Other than the Earth, a number of Solar System bodies have atmospheres dense enough for aerocapture. These include Venus, Mars, Jupiter, Saturn, Titan, Uranus, and Neptune.
Advances in aerocapture technology could quicken the development of aeroshells of lower mass and greater thermal tolerance. One can imagine advanced aerocapture missions decelerated by Neptune's atmosphere for rendezvous with Kuiper Belt Objects (KBOs) near the giant planet (Matloff, 2000b, Matloff and Taylor 2003). As aeroshell mass is reduced, the propulsion mass will also decrease since aerocapture greatly reduces the requirement for deceleration fuel.
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The Solar Photon Sail
ISP solar sail research concentrates upon near-term Earth-launched solar sails with a typical areal density of 0.015 kg/m2. Operational near-term sails will be stowed for launch and unfurled in space. Unlike the ultimate space-manufactured metallic sails, these are generally tri-layered. A plastic substrate is sandwiched between a reflective layer facing the Sun and a rear emissive layer that radiates absorbed solar energy.
As well as investigating low-mass materials and supporting structures, ISP sail researchers are considering methods of propellantless guidance, navigation and control, and developing relevant computer codes. Ground validation of deployment techniques for sub-scale sails is currently underway.
The solar sail requires no propellant (since thrust is provided by linear momentum transferred from impacting solar photons) and has no environmental impact. Unless efficient methods of power beaming are developed (Forward, 1984), sail technology will find most application on inner-Solar System missions where sunlight is most intense.
Near-term missions that may be enabled by the solar-photon sail include pole sitters permanently situated over high-latitude locations (McInnes, 1999) and constellations of solar observatories situated sunward of the Earth on long-duration missions to monitor space weather.
Although thin-film and inflatable structures have been unfurled in space, no dedicated solar sail mission has flown to date. The first NASA-launched sail may fly before 2010.
Solar Thermal Propulsion (STP)
Solar Thermal Propulsion is another in-space propulsion system that can live off the interplanetary land. This propulsion technology operates by focusing sunlight on a gaseous propellant, such as hydrogen (Shoji and Frve, 1988, and Grossman and Williams, 1990). Concentrated sunlight is focused upon an absorbing heat-exchange system for transfer to the propellant. For efficient operation, the propellant is heated to temperatures as high as 2780 Kelvin. Exhaust velocities of the heated fuel are intermediate between chemical and solar electric propulsion, typically 8 to 10 km/sec. Although STP does not have sufficient thrust for ground-LEO operations, the technology could transfer a payload between LEO and geosynchronous orbit (GEO) in about 30 days.
Research on this propulsion system deals with a number of issues, including solar-concentrator design. Both inflatable and rigid concentrators are under consideration, although inflatable concentrators are currently favored.
Tethers
Of all the near-term in-space propulsion technologies, the tether seems the most magical. Imagine—all an Earth-orbiting spacecraft has to do to raise its orbital height is to unwind an appropriately designed long, thin cable! Both electrodynamic (ED) and momentum exchange/electrodynamic reboost (MXER) tethers may be used for propulsion in the future.
Electrodynamic tethers have been described by Samanta et al (1992), Beletskii and Levin (1993), and Estes et al (2000). They have also been demonstrated in space by the NASA Tethered Satellite System mission in 1996. To boost a LEO spacecraft using an ED tether, a long conducting strand is deployed downward from the spacecraft. Electrons are collected from the Earth's upper ionosphere at the low end of the tether. Powered by energy obtained from the spacecraft's solar array, the collected electrons travel up the tether and are emitted at the spacecraft. The resulting electrodynamic force on the unidirectional current adds energy to the spacecraft's orbit, thereby raising the orbital height. Figure 2 describes the electrodynamic boost process.
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As described by Sorensen (2001), the MXER tether is a hybrid ED/momentum-exchange tether. A rotating momentum-exchange tether can increase a payload's orbital energy by grappling the payload at the low point of the tether's rotation and releasing it at the high point. However, the orbital energy of the tether itself decreases during this maneuver, and its orbital height is consequently lowered.
