The UFO (aka Flying Saucer or Mystery Ship) is a ship that will randomly pop up. If a player hits it, they will receive a random number of either 50, 100, 150, or 200 points. However, if they perform the secret of firing 22 shots, then hitting it on the 23rd shot, then on the 15th shot thereafter, they will receive 300 points every time. It is present in almost every Space Invaders game, official or otherwise.
Other characteristics / Other games
In the Atari 2600 version of Space Invaders, the saucer is called the Command Alien Ship. It has a set point value of 200 points, except for some game variations where it is only worth 100 points.
In Space Invaders Extreme, UFOs gain different abilities depending on color, like cyan being able to deploy new invaders like Part II, or blue saucers firing lasers. A new variant that initiates a Round is present.
Bootlegs and Inspired Games
In Cosmic Monsters, the saucer hides behind a hangar. If the player does not shoot it quickly enough after it leaves the hangar, then it will explode and add an entire row of monsters to their formation.
In Space Fever Color, the ship will turn into a smaller, faster-moving vehicle that is worth several hundred more points if shot. It usually flees in the opposite direction from where the first saucer came from.
In Astro Battles, the original saucer and a smaller, different-shaped one appear, as well as Gorf robots. (Note: this is from the original arcade version of Gorf; the Atari 2600 version did not have any saucers during this mini-game.)
In the More Invaders! game on the All Good Things Vectrex cart, the saucer will shoot at the player. This also happens on the Space Invaders Duel PC game, but if the saucer's shot hits a bunker then it makes a large explosion, scattering debris everywhere.
In the Cartoonic Space Invaders game there is a same sprite which it only flies from right to left which is true. When hit, the scream is played followed by the sound of a police siren. This is also true.