Comets (as per the definition) are small extraterrestrial frozen objects that have highly elliptical orbits around the sun, which show a tail-like thingy pointing away from the sun (coma), indicating the outgassing of gas & dust from its surface as it nears the sun. So, ISON is not a comet in that sense, because it’s in a hyperbolic orbit (just kidding, I should start the post someway, no?). It is a comet, but it’s probably our first candidate to have that kind of characteristic feature…
That’s our magnificent ISON (exposure by John Nassr at Stardust observatory)
It’s got nice names. C/2012 S1 is its astronomical name. Yet, it’s been called out as ISON because the two astro fellas who discovered the comet, used data from the ISON network to pinpoint it. It’s also been awarded the “comet of the century”. It was observed (on late September, 2012) when it was 585 million miles away from Sun. As it neared the orbits of Jupiter and then Mars, amateur astronomers started looking for the comet in telescopes, relative to the position of Mars in the night sky. There are millions of mind-boggling photographs and gigantic data, both from outer space and Earth (telescopes) in the internet. Anyways, Hubble telescope’s data (first satisfying observation) showed that the comet’s nucleus is only around 3 km and its orbital plane is almost perpendicular to that of Earth’s….
Hubble’s capture of the comet (when it was 380 million miles away from Sun)
256×256 patches of ISON, as seen by HiRISE in the Mars Reconnaissance Orbiter
In this way, the enthusiasm in photographing the comet started by the end of last year and we’ve speculated that the comet has come from the Oort cloud (a giant bunch of frozen objects orbiting the sun at a distance of about a light year), where most of the astronomical observations on the incoming space-buddies are made (observation timeline of ISON). Okay, this comet has its own specialties.
How special is this ISON?
The necessary scoop is this. As far as we’ve observed, it follows a nearly hyperbolic trajectory (something like the one I’ve drawn). Now, this animation has its flaws. Note the time period of the comet in the trajectory. The comet-like thingy is moving with an uniform velocity (except when it’s nearer to the perihelion). In reality, a comet accelerates as it nears the sun (and you should click that link, it shows LIVE info of ISON, where you’ll see the increment in velocity each second) and decelerates as it gets away from it. But never mind, that flaw isn’t a big issue (provided the other ones we gotta handle). Okay, the hyperbolic orbit explains why the comet hasn’t been observed in the past. Because, it’s paying its first visit to our solar system. We don’t know whether it’s a giant elliptical orbit (maybe it is), but as far as we know, it appears like a rare hyperbolic species.
That being said, it’s also worth phrasing that the comet is a sun-grazer. This means that the comet gets very close to the sun at its perihelion (just a fancy name for the orbit’s end nearer to the sun, the other end is called the aphelion). When the comet reaches its perihelion, its height (from the sun) would be around 700,000 miles (1.2 million km). The above animation is applicable to a large sun-grazing comet like our ISON.
How SOHO‘s LASCO would see ISON’s perihelion approach… (orbit-plot over its coronagraph)…
The number may appear as if it’s a giant value. But, it’s 125 times closer to the sun, than us (where temperatures may reach from 4000-5000 degrees). This means that the comet is gonna get cooked up as it grazes over the sun. But, it’s quite lucky. Astronomers say that it will only be half-baked. While small grazers melt and fragment into fine particles and rain into the sun like a shower, as they get closer into the corona (the same happens when anything enters Earth’s atmosphere – with the only difference that the former is due to radiative heat transfer, while the latter is due to heat from drag, which is convective) somewhat larger ones can make their way out.
Our comet ISON comes under those lucky ones. It’s quite larger in its size and its orbital velocity at the perihelion is sufficient enough (probably a few hundred kilometers per second) for it to get out of the sun’s atmosphere. So, it’s very likely for ISON to escape from sun’s rage with mild injuries (melting and fragmentation). Anyways, by the time it reaches the perihelion (28th of this November), it will appear bright enough (probably brighter than the Venus itself) to get caught in our eyes.
ISON viewers, get ready!!!
Over the last few weeks, the magnitude of ISON has decreased from +13 to +9. Once the magnitude reaches +6 or +5, we may get a chance to spot it out in the night sky. But still, you can look for ISON in the night sky starting from today, till 23rd of this November (an hour before dawn, towards southeast, some degrees above the horizon), near Spica in the constellation of Virgo (shown above). I mean, it will be visible to our naked eyes. The comet would be no different from a visible-dim star, but still you’ll be needing a pair of binoculars, or you’d have to go to suburbs to view it (and that’s because city-lights are one disturbing factor for our astronomical observations). In addition to that, astronomers have also calculated that the comet would’ve to be burnt like hell, in order for us to actually see it with our naked eyes (i.e) +3 or +4 magnitude would be good enough.
So, the best naked-eye visibility starts, once the comet has passed the perihelion which is, from the start of December (till the mid-January – everyday, an hour before the dawn, towards east, very few degrees above the horizon). You don’t have to memorize all that. I’ve found a wonderful simulation (embedded below) from Solar System Telescope, which is more than enough for you.
Adjust the simulation and you can find the position of the comet at your location. But, I’m quite sure it’s just before dawn for everyone…
My humble advice:
- Whenever you get a simulation, Rip it off..!!! Show no mercy, enjoy it to the core. For instance, the above simulation is interactive. It can work like a mini-Celestia or a mini-Stellarium. It has a timeline and a date & clock indicator, where you can play around and see the positions of ISON or other planets in the solar system. You can fast forward or backward using the orbits of objects, change the view (switch to Earth), and detect your location so that you can know where exactly from the horizon you’re gonna see the comet (or planet, or constellation), which direction you should look for them at a specific time, etc.
- And before looking out for the comet in your backyard, watch how one should watch the comet…
Now, set your alarm. Try spotting our 8-million kilometer tailed-guy everyday (we don’t know whether situation pleases us or not, because sometimes it can be cloudy, sometimes hazy, sometimes you won’t wake up, etc.) before dawn. You won’t miss him, as these six days (Nov. 17 to Nov. 23) are just the start. Major event is from the start of December. The simulation also shows that the comet would be closer to Earth (only about half the distance from Sun) at the end of December. At least, try not to miss him thereafter, and don’t forget to say “goodbye”…
(In case you’d like to know how it’s gonna appear in the sky…)