A rotating MXER tether has its rotation timed so that the tether tip is oriented below the tether-system center-of-mass and is swinging backwards at the perigee of its elliptical orbit. A payload from a LEO or sub-orbital launch is captured by a grapple on the lower tether tip at zero relative velocity and released at the high point of the tether's rotation. In theory, payloads could be accelerated to escape velocity in this fashion.
Left to its own devices, the MXER tether's orbit would decay after each payload capture and release. But if the MXER tether can also operate as an ED tether, electrodynamic forces on the unidirectional current flow can be used to raise the tether-station's orbit.
Much analytical work remains to be done to demonstrate the feasibility of this concept. But the MXER tether has the potential to revolutionize interplanetary space travel.
Ad Astra
Implementing an Interstellar Capability
At this point in space history, routine Earth-to-orbit travel remains a major challenge. But the Moon, Mars, and more remote destinations draw our attention outward. The propulsion technologies described will positively impact the development of the space infrastructure required to support an expanding interplanetary and, ultimately, interstellar human civilization.
Certain requirements for the expansion of human civilization beyond the Earth—understanding and mitigation of space-radiation effects, determination of optimum artificial gravity levels, development of closed-environment systems, etc.—will be satisfied by experiments aboard the International Space Station, or in conjunction with the next phase of exploratory missions above LEO. These will not be further discussed in this article.
Application of new propulsion technologies will have many positive effects in the development of an interplanetary (and ultimately interstellar) civilization. One requirement for such a civilization is expanded knowledge of the resource base of the Solar System. Advanced chemical rocketry, solar electric propulsion, and aerocapture should result in more massive and flexible scientific payloads to acquire this knowledge.
As well as reducing the cost of orbital transfer, development of solar thermal propulsion should assist the development of space mining and construction. Focused sunlight from the STP concentrator optics will provide an intense energy source for these applications.
Advances in chemical rocket technology may lead to the construction of spacecraft components directly from extraterrestrial resources. Such construction might be implemented by Rapid Prototyping (RP), which is the three-dimensional equivalent of a fax (Doyle, 2000). After a prototype is designed by a computer-aided design package, the RP machine quickly constructs the prototype layer by layer, conceivably using extraterrestrial resources as the feedstock. Perhaps this technique will be applied to the in-space construction of the ultra-thin solar-photon sails required for interstellar travel.
As discussed by O'Neill (1974, 1977), SEP research may lead to the development of the mass driver. These solar-powered electromagnetic catapults could transfer large quantities of material from space mines to space manufacturing facilities.
The ultimate design of robotic or crewed solar sail starships will be served by current research. In addition to the in-space fabrication of ultra-thin sail films, starship designers will require thin, strong cables connecting sail and payload and demonstration that the ship can operate in the high-temperature, high-acceleration environment of a close solar pass.
Finite-element computer models indicate that several sail configurat
ions remain stable for accelerations as high as 2.5 g (Cassenti et al, 1996). Tethers will yield experience with the operation of cable-like structures in space. Some aeroshell designs decelerating in planetary atmospheres will simulate the near-Sun acceleration of solar sail starships.
During the summer of 2005, the ISP team completed full deployment and thermal vacuum testing of two 20-m solar sails (Figure 3).
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To those who witnessed the deployment, it was clear that the idea of interstellar travel is beginning to emerge from the theoretical paper and the science-fiction story into the realm of system engineering. Perhaps within the lifetimes of many Analog readers, humanity's first robotic interstellar emissaries will be sailing the interstellar seas. Although we will not witness them, we can dream of the expeditions to follow, which will carry people to the stars.
Copyright © 2006 Les Johnson and Gregory L. Matloff
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References Cited:
Beletskii, V. V., and Levin, E. M., “Electrodynamic Tethers,” Dynamics of Space Tether Systems, Advances in the Astronautical Sciences, 83, Univelt, San Diego, CA (1993), pp. 267-332.
Bond, A., Martin, A. R., Buckland, R. A., Grant, T. J., Lawton, A. T., Mattison, H. R., Parfatt, J. A., Parkinson, R. C., Richards, G. R., Strong, J. G., Webb, G. M., White, A. G. A., and Wright, P. P., “Project Daedalus: the Final Report on the BIS Starship Study,” supplement to JBIS, 31, S1-S192 (1978).
