The Other Side of ISON: Here’s the Comet as Seen from Mercury

MESSENGER image of the approaching ISON from Mercury
MESSENGER image of the approaching ISON from Mercury

While many skywatchers, scientists, and astronomy enthusiasts around the world wait to see if comet ISON survives its perihelion — that is, its closest pass by the Sun — on Nov. 28, the MESSENGER spacecraft has captured an image of the incoming comet from its position in orbit around Mercury!

The image above, shared today on the MESSENGER website, shows ISON from a distance of 22.5 million miles, and just over 42 million miles from the Sun. At perihelion ISON will come within a scant 730,000 miles of the Sun. Whether or not it survives its Thanksgiving Day encounter has yet to be seen.

MESSENGER had actually first imaged ISON on Nov. 9. At that point it was just a barely-visible fuzzy spot. By the time the image above was acquired on Nov. 20 ISON had enlarged dramatically and its tail had become visible.

In addition to ISON, the periodic comet Encke was also captured by MESSENGER.

MESSENGER image of comet 2P/Encke, Nov. 17, 2013
MESSENGER image of comet 2P/Encke, Nov. 17, 2013

Encke reached the perihelion of its 3.3-year-long orbit around the Sun on Nov. 21. Encke is a very well-known comet and will return in another three years; ISON, on the other hand, has never visited the inner solar system before and there’s a chance that, even if it survives perihelion, it never will again.

MESSENGER obtained views of these comets that are very different from those of Earth-based observers. “MESSENGER imaged Encke only a few days before its perihelion when it was at its brightest,” explains Ron Vervack, of the Johns Hopkins University Applied Physics Laboratory, who is leading MESSENGER’s comet-observation campaign. “That we are so close to the comet at this time offers a chance to make important observations that could shed light on its asymmetric behavior about perihelion.”

In contrast, ISON did not pass as close to Mercury, but the comet was between the Earth and Mercury when it passed closest to MESSENGER, providing a unique perspective. “We saw the side opposite to that visible from Earth,” says Vervack, “so our images and spectra are complementary to observations from Earth made at the same time and could aid in understanding the variable activity of the comet as it approached the Sun.”

It’s also important to note that a planetary-research spacecraft like MESSENGER is not made to spot comets, yet it did so beautifully.

“Comet encounters were not considered when the MESSENGER mission was designed,” adds MESSENGER Principal Investigator Sean Solomon of Columbia University. “If Encke and ISON share a few of their secrets on the formation and evolution of the Solar System, the MESSENGER team will be delighted with the scientific bonus.” (Source/read more here)

Also, here’s an animation made from images of ISON and Encke acquired by NASA’s STEREO-A solar-observing spacecraft, currently positioned on the opposite side of the Sun:

Comet ISON entered the STEREO scene with Encke on Nov. 21 (Credit: Karl Battams/NASA/STEREO/CIOC)
Comet ISON entered the STEREO scene with Encke on Nov. 21 (Credit: Karl Battams/NASA/STEREO/CIOC)

Check out those tails! Flapping in the solar wind “like wind socks,” according to The Planetary Society’s Emily Lakdawalla during a comet-themed Hangout earlier today.

Read more about this in my article on Universe Today here.

MESSENGER image credits: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/Southwest Research Institute


  1. Jim in IA says:

    I was at the Messenger site only a few days ago. What a large change in brightness.


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