Space Tethers

Electrodynamic tether propulsion, using the free resources of sunlight and the Earth's magnetic field and ionosphere, could be implemented almost immediately with significant benefit to human explorers. As described in Chapter 15, fairly modest length tethers could be fielded in support of the International Space Station to provide reliable, low-cost reboost and orbit-maintenance propulsion. They can be used on virtually any system that is required to be in low Earth orbit for an extended period of time—from the massive International Space Station, to the more modest orbiting hotels proposed by entrepreneurs such as Bigelow Aerospace.

Momentum Exchange Electrodynamic Reboost tether systems may take much longer to achieve than their strictly electrodynamic cousins, but they are no less revolutionary. A network of MXER tethers surrounding the Earth, the Moon, and eventually Mars, may provide a reliable, low-cost and continuously operating transportation system for a robust and expanding human civilization. By initially implementing the system in the Earth—Moon system, human explorers will have the ability to get supplies, materials, and finished products to and from the lunar surface and Earth orbit—regularly, and at low cost.

The challenges to human exploration are significantly greater than those facing the precursor robotic explorers due simply to the complex and massive infrastructure needed to sustain human life in space. The abundant resources of space, fortuitously provided by Mother Nature, appear to offer us ways to reduce the burden we have assumed by carrying all our supplies with us as we leave the home planet. Unfortunately, we have not yet developed the technologies required to harness these resources to the level that they can be readily used without significant risk. Unless we begin to try, however, they will never be available! Any viable, long-term and sustainable plan for human exploration must begin to use solar-system resources or is surely doomed to failure.

DEFI

NOiN EAt&TH

Nor the comet that came unannounced out of the north flaring in heaven, Nor the strange huge meteor-procession dazzling and clear shooting over our heads,

(A moment, a moment long it sail'd its halls of unearthly light over our heads, Then departed, dropt in the night, and was gone;)

Walt Whitman, from Year of Meteors

NEAR misses have happened many times in Earth's long history. A fragment of asteroid or an errant comet approaches our world closely, skips through the upper atmosphere and back into space leaving a noiseless contrail, or bathes the evening sky in the shimmer of light reflected from its tail. But sometimes, a near miss or deflection shot morphs into a direct hit.

In the beginning, four and a half billion years ago, such impacts were a good thing. In those days, comet swarms criss-crossed the infant solar system. When they impacted a planet, lots of water, methane, and ammonia were released. Earth's oceans and atmospheric ingredients, necessary for the formation of life, are thought to have been deposited in this manner. It's not impossible that early life was actually transferred from Mars to Earth in the aftermath of an impact on the low-gravity Red Planet.

As the solar system evolved, potential impactors of the inner planets became rarer. And instead of being beneficial for terrestrial life forms, the impacts that occurred were destroyers rather than creators.

The fossil record reveals that many mass extinctions disrupted the terrestrial biosphere during the last billion years or so. The most famous of these, which occurred about 65 million years ago, ended the reign of the dinosaurs. Many believe that the impact of a celestial body in the 10-kilometer range is at least partially responsible for the fall of the dinosaurs and the rise of the mammals.

No humans existed on Earth at the time of the Great Fall, but we can imagine how the thunder lizards or our crude, early mammal ancestors might have reacted as the huge space mountain descended toward ground zero, in what is now the Yucatan.

Perhaps they had been aware of subtle shifts in skylight for a few days prior to impact, if the object was a comet. Perhaps the normal cycles of sleep and the hunt had been disrupted by these strange changes in the sky.

If the meteorite was instead of asteroidal origin, the first warning would have been flames in the sky and sonic booms. Perhaps some creatures craned their necks toward the intruder, perhaps others cowered in fear. It made no difference.

The Cretaceous-Tertiary (K/T) meteorite struck the Earth with enormous kinetic energy. To get some idea of the effects on Earth's environment, consider that the largest hydrogen bombs in the Cold War arsenals of the super powers had about 1,000 times the explosive yield of the weapons that destroyed Hiroshima and Nagasaki in 1945. The energy released in the Yucatan by the K/T impact had something like half-a-million times the yield of the largest H-bombs.

Such a large explosion, in one place and time, was certainly not a good thing for Earth's ecology. A huge fireball would have instantly extinguished all or most life within thousands of kilometers of ground zero. An enormous mushroom cloud would have risen to the stratosphere and ultimately dispersed to enshroud the entire planet. Firestorms would dart across the landscape; giant tsunamis would haunt the oceans and crash against coastal regions. It's not impossible that other seismic events— volcanoes and earthquakes—would be triggered globally by the impact.

Soon, Earth temperatures would decline as stratospheric dust blocked the Sun. Vegetation would die back, followed by the huge herbivores. Deprived of their food source, carnivores at the top of the food chain would soon follow them into extinction.

Among major land animals, only primitive mammals (who probably could hibernate in their burrows and sleep away the bad times) and flying, feathered dinosaurs—the ancestors of today's birds—survived the end of the Cretaceous era. Ultimately, their descendants would evolve and radiate to fill ecological niches left by the demise of the larger creatures.

Certainly, climate change, volcanism, and perhaps other factors contributed to the dinosaurs' extinction. But the enormous, rude visitor from space certainly helped to push them along.

Fine, you say, but all this occurred 65 million years ago. In all that time, the solar system has been cleared of at least some of the asteroids and comets that might threaten the Earth.

True, but lots of dangerous space rocks still exist in solar space. From time-to-time, they still whack the Earth. The last impact of consequence occurred in 1908, in Tunguska, a sparsely populated part of Siberia. With a diameter of about 100 meters, the Tunguska object stuck the Earth with the force of a large hydrogen bomb. If its trajectory had been slightly different, casualties would be numbered in the millions.

In 1998, john Remo published an inventory of near-Earth objects (NEOs) capable of some day impacting the Earth. Based on telescopic surveys, he estimates that there are about 20 that could cause mass-extinction events—those with diameters in excess of 5 kilometers. There are approximately 400 in the 2-kilometer size range, about 6,000 that are approximately 0.5 kilometer in size, and about 100,000 0.1-kilometer Tunguska-sized city killers.

Based on these numbers, which do not reflect random comet storms from the Oort Cloud, we can expect to lose a city once every few centuries and experience continent-scale devastation every couple of millennia. At million-year intervals, impacts with global consequences will occur and mass extinctions will be experienced at intervals of tens of millions of years.

Our planet seems to whirl through a cosmic shooting gallery. Is there anything we can do?

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