Manual Around the Sun Without a Sail

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Are Solar Sails the Future of Space Travel?

But their true potential may lie farther away: Experts believe solar sails are the best propulsion system for exploring the outer edges of the solar system and someday other stars. That is because unlike rockets, these spacecraft don? Potentially, solar sails can move spacecraft much faster than a rocket engine could. With solar sails, as long as you?

Other lightweight materials such as aluminum oxide or carbon fiber are also being tested. To capture as much pressure from sunlight as possible, sails need to be big. Going to the fringes of the solar system would require very large sails. Johnson imagines next-generation sails would have to be hundreds of meters on a side. They would be deployed close to the sun to gain thrust and build up immense speed so they could coast the rest of the way through the solar system.

In three years, a solar sail could reach speeds of , mph , kph , scientists estimate. At that speed, it could reach Pluto in less than five years. Friedman believes that a sail-propelled spacecraft for exploring the Kuiper Belt region of space rocks on the fringe of the solar system is possible within the next 10 years. But beyond the orbit of Jupiter, the energy from sunlight gets too weak to keep sails accelerating, so to go beyond our solar system, a craft could need added thrust. For that, he says, light could be provided by a solar-powered laser placed in orbit around the sun at a mid-point within the solar system.

NASA's first test of the concept of solar sailing came in with its Mariner 10 spacecraft, which was designed to fly by Venus and Mercury. Between and , NASA built two foot meter sails that were successfully tested on the ground under vacuum conditions. But funding for the projects fell through in and they never flew. At about that same time, affordable compact nanosatellites called CubeSats appeared on the scene, presenting a low-cost opportunity to launch a solar sail.

The result was NanoSail-D, a diamond-shaped sail 10 feet 3 meters on a side that was made of four triangular blades and was packed into a pound 4. Meanwhile, the Planetary Society is building LightSail-1, a square-foot square-meter sail that will weigh less than 11 pounds 5 kg. The radiant heat from the sail changes the temperature of the supporting structure.

Both factors affect total force and torque. To hold the desired attitude the ACS must compensate for all of these changes. Sail craft must operate in orbits where their turn rates are compatible with the orbits, which is generally a concern only for spinning disk configurations. Sail operating temperatures are a function of solar distance, sail angle, reflectivity, and front and back emissivities. A sail can be used only where its temperature is kept within its material limits.

Generally, a sail can be used rather close to the Sun, around 0.

Are Solar Sails the Future of Space Travel?

Potential applications for sail craft range throughout the Solar System , from near the Sun to the comet clouds beyond Neptune. The craft can make outbound voyages to deliver loads or to take up station keeping at the destination. They can be used to haul cargo and possibly also used for human travel. For trips within the inner Solar System, they can deliver loads and then return to Earth for subsequent voyages, operating as an interplanetary shuttle.

For Mars in particular, the craft could provide economical means of routinely supplying operations on the planet according to Jerome Wright, "The cost of launching the necessary conventional propellants from Earth are enormous for manned missions.

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Solar sail craft can approach the Sun to deliver observation payloads or to take up station keeping orbits. They can operate at 0. They can reach high orbital inclinations, including polar. Solar sails can travel to and from all of the inner planets. Trips to Mercury and Venus are for rendezvous and orbit entry for the payload.

Trips to Mars could be either for rendezvous or swing-by with release of the payload for aerodynamic braking. Minimum transfer times to the outer planets benefit from using an indirect transfer solar swing-by. However, this method results in high arrival speeds. Slower transfers have lower arrival speeds. For Saturn, the minimum trip time is 3. The Sun's inner gravitational focus point lies at minimum distance of AU from the Sun, and is the point to which light from distant objects is focused by gravity as a result of it passing by the Sun.

This is thus the distant point to which solar gravity will cause the region of deep space on the other side of the Sun to be focused, thus serving effectively as a very large telescope objective lens. It has been proposed that an inflated sail, made of beryllium , that starts at 0. Such proximity to the Sun could prove to be impractical in the near term due to the structural degradation of beryllium at high temperatures, diffusion of hydrogen at high temperatures as well as an electrostatic gradient, generated by the ionization of beryllium from the solar wind, posing a burst risk.

