A new propulsion method with interstellar implications recently emerged on the arXiv site, and in an intriguing video on David Kipping’s Cool Worlds channel on YouTube. Kipping (Columbia University) has built a video production process that is second to none, but beyond the imagery is his ability to translate sophisticated mathematical concepts into clear language and engaging visuals. So while we’re going to discuss his new propulsion concept using the arXiv paper, don’t miss the video, where this novel new idea is artfully rendered.

I was delighted to see the author invoking J.R.R. Tolkien in the video (though not in the paper), for he begins the Cool Worlds episode with some musings on interstellar flight and why it has come to engage so many of us. Tolkien devotees will already know the lovely term he used to explain our yearnings for something beyond ourselves: ‘sea-longing.’ It’s a kenning, to use the scholarly jargon, a metaphorical double construction that links two ideas. Anglo-Saxon poetry, about which Tolkien was a master, is rife with such turns of phrase.

Image: Columbia’s David Kipping, astrophysicist and guiding force of the Cool Worlds Lab.

Tolkien’s work on Beowulf was hugely significant to scholarship on that great poem, and The Lord of the Rings is peppered with linguistic echoes of the language. Here’s the relevant quote from The Two Towers, in which the elf Legolas invokes the things that drive his race:

And now Legolas fell silent, while the others talked, and he looked out against the sun, and as he gazed he saw white sea-birds beating up the river.

’Look!’ he cried. ‘Gulls! They are flying far inland. A wonder they are to me and a trouble to my heart. Never in all my life had I met them, until we came to Pelargir, and there I heard them crying in the air as we rode to the battle of the ships. Then I stood still, forgetting war in Middle-earth,; for their wailing voices spoke to me of the Sea. The Sea! Alas! I have not yet beheld it. But deep in the hearts of all my kindred lies the sea-longing, which it is perilous to stir. Alas! for the gulls. No peace shall I have again under beech or under elm.’

Sea-longing. If it was an innate component of Tolkien’s elvish personalities, it’s one common among all humans, I think, though clearly in greater or lesser amount depending on the person. I grew up in the American Midwest far from any ocean, but I had ‘sea-longing’ as a boy and have it still. It’s not just about oceans, of course, but about vast expanses that are partly real and partly a matter of the yearning imagination. It’s why some people have to explore.

Turning Yearning into Hardware

Kipping’s reputation is already secure as an innovator of a very high order. His work on exo-moons solidified the hunt for these objects, which surely exist but which have yet to be confirmed in the only two cases that look plausible so far. His vision of a ‘terrascope’ is reminiscent of gravitational lensing but draws on the Earth’s atmosphere to provide refractive lensing, a telescope concept that although it cannot compete with the gravity lens, nonetheless offers huge magnifications for a space-based telescope. His ‘Halo drive’ gathers energy from light boomeranging around a black hole while using no onboard fuel.

That latter idea is fully consonant with the laws of physics, but of course demands we find a way to get to a black hole to use its energies. By contrast, Torqued Accelerator using Radiation from the Sun (TARS) is a means of acceleration that could be built now. It offers no ‘warp drive’ type travel, and in fact in its most powerful iteration would weigh in at about 1000 kilometers per second. But interstellar flight pushes us to follow our leads, and we should keep in mind how huge a step 1000 km/s represents when weighed against the current defender of the velocity crown, Voyager 1 at about 17 km/s.

So let’s talk about this, because it’s a remarkable way to overcome a serious problem with solar sails, creating a way to push a payload beyond Solar System escape velocity with energy extracted from the Sun. As opposed to the Breakthrough Starshot concept, a politically impossible 100 GW laser array high in the Atacama, TARS offers us an exceedingly economical way to send not one but swarms of tiny probes. And if a journey to Proxima Centauri would take about a millennium, ask yourself what we could do with this in our own system.

