With ten minutes to contact, the diving head of the beanstalk reached the upper atmosphere. It entered the ionosphere and began to feel the first effects of frictional heating. Now it was starting to slow in its descent. The long tail, way out beyond synchronous altitude, was already tugging upward to provide a colossal outward tension that would slow all downward motion. The cup that hung at the very end of the beanstalk was moving higher and higher, sling-shotting out from the first approach spiral to stretch away from Earth. Eighty-five thousand kilometers above the surface, it formed the final point of a stalk that reared steadily closer to the vertical.
Looking down from the outer cup, an observer would see the shape of the beanstalk gradually straightening beneath, moving to make a clean line that dropped endlessly away to the distant Earth. The same observer, looking far out ahead of the swinging cable, would see the ballast asteroid, still thousands of kilometers away but rapidly coming closer.
The tension in the load-bearing cable had increased by two orders of magnitude in as many hours. It was still less than the final figure for the installed beanstalk, but already the stored energy exceeded that of any fusion weapon. Longitudinal waves of compression and tension rippled constantly along the length of the load cable, transmitting balancing forces from the out-flying higher end to the downward plummet of the lower cable.
Observers in Quito had heard the crack as the head passed through supersonic speed. Now they waited for the first sight of it. Along the equator, far to the west of Tether Control, a thin line of contrail at last became visible. It spread from the speeding head of the stalk in a wake of turbulent ice crystals. The shadow formed a dark swath on the equator, neatly bisecting the globe into north and south hemispheres. There was a steady rumble like approaching thunder.
High in the Andes, Indian peasants paused in their daily work of scratching the stubborn soil, long enough to offer their prayers to the old gods of the storm. Luis Merindo watched the scopes in Tether Control and sought the same reassurances from the newer deities of aerodynamics and electronics. The head of the beanstalk was a millisecond off at the first triangulation point. How much would that become when it reached the pit? He was relieved to see an estimate from Santiago flashing up onto his display. Just a few meters. They had more than enough margin for that at the pit.
As soon as atmospheric entry was initiated, Rob’s attention moved to the temperature sensors set throughout the length of the stalk. The change in gravitational potential as the beanstalk dropped would appear partly as kinetic energy and partly as dissipated energy within the stressed interior of the cable. That stretching and flexing would appear as adiabatic heating and cooling, driving the local temperature up and down differentially along the length. A thousand degrees was the limit. With ample strength at normal temperatures, the cable would weaken drastically above a thousand. The calculation had been one of the trickiest parts of stalk design, a bewildering maze of orbital dynamics, nonlinear elasticity and thermal diffusion.
Rob was relieved to see that his estimates were on the conservative side.