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Right: A "conventional" solar sail, fully deployed and cruising into interstellar space. Innovative ideas for "gray" and electromagnetic sails may leave this concept in the interstellar dust. (NASA) "A propellant-free system is very attractive because the main problem with interstellar travel is the weight of the propellant," said Geoffrey Landis of the Ohio Aerospace Institute at NASA's Glenn Research Center. He spoke Wednesday morning to the 10th annual Advanced Propulsion Research Workshop held by NASA, Marshall, the Jet Propulsion Laboratory, and the American Institute of Aeronautics and Astronautics being held Tuesday-Thursday at the University of Alabama in Huntsville.
Gray sails could provide a better ride Forward's Starwisp concept would have used a mesh of superconducting aluminum wires to receive its "push" from microwave photons, and then reflect to produce an equal magnitude thrust. This would propel the craft from Earth orbit past Neptune, at 1/20th the speed of light, in just a week. Since then, Forward and others have been rethinking the concept. "My major message is, that's wrong, don't use it" said Forward as he pointed at the equation he used in his initial studies. Since 1984, he has determined that the sail material would absorb a significant amount of the energy, weakening the structure and possibly letting it collapse.
Forward now proposes putting that absorption to work in a "gray sail" made of carbon. The sail would absorb the light, getting a push from it, and reradiate it as infrared energy. With the sail oriented properly to the source, this would generate a significant amount of thrust in the desired direction. A mission to interstellar space could be accomplished with a combination sail. An aluminum coating - just 70 atoms thick - would serve as a traditional reflective solar sail to boost the spacecraft out of Earth orbit, then cancel its solar orbital velocity so it plunges on a near-miss trajectory towards the sun.
As it passes just 3 solar diameters from the sun's visible surface, the aluminum would evaporate, exposing the carbon structure underneath. The carbon would absorb sunlight and heat to 2,000 K (almost 3,600 deg. F). Radiating infrared light would accelerate the craft at 14 times Earth's gravity (the Space Shuttle reaches a maximum of 3 G during launch). "The trajectory is nearly a straight line" away from the sun, Forward said. He is proposing a laboratory demonstration using a 1 kilowatt microwave beam to levitate a 2.5 cm (1 in.) square, 02.5 micron-thick carbon film in a vacuum chamber.
A precursor space mission, carrying a 1 kg (2.2 lb) payload on a 10x10-meter sail would take 20 hours to accelerate. In three weeks, it would pass the orbit of Pluto and continue outward to the Oort cloud of comets surrounding the solar system. Reaching a star would take 400 years, so it's only good as a demonstration. "It's still science fiction," Landis said, "but it's near-term science fiction." Even closer at hand is a concept to sail without a deploying a sail, but throwing a switch and generating one around the spacecraft. In an approach called Mini-Magnetospheric Plasma Propulsion - or M2P2 - a probe would imitate nature to get the solar wind to push it into deep-space. "The enabling technology is pretty much available today," said Dr. R.M. Winglee of the University of Washington Winglee works in the geophysics program which studies the magnetosphere, the region of space around the Earth where the solar wind is deflected by the Earth's magnetic field. Sailing in a bubble "What we're proposing to do is create a magnetic bubble to deflect the solar wind," Winglee explained. Magnetic sails were proposed by Robert Zubrin, inventor of the Mars Direct concept. Such sails are limited, so Wingless suggests injecting plasma (ionized gas) that would drag the magnetic field lines out and generate a bubble 30 to 60 km (18-36 mi) in diameter.
