One possibility considered by the Daedalus researchers was the interstellar ramjet (Figure 6.3). Conceived in I960 by the American physicist Robert Bussard, an ideal ramjet would use an electromagnetic (EM) field to scoop up protons from the interstellar medium and "burn" them in a fusion reactor to obtain helium and energy, thus allowing the ramjet to "live off the land" in interstellar space. Although it is the only physically possible interstellar propulsion scheme capable of reaching arbitrarily relativistic velocities, there are very major technological problems associated with the proton-fusing ramjet.
First is the comparatively minor point that most EM field configurations tend to reflect interstellar ions rather than collect them, thereby functioning as excellent drag brakes. A much more significant flaw is the
spacccraft acceleration and velocity
FIGURE 6.3 A proton-fusing interstellar ramjet.
fact that proton-fusion (the reaction that powers the Sun and most stars) may forever be beyond our technological reach.
In a nuclear fusion or fission reaction, less than 1% of the reactant mass is converted into energy. But what if we could increase this energy-conversion efficiency dramatically?
In theory, this can be done by combining fuel atoms with antimatter. The matter/antimatter reaction, made famous by Star Trek, converts all of the reactant mass into energy. A matter/antimatter rocket can theoretically achieve relativistic velocities without enormous mass ratios. For more information about antimatter propulsion, please refer to Chapter 20.
But (fortunately for terrestrial life) antimatter is exceedingly rare in the present-day cosmos. Producing it in specialized nuclear accelerators, or "antimatter factories,'' is enormously expensive. Perhaps it's a good thing in this terrorist-plagued world that production of even a gram of this substance would break the world's economy.
Another antimatter problem is long-term storage. This exceedingly volatile substance combines very rapidly and explosively with normal matter. If the electromagnetic containment field ever fluctuated (as it did occasionally in Star Trek), the starship might be momentarily visible across the galaxy as an exploding star.
The final exotic possibility to be considered here is space-time warping. A favorite of the science fiction author and movie producer, the space warp would locally alter the fabric of space-time so that a spacecraft could take a superluminal shortcut and voyage between stars on short-duration journeys.
Although general relativity theory indicates that space-time can be warped by sufficiently high mass densities, electromagnetic fields, and angular momentum (spin), it is not easy to put these principles into practice unless you can collapse a star's mass—perhaps several of them. Some studies indicate that creation of a non-gravitational "warp-bubble" around a spacecraft might require more energy than exists in the universe.
Even if we learn how to create such a "singularity" in space-time, there is another small problem. No one knows how to control the trajectory (if that is the correct word) ofa spacecraft traversing some higher dimensional hyperspace.
TAU: NASA's First Interstellar Probe Study
As the Daedalus study progressed, a number of other space agencies and organizations began to express an interest in robotic interstellar travel.
Foremost among these was the NASA Jet Propulsion Laboratory (JPL) in Pasadena, California.
In 1976, mission planners realized that Pioneers 10 and 11 and Voyagers 1 and 2, although they would escape the solar system, would not survive long enough to survey the interstellar environment more that 100 AU or so from the Sun. To obtain useful data regarding the near-interstellar environment, chemical rockets and planetary gravity assists had severe limitations. A radically new propulsion system was required.
At JPL, a number of engineers and scientists banded together to consider the type ofinterstellar mission that could be reasonably launched in the time frame 2000-2050. It was determined that the best that could be hoped for was a mission to a Thousand Astronomical Units (TAU). TAU would carry a suite of scientific instruments designed to measure electromagnetic fields and particle densities at and beyond the fringe ofthe solar system.
Optical-imaging equipment could also be included. By correlating photographs of star fields between TAU's telescopes and terrestrial instruments, it was demonstrated that accurate stellar distances could be determined for very distant stars.
TAU would be a large, fast spacecraft. It would have to attain 100 kilometers per second to reach 1,000 AU from the Sun within a human working lifetime. Only two near-term propulsion systems were deemed to be technologically and politically feasible. One was the nuclear-electric rocket (Chapter 3).
An onboard fission reactor would be utilized to ionize (cesium, argon, or mercury) fuel atoms and to accelerate these atoms to a velocity of 100 kilometers per second. One reason for TAU's size was fuel requirements—about two-thirds of the launch mass would be ion fuel.
Because this fuel would be exhausted over a period of years or decades, electric-engine reliability in the deep-space environment must be very high. TAU engineers therefore investigated a number of alternatives to the nuclear-electric rocket. Interestingly, their findings were identical to those of Daedalus investigators looking for an alternative to nuclear-pulse propulsion for interstellar colonization missions.
