The Ultra Atom is a rarely-encountered, super-charged version of the much more common atom balls found throughout the games.
Unlike the small blue atom balls, the larger Ultra Atom glows red or orange, with yellow or white lines encircling it, depending on the game or port of the game.
When collected, the Ultra Atom will instantly repair all damage to Jim's Super Suit, recharging it to 100%, and therefore Jim to full health.
In Earthworm Jim 2, if Jim has already collected a Chip Butty and gained 200% suit power, the Ultra Atom will charge the Super Suit back up to 200%, if it has subsequently lost any power between collecting the Chip Butty and Ultra Atom. Common atom balls do not have this effect, and will only begin repairing the Suit once it drops below 100% again.