TARS: Solar-Driven Torqued Accelerator

• TARS is a propulsion concept that converts solar photon momentum into rotational energy for accelerating gram- to kilogram-scale microprobes.
• It uses asymmetrically coated lightweight ribbons to generate torque and store energy over weeks in a near-solar quasite orbit.
• Optimization strategies include advanced CNT materials and orbital maneuvers to maximize achievable velocities while limiting structural stresses.

A torqued accelerator using radiation from the Sun (TARS) is a concept for interstellar payload acceleration that converts solar photon momentum into rotational kinetic energy, subsequently releasing that stored energy to propel a microprobe at high velocity. Unlike conventional solar sails, which continuously translate radiation pressure into linear propulsion, TARS accumulates rotational energy over extended intervals in a compact structure utilizing contrasting surface albedos. Payloads are then ejected tangentially, with attainable speeds governed by attainable material strengths and design optimizations. This approach enables sub-relativistic interstellar escape for gram- to kilogram-scale probes using commercially available nanomaterials and without reliance on large-scale directed-energy systems.

1. Fundamental Operating Principle

TARS exploits the differential photon pressure on asymmetrically coated surfaces to generate net torque. The typical configuration consists of two narrow, lightweight ribbons or “paddles” joined by a tether; each paddle has one side (“α-surface”) of high reflectance and the other (“β-surface”) possessing significant absorptance with strong thermal re-emission. When solar flux impinges nearly parallel to the device’s plane, the contrasting optical properties induce a torque, slowly spinning the structure up over a timespan of weeks to months in a near-solar orbit.

Angular acceleration is produced as

$$\dot{\omega} = \frac{3\,\epsilon_R\,S}{c\,\Sigma\,L}$$

where $S$ is incident solar flux, $c$ is the speed of light, $\Sigma$ the areal density, $L$ the ribbon length, and $\epsilon_R$ a quality factor encapsulating optical efficiency, with $\epsilon_R$ determined by the difference in albedos and contribution from thermal emission. After sufficient energy has accumulated in rotation, the payload is mechanically released from the rim, converting the local tangential velocity $v = L\omega/2$ into translational velocity for interstellar escape.

2. Materials and Structural Considerations

Attainable payload velocities are limited predominantly by the tensile strength of the ribbon material, as the centrifugal force during rotation imposes a maximum critical velocity:

$v_{\mathrm{crit}} = \sqrt{2\sigma/\rho}$

with $\sigma$ the ultimate tensile strength and $\rho$ the material density. TARS prototypes specify carbon nanotube (CNT) sheets—currently available at sub-20 g/m$^2$ areal densities and high strength—as the structural substrate. The reflective “α-surface” is enhanced with nanostructured metallic coatings (e.g., $\sim$10 nm Ag for $R_\alpha\sim0.9$), while the absorptive “β-surface” may utilize titanium nitride or comparable compounds for $R_\beta\sim0.3$. Ribbon cross-section is “tapered” (i.e., varies along $L$) so that material mass is distributed to withstand calculated radial and centrifugal stresses with minimal excess.

The entire assembly spans tens of meters and mass-budget calculations demonstrate phone-sized payloads (tens of grams) are compatible with total system masses $\mathcal{O}$(1 kg), including both structure and coatings.

3. Kinetic Performance and Velocity Limits

The achievable payload ejection velocity $v_\mathrm{max}$ in a TARS architecture is ultimately capped by the critical rotation rate compatible with the ribbon’s material tensile limit. For CNT-based constructs, $v_\mathrm{crit}$ can reach several km/s, with representative calculations indicating that, for areal densities $\Sigma \sim 10$–$20\,\mathrm{g/m}^2$, months-long photonic torque accumulation at 1 AU suffices to reach escape velocity from the Solar gravitational well.

The payload’s tangential velocity at the moment of release is roughly

$$v \simeq 3t\,\frac{\epsilon_R S_\oplus}{c \Sigma} \frac{1}{(a/\mathrm{AU})^2} \frac{1}{\sqrt{1-e^2}}$$

for a spin-up duration $t$, semi-major axis $a$, solar constant $S_\oplus$, and orbital eccentricity $e$. However, the structural dimensions required to continue augmenting $v$ grow rapidly: attempts to approach relativistic speeds ($\sim0.01c$ and above) render the devices unmanageably large for current material capabilities.

A summary of design scaling:

Parameter	Value (CNT)	Impact
Areal density	10–20 g/m$^2$	Higher ⇒ slower spin-up
Tensile strength	1–10 GPa	Higher ⇒ higher $v_\text{crit}$
Length	10–100 m	Longer ⇒ more total energy
Mass (total)	~1 kg	Sets feasible payload mass

4. Orbital Dynamics and “Quasite” Operation

Spin-up via radiation torque is more efficient when the ribbon remains in a high-flux region. TARS utilizes a sub-Keplerian “quasite” orbit—wherein the net radial radiation pressure is partially balanced by solar gravity and non-Keplerian path control—to keep the assembly relatively close (\textasciitilde~1 AU) to the Sun for the required weeks-to-months accumulator period. This orbital regime minimizes the loss of net flux due to device recoil and radial displacement, simplifying available energy calculations compared to continuously outward-bound solar sails.

Management of net recoil is necessary: unless the torque and total radiation pressure are carefully balanced, the device experiences an outward radial thrust that can otherwise de-optimize the usable solar flux delivered over the spin-up period.

5. Optimization Strategies for Enhanced Velocity

Several methods to surpass intrinsic velocity limits are outlined in the literature:

• Superior Material Selection: Replacement of CNT sheets with large-area graphene or other ultrahigh-strength, low-density nanomaterials increases $v_\text{crit}$.
• Gravity Assists: Incorporation of planetary flybys amplifies payload kinetic energy post-release.
• Oberth Effect: Orbits engineered with low-perihelion passes enable energy release (payload ejection) at the point of maximal orbital velocity.
• Electrostatic Confinement: Charging paddle tips creates repulsive Coulomb forces, partially counterbalancing tensile stress and permitting higher spin rates. An analysis estimates a velocity scaling

$$v \sim \left(\frac{3c^5}{32\pi} \frac{SL}{\Sigma}\right)^{1/8}$$

yielding up to 1000 km/s under optimal assumptions.

These approaches remain subject to materials limitations and introduce secondary engineering complexity, particularly with regard to device mass, stability, and charge management.

6. Comparative Context and Technological Significance

TARS is explicitly distinct from both conventional solar sails and high-power directed-energy propulsion (e.g., Breakthrough Starshot). Conventional sails must achieve extremely low areal densities ($\sim$0.8 g/m$^2$) and rely on continuous, radially outward photon pressure. Directed-energy schemes demand power scales currently beyond terrestrial infrastructure capabilities.

In contrast, TARS:

• Operates with existing nanomaterials and coatings (e.g., CNT, silver, titanium nitride).
• Requires only naturally available solar radiation, avoiding large-scale laser infrastructure.
• Enables a “flywheel” energy storage modality, potentially supporting multiple launch cycles (i.e., repeated charge-and-release for batch probe missions).
• Suits missions where payloads are of order grams—enabling initial interstellar forays or networked microprobe deployments—not high-mass cargo or relativistic travel.

The practical ceiling of achievable speeds remains sub-relativistic. Device mass and complexity increase rapidly for higher energy targets, making TARS well-matched to scenarios where cost, simplicity, and energy input are the principal constraints.

7. Prospects and Limitations

TARS demonstrates that mechanical design leveraging contrasting surface albedos and high-tensile nanomaterials can store and discharge solar-derived angular momentum for the acceleration of interstellar microscale payloads. The concept is technologically accessible—especially when high-power external energy injection is impractical—but bound by the mechanical properties of present-day materials. Improvements in areal density, optical efficiency, and ingenuity in structure–orbit coupling may yield further increments in performance. The approach is unsuitable for missions seeking relativistic transit times but is positioned as a precursor technology enabling low-cost, low-mass science probes to escape the Solar System on timescales of months to a year, using no onboard fuel and minimal supporting infrastructure (Kipping et al., 23 Jul 2025).