The concept is blindingly simple once it’s been thought of, and like Jim Bickford’s TFINER design (see TFINER: Ramping Up Propulson via Nuclear Decay) it’s almost jarring. Why hadn’t someone thought of this before? Kipping, pondering the dilemma of interstellar propulsion, asked whether a deep space sail necessarily has to be beam-driven. True, light from the Sun diminishes rapidly with distance, so that beyond Jupiter, a solar sail is getting little propulsive effect. But maybe pushing a sail is the wrong approach.

For that matter, does it have to be shaped like a conventional solar sail? Kipping began thinking about using sail materials to harvest the energy of solar photons, storing it in what could be considered a battery, and then using that stored energy, transformed into kinetic energy, to hurl a small spacecraft outwards. We thus get the huge advantage of harvesting abundant energy from a system that can be serviced because it remains relatively close to home, not to mention system reusability.

The notion is shown in the figure below, drawn from the paper. Imagine taking two light sails attached to each other by a tether, both identical and each coated on one side with highly reflective material and non-reflective material on the back. Now we can rotate one of them 180 degrees around, so that they are facing in opposite directions. The TARS unit begins to spin because of incident solar photons, and that spin gets faster and faster until the stresses on the tether close in on its design limits. Let me quote from the paper here:

At this point, one (or both) sails are detached (or a sail section) and will head off at high speed tangential to the final rotational motion. The light sail(s) will then continue to enjoy thrust from solar radiation in what follows, but crucially the initial high speed provides sufficient momentum to escape our solar system. The concept is attractive since it only involves two light sails and a tether, and is powered by the Sun. In practice, one might consider an initial spin-up phase with directed energy (but far less than 100 GW) or micro-thrusters, since TARS is more stable once rotation is established.

Image: This is Figure 1 from the paper. Caption: A simplified version of the TARS system. Here, the system comprises one tether and two paddles, which together are orbiting around the Sun, with an instantaneous velocity vector along the Y-axis. Incident solar radiation is largely reflected by the α-surface (the reflective surface) of the paddles, but largely absorbed by the β-surface. This leads to a radiation pressure torque that gradually spins up TARS. Note that both paddles experience both reflection and emission; we only show one of each for the sake of visual clarity in the above. Credit: Kipping & Lampo.

Below is an animation showing the basic concept, with the sails depicted here in the form of panels or paddles, with the same characteristics – a reflective side, a non-reflective side, and the two panels configured in such a way that the incident solar photons spin the system up. Now imagine a small payload at the end of one of these paddles being released just when the system has reached maximum spin-up, so that the craft, possibly the size of a small computer ship, hurtles away with enough force to achieve escape from the Solar System.

Image: This and the animations below are courtesy of David Kipping.

Spinning Up TARS

Don’t get wed to the idea of those sails as paddles; as we’ll see, other options emerge. The nod toward Breakthrough Starshot is evident in the choice of a payload built around microelectronics, but in this case we give up the laser array and use the power of the Sun rather than the collected energies of nuclear reactors to power up the craft. Also like Breakthrough Starshot, we can envision such tiny spacecraft being hurled in swarm formations so that they can network with each other during their journeys. After all, this is a remarkably economical system, capable of launching swarm missions to targets near and far.

So we’re talking about gathering rotational kinetic energy. As Kipping points out, even at 1 AU, Earth receives solar energy of 1.36 kilowatts per meter squared, so if we can tap that energy efficiently, we don’t need to beam our sail. The TARS concept gets around the inverse square law, the fact that solar photons push a sail outwards even as their efficiency plummets. Go twice as far from the Sun and solar energy is reduced not by two but four times. Whereas the spinning TARS stores energy in something analogous to a flywheel while remaining in its orbit. It then releases that energy in a single fling.

The question of TARS’ orbit is an interesting one. Kipping refers to the concept of a quasite, which he developed some years back, though only recently finding a use for it in this new idea. In an email this afternoon, he distinguishes his TARS orbit from the better known statite:

If we could engineer a sufficiently light (and reflective) sail, it is possible that the outward force caused by radiation pressure upon the sail precisely equals the inward gravitational force of the Sun. Such an object need not rely on orbits for stability, it could be placed wherever you want – hanging out in inertial space just motionless. A quasite is not quite so extreme as this. Yes it’s still a sail, but now the gravitational force exceeds the radiative force. Hence, it wants to fall into the Sun (but less so than a non-sail object).

