The Science Vessel is an airborne detector caster unit. In the Terran army, it generally serves a support role. The Science Vessel is produced at the Starport with Control Tower add-on and it requires a pre-existing Science Facility.
It possesses three abilities. It comes with Defensive Matrix, an ability which adds 250 HP to the unit. Irradiate and EMP Shockwave can be researched at the Science Facility. Irradiate deals 250 damage over 25 seconds to biological units, and deals splash. EMP Shockwave reduces shields and energy of the affected units to 0. An energy upgrade can also be upgraded for 50 extra energy.
These are researched at the Science Facility.
The Science Vessel is considered a staple in TvZ. It provides reliable detection for the Terran's Marine, Medic forces, and supplements it with Defensive Matrix, while damaging or killing Zerg units with Irradiate. Irradiate is generally used to kill stacked Mutalisks, Lurkers, Guardians, and especially Defilers, and it is used to soften Ultralisks. Defensive Matrix can be used to break Zerg defenses. A Marine with Defensive Matrix will soak damage, while the other forces destroy the defenses. A Terran with a "Cloud" of Science Vessels is a formidable force, as Irradiate is capable of killing off almost every essential Zerg tech unit quite quickly, while losing only energy.
The Science Vessel Eraser trick can be performed with two or more Science Vessels that Irradiate each other and move over a Zerg's Drone line or over a Zerg's army. The units under the Science Vessel will take splash damage. The technique can also be used as a limited defense against Scourge, as the Scourge will take damage from the splash. 2-3 irradiated Vessels with good micro are near to impossible to kill with Scourges until radiation wears off. When using the Eraser trick, one helpful tip is to Patrol the 2 Science Vessels so you can do other things, like macroing.
The Science Vessel, being powerful and expensive, will be the target of Zerg Scourge and the Defiler's Plague. Terran players must keep their Vessels alive, and the Zerg needs to thin out the Science Vessel count. Plagued Vessels can be easily dispatched by a lone Mutalisk.
The Science Vessel sees very limited play in TvT. It has no real role, as Irradiate is useless against mechanical units, Defensive Matrix sees limited use, and EMP Shockwave is very situational. EMP Shockwave can be used to deplete the energy of Battlecruisers, so that they are unable to use their Yamato Cannon. In rare cases EMP Shockwave can be also used to support cloaked Wraiths - it will deplete the energy of a ComSat scanner.
The Science Vessel is an effective unit in TvP. Along with providing detection against Dark Templar, the Vessel's EMP is a solid counter to the Arbiter and the High Templar. EMPed Arbiters are unable to Recall or Stasis Field, and EMPed High Templar are unable to use their Psionic Storm. Science Vessel's detection ability is also very useful to help Goliaths to snipe Observers.
EMP can also be used with limited effectiveness against Protoss shields. Due to its small area of effect, and the effectiveness of Vultures, it is generally considered negligible in usefulness. The Science Vessel's EMP is better used against Arbiters.
Since Protoss buildings are also protected by shields, an EMP can instantly halve the building's health. Normally, buildings would take two Nukes to destroy, but an EMPed building is destroyed by only one direct Nuke. However, Ghosts are rarely seen in this match-up, so EMPing buildings tend to be of little use.
In the rare case that you see a Dark Archon, be aware that Feedback can kill a Science Vessel with full energy, so it is important to spend your energy often; also beware of Mind Control.