Centauri Battlecruisers
The largest Centauri vessel seen in the series is referred to as a "battlecruiser" by characters.  This vessel represents the peak of technological advancement made during hundreds of years of space faring by the Centauri.  It is one of the most advanced and powerful of the front-line warships used by any of the younger races.

Centauri battlecruisers are easily capable of forming their own vortex into hyperspace, and are often used to ferry ambassadors or rulers, such as the Emperor in The Coming of Shadows.  It is unknown if these vessels can carry starfighters, but they are often escorted by fighters and smaller warships.

Size
Firepower
This image is from The Coming of Shadows.  The Centauri Emperor is being transported to Babylon 5 onboard his battlecruiser, escorted by several medium cruisers and starfighters.  The medium cruisers are surrounding the battlecruiser, three in front, and three behind.  The fleet is in a small space, as all of the ships come through the jumpgate together.  The ships are much closer together than the camera is to any of them.

The medium cruisers measure 25, 34, 30, 30, 28, and 36 pixels wide in order from left to right, top to bottom, for an average of 30.5 pixels in width of each vessel.  The battlecruiser measures 29 pixels wide.  If a battlecruiser is wider than a medium cruiser, it can't be by much, as it is narrower than the average of the medium cruisers in this slightly off-center image.
In this image from the same episode, a medium cruiser is almost directly behind and below the battlecruiser from the camera's point of view.  The medium cruiser is 56 pixels wide, and the battlecruiser is 83 pixels wide.  Since the medium cruiser is more distant, the heavy battlecruiser is no more than 150% as wide as the medium cruiser.
This is another image from the same episode, as the Centauri fleet is leaving Babylon 5.  The battlecruiser is measured at almost exactly twice the width of the medium cruiser behind it and to the left (from the camera's point of view).
Here, the fleet is immediately beside Babylon 5.  The width of the front of the station's rotating section is 68 pixels, and the battlecruiser is 40 pixels wide.  The fleet is more distant than the face of the station, giving the battlecruiser a lower limit of approximately 196 meters in width.

The widest part of the station is 126 pixels wide, and is more distant than the fleet, giving the battlecruiser an upper limit of approximately 235 meters in width.
This image is from The Fall of Night.  The view is not from directly overhead, but does provide an approximate representation of the ship's length/width ratio.  The warship is 184 pixels wide and 211 pixels long.  Using the limits calculated above, this class warship is at least 225, but no more than 270 meters in length.
    This image was taken during the bombing of Narn.  The window Londo Mollari was staring out is approximately 1 meter wide, and is measured here at 14 pixels.  The entire observation dome is 202 pixels, or approximately 14-15 meters diagonally.
    This is the same ship from a few frames earlier than the image above.  The observation dome is 41 pixels in its diagonal measurement, and the entire image is 358 pixels wide.  Thus, the portion of the battlecruiser seen in this image is on the order of 125 meters wide.  The image does not show the entire battlecruiser, but this is a significant fraction of the vessel's total size.
    This image is from The Coming of Shadows, and shows the vessel that carried the Centauri Emperor.  The front of the rotating section of Babylon 5 is approximately 333 meters in diameter, and measures here at 54 pixels.  The Centauri battlecruiser measures at 58 pixels diagonally.  The vessel is closer to the camera than the station, so it has an upper limit of 360 meters in this diagonal measurement, which is similar to the ship's length and width.
These images are also from The Fall of Night.  When the Centauri vessel attacked Babylon 5, the station's defending Starfuries approached and returned fire at point-blank range.  In these images, Starfuries are in close proximity to the attacking Centauri vessel; so close in fact, that they cast shadows on the warship's hull.  This provides support for the upper and lower limits calculated above.

Size Overview:
Length 225-275 meters
Width 190-240 meters


 
    This class of Centauri battlecruiser is apparently the most powerful warship in the Centauri war machine.  This class was chosen to bomb the Narn homeworld, to ferry the Emperor to Babylon 5, and was stated to be the largest battlecruiser by Reefa.  Each vessel carries at least 8 heavy twin turrets, 4 on the dorsal surface, and 4 on the ventral surface.
    Each turret has a wide field of view, thus a wide firing arc.  They are capable of high rates of fire, delivering staggering firepower per bolt of energy.  In The Fall of Night, one of these vessels was able to attack Babylon 5, a squadron of Starfuries, and a Narn heavy cruiser simultaneously on equal terms.  The battlecruiser's weapons severed one of the station's forward stabilizers.
    To avoid the heavy casualties that would be suffered in invading the Narn homeworld, the Centauri decided to bombard the planet from orbit until the Narns surrendered.  This bombardment involved a dozen Centauri battlecruisers and support ships, and lasted for at least 4 days, causing large scale destruction worldwide.  We know that the Centauri have conventional weapons capable of ground bombardment, as seen in In the Beginning.  Furthermore, the Narns and Drazi bombarded Centauri Prime from orbit with conventional energy weapons.  Given the uncomfortable "nuclear winter" effect of an asteroidal bombardment, the fact that mass drivers are illegal, and the fact that the Centauri intended to occupy the Narn homeworld, bombardment by mass drivers must present significant advantages over bombardment by conventional weapons.

However, several disadvantages also present themselves.  Since the planet's atmospheric conditions are likely to change after a large-scale orbital bombardment, plant life likely declines rapidly, followed by herbivores, and finally carnivores.  The Narns would have great difficulty producing significant harvest, and would soon have equal difficulty raising livestock.  Thus, the entire Narn population would be essentially dependent on the Centauri to provide them with food.

