Discovered in 1848, Hyperion is one of the solar system’s strangest objects. Although classed as a larger moon, Hyperion measures just 270 kilometers (168 miles) across. But it’s not its size, rather Hyperion’s appearance and behavior that interest scientists.
Hyperion rotates chaotically as it orbits its parent planet, Saturn. It’s the only moon in the solar system known to behave in this way. Hyperion’s axis of rotation wobbles to such a degree that the moon’s orientation in space is unpredictable. Whether for terrestrial observers using telescopes or cameras on board spacecraft such as Cassini, this presents certain difficulties since it’s impossible to predict which face of Hyperion will be orientated towards them at any given time. To date, Hyperion has presented pretty much the same face towards Cassini during the spacecraft’s previous close approaches to the moon. Hyperion is also unique in being the solar system’s only regular planetary natural satellite that isn’t tidally locked.
Aside from its odd rotation, Hyperion also has a strange sponge-like appearance, first revealed in full during Cassini’s closest targeted approach to the moon in September 2005 at a distance of just 505 kilometers (314 miles).
The surface of the moon, sometimes designated as Saturn VII, is covered with deep, sharp-edged craters giving Hyperion an appearance reminiscent of a giant sponge. Dark reddish material fills the bottom of these craters. Scientists discovered in 2007 that this substance contains hydrocarbons — long chains of carbon and hydrogen similar to material found on other Saturnian satellites.
Hyperion’s unusual, sponge-like appearance is attributed to the moon having an unusually low density for such a large object. In planetary terms, Hyperion is very much a lightweight. As Digital Journal reported after Cassini’s 2005 and 2006 flybys, Hyperion was discovered to have a density just 0.5 times that of water, much lower than rocky planets such as the Earth and Mars. To put that in context, rocks have a density of between two and three times that of water. Even water-ice, with a density of 0.9 times that of water, weighs in “heavier” than Hyperion.
These earlier flybys revealed that 40 percent of Hyperion consisted of, remarkably, empty space. Its solid material comprises, in the main, water-ice rather than rock.
Hyperion’s low density makes the moon highly porous, coupled with weak surface gravity. These factors mean any objects such as asteroids and meteorites colliding with Hyperion make something of a soft landing, tending to compress the moon’s surface rather than gouging out material. Hyperion’s weak gravity also means that any material that does find its way into space as a result of such impacts never returns to the moon.
Today, Cassini’s close encounter with Hyperion, at a distance of about 34,000 kilometers (21,000 miles), occurs at approximately 6:36 a.m. PDT (9:36 a.m. EDT). NASA expects to have images from this latest close pass in one or two days.
Once Cassini leaves Hyperion’s neighborhood, it’s next notable flyby is scheduled for June 16 when the spacecraft will rendezvous, at a distance of 516 kilometers (316 miles), above another of Saturn’s icy moons, Dione. Dione is a small moon, 1,123 kilometers (698 miles) in diameter, that whizzes around Saturn taking just 2.7 Earth days to complete each orbit.
Enceladus, the biggest snow-blower in the solar system
In October 2015, Cassini gets up close to another Saturnian moon, Enceladus. When it comes to weird moons, Enceladus is upsides with Hyperion but in completely different ways. At just 500 kilometers (310 miles) across Enceladus is tiny but despite its small size, it’s one of the most intriguing bodies in the solar system.
In 2005 Cassini’s onboard instrumentation discovered that Enceladus was spewing water vapor from geysers out to a distance three times the moon’s radius. These tiny icy particles, about the width of a human hair, escape Enceladus surface at speeds of up to 400 meters per second (800 mph). The snowy eruptions, which appear to be continuous, deposit a fine layer on Enceladus’ surface and give the moon a halo of fine ice dust. Enceladus’ smooth, almost white surface means that it reflects almost 100 percent of sunlight that falls on it.
Cassini will make two close flybys of the Enceladus, with an intimate closest approach of just 48 kilometers (30 miles).
After departing Enceladus, Cassini will start preparations for its mission finale in 2016. To wind up a mission that will have stretched over almost two decades, Cassini will repeatedly dive through the space between Saturn and its rings.