A revised perihelion of 0. Forward has commented that a solar sail could be used to modify the orbit of a satellite about the Earth. In the limit, a sail could be used to "hover" a satellite above one pole of the Earth. Spacecraft fitted with solar sails could also be placed in close orbits such that they are stationary with respect to either the Sun or the Earth, a type of satellite named by Forward a " statite ". This is possible because the propulsion provided by the sail offsets the gravitational attraction of the Sun.

Such an orbit could be useful for studying the properties of the Sun for long durations. In his book The Case for Mars , Robert Zubrin points out that the reflected sunlight from a large statite, placed near the polar terminator of the planet Mars, could be focused on one of the Martian polar ice caps to significantly warm the planet's atmosphere. Such a statite could be made from asteroid material. Minor errors are greatly amplified by gravity assist maneuvers, so using radiation pressure to make very small corrections saved large amounts of propellant.

In the s, Robert Forward proposed two beam-powered propulsion schemes using either lasers or masers to push giant sails to a significant fraction of the speed of light. In the science fiction novel Rocheworld , Forward described a light sail propelled by super lasers.

As the starship neared its destination, the outer portion of the sail would detach. The outer sail would then refocus and reflect the lasers back onto a smaller, inner sail. This would provide braking thrust to stop the ship in the destination star system. Both methods pose monumental engineering challenges.

The lasers would have to operate for years continuously at gigawatt strength. Forward's solution to this requires enormous solar panel arrays to be built at or near the planet Mercury. A planet-sized mirror or fresnel lens would need to be located at several dozen astronomical units from the Sun to keep the lasers focused on the sail. The giant braking sail would have to act as a precision mirror to focus the braking beam onto the inner "deceleration" sail.

A potentially easier approach would be to use a maser to drive a "solar sail" composed of a mesh of wires with the same spacing as the wavelength of the microwaves directed at the sail, since the manipulation of microwave radiation is somewhat easier than the manipulation of visible light.


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The hypothetical " Starwisp " interstellar probe design [26] [27] would use microwaves, rather than visible light, to push it. Masers spread out more rapidly than optical lasers owing to their longer wavelength, and so would not have as great an effective range. Masers could also be used to power a painted solar sail, a conventional sail coated with a layer of chemicals designed to evaporate when struck by microwave radiation.

To further focus the energy on a distant solar sail, Forward proposed a lens designed as a large zone plate. This would be placed at a location between the laser or maser and the spacecraft. Another more physically realistic approach would be to use the light from the Sun to accelerate. Acceleration will drop approximately as the inverse square of the distance from the Sun, and beyond some distance, the ship would no longer receive enough light to accelerate it significantly, but would maintain the final velocity attained.

When nearing the target star, the ship could turn its sails toward it and begin to use the outward pressure of the destination star to decelerate. Rockets could augment the solar thrust. Similar solar sailing launch and capture were suggested for directed panspermia to expand life in other solar system. Small solar sails have been proposed to accelerate the deorbiting of small artificial satellites from Earth orbits. Satellites in low Earth orbit can use a combination of solar pressure on the sail and increased atmospheric drag to accelerate satellite reentry.

The sail's purpose is to bring the satellite out of orbit over a period of about 25 years. As of , it was still under thrust, proving the practicality of a solar sail for long-duration missions. The sail is made of thin polyimide film, coated with evaporated aluminium. It steers with electrically-controlled liquid crystal panels. The sail slowly spins, and these panels turn on and off to control the attitude of the vehicle.

When on, they diffuse light, reducing the momentum transfer to that part of the sail. When off, the sail reflects more light, transferring more momentum. In that way, they turn the sail. The design is very reliable, because spin deployment, which is preferable for large sails, simplified the mechanisms to unfold the sail and the LCD panels have no moving parts.

Parachutes have very low mass, but a parachute is not a workable configuration for a solar sail. Analysis shows that a parachute configuration would collapse from the forces exerted by shroud lines, since radiation pressure does not behave like aerodynamic pressure, and would not act to keep the parachute open. The highest thrust-to-mass designs for ground-assembled deploy-able structures are square sails with the masts and guy lines on the dark side of the sail.

Usually there are four masts that spread the corners of the sail, and a mast in the center to hold guy-wires. One of the largest advantages is that there are no hot spots in the rigging from wrinkling or bagging, and the sail protects the structure from the Sun. This form can, therefore, go close to the Sun for maximum thrust. Most designs steer with small moving sails on the ends of the spars.

In the s JPL studied many rotating blade and ring sails for a mission to rendezvous with Halley's Comet. The intention was to stiffen the structures using angular momentum, eliminating the need for struts, and saving mass. In all cases, surprisingly large amounts of tensile strength were needed to cope with dynamic loads. Weaker sails would ripple or oscillate when the sail's attitude changed, and the oscillations would add and cause structural failure.

The difference in the thrust-to-mass ratio between practical designs was almost nil, and the static designs were easier to control. JPL's reference design was called the "heliogyro".

It had plastic-film blades deployed from rollers and held out by centrifugal forces as it rotated. The spacecraft's attitude and direction were to be completely controlled by changing the angle of the blades in various ways, similar to the cyclic and collective pitch of a helicopter. Although the design had no mass advantage over a square sail, it remained attractive because the method of deploying the sail was simpler than a strut-based design. Heliogyro design is similar to the blades on a helicopter.

The design is faster to manufacture due to lightweight centrifugal stiffening of sails. Also, they are highly efficient in cost and velocity because the blades are lightweight and long. Unlike the square and spinning disk designs, heliogyro is easier to deploy because the blades are compacted on a reel.

The blades roll out when they are deploying after the ejection from the spacecraft. As the heliogyro travels through space the system spins around because of the centrifugal acceleration. Finally, payloads for the space flights are placed in the center of gravity to even out the distribution of weight to ensure stable flight.

JPL also investigated "ring sails" Spinning Disk Sail in the above diagram , panels attached to the edge of a rotating spacecraft. The panels would have slight gaps, about one to five percent of the total area.

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Lines would connect the edge of one sail to the other. Masses in the middles of these lines would pull the sails taut against the coning caused by the radiation pressure. JPL researchers said that this might be an attractive sail design for large manned structures. The inner ring, in particular, might be made to have artificial gravity roughly equal to the gravity on the surface of Mars.

A solar sail can serve a dual function as a high-gain antenna. Pekka Janhunen from FMI has invented a type of solar sail called the electric solar wind sail. The sails are replaced with straightened conducting tethers wires placed radially around the host ship. The wires are electrically charged to create an electric field around the wires. The electric field extends a few tens of metres into the plasma of the surrounding solar wind.

The solar electrons are reflected by the electric field like the photons on a traditional solar sail.

Years Around The Sun - Miles Away

The radius of the sail is from the electric field rather than the actual wire itself, making the sail lighter. The craft can also be steered by regulating the electric charge of the wires. A magnetic sail would also employ the solar wind. However, the magnetic field deflects the electrically charged particles in the wind. It uses wire loops, and runs a static current through them instead of applying a static voltage.

Magnetic sails bend the path of the charged protons that are in the solar wind. By changing the sails' attitudes, and the size of the magnetic fields, they can change the amount and direction of the thrust. The polymer provides mechanical support as well as flexibility, while the thin metal layer provides the reflectivity. Such material resists the heat of a pass close to the Sun and still remains reasonably strong.

The aluminum reflecting film is on the Sun side. Eric Drexler developed a concept for a sail in which the polymer was removed. His sail would use panels of thin aluminium film 30 to nanometres thick supported by a tensile structure. The sail would rotate and would have to be continually under thrust. He made and handled samples of the film in the laboratory, but the material was too delicate to survive folding, launch, and deployment.

