altar has been destroyed by the Pwnhammer in a message "Rocks are falling from the sky".

Butterfly ore sometimes can merge with an Asteroid, so stay on the lookout for Asteroids near butterfly ore.

Asteroids have their own gravity, and you will be attracted to them with only 1/4 of the gravity if you get close enough.

Very rarely, an asteroid may have a dead tree on it.

In Outer Space
Asteroids can be used for mining resources to craft new upgrades for your Rocket/Frigate.

Very rarely, albeit more common than on Terraland/Earth/the new world (see Infinity Void), dead trees may be on the asteroids for Wood.

Asteroid Fields
Asteroid fields such as the Asteroid Belt, Jovians, Kuiper Belt, Comet Network, etc. will have multiple asteroids. They will also have comets (Comet asteroid fields, such as Oort Cloud and Comet Network, have multiple of these) which have more resources, such as Wood when no trees are present, Ice, Chlorophyte, Luminite, and others.

Asteroid Merging
If you wait long enough in outer space, especially in the Battle Mode (see Frigate), asteroids might attract and merge. The larger asteroid will add the amount of kilometers the smaller one plus its original size, and expand. The smaller asteroid will be destroyed into Iron and Copper particles which can be extracted to make ore. Sometimes, asteroids might become large enough to become planetary moons, which will attract them to Jupiter immediately if this happens.