Rosetta’s Shadow Darkens An Unfamiliar Landscape

Rosetta took this photo of Comet Churyumov-Gerasimenko from an altitude of just 3.7 miles (6 km) on Feb. 14. Details as small as 4-inches across are visible as is the spacecraft's shadow (bottom). Click to enlarge. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Rosetta took this photo of Comet Churyumov-Gerasimenko from an altitude of just 3.7 miles on Feb. 14. Details as small as 4-inches (11 cm) across are visible as is the spacecraft’s shadow (bottom). Click to enlarge. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Can you imagine how close Rosetta must be to comet 67P/C-G to see its own shadow? Try 19,500 feet. This photo was taken during its closest flyby yet on Valentine’s Day and reveals an exotic landscape of rock, ice and dust from an altitude of just 3.7 miles (6 km), about half the altitude of a typical jet.

Picture yourself looking out the window of a plane well before it reaches cruising altitude. While the landscape resembles the barren cliffs, canyons and hills of the southwestern U.S. it has its own unique platy cragginess. To the left, the rugged cliffs give way smoother, dust-covered terrain.

The OSIRIS narrow-angle camera image from the 14 February close flyby (bottom left) shown here in context with Navigation Camera images (top left, top right and bottom right). Credit: NAVCAM: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0; OSIRIS: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
The OSIRIS narrow-angle camera image from the February 14 close flyby (bottom left) is shown here in context with Navigation Camera images (top left, top right and bottom right) so you get a better idea of where on the comet the photo was made. Credit: NAVCAM: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0; OSIRIS: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The shadow at bottom measures about 65 x 160 feet (20×50 meters), and the frame covers an area of just 750 feet square (250-m).

During the flyby, Rosetta passed through a unique observational geometry; for a short time, the Sun, spacecraft, and comet were exactly aligned. As seen from above, surface structures on the comet cast almost no shadows, allowing scientists to precisely measure the reflection properties of the materials, including determining the sizes of the mineral and ice grains.

Graphic to illustrate the difference between how a sharp shadow is generated by a point source (left) and a fuzzy shadow by a diffuse source (right). Credits: Spacecraft: ESA/ATG medialab. Comet background: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
Graphic showing difference between how a sharp shadow is generated by a point source (left) and a fuzzy shadow by a diffuse source (right). Credits: Spacecraft: ESA/ATG medialab. Comet background: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

Coming back to the shadow, notice how fuzzy it is. Even though the Sun appeared only 0.2° across (0.5° from Earth) from Rosetta’s distance of 215.6 million miles at the time, it’s still not a point source. Only point sources of light create precisely sharp shadows identical in size to the objects that produce them. Rosetta’s shadow has a fuzzy, soft-edge “penumbra” that adds an additional 65 feet (20-m) to its size.

If you look again at the first image at the start of this blog, you’ll see a faintly bright halo around Rosetta’s shadow. This is primarily caused by the “opposition effect” or shadow hiding. Grains directly below Rosetta in line with the Sun cast no shadows and so appear brighter than other grains nearby casting very short shadows.

8 Responses

  1. Edward M. Boll

    I again looked at the bright comet chronicles by John Bortle which came out in 1998. Hale- Bopp was the last one mentioned. I can only think in the last 17 years 2 of which may have qualified for that. McNaught in January of 2007 and Panstaars L4 in 2013. We hope that US Catalina makes it this Fall.

      1. Ray

        From its absolute magnitude it seems to be in the larger size range…so even though ‘new’ comets are supposed to be unpredictable, most of the new comets at comparable perihelia and of similar absolute magnitude (C/2012 K1, C/2009 P1) performed basically as expected. I eagerly await the first observations from the Southern Hemisphere in April!

          1. Ray

            Yes, that will be quite exciting! I’m hoping there will be an outburst so that we can see the direct cause from Rosetta. Too bad we will never get a spacecraft to 29P. I would very much like to know why some comets outburst more frequently than others.

          2. astrobob

            Ray,
            I find 29P fascinating as well. I’ve seen many comets, some multiple times, but 29P’s outbursts are so regular and dramatic I’ve seen it nearly every year since the mid-80s.

  2. Edward M. Boll

    Comet Lovejoy would still be very easy to find with binoculars if it were not for brilliant moonlight.

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