Left: An artist's concept shows how Earth's magnetic field deflects the solar wind and forms the immense magnetosphere around the planet. Scientists may imitate nature and generate a mini-magnetosphere around a space probe and let the solar wind accelerate it into deep space. The solar wind exerts no appreciable push on the Earth because of the Earth's great mass. (NASA) Winglee calculates the specific impulse (a measure of efficiency), would be tens of thousands of seconds. That's 10 to 20 times better than the Space Shuttle Main Engine. "We can go faster and lighter than anyone else," Winglee said. How fast? If launched in 2003, M2P2 would go past the heliopause, where the solar wind runs into the interstellar wind, by 2013. That's a distance of more than 150 times the distance from the sun to the Earth. Voyager 1, launched in 1977, will get there in 2019. Winglee said that adding dust particles to the magnetic bubble would enhance the thrust, and accelerate the M2P2 even faster for a mission to another star. After giving his briefing, Winglee received a glowing recommendation from sail advocate Forward: "I just love the audacity of that concept." Crack the whip to Mars and back Forward also is closely involved in developing a precise interplanetary game of "crack the whip" that could send payloads to the Moon or Mars. "Our goal is to develop a public transit system in space," said Robert Hoyt, president of Tethers Unlimited. Hoyt and Forward believe that an interlocking, well-timed series of rotating tethers could carry payloads from low Earth orbit to the surface of the Moon with almost no rocket power involved.
Right: At artist's concept traces the trajectory for a payload dispatched from the HEFT tether orbiting Earth to the Lunavator Facility that will place it on the Moon. (Tethers Unlimited) Under a contract to the NASA Institute for Advanced Concepts, Tethers Unlimited is defining a Cislunar Tether Transport System. The first step is appropriately named HEFT - High-strength Electrodynamic Force Tether, 90 km long in orbit around the Earth. At the other end of the line is the Lunavator Facility, a 200 km tether - plus counterbalance and central mass - in orbit around the Moon. At the start, HEFT has a 1,000 kg payload at one end of the tether. It can start rotating by momentum exchange with payloads coming back from the Moon, reeling the tether in, or by using the tether itself as part of an electric motor (explained in a few paragraphs). When the payload is swung out from Earth, HEFT releases and the payload sails on to the moon. A little bit of rocket power is used on the way since tidal forces and other effects will usually require midcourse corrections. |
April 8, 1999:
A fixture in any movie involving a trek across the desert is
a store with a sign warning, "Last chance for gas, next
200 miles." Right now, the sign at the edge of the solar
system reads, "Last chance for gas, next 4.3 light years."
If you're using an interstellar sail, you can ignore the sign
and coast to the next solar system. Since the mid-1980s, scientists
have been considering sailing to the stars with a "breeze"
produce by lasers or microwave transmitters.
Right:
The sunshade for the Next Generation Space Telescope is not as
large as a sail for space propulsion, but will provide valuable
technical lessons on how to build one. (NASA)
The magnetic field
of 0.1 Tesla could be generated by a conventional solenoid, and
the helicon plasma source "is amazingly simple." With
a bottle of just 3 kg (1.5 lb) of helium as the plasma fuel,
the magnetic bubble could be operated for three months. The size
of the bubble would expand and contract with variations in the
solar wind, so the force on the 100 kg spacecraft would stay
constant at 1 Newton (about a quarter pound). The 3 kilowatts
of power to run the magnet and plasma generator would be powered
by solar cells.
People used to the
smoke and fire of rockets may try to decode tether as an acronym
for an exotic rocket. It's not. A tether is a flexible line or
rope connecting two objects. Scientists have known for some years
that if two bodies were rotating at opposite ends of a tether,
they will snap into different orbits if the tether is broken.
That happened with the unfortunate loss of the Tethered Satellite
System when it was flown on the Space Shuttle in 1996. If the
bodies are moving fast enough, one could be sent on its way to
the planets.|
The payload arrives at the Moon, just in time to meet the Lunavator Facility as its tip is swinging outward. At this point, the Lunavator orbits well clear of the Moon. To deliver the payload, the central mass winches its way to the end with the counterbalance. Now the center of mass is very close to one end of the tether. The other end, with the payload, swings down to the surface and deposits the package at zero velocity. That may sound impossible, but think of the edge of an automobile tire. It meets the road at zero speed (but is traveling at twice your car's speed when it rotates to the top). With the package delivered, the Lunavator Facility redistributes its masses in preparation for the next arrival. Or, it can pick up a package, at zero speed, and sling it back to Earth. Early work on the Cislunar Tether Transport System led Forward to extend the idea to Mars. "When Rob Hoyt first started his calculations, he was throwing the payloads too hard," Forward said. "He had to slow them down or otherwise they would escape from the combined Earth-Moon gravity field. After doing some calculations, I found that ordinary Spectra [the tough, light-weight fishing-line material used in the tethers] could throw payloads to Mars."
