We’re all used to seeing fantastic images of Saturn and its family of moons from NASA’s Cassini spacecraft, which has spent the last decade in orbit around the ringed world. But every now and then Cassini aims its cameras outwards, capturing images of the sky beyond Saturn – just like we might look up at the stars from here on Earth. And while it’s not designed to be a deep-space observatory like Hubble or Subaru (or even like a modest backyard telescope, really) Cassini can still resolve many of the same stars we can easily see in the night sky… and, on April 12, 2015, it spotted something much farther away: the Sombrero Galaxy (M104), 29 million light-years distant!
Coincidentally Cassini grabbed its image of M104 exactly five years after it was imaged with Japan’s Subaru Telescope, located atop Mauna Kea.
In the original raw image (N00238002) posted to the Cassini mission site the field of view is 1. rotated 180º from what we’d typically consider “north” (common in Cassini raw data) and 2. chock full of pixel noise. The latter is due to the long exposure time of the acquisition – stars, of course, are much dimmer than a giant planet or its icy, reflective moons.
But with a rotation of the image and a removal of noise, what’s left is the general shape of the bright blurry object at center and remaining stars, slightly elongated due to spacecraft movement during exposure.
Compare that to astrophotos of M104 and it’s clear that everything lines up – this is a galaxy, seen from Saturn! Even the Sombrero’s signature dark dust lines, seen nearly edge-on, can be made out.
Of course when an object is 28 million light years away from Earth getting a view from a mere 900 million miles closer doesn’t make it any more detailed – and again, Cassini isn’t a deep-space telescope. But with an apparent magnitude of +8 M104 is quite bright for astronomical objects, although just outside the boundary of what we can spot with our naked eyes (but it is visible with binoculars.)
(A tip of the old sombrero to Mr. Kevin M. Gill, who noted the raw image and commented on its interesting nature on Twitter, sparking further investigation.)
Update 5/10/18: I’ve recently been playing around with the raw data available on NASA’s PDS and have put together an image of M104 using that, which is much better than the preview-quality JPEGs. See below, which combines clear and color-channel images taken on April 12, 2015: