Propulsion Trades for a 2035-2040 Solar Gravitational Lens Mission

The Solar Gravitational Lens (SGL) enables multipixel imaging and spatially resolved spectroscopy of a nearby terrestrial exoplanet from heliocentric distances $z\simeq 650$-$900$ AU, where solar power is negligible and transportation largely sets time-to-first-science. Reaching 650 AU in 20 yr implies a ballistic lower bound $\bar v_r \simeq 32.5~{\rm AU/yr}\simeq 154$ km/s, motivating propulsion beyond chemical and gravity-assist-only options. We compare close-perihelion solar sailing, fission-powered nuclear electric propulsion (NEP), and Oberth-enabled hybrid injection using simple time-to-distance models that isolate the long outbound leg (i.e., excluding architecture-dependent inner-solar-system injection overhead). For solar sailing, $r_p=0.05$ AU requires $σ_{\rm tot}\simeq 4.9~\mathrm{g,m^{-2}}$ for $v_\infty\simeq 105$ km/s and $σ_{\rm tot}\simeq 2.3~\mathrm{g,m^{-2}}$ for $v_\infty\simeq 155$ km/s, placing sub-20 yr sail-only access in an ultra-low-areal-density, deep-perihelion survivability regime. For NEP, a constant-power stage closure shows that a $m_0=20$ t spacecraft with $m_{\rm pay}=800$ kg and $η=0.7$ reaches 650 AU in 27-33 yr for $α_{\rm tot}=10$-$20\mathrm{kg,kW_e^{-1}}$ (typical optima $P_e\simeq 0.18$-$0.30~\mathrm{MW_e}$, thrust of a few newtons). NEP-only sub-20 yr transfers require extremely aggressive assumptions ($α_{\rm tot}\lesssim 3~\mathrm{kg,kW_e^{-1}}$ and very-high-$I_{\rm sp}$, long-life EP), whereas hybrid architectures become plausible for $α_{\rm tot}\sim 10$-$15~\mathrm{kg,kW_e^{-1}}$ if an injection stage supplies $v_0\gtrsim 50$-70 km/s prior to NEP cruise. We map these requirements to technology readiness and identify system-level demonstrations needed by the early 2030s for a credible 2035–2040 start.

💡 Research Summary

The paper presents a systematic propulsion trade study for a Solar Gravitational Lens (SGL) mission slated for a 2035‑2040 launch. The SGL offers unprecedented gain and angular resolution, enabling multi‑pixel imaging and spectroscopy of an Earth‑like exoplanet at 10‑30 pc, but requires the spacecraft to travel to heliocentric distances of 650‑900 AU where solar power is negligible. The authors focus on the outbound leg, isolating the time‑to‑distance problem, and compare three propulsion families: (1) close‑perihelion solar sailing, (2) fission‑powered nuclear electric propulsion (NEP), and (3) an Oberth‑enabled hybrid injection that supplies an initial hyperbolic excess speed (v₀) before a long‑duration NEP cruise.

Kinematic baseline
Reaching 650 AU in 20 years demands an average radial speed of 32.5 AU yr⁻¹ (≈154 km s⁻¹). Even a purely ballistic cruise at this speed would take 20 years; any realistic powered trajectory requires a higher v∞ because a substantial fraction of the distance is covered while still accelerating. Consequently, missions targeting ≤30 yr need v∞ ≈ 95 km s⁻¹, while ≤20 yr missions need v∞ ≈ 155 km s⁻¹. Chemical propulsion cannot meet these Δv requirements; with Isp ≤ 450 s the mass ratio would be 10⁹‑10¹⁵, far beyond feasible launch vehicle capabilities.

