Propulsion Trades for a 2035-2040 Solar Gravitational Lens Mission

Abstract

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.

Figures (5)

• Figure 1: Required mean radial speed versus time-of-flight (TOF) for reaching $z=650$ and $900~\mathrm{AU}$. The $20\mathrm{yr}$ requirement corresponds to $\bar{v}_r \approx 32.5~\mathrm{AU}/\mathrm{yr} \approx 154\mathrm{km\,s^{-1}}$ to $650~\mathrm{AU}$.
• Figure 2: Idealized system areal density $\sigma_{\rm total}$ required to achieve a target solar hyperbolic excess $v_\infty$ using the photonic-assist scaling $v_\infty \approx \sqrt{2\mu_\odot\beta/r_p}$ with $\beta \simeq p_0/(\sigma_{\rm total}g_\odot(1~\mathrm{AU}))$ for a perfect reflector. Lower perihelion or lower areal density increases achievable cruise speed.
• Figure 3: Optimized NEP time of flight to $z=650\,\mathrm{AU}$ versus NEP system specific mass $\alpha_{\rm tot}$ for $I_{\rm sp}=9000~{\rm s}$, $\eta=0.7$, and payload mass $m_{\rm pay}=800~{\rm kg}$. Each point is optimized over propellant fraction under the constant-$P_{\rm e}$ model.
• Figure 4: Optimized NEP time to $z=650\,\mathrm{AU}$ for a $m_0=20$ t wet-mass spacecraft with $m_{\rm pay}=800$ kg and $\eta=0.7$. Contours show the coupled requirements on thruster $I_{\rm sp}$ and system specific mass $\alpha_{\rm tot}$ under the constant-$P_{\rm e}$ model.
• Figure 5: Effect of initial hyperbolic excess speed $v_0$ at the start of NEP cruise on optimized time of flight to $z=650\,\mathrm{AU}$ ($m_0=20$ t, $m_{\rm pay}=800$ kg, $\eta=0.7$, $I_{\rm sp}=9000$ s). A 20 yr transfer is inaccessible for $v_0\simeq 0$ unless $\alpha_{\rm tot} \lesssim 3~{\rm kg~kW}^{-1}_{\rm e}$, but becomes feasible for $\alpha_{\rm tot} \sim 10$--$15~{\rm kg~kW}^{-1}_{\rm e}$ if $v_0$ is boosted to $\sim 50$ km s$^{-1}$ by a high-thrust injection stage.