SpaceX CEO Elon Musk has proposed an unusual approach to conducting a robotic survey of the Solar System’s major outer planets, asteroids, and comets, requiring a stripped-down Starship with a minimalist payload of Starlink satellites modified for interplanetary cruises and high-resolution cameras.



To enable this arrangement, it sounds like an expendable variant of Starship would have to be designed and built, cutting as much extraneous mass as possible to put as much energy as physically possible into its payloads. Outer planets – those lying beyond the Solar System’s main asteroid belt – are a minimum of 400 million miles (~650 million km) from Earth and stretch out to bodies like 2014 MU69 (below) at 4+ billion miles (6.8+ billion km) beyond Earth’s orbit. To travel those truly absurd distances, the time-to-destination can often be measured in decades, a timeframe that is physically impossible to shrink without hugely powerful rockets like BFR. Even then, SpaceX would face major hurdles to pull off Musk’s impromptu mission design.

We should send probes to outer solar system & get high res, true color images of these strange things. Maybe there’s an alien artifact out there … — Jung Musk (@elonmusk) March 29, 2019

New Horizons, the tiny but amazing spacecraft responsible for the first-ever close-up photos of Pluto and (more recently) the bizarre MU69 comet/asteroid, is perhaps the best categorical example of what Musk is proposing. Weighing less than 480 kg (1060 lb) and powered by a radioisotope generator (RTG), the spacecraft was launched in January 2006 and – after a single gravity assist around Jupiter – flew by Pluto a bit less than ten years later in July 2015, traveling a blistering ~13.8 km/s (8.6 mi/s).

After traveling several billion miles over nearly a decade, New Horizons completed its main mission, returning spectacular views of the unexpectedly exotic Pluto. (NASA/JPL)

Coincidentally, at least the first prototypes of SpaceX’s Starlink satellite constellation weighed around 400 kg (880 lb) during their March 2018 launch, just shy of New Horizons’ own dry mass. Major differences abound, however. Most notably, Starlink satellites will be powered by solar arrays optimized for energy generation at Earth’s distance from the sun, compared to New Horizons’ RTG reactor. At distances beyond Saturn, reliance on solar power would be an extraordinary challenge for any spacecraft hoping to do more than simply survive. For example, due to certain unforgiving laws of physics, New Horizons would receive – quite literally – 0.06% the solar energy per unit of area at Pluto.



To produce the scant ~300 Watts New Horizon receives from its nuclear power source, a single Starlink satellite would need a minimum of 1400 m^2 (~15,000 ft^2) of high-efficiency solar panels to survive and power a minimal suite of instruments and communications hardware. Assuming an extraordinary 170 g/m^2 solar array as proposed by Alta Devices, a Starlink satellite would need solar cells weighing no less than 250 kg (550 lb) total to operate at Pluto, a mass that absolutely does not factor in the complex mechanisms necessary to deploy a third of an acre of solar panels from an area of just a few cubic meters.

Probably no fairing either & just 3 Raptor Vacuum engines. Mass ratio of ~30 (1200 tons full, 40 tons empty) with Isp of 380. Then drop a few dozen modified Starlink satellites from empty engine bays with ~1600 Isp, MR 2. Spread out, see what’s there. Not impossible. — Jung Musk (@elonmusk) March 30, 2019

Frankly put, solar-powered exploration beyond the orbit of Jupiter and perhaps Saturn becomes almost inconceivably difficult. Further, the above numbers don’t even take into account each Starlink spacecraft’s electric thrusters, which would need several times more solar panels or massive batteries (themselves needing heaters) to operate at an optimal power level for long, uninterrupted periods of time, a necessity for electric propulsion. Several billion miles closer to the sun, in the main asteroid belt or around the gas giants Jupiter and Saturn, solar power is still extremely challenging but not impossible. NASA’s Juno spacecraft, the first solar-powered vehicle to visit the outer planets, uses solar arrays with an area of 72 m^2 (800 ft^2) to produce less than 500 Watts of power around Jupiter, compared to the ~14 kW they could produce around Earth.

Juno’s solar arrays are an impressive ~28% efficient but still weigh 340 kg (750 lb) and produce less than 500 Watts of power around Jupiter. (NASA)

At the end of the day, SpaceX’s Starlink satellites and Starship-based boost stage would need to undergo radical (and thus expensive) redesigns to accomplish such an ambitious ‘tour’ of the Outer Solar System, quite possibly also requiring the development and integration of wholly new technologies and exploration strategies to get off the ground. While the challenges are immense, the fact that Mr. Musk is already expressing interest in supporting such an exploratory, science-focused mission inspires confidence in the many future benefits that could soon be derived from Starlink and Starship, if successfully developed. Assuming missions that remain within the Inner Solar System, an exploration architecture as described by Musk is already readily doable and wouldn’t need the major modifications and leaps necessary for Outer Solar System ventures. Possible destinations where it could be practical include the Moon, Mars, Venus, the main asteroid belt (i.e. Ceres, Vesta, etc.), and many others.



If SpaceX can find a way to get both Starlink and Starship off the ground and into operational configurations, the future of space exploration – both human and robotic – could be extraordinarily bright.

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