Using Magnetic Sails for Interplanetary Travel

Though the dream of many, at one time, the idea that anyone other than highly trained astronauts travelling to space seemed like it would be impossible, concept that would only be possible in Hollywood movies or on kids’ cartoons, such as The Jetsons; however, with the recent SpaceX launches, the discussion of civilian interplanetary travel has been a hot topic of conversation. Of course, in order to do so safely and successfully, NASA, SpacX, and other organizations that are involved in the space industry are exploring the options that are available. One of those options is exploiting the solar wind to power spacecraft using the magnetic sail, or “Magsail”.

What is “Magsail”?

It’s no secret that space travel is extremely complex, and as such, it would take tens of thousands of years to reach even the closest of stars if using traditional spacefraft; however, there have been several ideas and concepts that have been proposed to enhance interspace travel, such as the magnetic sail. A relatively new idea, “Magsail” is the shortened term for “magnetic sail”, a concept that has been proposed in order to propel spacecraft by making use of the magnetic field that is created by a loop of superconducting cables in order to bounce interplanetary or interstellar plasma winds. It’s been determined that when sailing on the solar wind at a radius of one astronautical unit (A.U.), a magnetic sail can achieve an acceleration of up to 0.01 m/s², which is significantly larger than the accelerations that can be achieved via a conventional solar lightsail. When utilized as a brake, so-to-speak, on an interstellar spacecraft, the magnetic sail can minimize the velocity of the craft by a factor of e every half-decade (five years). In order to analyze the use of the magnetic sail, or “Magsail”, when combined with a laser lightsail or a fusion rocket accelerated interstellar spacecraft, a performance system code was used, and it was determined that the Magsail could effectively slow down the time of flights by 40 to 50 years, and could reduce the requirements for propellant by 30% for fusion rockets that propelled 10 lightyear missions. Additionally, the Magsail was determined to be an efficient way to decelerate laser lightsail propelled missions that, prior to the use of the magnetic sail, was not possible.

To further explain, the Magsail is an instrument that can be used to increase or decrease a spacecraft’s speed by utilizing a magnetic field in order to increase the plasma that the solar wind and interstellar medium naturally contains. A look of hundreds of kilometers of superconducting cable is stowed in a drum that’s connected to a payload spacecraft. When it’s time to be used, the cable is sent out into space and a current is initiated within the loop. Once commenced, the current is indefinitely maintained within the superconductor without any additional power necessary. The magnetic field that is generated by the current will impart what is referred to as hoop stress to the loop, which aids in deployment, and it will be forced into a rigid circular shape eventually. The loop functions at low field strengths, which means that minimal structural strengthening is needed.

When operational, charged particles that enter the field are deflected as per the B-field they encounter, and as such, they impart the momentum to the loop. If the solar wind or any other net plasma wind occurs relative to the spacecraft, the loop of the magnetic sail will always produce a drag, and as such, the spacecraft it is attached to will be accelerated in the direction of that relative wind. The solar wind that is near the earth is a flux of protons and electrons in an order of magnitude of several million per cubic meter and at a velocity of 300 to 600 km/sec, which can be utilized to advance the speed of a spacecraft radially away from the sun and the max amount of speed that would be available would be close to that of the solar wind itself. Though it would not be enough for interstellar space travel, those velocities would be more than enough for interplanetary travel. 

The normal configuration of the dipole field also produces a force that is perpendicular to the solar wind. Though lift is not vital for interstellar applications, it does significantly improve the effectiveness of the Magsail for interplanetary operations. Further maneuvering possibilities could be achieved via gravitational, so-called, “swing-by’s”, of significant planes. The secondary application is a brake that can be used for a spacecraft that is traveling on an interstellar basis at fractions of the speed of light. The magnetic field that is so quickly moving around the Magsail ionizes the interstellar medium and then bounces the plasma that results, which then creates a drag and slows down the craft.

The possibility to slow down the speed of the spacecraft from interstellar to interplanetary velocities without rocket propellant expenditure reaches a significant reduction of rocket mass ratio and the mass of the total mission. The current conception of the Magsail is dependent on functioning the superconducting loop of material at high current densities of ambient temperature. The ambient temperature in interstellar space is 2.7 degrees Kelvin, and present superconductors of low temperature NbTi and NbȝSn with critical currents of around 1.0 x 10 and 2.0 x 10 Amps/m². The ambient temperature in interplanetary space are higher than the crucial temperatures of low temperature superconductors, and as such, the materials demand costly refrigeration. With that said, though, the newly developed high temperature ceramic superconductors have been shown to have similar critical currents at temperatures that can be maintained in interplanetary space by utilizing basic radioactive thermal control mechanisms.

Given that this performance would be available in mass amounts of cable one day, parameterizing the issue has been assumed a near term high temperature superconductor with a critical current of 10 to the 10th power amp/m, and state-of-the-art superconductors with a critical current of 10 amps/m. Since the magnets that are used with the Magsail are only functioning in an ambient environment that is below the critical temperature, no substrate material past what was necessary for mechanical support, and as such, a fix magnet density of 5000 kg/m³, the current mass density ratio of the magnets is (j/p) of 2 x 10 and 2 x 10 amp m/kg.

Conclusion

Looking toward the future, if regular space travel were to occur, a way to protect travelers, as well as the technology, from the elements of the protective magnetic field of the earth, will be necessary in order to ensure safety. Though the weather in space may be dangerous, the above-mentioned information does suggest that there is a possibility to exploit solar wind in order to power spacecrafts via magnetic or electric sails; or, even by harvesting some of the energy utilizing the Dyson-Harrop satellite. With that said, the possibilities for safe and successful space travel in the future may really be a possibility that we will be seen in the future. Scientists will continue to explore and preform studies in order to further prove whether or not the use of magnetic sails would make space tourism safe.

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