Theoretical Research on Magnetic Sails for stopping interstellar probes at other stars

Research in interstellar travel technologies has seen a paradigm change over the last few years. Early concept studies, like Project Orion and the Daedalus project, envisioned gigantic fusion based starships. The perspective changed however when efforts to develop miniaturized satellites, i.e. waferSats, were expanded to designs of spacecraft-on-the-chip probes suitable for deep space exploration.

Low weight of nanocrafts would allow to accelerate them at launch to a substantial fraction of the speed of light by ground- or space based arrays of lasers with an overall power of about 100 GW.

Other aspects of long-duration interstellar missions, such as the feasibility of self-healing electronics based on silicon nanowire gate-all-around FET and the interaction of relativistic spacecrafts with the interstellar medium are being addressed.

Decelerate the interstellar probe passively on arrival could be done with magnetic sails.

The recent progress in laser propulsion research has advanced substantially the prospects to realize interstellar spaceflight within a few decades. Here we examine passive deceleration via momentum braking from ionized interstellar media. The very large area to mass relations needed as a consequence of the low interstellar densities, of the order of 0.1 particles per cm3, or lower, are potentially realizable with magnetic sails generated by superconducting coils. Integrating the equations of motion for interstellar protons hitting a Biot Savart loop we evaluate the effective reflection area in terms of the velocity v of the craft. A researcher has found the scaling relation of bare sail area and the current for magnetic sails. The resulting universal deceleration profile can be evaluated analytically and mission parameters optimized for a minimal craft mass.

Magnetic sail need superconductos with currents of a million Amperes or more to generate a magnetic field capable to reflect protons at relativistic speeds, and because of low particle density of typically less than one particle per cm3 of the interstellar medium.

A single universal scaling function describes the dynamics of magnetic sails to an astonishing accuracy.

The interstellar neighborhood of the sun is characterized to a distance of about 15 lightyears by a collection of interstellar clouds, with the local interstellar cloud interacting with the G cloud. Together they are embedded into the local bubble, which reaches out to roughly 300 lightyears.

For the case of a sample high speed transit to Proxima Centauri we find that magnetic momentum braking would involve daunting mass requirements of the order of 103 tons. A low speed mission to the Trappist-1 system could be realized on the other side already with a 1.5 ton spacecraft, which would be furthermore compatible with the specifications of currently envisioned directed energy launch systems. The extended cruising times of the order of 104 years imply however that a mission to the Trappist-1 system would be viable only for mission concepts for which time constrains are not relevant.

Magnetic sails fail to operate when the magnetic field generated by the current through the loop is too weak to transfer momentum to the interstellar protons. The technical feasibility of magnetic sails is hence dependent in first place on the availability of materials able to support elevated critical currents. Using the properties of state of the art 2nd generation high temperature superconducting tapes and otherwise conservative estimates for the mission parameters we find that magnetic sails need to be massive, in the range of 1000 tons, in order to be able to decelerate high speed interstellar crafts.

Optimizing the requirements for a trajectory to the Trappist-1 system we found importantly that magnetic braking is possible for low-speed crafts even when the target star is located within the local bubble, that is when the particle density of the interstellar medium is as low as 0.005 cm´3 . In this case a 1.5 ton craft could do the job. The launch requirements of such a craft would be furthermore compatible with the specifications of the directed energy launch system envisioned by the Breakthrough Starshot project for Centauri flybys, which could hence see dual use. The extended cruising time of the order 10,000 years implies however that slow cruising trajectories are only an option for mission, such as for life-carrying Genesis crafts, not expected to yield near-term results in terms of a tangible scientific return.

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1

Hi Brian
Claudius's paper is very useful for studying mag-sails, especially his fitting function. And he mentions me in his acknowledgements (albeit spelling my surname wrong.) In turn I put his function to good use in my IAC 2017 paper as well as found the optimum current for launching via the Solar Wind (it's e^3 times the critical current, while for braking it's just e.) Whether anyone will send a panspermia mag-sail to TRAPPIST-1, I can't say, but it's an elegant idea.

2

"The perspective changed however when" we were denied the technologies that were actually equal to the task. And now we're stuck hoping that we can fire itty bitty little probes across the space between the stars, WHEN WE ALREADY KNOW HOW TO LAUNCH STAR SHIPS THE SIZE OF OCEAN LINERS.

There's no happy gloss we can put on this. Our civilization looked at space, had a failure of nerve, and gave up. We burned our "treasure fleet" before we ever laid the first keel.

In another generation or two, maybe our descendants will take up the baton my cowardly generation threw down. At least, I hope so.

3

Hey it worked for the Ming Chinese!

Well, OK, not "worked" so much. It did give a temporary bit of stability free from evil foreign influences. Until the next group of horse barbarians from the North took over and killed 10 million people. And then the foreign lands over the seas that were successfully ignored for a couple of centuries turned up anyway.