The moons of Saturn

In honour of the Cassini probe, the diagram above shows (to scale) the larger moons of Saturn (those over 100 km in diameter). The moons are shown in order of distance from Saturn, with Janus and Epimetheus being the closest. Titan comes between Rhea and Hyperion.

Janus, Epimetheus, Mimas, Rhea, and Phoebe are shown in black-and-white. Enceladus, Tethys, and Dione are shown in enhanced colour (combining infrared, visible, and ultraviolet). Titan is shown in infrared, which penetrates the atmospheric haze. Hyperion is in true colour, and Iapetus in false colour. Click on the image to zoom.


Silly Season in the Sky (again)

It’s apparently time for lunatic end-of-the-world prophecies again. The latest relates to a “great red dragon” in the sky (a reference to Revelation 12):

Turns out that this is a double image of Saturn, taken way back in 1983 by the Infrared Astronomical Satellite (IRAS). Saturn was then at RA 13h45m18.2s, DEC -08d13m07s.

It’s a false-colour image (i.e. not red at all), being taken at 100 microns, in the infrared region of the spectrum. But with enough spin, apparently it can be made to sound scary.

In the final version of the infrared sky survey, this artefact was blacked out, since it doesn’t reflect any actual stellar infrared sources (just a planet that moves around). Of course, that removal got the conspiracy-theory nutters going.

Spaceprobes in the Solar System

The diagram above (click to zoom) and list below show currently active spacecraft in the Solar System, not including those operating close to the Earth (and I’ve probably missed a few):

  • Voyager 1 and Voyager 2, launched in 1977, now heading into the darkness and still reporting back, even though they are over 17 billion km or 16 light-hours away (not shown above).
  • Cassini–Huygens, launched in 1997, now in the final stages of its exploration of Saturn (see fact sheet).
  • 2001 Mars Odyssey, launched in 2001, and still orbiting Mars.
  • New Horizons, launched in 2006, now en route from Pluto to the Kuiper belt object 2014 MU69, which it should reach in January 2019.
  • Dawn, launched in 2007, currently orbiting Ceres (image shown above the Earth).
  • Akatsuki, launched in 2010, currently orbiting Venus, which it will do until 2018.
  • Juno, launched in 2011, currently orbiting Jupiter, which it will do until February 2018.
  • Hayabusa 2, launched in 2014, currently en route to asteroid 162173 Ryugu, which it should reach in 2018. It will then take a sample which should arrive back home in 2020 (not shown above).
  • ExoMars Trace Gas Orbiter, launched in 2016, currently orbiting Mars and mapping the Martian atmosphere (the associated Mars lander was lost in 2016).
  • OSIRIS-REx, launched in 2016, currently en route to asteroid 101955 Bennu, which it should reach in 2018. It will then take a sample which should arrive back home in 2023 (image shown below the Earth).

In addition to the above, BepiColombo is scheduled to launch for Mercury in October 2018, and SolO is scheduled to launch for the Sun that same month. Also, InSight is scheduled to launch for Mars in May 2018, and Solar Probe Plus is scheduled to launch for the Sun in August 2018.

Metallic hydrogen

It has long been known that hydrogen forms a metallic solid at moderate to low temperatures and extremely high pressures. Last year, Ranga Dias and Isaac F. Silvera at Harvard University were the first to actually produce this metal, using the enormous pressure of 495 gigapascals (see their paper).

At higher temperatures, hydrogen forms a metallic liquid, and this is believed to exist at the heart of Jupiter and Saturn. This liquid has yet to be observed.

Cassini mosaic of Saturn

This wonderful mosaic image of Saturn (click for more details and larger image) is derived from 141 wide-angle images taken by the hard-working Cassini spacecraft. The image covers a region about 651,591 km (404,880 mi) across. As in this previously released image, Earth is clearly visible.

A glimpse of home

This superb NASA/JPL/SSI photograph from 19 July, taken by the hard-working Cassini spacecraft, looks back 1,440,000,000 km to our home planet – the blue dot below Saturn’s rings. Click on the photograph for details, and for larger versions of the image.

I pray for one last landing
On the globe that gave me birth;
Let me rest my eyes on the fleecy skies
And the cool, green hills of Earth.
” – Robert A. Heinlein

The lakes of Titan

The lakes of Saturn’s moon Titan (seen in a synthetic-aperture radar montage above) are most likely composed of methane and ethane: two natural gases which boil at a chilly −161.5 °C and −89 °C respectively. Compared to that, Australian winter weather is warm!

Unlike the mountains of Titan, the lakes are named after lakes on Earth, while the larger maria are named after sea-monsters like the Punga and the Kraken. In reality, there are no sirens on these seas. Well, probably not.