Solar sail analysis
The sail performance follows v∞ ≈ s · 2 · μ⊙ · β · rₚ⁻¹/², where β = a₀/g⊙ and a₀ = p₀/σ_tot (σ_tot = total areal density including sail, structure, payload, power source, avionics, etc.). For a perihelion distance rₚ = 0.05 AU, achieving v∞ ≈ 105 km s⁻¹ requires σ_tot ≈ 4.9 g m⁻²; reaching 155 km s⁻¹ pushes σ_tot down to ≈ 2.3 g m⁻². These are ultra‑low values, an order of magnitude lighter than current high‑performance solar‑sail demonstrators. The required sail area would be on the order of 10⁵ m² (hundreds of meters on a side). Moreover, the sail must survive intense solar flux (>2 MW m⁻²) at 0.05 AU, demanding advanced high‑temperature, high‑reflectivity coatings, micrometeoroid shielding, and an integrated non‑solar power source (e.g., a 100 W radioisotope thermoelectric generator). The authors conclude that sub‑20‑year sail‑only access lies in a regime of ultra‑low areal density combined with deep‑perihelion survivability that is currently at TRL ≈ 4‑5 for materials and TRL ≈ 3‑4 for system integration.

NEP analysis
A constant‑power NEP model is used: thrust T = 2 η Pₑ g₀ Isp, with η ≈ 0.7. The key metric is the power‑to‑mass ratio α_tot = m₀/Pₑ. For a 20 t spacecraft (including an 800 kg payload) and α_tot = 10‑20 kg kWₑ⁻¹ (typical of near‑future fission‑powered systems), the cruise to 650 AU takes 27‑33 years, with electrical power in the 0.18‑0.30 MWₑ range and thrust of only a few newtons. Achieving ≤20 yr would require α_tot ≲ 3 kg kWₑ⁻¹, Isp > 10 000 s, and η > 0.8—parameters far beyond current electric‑thruster and power‑system capabilities (TRL ≈ 6‑7). The analysis also includes system‑closure checks: total impulse, thrust‑to‑mass ratio, radiator area (≈ 300 m² for 0.3 MWₑ), and thruster lifetime (≥ 15 yr). The authors note that NEP, while not the fastest, provides continuous electrical power for the science payload at the SGL focal region, making it attractive for mission operations even if combined with a faster injection stage.

Hybrid Oberth‑NEP concept
The paper evaluates a two‑step architecture: an initial high‑Δv injection (via an Oberth maneuver using a nuclear thermal rocket (NTP) or a laser‑driven electric thruster) that imparts v₀ ≈ 50‑70 km s⁻¹, followed by a long‑duration NEP cruise. With this head‑start, α_tot ≈ 10‑15 kg kWₑ⁻¹ and Pₑ ≈ 0.3‑0.5 MWₑ are sufficient to meet the 20‑year deadline. NTP alone cannot meet the Δv budget because of heavy cryogenic propellant and low specific impulse, but it can serve as a “boost” stage. Laser‑driven electric propulsion, still at TRL ≈ 3, could provide the necessary Oberth boost if demonstrated by the early 2030s. The hybrid approach thus reduces the overall mass‑fraction penalty and relaxes the extreme NEP performance requirements.

Technology readiness and roadmap

• Solar sail: ultra‑low‑density graphene‑nanotube composites, high‑temperature dielectric mirrors, and integrated RPS are at TRL ≈ 3‑4. The authors propose a 1 AU‑scale sail demonstration by 2028‑2030, followed by a 0.1 AU perihelion test by 2032.
• NEP: high‑efficiency (>70 %) power conversion, high‑voltage Hall or ion thrusters (Isp ≈ 4000‑5000 s), and large radiators are at TRL ≈ 6‑7. A 0.5 MWₑ prototype flight is slated for 2032‑2034.
• Hybrid Oberth: NTP is at TRL ≈ 5, while laser‑driven electric propulsion is at TRL ≈ 3. Integrated Oberth‑NEP flight demonstration is recommended for 2034‑2036.

The authors argue that a realistic path to a 2035‑2040 SGL mission hinges on completing these milestones, thereby de‑risking the propulsion system and ensuring that the mission can meet the ≤20‑year time‑to‑first‑science requirement. The paper concludes that a hybrid architecture—initial Oberth boost plus sustained high‑power NEP—offers the most credible solution, while solar‑sail‑only concepts remain attractive for science payloads that can double as a starshade during operations.