Issue No. 123

The Orbital Index

Issue No. 123 | Jun 30, 2021

🚀 🌍 🛰

Guest Contribution

Sailing on solar protons. The electric solar wind sail (E-sail) is a novel interplanetary propulsion concept that employs the solar wind to create a propulsive force. When long, thin, charged tethers are placed in a natural plasma stream, such as the solar wind, the electric field around the tethers deflects solar wind particles, transferring part of their momentum to the spacecraft (paper). Effectively, the spacecraft sails on solar protons instead of the photons of traditional solar sails. The E‑sail tethers are charged with a high-voltage source. For the tether to maintain a positive voltage, excess electrons gathered from the surrounding plasma are continuously pumped out using an electron emitter (like the electron gun in an old-school CRT TV). With one or multiple E‑sail tethers attached, a spacecraft can be propelled away from the Sun, or it can ‘tack’ (i.e. sail against the wind) towards the Sun, inclining the sail to brake the craft’s orbital motion such that the Sun’s gravity pulls it inward. The E‑sail does not work as a source of propulsion inside a magnetosphere, such as the one that protects the Earth from solar wind and cosmic radiation. Therefore, an E‑sail spacecraft requires an orbital insertion beyond the magnetosphere—into cislunar space for example. While it cannot provide propulsion within the Earth’s magnetosphere, the E‑sail can be reconfigured with negatively charged tethers to act as a plasma brake for deorbiting spacecraft within the Earth’s ionosphere (paper). (The tethers could also be charged positively, but in the ionosphere, the negative polarity consumes less power and does not require an electron emitter.) Collectively, the E-sail and plasma brake configurations are called Coulomb Drag Propulsion (CDP). The first missions of Estonia and Finland, ESTCube-1 and Aalto-1, carried CDP experiments, but in both cases, their tethers failed to deploy (paper & paper). Three CDP demo missions, AuroraSat-1, ESTCube-2, and FORESAIL-1, will be launched in 2021/2022. Promising mission concepts under study include Multi-Asteroid Touring (design paper) to image 100s of asteroids with 10s of nano-spacecraft, and the North Star micro-craft to exit the Solar System above the ecliptic. 

contributed by Andris Slavinskis, associate professor at UT Tartu Observatory & ESTCube-2 PI. (Related: Andris also edits the Space Travel Blog, which just published two detailed articles about ESTCube-1’s development and operation.)

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Starlink proceeds to global rollout. With the last satellites of Starlink’s initial orbital shell launched and deployed, SpaceX’s large-scale satellite internet service is moving into its next stage. Musk recently announced that over 69,000 subscribers were simultaneously active on the service, which is available in ten countries. (Musk’s goal of 500k subscribers in 12 mos remains ambitious.) Reports from a German user have set a new upper bound on the service’s speed, delivering 649 Mbps (with an average of ~300 Mbps). While the current constellation of 1,578 active satellites (just look at how crowded our coverage map simulator is), and the future deployment of 2,814 more, promises to provide internet access to many rural and impoverished areas, it’s not without downsides. While in a much-improved state from this time last year (Ed.: we’ve seen very few recent critiques from the astronomy community and would love any feedback on why this is.), Astronomy continues to be impacted by increasing satellite interference—this leads to reduced capacity for many of our largest science telescopes. Additionally, the currently high cost of the service makes the promise of internet access for the developing world a questionable one. There have also been plenty of mixed experiences in the beta program, although many of these clearly come down to users not following SpaceX’s warnings regarding line-of-sight obstructions and their impacts on performance and reliability. (This Verge piece, while containing some great info, is a particularly good example of complaining about results being exactly what it said on the tin. 🥫) Global coverage, excepting polar regions, should technically be available in late August, with availability continuing to roll out on a country-by-country basis. On the hardware front, satellite-to-satellite laser links are still being developed, a new lower power antenna design is in the works, as well as improvements to performance in high heat conditions. This only represents the first stage of Starlink’s growth, and Musk believes that to fully realize the constellation’s value, it may require $30 billion more in investment.


A simplified and exaggerated model of how the bell-like seismic ringing of Saturn is transmitted gravitationally to the planet’s icy rings. Credit: Christopher Mankovich.

News in brief. Virgin Orbit may launch an orbital LauncherOne today with seven satellites (for the US DoD, The Netherlands’ Air Force, and SatRevolution)—and this time the launch will be televised; the FAA has approved commercial Virgin Galactic flights from New Mexico; ULA continues to deny that Blue Origin’s BE-4 engine is causing Vulcan’s timeline slippage; SpaceX scrubbed its launch of their Transporter-2 rideshare mission due to an airplane entering the range no-fly zone just prior to launch; Rocket Factory Augsburg successful tested their full-scale, staged-combustion engine for 3 seconds; and, China released excellent videos of its Zhurong Mars rover landing and roving from its wireless camera.

These are not stars. This is an ultra-low frequency image of the night sky taken by LOFAR and the dots are black holes. LOFAR is a radio telescope distributed over 52 stations in nine European countries and is the “largest radio telescope operating at [the] lowest frequencies that can be observed from Earth”.

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