Given the strength of Trek weaponry, one photon would do. I remember reading somewhere that a standard photon torpedo carried 1.5 kg of antimatter on board, which is yield-equivalent to a Tsar bomba (almost 60 megaton). However, phasers could have been easily used for close air support, even from orbit - AFAIR Enterprise-D used them once as surgical tools for geoengineering.
Most episodes seem to script around the need for air support, but I remember one case where it was at least acknowledged on-screen - one DS9 episode had a ground assault at an enemy position to destroy a under-repair gateway in it, and it was said early on that the surrounding building was strong enough that a torpedo strike from orbit would not necessarily destroy the target.
Edit:
My personal favorite case of missed opportunity is the use of transporters to beam armed torpedoes directly onto enemy ships. It was used once in a Voyager episode. It's an obvious, but sadly plot-killing tactic, that hasn't been sufficiently hand-waved away in the lore, so one has to wonder why nobody uses it.
The StarGate series actually took that option seriously (the series was much smarter when it came to combat). In it, to the dismay of the species that provided humans with transporters, human ships often beamed nuclear warheads onto unsuspecting enemy vessels, achieving seemingly impossible victories.
"My personal favorite case of missed opportunity is the use of transporters to beam armed torpedoes directly onto enemy ships. It was used once in a Voyager episode. It's an obvious, but sadly plot-killing tactic, that hasn't been sufficiently hand-waved away in the lore, so one has to wonder why nobody uses it."
Akshually...
Transporters cannot penetrate shield, usually either incoming or outgoing. Usually.
> Transporters cannot penetrate shield, usually either incoming or outgoing. Usually.
Except when they can, due to frequency matching, "technology generation" mismatch between shields and transporters, and countless of other one-off workarounds that happened. Think of every episode in which the Enterprise/Defiant/Voyager was boarded while in combat.
But even when considering the regular case of shields blocking incoming and outgoing transport, there's plenty of opportunities to use transporters offensively. They could be used as surprise first strike weapon to initiate hostilities by immediately taking enemy ship out. They could be used to finish off an enemy in a one-to-one fight. They could be used in coordinated fashion in fleet battles, where whenever an enemy ship loses shields, one ship drops their own shields and transports some photons over while other ships cover the vulnerable friend from immediate fire. Etc.
All in all, there are plenty of opportunities for offensive transporter use which were never explored in the series.
Akshually... it depends on the writer, series and episode.
The shields have the fatal flaw that anything operating at the same frequency can go right through them, including a transporter beam. That's come up as a plot point more than once.
Most episodes seem to script around the need for air support, but I remember one case where it was at least acknowledged on-screen - one DS9 episode had a ground assault at an enemy position to destroy a under-repair gateway in it, and it was said early on that the surrounding building was strong enough that a torpedo strike from orbit would not necessarily destroy the target.
Edit:
My personal favorite case of missed opportunity is the use of transporters to beam armed torpedoes directly onto enemy ships. It was used once in a Voyager episode. It's an obvious, but sadly plot-killing tactic, that hasn't been sufficiently hand-waved away in the lore, so one has to wonder why nobody uses it.
The StarGate series actually took that option seriously (the series was much smarter when it came to combat). In it, to the dismay of the species that provided humans with transporters, human ships often beamed nuclear warheads onto unsuspecting enemy vessels, achieving seemingly impossible victories.