Comets are usually named for the person or people who discover them. Comet ISON, however, is named for the International Scientific Optical Network, a group of telescopes and observatories headquartered in Russia.
ISON is a non-periodic comet making its first-known visit to the solar system. It is also a sungrazer, which means it will pass very close to the sun at its perihelion* (pair-eh-HEEL-yehn) on Nov. 29 – fewer than 1 million miles above the sun’s surface. Temperatures there could reach close to 5,000 degrees Fahrenheit. With the combined heat and gravitational pull of the sun, there is a chance that comet ISON will disintegrate during this period.
* An object orbits closest to the sun at a point called the perihelion, meaning “near the sun.” It is farthest from the sun at a point called the aphelion (a-FEEL-yehn).
Orbits and the ecliptic
Most of the objects in our solar system – and each of the eight planets – reside in what’s known as the plane of the ecliptic. In this case, a plane is a flat surface. If you think of the solar system as a DVD or CD, the plane of the ecliptic is the disc itself. The sun is located in the center, and the planets orbit the sun at various distances, while still being on the disc.
For example, Neptune’s orbit, as the solar system’s outermost planet, would be at the disc’s outer edge. Earth’s orbit would be located much closer to the center.
Most of the comets we know of also orbit the sun like the Earth does, but their orbits are much different than our own. Rather than rotating around the sun at roughly the same distance, like the planets, comet orbits can be eccentric, or irregular. They often pass much closer to the sun than Earth does and also travel much farther away. In addition, many long-period comets have orbits that do not belong in our ecliptic plane. Instead, their orbits may take them far above the plane, below it or both. This can affect who on Earth is able to see such a comet.
Looking at ISON
Here are some tips for observing this once-in-a-lifetime visitor:
• View the comet from the darkest area possible, away from the lights of towns or cities. If you live in a city, turn off or block out as much light as possible.
• ISON will be very close to the horizon, where the sky meets the land, at sunset and just before sunrise. Try to find an area where you can see the sun set fully below the horizon. ISON will be close by.
• Do not look directly at the sun!
ISON will be close to the sun, and the sun’s light can harm your eyes. Be safe!
• Local astronomy clubs and observatories may offer opportunities to observe ISON. Check to see if there are any groups in your area.
Fun fact: Though we often think of the shape of Earth’s orbit as a circle, it is actually an ellipse – which is more like an oval.
This computer graphic shows how comet 2013 A1 (Siding Spring) will pass close to Mars in October 2014. Siding Spring is approaching Mars and the rest of the inner solar system from below the ecliptic, and will exit far above the inner planets’ orbits.
A Visit From ISON