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The Orbital Index

Issue No. 265 | Apr 17, 2024

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The cosmic distance ladder. In order to estimate distances to astronomical objects (and attempt to solve the Hubble tension), astronomers use what is known as the cosmic distance ladder, a set of distance measuring techniques, each building off the previous technique. Comparatively close objects can be gauged via parallax, using shifts in apparent position caused by the Earth’s orbital movement to triangulate distance (as is done with exquisite accuracy by Gaia). More distant objects like galaxies can be measured by looking at the brightness of "standard candles" within them. Standard candles are objects whose actual luminosity is known based on physical constraints, so their distance can be gauged by comparing how bright they appear from Earth with their known actual luminosity. An example is Cepheid variables, stars whose luminosity is correlated with their pulsation periods. Over even more vast distances, Type Ia supernovae are used, again due to their predictable peak luminosity. There are many other techniques within the ladder. Similar to Cepheid variables, the predictable peak brightness of red giant stars at what is known as the tip of the red giant branch (TRGB) is often used. This technique is now even better, as a recent paper found that known acoustic oscillations of red giants at the TRGB can be used to more accurately determine their ages, which tells us how bright they should be, which, again, lets us infer their distance.

The cosmic distance ladder, illustrated. Credit: David Darling

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The first Artemis lunar surface instruments. Paired with the recent LTVS announcement, NASA released plans for the first candidate instruments for astronauts to deploy when they return to the lunar surface as part of the Artemis III Deployed Instruments (A3DI) program. According to the agency, “the instruments will address three Artemis science objectives: understanding planetary processes, understanding the character and origin of lunar polar volatiles, and investigating and mitigating exploration risks.” Here’s a pdf that goes deep into Artemis’s science objectives. The first selected payload, the Lunar Environment Monitoring Station (LEMS), is focused on the seismographic exploration of the lunar south pole region, measuring moonquakes and characterizing the region’s crust and mantle. LEMS (not to be confused with the LEM) will have a lifespan of between three months and two years, and is envisioned to form the foundation of a network of lunar seismic monitoring stations. Lunar Effects on Agricultural Flora (LEAF) will observe the effects of the lunar environment on plant growth, particularly looking at photosynthesis, radiation exposure, and the impacts of partial gravity. LEAF hopes to return the first lunar-grown plant samples with Artemis III’s return trip—it will grow Brassica rapa, Duckweed (the smallest flowering plant), and Thale cress (the first plant to have its genome sequenced). If successful, samples will provide more useful data on off-world plant growth than Chang’e 4’s cotton seedling, which germinated and was photographed but then died due to plunging temperatures. The Lunar Dielectric Analyzer (LDA), a JAXA-funded instrument to measure the lunar regolith’s ability to propagate electric fields, will aid in our ability to find subsurface volatiles, especially ice. Artemis III is currently scheduled to fly in late 2026, and while final mission science manifests are subject to change, these three instruments are currently among the most likely to fly along with the first humans to land on the moon in 55+ years. Related: In exchange for the contribution of JAXA’s pressurized Lunar Cruiser rover to the Artemis program, the first non-American to walk on the Moon will be Japanese (unless China gets there first).

Alan Shepard’s shadow next to the Apollo 14 SNAP-27 RTG, which powered the mission’s ALSEP surface experiments package, shown here wired to a central relay station that sent data to Earth. ALSEPs were part of all Apollo missions starting with Apollo 12 (November 1969), and all five were still actively transmitting when the program was canceled in September 1977. 

News in brief. ULA’s Delta IV Heavy finally launched for its 16th and final time NASA’s TESS spacecraft unexpectedly went into safe mode, but is expected to resume science operations soonColorado startup Max Space announced plans to build large inflatable space station modules, with their first one launching in 2025South Korea launched the second (out of five planned) military SAR spy satellite (primarily to monitor North Korea) SpaceX launched and landed a booster that has flown for a record 20 times, doubling its original goal of reflying boosters 10 times An FAA official stated that they have no current plans to tax commercial launches NASA released the first part of their Space Sustainability Strategy (doc) that initially focuses on maintaining and assessing sustainability in Earth orbit After two last-second aborts, Russia successfully conducted a fourth test launch of their Angara A5 rocket (24.5 tons to LEO)—the Angara program was started 30 years ago after the collapse of the Soviet Union as an all-Russian launch vehicle China’s lunar orbiting satellite Queqiao-2 completed performance testing that included communication with the Chang'e-4 lander on the far side of the moon, and is ready for use in the upcoming Chang'e-6 sample return mission set to launch in May Astroscale’s ADRAS-J mission successfully completed its rendezvous maneuver by raising orbit several times to fly near a defunct rocket upper stage, and is now gearing up for a close approach.

A render of Astroscale’s ADRAS-J mission, which is now just “paparazzi distance” away from an H-2A rocket second-stage that has been floating in orbit since 2009. Credit: Astroscale 


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The Vera C. Rubin Observatory’s LSST camera, built by SLAC and collaborators, is complete. The massive 3,200-megapixel camera, composed of 201 custom-designed CCD sensors, will survey the southern night sky for at least a decade and generate huge quantities of wide field data for mapping dark matter, tracking asteroids, and broadly surveying the Universe using computational astronomy. It is the size of a small car, weighing 3,000 kg and featuring a 1.5 m front lens—making it the largest digital camera ever constructed for astronomy. “Its images are so detailed that it could resolve a golf ball from around [24 kilometers] away, while covering a swath of the sky seven times wider than the full moon.