To avoid TARS indeed migrating inwards, we give it a well-calculated nudge such that its tangential velocity is sufficient to keep a constant altitude from the Sun at all times. Although all conventional orbits do this too, the tangential velocity here is less than that of the Earth or indeed any other orbiting object. Hence it’s in what we’d call a sub-Keplerian orbit, and indeed dust particles can do this too since they too can feel strong radiative forces. This engineered quasite thus is a Solar sail which doesn’t recede (or migrate) from the Sun, it stays at the same separation which is crucial for TARS being able to build up angular momentum over time. A consequence of its slower tangential nudge is that it orbits the Sun slower than the Earth does (if at 1AU).

Shape and Material

TARS in its simplest form can be reduced to a single ribbon-like structure, where there is no tether, and the two paddles simply meet at the midpoint. The shape arrived at in the image below is optimum for ensuring rotational stability. The paper considers the use of carbon nanotube sheets, given that this material is more readily available in the market. Tapering the ribbon improves performance, with a segment at the end containing the payload, which can be reflective enough to gain an additional boost as it recedes from the Sun.

Image: For the purposes of calculation, Kipping works with a TARS that is seven meters wide and 63 meters long. The thickness is 2.8 microns, using carbon nanotube sheets, sprayed on one side with nanostructure silver and carbon deposition on the other. This thickness allows a microchip to be attached flush at the two ends, as per the illustration. This is light in weight (1.6 kilograms), so rideshare payloads are hardly a problem. As with solar sails, the device would have to be unfurled once it reaches space. Animation credit: David Kipping.

The calculations referred to above see a three-year spin up time and ejection of the payload at 12.1 kilometers per second – this limit is dependent on the tensile strength of the TARS nanotube sheets. Moving in its quasite orbit, TARS already has 28.3 kilometers per second. Kipping calculates in this configuration that the payload chip would leave TARS at 40.4 kilometers per second. This is just over Solar System escape velocity, making TARS an interstellar option. No beamed energy, no onboard propulsion, just solar energy collected and deployed.

So we have a payload roughly the size of a smartphone that can escape from the Solar System, but velocities can be increased depending on materials used – graphene creates a clear improvement, one that could be further tweaked with a gravity assist. An Oberth effect ‘sundiver’ maneuver is a possibility. And as Kipping notes, the payload can be reflective enough to serve as a small solar sail, acquiring additional velocity as it departs the inner Solar System.

A Magnetic Option to Boost Velocity

To go beyond these tweaks, applying an equal and opposite charge to each tip would create a rotating magnetic dipole. Out of this we get a magnetic field, which in turn yields electromagnetic radiation. A system like this, calculated in the paper, is capable of a critical speed in the range of 1000 kilometers per second. 0.3 percent of c. Meanwhile, the use of TARS to create magnetic shielding for uses in the Solar System can hardly be discounted. Kipping mentions in his video the prospects of using numerous TARS orbiters at Mars to provide radiation shielding for colonies on the surface.

I sometimes hear from readers frustrated by the magnitude of the interstellar challenge. Even Breakthrough Starshot’s 20 percent of lightspeed takes too long for them, and they think we should put all our efforts into attempts to move faster than light. But progress is incremental in most cases, and whether or not we ever achieve breakthroughs like Alcubierre warp drive, we still push the envelope of what is practical today.

Progress is not just individual but civilizational. This is valuable near-term thinking that extends our capabilities one step at a time, and like TARS offers multiple uses within our System and beyond. One step at a time is the nature of the game, and these steps are taking us slowly but inexorably toward the sea.

The paper is Kipping & Lampo, “Torqued Accelerator using Radiation from the Sun (TARS) for Interstellar Payloads,” accepted at Journal of the British Interplanetary Society (preprint).