The Centauri would also experience the harsh, dusty atmosphere during the occupation.  With little sunlight, the entire planet would be dark and cold.  With the entire planet now a wasteland, the Centauri would have difficulty producing any profit from this occupation.  It is unlikely that bombardment by energy weapons and explosives would result in these disadvantages.  Thus, bombardment by mass drivers must be much more efficient than bombardment by conventional weapons, causing much more destruction per time period than the fleet could otherwise manage.



The Centauri make plans to bomb the Narn homeworld.  Note that 12 heavy battlecruisers were used, the intent to "flatten" the planet, the intent to occupy the planet, and the bombing was projected to last for days.

ISN's report of the bombing.  Note that the bombing lasted at least 4 days.  Casualties are reported in the "hundreds of thousands."
    In this image, the asteroid is 124 pixels long and 91 pixels tall, for an average diameter of 107.5 pixels.  This is approximately 55% of the diameter of the lateral sections of the mass driver mechanism.

 
This image shows an asteroid that has just been launched from a mass driver.  The entire warship is not seen in this image, but half of the vessel's width is approximately 300 pixels.  The mass driver mechanism is 64 pixels tall and 131 pixels wide.  Therefore, the mass driver is on the order of 23 meters in diameter and 47 meters wide (averaging the ship's upper and lower limits in width).  If the asteroid in the image above this one has a diameter 55% of that of the mass driver, then it is on the order of 13 meters in diameter.  The asteroid in this image is 33 pixels wide and 30 pixels tall.  This gives it an average of 31.5 pixels in diameter, thus it is approximately 11 meters in diameter.  Since this asteroid has already been launched, this is a lower limit, but close to the actual value as the asteroid left the mass driver approximately 1/10 second ago.

 
This image sets a strict upper limit on the size of this asteroid.  A fighter is on the far side of the warship, and appears to be a significant fraction of the size of the asteroid.

Also note in this image, what appears to be a rack of some kind in close proximity to the mass driver device.  It seems likely that this is used to store asteroids until they can be launched.  It appears that this warship is currently launching it's last available asteroid, until more can be gathered from nearby space.  In The Long Night of Londo Mollari, G'Kar yells "You plucked asteroids out of near orbit and sent them plunging down onto our cities!"

Assuming the asteroids were 15 meters in diameter and were mostly iron, they had a mass of approximately 14 million kilograms.  The mass drivers only accelerated the asteroids enough to direct their path, so that the planet's atmosphere would not deflect them.  If an asteroid was launched at a severe angle, it may stray off course when it meets resistance from the atmosphere, and miss the target completely.  These asteroids were launched at an almost perfect right angle to the surface of the planet.  One asteroid that had just been launched passed right over the battlecruiser carrying Londo Mollari.  This asteroid took no less than 8 frames to travel the length of the ship, setting an upper limit of around 1200 meters per second for initial velocity.

A first glance at the scene appears to demonstrate the asteroids traveling at a much higher rate, as they appear to be visible during their entire journey to the planet's surface.  However, it is impossible for the naked eye to observe objects of this size from such great distances.  Thus, the asteroids seen heating up in the atmosphere shortly after a launch must have been launched a few moments before in a similar direction.

Viewing Distance Calculator
The primary factor in the final velocity of these asteroids is the planet's gravity.  Since Centauri and Humans can walk around comfortably on the surface of Narn, it can be assumed that it has comparable gravity to Earth.  In this case, the asteroids would be traveling at approximately the planet's escape velocity when they hit the ground.  For Earth, this is approximately 11,000 meters per second.  Since the asteroids had already been accelerated to around 1 km/sec, the asteroids were traveling at around 12,000 meters per second when they hit the ground.  The total energy delivered would be approximately 1000 terajoules, or about 240 kilotons.  For comparison, the bomb dropped on Hiroshima in the final days of World War II had a yield of approximately 15 kilotons, and leveled 60,000 buildings.

These asteroids were launched at a slow rate by each ship - no more than once every three seconds.  As logic suggests that the mass drivers are more destructive than conventional weapons, the upper limit on a battlecruiser's conventional firepower is on the order of 350 terawatts.

NASA reports that an impact on the order of a million megatons will cause a drastic change in a planet's climate, which causes massive loss of plant life and starvation on a global scale.  An impact on the order of 100 million megatons can cause extinction of a planet's population.  Such an event occurred 65 million years ago when an asteroid impacted what is now Mexico, causing extinction of 2/3 of all animal life on Earth.
 
 
The Narns were not driven to extinction.  The actual bombing only caused several million Narn casualties.  ISN estimated "hundreds of thousands" of casualties in the fourth day of full-scale bombing.  But the bombing did result in a change in climate and massive starvation planet-wide.  If the fact that the planet was barren before the bombing is ignored, the total energy delivered could be on the order of a million megatons (1,000 gigatons).  A bombardment on this scale would cause a change in climate of the entire planet, and would kill a significant fraction of the Narn population regardless of any kind of shelter they used during the bombing itself.  If the bombing was on this scale, lasted exactly 4 days and was conducted by 12 ships, each warship had a sustained "firepower" of approximately 1010 terawatts.  This is a few times higher than the upper limit calculated above, but since the planet was barren before the bombing, and most of the Narn population survived, this estimate is somewhat generous.  Also, when G'Kar visited his home planet in a later episode, a moderate amount of sunlight easily passed through the atmosphere, and the sun was visible in the sky.

Londo tells the Narns of Reefa's guilt in planning the bombing.  It seems that only a few million Narns were killed in the 4 days of bombing and the occupation afterwards.


G'Kar discusses the post-bombing state of Narn with an associate.


Mass driver scene from start to finish.

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