Bond, A., and Martin, A. R., “Worldships: an Assessment of the Engineering Feasibility,” JBIS, 37, 254-266 (1984).
Cassenti, B. N., Matloff, G. L., and Strobl, J., “The Structural Response and Stability of Interstellar Solar Sails,” JBIS, 49, 345-350 (1996).
Doyle, A., “Pioneering Prototypes,” Computer Graphics World, 23, No. 9, 39-47 (September, 2000).
Dyson, F., “Interstellar Transport,” Physics Today, 21, No. 10, 41-45 (October, 1968).
Estes, R. D., Lorenzini, E. C., Sanmartin, J., Pelaez, J., Martinez-Sanchez, M., Johnson, C. L., and Vas, I. E., “Bare Tethers for Electrodynamic Space Propulsion,” Journal of Spacecraft and Rockets, 37, 205-211 (2000).
Forward, R. L., “Round-Trip Interstellar Travel Using Laser-Pushed Lightsails,” Journal of Spacecraft and Rockets, 21, 187-195 (1984).
Grossman, G., and Williams, G., “Inflatable Concentrators for Solar Propulsion and Dynamic Space Power,” Journal of Solar Energy, 112, 229-236 (1990).
Jaffe, L. D., Ivie, C., Lewis, J. C., Lipes, R., Norton, H. N., Sterns, J. W., Stimpson, L. D., and Weissman, P., “An Interstellar Precursor Mission,” JBIS, 33, 3-26 (1980).
Johnson, L., and Leifer, S., “Propulsion Options for Interstellar Exploration,” AIAA 2000-3334.
Johnson, R. D., and Holbrow, C., Space Settlements: A Design Study, NASA SP-413, NASA, Washington, D.C. (1977).
Lai, Gary, “Hot-Air Ballooning Through Space: The Promise of Mini-Magnetospheric Plasma Propulsion, Analog Science Fiction & Fact (January/February 2004).
Mallove, E. F., and Matloff, G. L., The Starflight Handbook, Wiley, NY (1989).
Martin, A. R., “World Ships—Concept, Cause, Cost, Construction, and Colonization,” JBIS, 37, 243-253 (1984).
Matloff, G. L., and Mallove, E. F., “Solar Sail Starships—The Clipper Ships of the Galaxy,” JBIS, 34, 371-380 (1981).
Matloff, G. L., and Mallove, E. F., “The Interstellar Solar Sail: Optimization and Further Analysis,” JBIS, 36, 201-209 (1983).
Matloff, G. L., Deep-Space Probes, Springer-Praxis, Chichester, UK (2000).
Matloff, G. L., “Persephone: A Non-Nuclear Rendezvous Mission to a Kuiper Belt Object,” in Proceedings of Space Technology and Applications International Forum-STAIF 2000, ed. M. S. El-Genk, American Institute of Physics (2000).
Matloff, G. L., and Taylor, T., “The Solar Sail as Planetary Aerobrake,” IAC-03-S.6.02.
O'Neill, “The Colonization of Space,” Physics Today, 27, No. 9, 32-40 (September, 1974).
O'Neill, G. K., The High Frontier, Morrow, NY (1977).
Samanta, R. R. I., Hastings, D. E., Ahedo, E., “Systems Analysis of Electrodynamic Tethers,” Journal of Spacecraft and Rockets, 29, 415-424 (1992).
Shoji, J. M., and Frve, P. E., “Solar Thermal Propulsion for Orbit Transfer,” AIAA 88-3171.
Sorensen, K. F., “Conceptual Design and Analysis of an MXER Tether Boost Station,” AIAA 2001-3915.
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About the Authors:
When he is not managing NASA's Science Programs and Projects Office, Les Johnson follows his alternate path as a science-fiction fan.
Greg Matloff, who consults for ISP, is an assistant professor at New York City College of Technology. He has published widely in the field of space propulsion.
The authors will soon see their book, “Living Off the Land in Space,” published later this year by Praxis and Copernicus.
The preparation of this document was partially supported by SAIC sub-contract 440055739, from NASA MSFC.
[Back to Table of Contents]
PREVENGE by Mike Resnick & Kevin J. Anderson
Being a person of firm principles has its pitfalls....
