Let's take a moment to marvel at the design of the Voyager spacecraft. Built using 1970s technology - computing hardware/software, instruments, RTGs, thrusters, comms, etc. - most of which are still operational after 47 years in the harsh environment of space, 23:14 and 19:30 light hours away resp., with no maintenance or repairs, just workarounds around few failures, quietly exploring where no spacecraft has explored before.

https://science.nasa.gov/mission/voyager/where-are-voyager-1-and-voyager-2-now/
#Voyager
3/n

@ClaireLamman IMG! That’s fantastic! My holidays are better for this #gingerbread #voyager Is the plaque on there?

Cassini surmised, correctly, that Iapetus had a dark side and a bright side, and could be seen easily only when the bright side was facing the observer. As the visibility was always good on the same side of the planet he concluded Iapetus was in synchronous rotation like our Moon.

Here is one of the Voyager 1 images of the area I want to examine. Not much to see... that dark ring is presumably a crater but we don't see it very clearly. But we will see more... #Saturn #Voyager #iapetus

I am now turning to Iapetus, a moon of Saturn which was not well seen by Voyager. Let's start with a map from the USGS based on the Voyager images of Iapetus. The most prominent thing is the vast black area now called Cassini Regio. J. D. Cassini discovered the moon in 1671 but he could only see it when on one side of Saturn. Later with a better telescope he could see it faintly on the other side of the planet. What's going on? #Saturn #Voyager #Iapetus

This observation of Mimaldi shows that there can be unexpected finds in Voyager images. That's not so important when we have Cassini images, but what about places where we don't have great images and not much chance of getting better ones for a long time? We've seen a few moons of Uranus here, but what about Neptune's Triton, especially the side not seen closely. Or the other sides of Pluto and Charon? Tomorrow, something much more dramatic than Mimaldi. #Saturn #Voyager #Rhea

Let's have a look for Tirawa. Here is an image - actually a composite of a few frames of a multispectral sequence - showing the right part of Rhea. It's near a full phase, so not much topographic shading. The black arrows show the full outline of the Tirawa basin. But what about the white arrows? They outline another basin called Mimaldi (see yesterday's map with names). It wasn't noticed at the time of Voyager and I wasn't confident enough to argue for it. #Saturn #Voyager #Rhea

Here is the set of Voyager images of Saturn's moon Rhea. The blank square separates Voyager 1 images (top) from Voyager 2 images, just 6 of the latter. The closer images show lots of craters - wall to wall craters. That one crescent image of Rhea shows an arc of shadow which is something bigger, an impact basin which was named Tirawa. It's not easy to find in the other images but it is there. Is it alone? #Saturn #Voyager #Rhea

Evander is located in the unmapped southern section of Dione in that USGS map. The map could have been extended towards both poles using low resolution images. This isn't the only large impact basin which escaped notice in Voyager images. I already mentioned Tethys in that respect. Tomorrow we will see another example... but I'm saving the best until last. #Saturn #Voyager #Dione

Those images of Dione are hiding something. Here is one of the distant images enlarged to show an oblique view of a 300 km diameter circular impact basin. It's now called Evander. Here is a Cassini map with names:

https://asc-planetarynames-data.s3.us-west-2.amazonaws.com/dione_comp.pdf

And here is my 2002 LPSC abstract on its discovery:

https://www.lpi.usra.edu/meetings/lpsc2002/pdf/1553.pdf

I presented this but never wrote it up, being bogged down in much bigger projects at the time (and ever since).

This is the full set of useful images of Dione (by useful I mean useful for geology, not distant navigation images or very narrow crescents). The top set of 13 images are Voyager 1, the last 5 are Voyager 2. It's a beautiful world with craters, fractures and bright streaks. One image does not have a typical dark background - it was imaged against Saturn itself. #Saturn #Voyager #Dione

Moving on to Dione - I didn't make a map but here is a US Geological Survey airbrushed map made using Voyager images. I loath this map projection but 40 years ago it was very fashionable because it made it easy to compress digital files, back when it mattered how big a file was. The distortions along the diagonal sides are horrible. But it shows image coverage. The poles are missing. That bit of the equator not shown is not really missing, as we will see. #maps #Saturn #Voyager #Dione

Here is the northern hemisphere of Mimas in that full Voyager mosaic.

These maps are never going to look great compared with Cassini imaging. My intention here is to show how some details not noticed at the time were indeed present in Voyager images. We will see some more of these in the coming days. #maps #Saturn #Voyager #Mimas

At last, here is the composite map using Voyager 1 and 2 images of little Mimas. Most of the surface is covered. You might like to compare it with this Cassini map:

https://www.jpl.nasa.gov/images/pia17214-mimas-global-map-june-2017/

I pointed out that my map and the USGS map don't agree in crater positions in places. Neither of them match this very well. Pointing uncertainties, an incorrect shape model of Mimas in early mapping (not really a sphere) and other errors affect both of them. #Saturn #Voyager #Mimas