Voyager - to Uranus

These images are from a Sky at Night programme from 1986, when Voyager 2, which was launched in August 1977, had travelled beyond Saturn to this distant "ice giant" (temperature about -216 degrees). (Watch it and others on the BBC iplayer, here.)

Uranus has a small rocky core, but mostly consists of a hot, dense fluid of  water, methane and ammonia. It orbits the sun every 84 years, and rotates on its axis every 17 hours.

Interestingly, its axis is perpendicular to its orbit - the "hot spot" in the image is the south pole - and even more interestingly, because of the retention of heat by the atmosphere, and convection in its fluids, the temperature is much the same as at the north pole. The magnetic axis is tipped at 55 degrees from the axis of rotation, and the magnetic field is about a third the strength of Earth's - the planet's fast rotation creates a dynamo effect. 

Images show that Uranus has banding - the red spot is the south pole

The "donut" circles in the images are artefacts, due to the camera, and the pink crescent is an artefact of a different sort. The colours arise from the different filters used in separate photographs, for each of which the radius is different; combining them leaves "bare areas" around the edge -

 Special methane filters were used to look at the atmosphere. The blue crescent indicates the area of the planet that's free of high haze layers; the red ring shows areas of haze around the equator -

The atmosphere is now known to contain hydrogen, helium, and a little methane.

It was known from telescopic observation that Uranus had five moons, and Voyager discovered two more; now, it is known to have 27 moons, all named after characters in the works of Shakespeare and Alexander Pope.

Miranda "has canyons like Mars, grooves like Ganymede,
and compressional forms like Mercury" - a range of terrain

Umbriel has an old surface with very large craters

Titania has small craters and lots of rifts

The first picture of the 9 rings - some have "shepherd satellites" that
keep the fine particles of the rings organised

A longer exposure (96 seconds) in scattering light;
the long specks are due to the length of the exposure
as Voyager sped past at about a million miles a day,
50,000 miles above the planet

Next planet: Neptune

Having visited four planets,Voyager 2 is now - more than 40 years after launch - beyond the solar system, in interstellar space, having visited Neptune in 1989. The last solid body it studied was Neptune's moon Triton. It's travelling at 470 million kilometers a year, and Voyager 1, which visited Jupiter and Saturn, has also left the solar system, travelling 520 million kilometers a year on a different trajectory.

In those four decades, technology has made leaps and bounds - state of the art in 1986 still included computers with green screens, remember those? Digital manipulation - making false-colour images - took "a day or so", partly because of the low light levels at that distance from the sun.