The Death of a Comet: ISON's Post-Mortem
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
When the “Comet of the Century” died to a solar roasting last year, the world was sad. Hopes of a beautiful cometary display rapidly evaporated as space observatories relayed images of the once-proud dirty snowball become a pulverized cloud of sublimating ice and scattered dust.
So what happened to Comet ISON and its promise of a celestial spectacular?
According to new research published in the journal Astronomy & Astrophysics, it seems the fate of the icy mass was sealed before it made its suicidal 1.8 million kilometer close approach of the sun (an event known as perihelion).
“Our measurements and calculations indicate that ISON ran out of steam before perihelion,” said lead researcher Werner Curdt, of the Max Planck Institute for Solar System Research (MPS).
Using data from the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument on board the joint NASA/ESA Solar and Heliospheric Observatory (SOHO), Curdt and his team were able to get a very privileged view of ISON as it barreled deep into the solar corona, the sun’s multimillion degree atmosphere.
Only hours before ISON’s Nov. 28, 2013, close approach, the comet stopped producing a tail of vapor and dust, which is strange considering its surface would have been rapidly heating up. According to analysis of SUMER observations, around 8.5 hours before perihelion, the comet released a “short and violent outburst” that extinguished any further release of gas and dust.
Before the outburst, ISON was acting just as it should during solar approach — a beautiful, long tail created a bright arc in observations by the Large Angle and Spectrometric Coronagraph (LASCO) also on board SOHO. LASCO is used by solar astronomers to observe the region surrounding the sun to track space weather events such as coronal mass ejections (CMEs). But LASCO also has a dazzling track record of tracking ‘sungrazing comets’ that regularly drop deep into the corona.
As ISON dropped closer to the sun, the LASCO view was lost as the comet disappeared behind the instrument’s occulter — a component of LASCO that blocks the glare of the sun so faint structures in the corona can be resolved. So the only instrument that could track ISON’s final moments was SUMER.
Although SUMER isn’t designed to analyze disintegrating comets, it was able to recover images of ISON’s tail in ultraviolet light. “The only instrument that could obtain serviceable data at this time was SUMER,” said Curdt. “For everyone involved, this was a huge challenge.”
During perihelion, SUMER revealed a curved, pointed tail at least a quarter of a million kilometers long. But there was no sign of the comet’s nucleus. The Max Planck team simulated various scenarios that could replicate the tail they were seeing, assuming the nucleus was still intact and active.
“We were not able to reconstruct anything resembling our images, assuming that ISON was still active during the SUMER observations,” said co-investigator Hermann Böhnhardt, also from the MPS.
This has led the researchers to believe that the SUMER observations most likely fit computer simulations of a cometary nucleus that ceased producing gas and dust hours before perihelion.
Although the researchers cannot definitely say that the nucleus disintegrated before close approach, the fact that there was a rapid outburst 8.5 hours before is indicative of a disintegration event. The outburst generated a cloud of approximately 11,500 tons of dust in a sphere 280 meters across.
So these observations support the idea that ISON broke apart before its closest approach, leaving just a cloud of debris to continue ISON’s orbit like an eerie cometary apparition, and certainly nothing resembling the Comet of the Century.