Comets ISON and Encke as seen by NASA's Solar TErrestrial RElations Observatory (STEREO-A) on Nov. 22.
Comet ISON is currently diving through the inner solar system, soon to make a close approach with the sun on Nov. 28. Known as a "sungrazer comet," ISON will dive deep into the sun's corona, enduring the intense multi-million degree plasma and powerful solar energy. Since the "pristine" comet was discovered in September 2012, ground-based observatories, space probes and space telescopes alike have been trying to grab a glimpse of what could be the "Comet of the Century" should it survive its solar close approach. Although many of the space missions were not designed to take snapshots of comets, it hasn't stopped many from trying and contributing valuable science as we continue to track ISON's progress. With the help ofthe NASA Comet ISON Observing Campaign
, nine space missions are listed as providing observations of Comet ISON. More will inevitably join the fun, but here's the story so far.PHOTOS: Comet ISON: 5 Things You Should Know
On Jan. 17-18, 2013, NASA's Deep Impact spacecraft imaged Comet ISON during its deep space sojourn from a distance of nearly 500 million miles. From this early observation mission scientists were able to deduce the comet tail was already more than 40,000 miles long, despite the fact it was around the orbit of Jupiter.ANALYSIS: NASA Probe Spies Incoming Comet ISON
Shortly after NASA's Deep Impact (EPOXI) mission imaged ISON, mission managers of NASA's Swift space observatory commanded the mission to take a look. Although Swift is not designed to observe comets (it is, after all a gamma-ray observatory!), it used its Ultraviolet/Optical Telescope (UVOT) to gain a valuable insight to the comet and its growing coma and tail.
NASA's Spitzer Space Telescope is more at home taking snapshots of the infrared emissions of distant star-forming regions, but that doesn't mean it can't be used to observe comets. Seen here, Comet ISON was observed in two infrared wavelengths on June 13
NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
Of all the space observatories, the Hubble Space Telescope is able to take some of the most spectacular images of the incoming comet, monitoring ISON as it approaches the sun, watching for any sign of breaking up.
On Oct. 8, 2013, NASA's Mars Reconnaissance Orbiter was commanded to slew away from it's Mars-facing orientation and direct its HiRISE camera at ISON. Although the comet was exhibiting little activity, HiRISE managed to pick it out of the dark.
During close approach with Mars, it wasn't only the MRO that snapped Comet ISON. The veteran Solar and Heliospheric Observatory (SoHO) was able to detect the comet with its wide-angle Solar Wind ANisotropies, or SWAN, instrument.
On Oct. 28, 2013, Comet ISON drifted into the field of view of one of NASA's twin solar observatories, STEREO B. One of two solar satellites, STEREO B is following Earth in its orbit (while STEREO A is leading) with a unique view from the far side of the sun.
On Nov. 4, NASA's Chandra X-Ray Observatory got in on the ISON imaging act, analyzing the structure of the comet. Overall, ISON's X-ray revealed it's a pretty typical comet, shedding 16 Olympic-sized swimming pools worth of water per day.
On Nov. 18 and 19 it wasn't only Comet ISON that passed the orbit of Mercury, Comet Encke did the inner solar system dash, too. Earlier in the month, NASA's Mercury orbiter MESSENGER was able to image ISON and Encke as they approached, reminding us there is usually more than one comet flying through the solar system at any given time.ANALYSIS: Fly With ISON In Stunning Comet Simulation
Comets that stray too close to the sun have a very uncertain future. Some live to tell the tale, while others literally lose their tails. And as we get excited for ISON's hellish dive deep into the solar corona, astronomers are hoping that the potential "Comet of the Century" will also get the chop.
Comets are composed of rock and ice that form their nucleus. As they approach the sun, their icy masses begin to sublimate into space, creating a large cloud of gas known as a coma. As they fall ever deeper into the sun's gravitational well, the solar wind "blows" back these gases to produce a bright tail. In actuality, comets usually have two tails; one that is composed of neutral particles and created purely by solar wind pressure and another that is composed of charged particles and blown along the sun's interplanetary magnetic field (IMF).
As one would expect, while comets travel through the tumultuous stream of solar gas that flows throughout the solar system, any transient bursts in energy or changes in solar wind density can impact a comet's tail.
In 2007, solar astronomers tracking the progress of Comet Encke as it pushed deeper into the corona (our sun's hot atmosphere) saw a rare event. A coronal mass ejection (CME) erupted from the sun and collided with the comet. The energized gas embedded inside the CME's magnetic bubble wasn't dense enough to impact the comet's nucleus in any measurable way, but it did have a dramatic effect on Encke's tail: it was ripped off.
Now the sun is at solar maximum, solar activity is much higher than it was in 2007 (despite the fact that this solar cycle is fairly weak in comparison to previous cycles), so astronomers are waiting with bated breath just in case our nearest star should hurl a magnetic tantrum in Comet ISON's direction.
"I would absolutely love to see Comet ISON get hit by a big CME," said astronomer Karl Battams, of the Naval Research Lab and NASA's Comet ISON Observing Campaign (CIOC). "It won't hurt the comet, but it would give us a chance to study extreme interactions with the comet's tail."
Space weather activity is one factor, but ISON will also dive much deeper into the corona than Encke did -- some 30 times closer, with a close approach (perihelion) of a little over a million kilometers from the sun's 'surface' (photosphere) on Nov. 28.
"The CME that ran over Comet Encke back in 2007 was slow, barely creating a pressure pulse by compressing the solar wind ahead of it," said Angelos Vourlidas also of the Naval Research Lab and CIOC. "It was this compression which caused the Encke's tail to fly off."
In ISON's case, a CME impact will likely be far more dramatic. "Any CME that hits Comet ISON close to the sun would very likely be faster, driving a shock wave with a much stronger magnetic field," added Vourlidas. "Frankly, we can't predict what would happen."
Coincidentally, Comet ISON has the veteran Comet Encke for company on its first dive into the corona and a fleet of solar observatories will be watching the pair's tails being buffeted by the solar wind.
We currently have no direct means to probe the inner corona, so observing any object interacting with the superheated plasma deep inside the sun's atmosphere is a valuable science opportunity. But will the sun oblige and fling a CME at ISON? That will be left to chance, but both comets are in a prime spot for a CME collision.