Maps established the theory of subspace (or warp) highways. In certain regions, a spaceship can fly at a multiple of the speed that corresponds to the current warp factor. In The Original Series, warp factor6 was established as the common speed of the USS Enterprise NCC-1701. In some cases, the starship traveled at warp7 or above, but with risk of damaging the ship or the engines. Warp8 in The Original Series was the "never exceed" speed for the hulls and engines of Constitution-class starships. Warp6 was the maximum safe cruising speed for that vessel class. Later on, a prequel series titled Star Trek: Enterprise describes the warp engine technology as a "Gravimetric Field Displacement Manifold" (Commander Tucker's tour, "Cold Front") and describes the device as being powered by a matter/anti-matter reaction which powers the two separate nacelles (one on each side of the ship) to create a displacement field. Enterprise, set in 2151 and onward, follows the voyages of the first human ship capable of traveling at warp factor 5.2, which under the old warp table formula (the cube of the warp factor times the speed of light), is about 140 times the speed of light (i.e., 5.2 cubed). In the series pilot episode "Broken Bow", Capt. Archer equates warp 4.5 to "Neptune and back [from Earth] in six minutes" (which would correspond to a distance of 547 light-minutes or 66 au, consistent with Neptune's being a minimum of 29 au distant from Earth). Modified warp scale (The Next Generation, Deep Space Nine, Voyager, and Picard) For Star Trek: The Next Generation and the subsequent series, Star Trek artist Michael Okuda drew up a new warp scale and devised a formula based on the original one but with an important difference: In the half-open interval from 9to 10, the exponentw increases toward infinity. Thus, in the Okuda scale, warp velocities approach warp 10 asymptotically. According to the Star Trek: The Next Generation Technical Manual there is no exact formula for this interval because the quoted velocities are based on a hand-drawn curve; what can be said is that at velocities greater than warp 9, the form of the warp function changes because of an increase in the exponent of the warp factorw. Due to the resultant increase in the derivative, even minor changes in the warp factor eventually correspond to a greater than exponential change in velocity. Warp factor 10