The exterior of a TARDIS is made up of a
Chrono
plasmic Shell composed of atoms
of validium, mercury, and inorganic polymers.
It exists as a
Real World Complex
Space-Time Event that is
Time
Mapped onto the interior using the
Real World
Logic Gearing, made up of
Complex
Block-Transfer equations. As such, the shell holds the dimensional threshold that connects to
the Micro-Universe
within.
The
Chrono
plasmic Shell is driven by the Chameleon Circuit. The default state for the shell of a
Type 15-52
TARDIS is a silver-grey metallic cube
3 meters on each side.
For a later Types the exterior is a white domed topped cylinder 2 meters tall and 1.2 meters in diameter.
The Exo-Shell's mass is distributed over the first 5 dimensions.
This means that, from a 3 dimensional point of view, the outer shell has variable mass
that tends to fluctuate over time. Thus the density of the shell tends to be different every time it is measured.
The shell can reach a density of up to 2,300 grams per cubic centimetre. It is possible to use the chameleon circuit to create a small cubbyhole compartment to store a spare TARDIS Key. The specific Type of a TARDIS can be determined by checking the molecular patina of its Shell.
The exterior does emit low levels of Tachyons, which can be detected using the proper equipment. The telepathic circuits create a field that makes people passing by the TARDIS more likely not to notice it. This field is only effective on people who aren't looking for something unusual. The internal energy of a TARDIS can be diverted to the outer shell, delivering a massive blast of energy anything it comes in contact with.
Since the shell's atoms are merely Block-Transfer constructs, the exterior of a TARDIS is immune to all attacks and is 'technically indestructible' (even without the Force Field).
The exterior is also not radar-reflective,
making it invisible on some tracking systems.
However,
spatial elements
(such as atoms and molecules) from each destination will often become embedded in the Shell. While this does not weaken the Shell,
it does create a time thickening effect on the underlying cosmological foam-structures of space. This slight gravitational change can be measured and used to detect the operation of materialization point of a TARDIS. Exposure to Zero Time (not developed until the Time War) removes the spatial elements from a TARDIS's Shell.
While the entire Exo-Shell can be transmatted or transducted without difficulty, the Interior cannot be penetrated by transmat beams unless the beam's power has been increased by a factor of 50 million. Certain gravitic, thermal, telepathic, and radioactive forces will be transferred to the inside (if the force fields are deactivated).
Most of these forces (including gravity) can be negated by isolating this exterior shell from the interior Micro-Universe. However,
a massive exterior gravitational force can warp the inner dimensions against the outer shell until a catastrophic schism develops in the outer shell. If this were to occur the interior of a TARDIS would break out and be randomly mapped into the area. This situation can be rectified with a sufficiently complex and powerful gravitational field that can draw the interior segments together, allowing the Endo and Exo-Shells to reform.