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. |
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¶Papers- We’ve never seen activity on Venus that looks similar to the motion of Earth’s tectonic plates. However, by re-analyzing 30-year-old radar data of surface deformation from the Magellan mission with new algorithms (paper), researchers have inferred large crustal block structures that are “sluggishly jostling against each other like broken pack ice on a pond or lake.” Each lithospheric block is about the size of Alaska. While very different from plate tectonics on Earth today, these dynamics may be similar to Earth 2.5-4 billion years ago and provide evidence for a geologically active and currently-molten Venusian interior.
- A detailed map of the Milky Way’s outermost galactic halo, lovingly crafted by Gaia and NEOWISE, shows a wake left by the Large Magellanic Cloud as it heads towards an eventual collision/merger with our galaxy (paper). This data provides further support for our models of dark matter and how it affects galaxy formation and mergers. Here’s a video explainer.
- Also enabled by Gaia’s prolific data: In 1.32 million years, the K-type orange dwarf star Gliese 710 will pass within 0.02 parsecs (~24 light days) of our Sun (paper), well within the Oort Cloud (which is uncertainty estimated to extend between 0.15-1.58 light-years from the Sun). If we’re still around, this would be a great time to launch an interstellar mission. Meanwhile, about a million years later, comets whose orbits were disturbed by Gliese 710’s passing will arrive in the inner solar system, likely with a flux of a dozen comets a year. Close stellar encounters like this may well be the origin of past periods of cometary bombardment. ☄️
- Seismic waves inside Saturn cause gravitational oscillations in the planet’s rings, allowing us to estimate the properties of its core. The core takes up approximately 60% of the planet’s radius and is composed of 17 Earth-masses worth of ice and rock (paper).
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¶Etc.- How Risky Is It to Send Jeff Bezos to the Edge of Space?
- Five revealing satellite images show how fast our planet is changing. Relatedly, satellite images show extreme drought drying up California’s reservoirs.
- Discovered in older data from DES, 2014 UN271 (Bernardinelli-Bernstein) is a Trans-Neptunian Object (TNO), likely from the Oort Cloud, with a very elliptical out-of-plane orbit that takes it from 40,000 AU (0.6 ly!) to around the orbit of Saturn in about 1.5 million years. Right now it’s 20 AU away, inbound toward the Sun at ~9 km/s, with periapsis in 2031. It’s also potentially 200 km wide, making it almost a dwarf planet and possibly the largest cometary nucleus ever seen—Hale-Bopp was 50 km wide. We don’t know yet if it will develop a tail.
- How does China's urine recycling system work in space?
- Is progress in science and technology slowing down? There has been much recent discussion. Matt Clancy explores this and related questions in his excellent New Things Under the Sun newsletter. A recent issue looked at micro and macro evidence on the productivity of R&D over time and concluded that “looking at the rate of technological advance across a variety of sectors - computer chips, agricultural yields, health, and machine learning - we see a strong tendency for a constant rate of advance to be only sustainable by significantly increasing research efforts.” As a general tendency, innovation gets harder over time. This isn’t particularly surprising, but it presents a reality check for proponents of ideologies like the singularity.
- A Twitter thread on taking apart an Apollo-era 20-watt Traveling Wave Tube Amplifier used to communicate with Earth from lunar orbit.
- And a natural follow-up: the 11 greatest vacuum tubes you’ve never heard of. 💡
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