It wasn't the murders themselves that broke his heart. They weren't permanent. He had the ability to un-do something as simple and straightforward as a murder.
No, the maddening part was that people never stopped trying. Why was the total, cold-blooded obliteration of a human life the preferred problem-solving method for so many men and women? It offended his deep moral sense.
His name was Kyle Bain, and he and the other members of the Knights Temporal had to make things right, either before or after the fact.
Kyle couldn't help wondering about the killers whose crimes he was assigned to negate. You had a good start in life; you had money, education, opportunity. Where did it all go wrong?
Or you—you had love, and now you'll never have it again. Do you know what a rare gift you threw away, just like you'd throw out the garbage every morning?
Or this current case: Vincent Draconis, a major industrialist who controlled an empire on three continents. He had been/would be murdered in one unguarded moment, leaving a widow and three fatherless children. The confusion in the aftermath would cost almost twenty thousand people their jobs. So much suffering.
That couldn't be allowed. It was a situation made to order for the Knights Temporal....
Kyle arrived in the afternoon, ten hours before the murder was due to occur. According to the file, Draconis was going to be shot down in cold blood just before midnight while working late at his office. Even without witnesses, the man immediately fingered for the crime was Jason Bechtold, vice president of one of Draconis's companies.
Dressed just like the hundreds of other businessmen entering Bechtold's suite of offices, Kyle gained access to the correct, bustling floor. If he followed Bechtold as he left for the day, he could be there later on in time to deflect the murder. A simple enough job, one for which he was well trained.
The Knights Temporal had been founded by Harvey Bloom, a name that hardly seemed destined to go down in history, though Bloom had already placed his name in a thousand alternate histories, maybe more. A theoretical mathematician, Bloom spent the first half of his professional life finding the secrets hidden in Einstein's Special Theory of Relativity, and the second half acting upon them.
Bloom was also a moralist, more interested in Doing Good than in Making A Fortune. Instead of taking out patents or using his privately-funded “temporal displacement” work to study the past, Bloom knew in his bones that he had an obligation to right wrongs. And the most unforgivable wrong, commandment number one, was Thou Shalt Not Kill. When he recruited twenty-five Right-Thinking young men and women to be his crusaders, he made sure they all shared his moral values.
Thou shalt UN-kill, whenever possible.
After reviewing the file of Vincent Draconis's murder, Kyle waited and watched. When Bechtold emerged from his office late in the day, Kyle di
screetly followed him out of the building, then in a cab, to where the executive met an elegantly dressed young woman at an expensive restaurant; they kissed, and a head waiter led them to a table.
Kyle holed up in a coffee shop across the street, nursing his small coffee and nibbling a cheese Danish. After an hour he could sense some irritation from the waitress, so he tipped her twenty dollars to leave him alone. He knew that right now, in the restaurant across the street, Jason Bechtold must be planning the murder of his boss, though he didn't seem particularly agitated. Cool customer. Establishing an alibi.
Kyle would block him before he could get to Draconis's office late at night. A subtle intervention was best, and if done properly no one would even notice. Waiting too long, cutting too close to the murder event, often raised awkward questions and suspicions.
Bechtold and his lady emerged from the restaurant at ten, and Kyle hastily tagged along, invisible in the street crowds. The woman was swaying slightly as they casually entered a five-star hotel. Nothing in the executive's behavior gave any hint of his murderous intent.
According to the file, Bechtold's defense was that he'd spent the night with this woman in room 2145. If she was sound asleep—from a tranquilizer slipped in her drink, perhaps?—and Bechtold was back in bed before she woke up in the morning, she'd corroborate his story.
After giving them ample time to reach the room (considering the way the lady was hanging on Bechtold's arm, Kyle didn't think the executive would be leaving soon) he rode the elevator up to the twenty-first floor and posted himself a few feet from the door to 2145. He sat down on the corridor's plush carpeting and waited. And waited.
And waited.
Past time for the murder. Kyle hadn't done anything at all; had he somehow intervened without knowing it? He knew Bechtold was still in the hotel room, but Draconis was supposed to be dead by now.
Analog SFF, November 2006 Page 9