The design planned to rely on space-based production of the film panels, joining them to a deploy-able tension structure. Sails in this class would offer high area per unit mass and hence accelerations up to "fifty times higher" than designs based on deploy-able plastic films. Drexler used a similar process to prepare films on the ground. As anticipated, these films demonstrated adequate strength and robustness for handling in the laboratory and for use in space, but not for folding, launch, and deployment.

Research by Geoffrey Landis in —, funded by the NASA Institute for Advanced Concepts , showed that various materials such as alumina for laser lightsails and carbon fiber for microwave pushed lightsails were superior sail materials to the previously standard aluminium or Kapton films. In , Energy Science Laboratories developed a new carbon fiber material that might be useful for solar sails. The rigidity and durability of this material could make solar sails that are significantly sturdier than plastic films.

The material could self-deploy and should withstand higher temperatures. There has been some theoretical speculation about using molecular manufacturing techniques to create advanced, strong, hyper-light sail material, based on nanotube mesh weaves, where the weave "spaces" are less than half the wavelength of light impinging on the sail. While such materials have so far only been produced in laboratory conditions, and the means for manufacturing such material on an industrial scale are not yet available, such materials could mass less than 0.

The least dense metal is lithium , about 5 times less dense than aluminium. Fresh, unoxidized surfaces are reflective. It would have to be fabricated in space and not used to approach the Sun. In the limit, a sail craft might be constructed with a total areal density of around 0. Magnesium and beryllium are also potential materials for high-performance sails. These 3 metals can be alloyed with each other and with aluminium.

Aluminium is the common choice for the reflection layer. Chromium is a good choice for the emission layer on the face away from the Sun. It can readily provide emissivity values of 0. Usable emissivity values are empirical because thin-film effects dominate; bulk emissivity values do not hold up in these cases because material thickness is much thinner than the emitted wavelengths.

Sails are fabricated on Earth on long tables where ribbons are unrolled and joined to create the sails. Sail material needed to have as little weight as possible because it would require the use of the shuttle to carry the craft into orbit. Thus, these sails are packed, launched, and unfurled in space. In the future, fabrication could take place in orbit inside large frames that support the sail.

This would result in lower mass sails and elimination of the risk of deployment failure. Sailing operations are simplest in interplanetary orbits, where attitude changes are done at low rates. For outward bound trajectories, the sail force vector is oriented forward of the Sun line, which increases orbital energy and angular momentum, resulting in the craft moving farther from the Sun. For inward trajectories, the sail force vector is oriented behind the Sun line, which decreases orbital energy and angular momentum, resulting in the craft moving in toward the Sun.

It is worth noting that only the Sun's gravity pulls the craft toward the Sun—there is no analog to a sailboat's tacking to windward. To change orbital inclination, the force vector is turned out of the plane of the velocity vector. In orbits around planets or other bodies, the sail is oriented so that its force vector has a component along the velocity vector, either in the direction of motion for an outward spiral, or against the direction of motion for an inward spiral. Trajectory optimizations can often require intervals of reduced or zero thrust.

This can be achieved by rolling the craft around the Sun line with the sail set at an appropriate angle to reduce or remove the thrust. A close solar passage can be used to increase a craft's energy. The increased radiation pressure combines with the efficacy of being deep in the Sun's gravity well to substantially increase the energy for runs to the outer Solar System.

Your Answer

The optimal approach to the Sun is done by increasing the orbital eccentricity while keeping the energy level as high as practical. The minimum approach distance is a function of sail angle, thermal properties of the sail and other structure, load effects on structure, and sail optical characteristics reflectivity and emissivity.

A close passage can result in substantial optical degradation.


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  7. Required turn rates can increase substantially for a close passage. A sail craft arriving at a star can use a close passage to reduce energy, which also applies to a sail craft on a return trip from the outer Solar System. A lunar swing-by can have important benefits for trajectories leaving from or arriving at Earth. This can reduce trip times, especially in cases where the sail is heavily loaded.

    A swing-by can also be used to obtain favorable departure or arrival directions relative to Earth. A planetary swing-by could also be employed similar to what is done with coasting spacecraft, but good alignments might not exist due to the requirements for overall optimization of the trajectory.