Arrival at Mars is the reverse, with the MarsWhip stage dropping the payload into the Martian atmosphere to glide or parachute to its destination. "It will get you in," Forward said, "You don't need a deorbit propellant." The Martian atmosphere rules out tethers going directly to the surface, at least for the foreseeable future. The trip to Mars could be made in 116 to 162 days, depending on the speed of the whip tip. With aerobraking to slow the craft on arrival at Mars and just before contacting the MarsWhip, the craft can make the trip in as little as 94 days by increasing the speed of the EarthWhip. "We have a new idea," Forward said. "It looks pretty solid, and it looks pretty promising." A step down before stepping up
Left: NASA/Marshall engineer Les Johnson inspects part of the deployment mechanism for the ProSEDS tether mission. (NASA) "We believe that an electrodynamic tether has a lot of applications," said Les Johnson, the principal investigator at NASA/Marshall. ProSEDS' tether will expose the last 5 km of wire to make an electrical connection to the plasma (electrified gas) surrounding the Earth. As the rocket stage (the second stage of a Delta II that will launch an Air Force satellite) orbits the Earth, the wire cuts through the Earth's magnetic field. With electronics on the stage completing the circuit, the tether thus generates an electrical current at the expense of its speed, thus lowering its altitude. "After ProSEDS, there may be a commercialization of this concept," Johnson said, "with operators putting these onboard spacecraft to deorbit rocket stages without using fuel." NASA is also studying an Electrodynamic Tether Upper Stage that - by proper control of the electrical current - could boost satellites to higher orbits, then return itself to a lower orbit to deliver more satellites. A highly profitable application could be on the International Space Station where a low-cost electrodynamic tether could save about $1 billion a year in the cost of supplying reboost propellants. |
So he started designing for the
next phase, the Mars-Earth Rapid Interplanetary Transport Tether
(MERRITT) system. Forward has made another advance in that the
outbound payload need not reach orbit. It can make an atmospheric
ascent to 150 km (90 mi) where the EarthWhip picks it up and
throws it outward to Mars. Like the Lunavator, the EarthWhip
touches the payload at nearly zero relative speed, the center
of mass shifts to balance the arrangement, and the tether releases
the payload at the right instant to send it to Mars.
The first step on
this trip will be taken in August 2000 when Marshall Space Flight
Center flies the Propulsive Small Expendable Deployer System
(ProSEDS), a 15 km tether that will act like a small electric
motor to lower the orbit of an expended rocket stage faster than
natural atmospheric decay.|
Far
out propulsion conference blasts off -- Apr. 6, 1999. The 1999 Advanced Propulsion
Research Workshop begins this week. Setting Sail for the Stars -- April 8, 1999. Cracking the whip and unfurling gray sails are among new techniques under discussion at the 1999 Advanced Propulsion Research Workshop Reaching for the stars -- April 12, 1999. Antimatter and fusion are the subjects of new fuels for rocket engines of the future. MSFC Advanced Space Transportation Programs Office. Leftover Instruments Will Pave Way for New Propulsion Test (March 22, 1998) Well understood and well used scientific insturments will help verify a new instrument as they all fly on JAWSAT. Spacecraft may fly on "empty" (Jan. 22, 1999) Using a propulsive tether concept, spacecraft may be able to brake or boost their orbits without using onboard fuel. A NASA/Marshall project, named "ProSEDS," is slated to demonstrate braking, by accelerating an expended rocket toward re-entry. Lecture series to cover Physics for the Third Millennium (Feb. 2, 1998) Lectures on science in the next century will be held at Marshall Space Flight Center during February 9-12, 1998. Relativistic physics, and next generation propulsion techniques are among the topics. |
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