If we construct in space the thinnest, most highly reflective and temperature tolerant possible solar sail, attach it to its payload with the sail sail cable
spacecraft acceleration and velocity direction
strongest possible cables, we can use this device to conduct interstellar missions. (As described in Chapter 13, solar sails are propelled by the force of solar photons reflected from the sail.) It's necessary to unfurl the sail as close to the Sun as possible, from an initially parabolic or hyperbolic solar orbit, to achieve a high interstellar velocity (Figure 6.4). But interstellar solar sails are slow—the best possible crossing time to the Alpha Centauri system is about 1,000 years. This is the approximate performance of a star ark propelled by thermonuclear-pulse propulsion.
Now if we downgrade our sail and structure to the best possible Earth-launched configuration projected for the first few decades of the twenty-first century, performance is degraded. But it is not impossible that such a reasonably near-term sail could approximate the performance of the TAU nuclear-electric rocket.
Extrasolar mission planners began around 1990 to consider the solar sail. They were attracted by the fact that utilizing the sail rather than a nuclear rocket would be far easier from a sociopolitical standpoint.
Also, unlike the nuclear drives, the solar sail is scalable. Early extrasolar missions with payloads in the 10- to 30-kilogram range utilizing sails less that 200 meters in radius could yield information valuable to the designers of true interstellar missions utilizing sails 100 kilometers or larger in radius.
In 1992, a team of European and American engineers and scientists began to consider a sail mission to the Sun's gravitational focus. Gravitational lenses, many of which have been discovered in intergalactic space, occur when a massive celestial object is between a more distant object and the observer.
The gravitational field of the closer object greatly amplifies the light from the more distant object. The image of the occulted object will be distorted: some intergalactic gravitational lenses have multiple images of the more distant object, appearing to form a cross in space.
Although the amplified beam from such an "Einstein cross'' continues to infinity, the focus begins a distance from the closer object. If one disregards solar coronal effects, which may push the gravitational focus farther out, the Sun's gravitational focus begins about 550 AU from the Sun.
If astronomers wish to obtain greatly amplified images of a celestial object, say the super-massive black hole at the center of the galaxy, they might launch "ASTROsail" in the direction opposite the Milky Way's center. After passing the 550 AU inner solar-gravitational focus, greatly amplified images would be obtained and relayed back to Earth. If radio astronomers succeed in their searches for radio transmissions from extraterrestrial intelligence (SETI), a SETI sail operating beyond 550 AU could provide excellent data on ET's home solar system, if that system were occulted by the Sun.
But even with payload micro- (or nano-) miniaturization, and even assuming reasonable advances in sail technology and supplemental use of giant-planet gravity assists, the duration of such a mission would be many decades. A nearer extrasolar target than the Sun's gravity focus would be desirable, at least for a preliminary technology-demonstration mission. If the focal probes could be considered as interstellar precursor missions, a less-demanding extrasolar sail might be considered an interstellar-precursor precursor!
Such a concept was the European Aurora study of the mid-1990s. Instead of aiming for the Sun's gravity focus, Aurora was to target the much nearer heliopause.
The interaction between the Sun and its surrounding galaxy is complex. When the radioactive batteries on board the Voyagers finally run out of juice around the year 2020, these two small robots will be perhaps 150 AU from the Sun. If they could hold out another few decades, scientists could receive particles and fields data from the Sun's galactic vicinity.
The particles/fields boundary between the Sun's influence and that of the galaxy is called the "heliopause." Located at approximately 200 AU from the Sun, the heliopause was to be the goal of the conceptual Aurora probe.
This craft would be equipped with a microns-thin solar sail capable of withstanding a pass to within 0.3 AU from the Sun. Owing to the difficulty of launching a very thin solar sail from Earth and unfurling it in space, some Aurora researchers proposed that the sail could be deposited on a thicker plastic substrate that would be sensitive to ultraviolet radiation and would quickly sublimate when exposed to sunlight.
Other mass-reducing innovations for the proposed 250-meter Aurora sail included inflatable sail beams and supports. The total mass of the Aurora spacecraft was estimated to be 150 kilograms. It would likely be placed in an Earth-escape trajectory by a chemical upper-stage rocket prior to unfurlment.
Because of its low mass and close solar approach, Aurora was projected to exit the solar system with a velocity in excess of 12 AU per year. Several times faster than Voyager, Aurora would pass the heliopause less than two decades after launch and might well survive to transmit data from the Sun's gravitational focus.
As is the case with many studies, Aurora ran its course and was never launched. However, many of the technological innovations projected for this craft have been folded into the NASA's hoped-for Interstellar Probe project—a more advanced sailcraft that NASA hopes to launch toward the heliopause around 2020. This extrasolar probe concept is further described in Chapter 8.
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