HT Games
History
In the original "StarCraft," released in 1998, the Science Vessel was a flying detector unit and ability caster for the Terran race. Their skill set included Defensive Matrix, EMP Shockwave and Irradiate. This unit was helpful for detecting invisible units, defending attacks and harassing the enemy. In "StarCraft 2," some of the Science Vessel's skills were moved to other units, so while the unit is gone, some of its functionality remains.
Reappearance
The Science Vessel appears as an unlockable unit in the "StarCraft 2: Wings of Liberty" campaign. "StarCraft 2" is set up in three separate games, with the Terran race being the race of the first game, so as you progress through the campaign and earn points, you can eventually use this vessel and have access to its skills. Science Vessels are built from the Starport with an attached Tech Lab.
Skills
Science Vessels in "StarCraft 2" have one fewer abilities than their original "StarCraft" counterparts, and only one skill from the original game. However, they still come in useful, especially in the later missions. Their skills are Nano-Repair, a skill that quickly repairs nearby mechanical units, and Irradiate, a skill that does damage to biological units over time.
Strategy
Both skills are equally useful depending on what build you are using and what enemy you are facing. When you end up facing the Zerg, as is inevitable, Irradiate is a powerful skill to do damage to numerous enemies at the same time. When you have a large number of expensive mechanical units, keep your Science Vessels behind them as they attack or defend to keep them alive. They can use their repair skill even while in battle, so your mechanical units will enjoy a steady stream of recovering hit points as they fight. However, each vessel can only repair one unit at a time, so prepare to build more than one or two as you get ready for large fights.