Comet ISON is a sungrazing comet whose closest approach to the Sun (perihelion) will happen on November 28, 2013. Comet ISON was discovered last year by two Russian observers when it was further from the Sun than Jupiter. This usually means the comet is large and will become bright. Comet ISON's nucleus is thought to be about 2 miles across (no more than 4 km from HST observations). It is currently approaching the frost line at 2-3 AU where water ice will start to evaporate and the comet will brighten. We had a Comet ISON workshop last week at APL in Maryland. You can go to that site for information about Comet ISON and the many ways people will be observing it.
Comet ISON may become a spectacular comet in the night sky but SDO cannot see a comet until it is passing very close to the Sun. This graphic was developed to show what we learn from Comet ISON that is different from the previous comets that were seen by the EUV telescopes on SDO and STEREO.
The numbers on the left are the distances from the center of the Sun, with curves drawn at each integer distance from 1-4 solar radii (1-4 Rsun).
Next are the perihelion distances of the two sungrazing comets seen in the EUV and Comet ISON, also extended as curves across the diagram. Sungrazing comets do not dive into the Sun, they appear to skim along the surface of the Sun. That allows for a lot of heating from the hot Sun but also means they don't burn up from friction like a meteor in the Earth's atmosphere. Many things change as you adjust the perihelion distance. For example, Comet Lovejoy was moving at 575 km/s (1.3 million mph) at a perihelion distance of 1.15 Rsun. Comet ISON will be sailing along at a mere 375 km/s (840,000 mph) at its perihelion. Comet Lovejoy took about 1 hour to go last part of its orbit (from 2.3 Rsun to 1.15 Rsun). Comet ISON will take over 3 hours to do the same, but this time moving from 5.4 Rsun to 2.7 Rsun.
"Breaking Up" shows the locations of the Roche limits for fluid and solid spheres. If a fluid ball flies closer to the Sun then the top of this bar it will break up into fragments, if a solid ball flies closer than the bottom of the bar it will break up. The first two comets were likely to fall apart because they flew so close, well below the Roche limit for a solid ball, but Comet ISON will fly through a region where it may or may not break up.
"Ice Evaporation" is an indication of the heating by solar radiation compared to the heating at the photosphere (or 1 Rsun). The heating of the comet by the Sun increases as the comet gets closer to the surface, but another effect that I call the two-pulley problem increases the heating even more. The first is the proximity effect but the second only happens when a small body gets close to a large body. More than half of the small body gets heated at once. That means the heating rate at 4 Rsun is 30 times less than the rate at the Sun's surface rather then the 16 ([1/4]4) times less you would think from the proximity effect alone. For comparison, the heating rate at the Earth is 50000 times smaller than the heating just above the surface of the Sun!
"Melting Sand" shows the region below 4 Rsun where micrometeoroids about 1 micron in size rapidly evaporate. Micrometeoroids are small sand particles in the comet ice. As the ice evaporates the particles are released and they also evaporate as they are heated by the Sun. The location of this boundary varies a great deal with the model of the micrometeoroid and will be tested by ISON where it leaves the material behind.
The "Visibility" grey bar, is the focus of our work on ISON into what makes the comet debris bright in the EUV. It is our primary known unknown at this time.
The "Escape" bar shows the solar wind acceleration region extending from 1.1 to above 4 Rsun. Debris from Comet ISON may be swept out into the heliosphere by the same acceleration that removes the solar wind from the Sun. Observations of Comet ISON may allow us to watch this happen. A lot of debris from the earlier two comets was left in the coronal loops and will slowly become part of the solar wind.
The coronal loops and helmet streamers show the types of magnetic fields the comets will fly through. At Comet ISON the magnetic field is mostly up and down, only the helmet streamers stick up this high in the corona. Closer to the Sun the field forms coronal loops and the debris is trapped near the surface. This is why we saw the tail of Comet Lovejoy wiggling. The tail of Comet ISON will probably look like swaying stalks as it will leave debris in a much simpler magnetic field.
Have a comet for Thanksgiving!