Total Lunar Eclipse, 2011-06-15
Russia’s Mir space station and the moon share a 70mm frame exposed by one of the STS-91 crew members aboard the Earth-orbiting Space Shuttle Discovery as it passed over a line of heavy thunderstorms on Earth. Credit: NASA
Created using still images taken by the Cassini spacecraft during it’s flyby of Jupiter and while at Saturn. Shown is Io and Europa over Jupiter’s Great Red Spot.
NASA/JPL-Caltech/SSI/CICLOPS/Kevin M. Gill
Saturn and Titan
Credit: NASA/JPL-Caltech/Space Science Institute
Can a Natural Satellite Have Its Own Natural Satellite?
In all known planetary systems, natural satellites occur in a restricted dynamical phase space: planets orbit stars and moons orbit planets. It is natural to ask, can submoons orbit moons? If so, why don’t any of the known moons of the Solar System have their own submoons? One possibility is that the formation mechanism of planetmoon systems precludes their formation. Another possibility, is that these bodies are dynamically unstable and are rapidly scoured from their system after formation. Here, we investigate the latter hypothesis.
What are the requirements for stability of a submoon? To ensure dynamical stability, the host moon must have a Hill sphere that is larger than its physical radius as well as its Roche limit. The submoon must also survive any long-term dynamical effects such as tidal evolution.
Only large moons on wide-separation orbits can host long-lived submoons. This is mainly because massive, distant moons have larger Hill radii that provide more stable real estate for submoons
The researchers calculated that in the Solar System, 4 natural satellites could have their own natural satellite.
Our Moon, Callisto of Jupiter, and Titan and Iapetus of Saturn.
These satellites are relatively large, are relatively distant from their planets, ie there is a small area around them that could have a natural satellite and that satellite would not be stolen by the planet.
On this day in 1846 the astronomer William Lassell discovered the largest moon of Neptune, Triton.
It is the only large moon in the Solar System with a retrograde orbit, an orbit in the direction opposite to its planet’s rotation. At 2,710 kilometres in diameter, it is the seventh-largest moon in the Solar System. Because of its retrograde orbit and composition similar to Pluto’s, Triton is thought to have been a dwarf planet captured from the Kuiper belt. Triton has a surface of mostly frozen nitrogen, a mostly water-ice crust, an icy mantle and a substantial core of rock and metal. The core makes up two-thirds of its total mass.
All detailed knowledge of the surface of Triton was acquired from a distance of 40,000 km by the Voyager 2spacecraft during a single encounter in 1989. The 40% of Triton’s surface imaged by Voyager 2 revealed blocky outcrops, ridges, troughs, furrows, hollows, plateaus, icy plains and few craters.
Triton is geologically active; its surface is young and has relatively few impact craters. Although Triton’s crust is made of various ices, its subsurface processes are similar to those that produce volcanoes and rift valleys on Earth, but with water and ammonia as opposed to liquid rock.
Triton’s entire surface is cut by complex valleys and ridges, probably the result of tectonics and icy volcanism. The vast majority of surface features on Triton are endogenic—the result of internal geological processes rather than external processes such as impacts. Most are volcanic and extrusive in nature, rather than tectonic
The Voyager 2 probe observed in 1989 a handful of geyser-like eruptions of nitrogen gas and entrained dust from beneath the surface of Triton in plumes up to 8 km high.
Triton is thus, along with Earth, Io, and Enceladus, one of the few bodies in the Solar System on which active eruptions of some sort have been observed. (source)
Full moon over the Italian Alps
Image Credit: Stefano De Rosa
On that day in 1957 was launched the satellite Sputnik 1, the Earth’s first artificial satellite.
The Soviet Union launched it into an elliptical low Earth orbit on 4 October 1957, orbiting for three weeks before its batteries died, then silently for two more months before falling back into the atmosphere. It was a 58 cm (23 in) diameter polished metal sphere, with four external radio antennas to broadcast radio pulses. Its radio signal was easily detectable even by radio amateurs and the 65° inclination and duration of its orbit made its flight path cover virtually the entire inhabited Earth. This surprise success precipitated the American Sputnik crisis and triggered the Space Race, a part of the Cold War. The launch ushered in new political, military, technological, and scientific developments.