There were some interesting developments involving comet C/2012 S1 (ISON) over the weekend.

The View From the Far Side of the Sun

ISON is being observed by many spacecraft. One of them is NASA’s STEREO A, one of a pair of spacecraft orbiting the Sun ahead and behind the Earth, to provide a view of the part of the Sun we can’t see from home (the other is, of course, STEREO B). The animation here (by NASA / Karl Battams at the Comet ISON Observing Campaign; click to embiggen) shows the view from Nov. 20 - 23, 2013. There's a lot happening: The Earth is in the field of view (right), as is Mercury (left); the vertical lines coming from them are detector artifacts called blooming. As you can see, there are two comets visible: ISON to the lower left, and 2P/Encke to the right.

Encke has a 3.3-year orbit that takes it as close to the Sun as Mercury and back out well past the orbit of Mars. It just so happens to be near the Sun now, and is seen close to ISON from STEREO’s point of view. The Sun is off to the right, with waves and puffs of solar wind moving right-to-left (note that there's a gap in the data, making the animation jump in the middle). This material is making Encke’s tail flap like a flag in the wind. ISON’s tail is moving too, but not as much; although they look near each other here, the two comets are actually a fair distance apart in space and experiencing different aspects of the wind. 

Interestingly, Encke is physically larger than ISON (5 kilometers versus 2), but appears fainter. Its short orbit means it's been near the Sun many, many times, so it's lost a great deal of the icy material that turns into a gas and makes comets bright when they're young.

I’ll note that viewed from Earth, ISON is only about 15° from the Sun now (it's physically well inside the orbit of Mercury), and is essentially impossible to observe from the ground without a great deal of experience; it rises so close to sunrise that by the time it’s up high enough to see, the sky is too bright to spot it.

Will ISON Ever Come Back?

On Friday last week I wrote a bit about escape velocity, and how that means ISON may be on its first and only pass of the Sun. However, now I’m not so sure.

Most comets we see are on elliptical orbits around the Sun. That means they don’t have enough energy (velocity) to escape from the Sun. Like a circle, an elliptical orbit is closed, so the comet stays bound the Sun. But if the comet has enough velocity it could be on a hyperbolic orbit, which is open-ended. In that case, it makes one pass of the Sun and then heads into deep space, never to return.

Mathematically, those shapes are defined by their eccentricity, which in a sense is how different they are from a circle. The higher the value of the eccentricity, the more an orbit deviates from being circular.

A circle has an eccentricity of 0, while an ellipse can have an eccentricity up to (but not equaling) one. If the eccentricity is exactly one, the orbit is a parabola, and the comet is moving at escape velocity. If it gets an extra kick, the orbit becomes a hyperbola, with an eccentricity greater than one, and the comet is moving faster than escape velocity.

And that’s where things get interesting for ISON. The orbit is determined by making multiple observations over time. The positions are then fed into a series of complex equations that solve for the shape of the orbit — these include how big the orbit is, the eccentricity, and more.

You’d think this is an exact science, but in fact it isn’t. Some observations are better than others; perhaps the air was turbulent for one set of observations, distorting the images taken. Maybe the stars in one set of images are overexposed, throwing off their positions. There are quite a few variables. At some point, a human has to decide if an observation is included in the orbital calculation or not.

When astronomers observe a comet, they can send their findings to various clearing houses of information which compile the data and calculate the orbit. One such group is the International Astronomical Union’s Minor Planet Center, and another is NASA’s Jet Propulsion Laboratory. While both are excellent, they use slightly different methods to calculate orbits, and can get different shapes. Usually they are very close, and it’s not a big deal.

However, ISON’s orbit is really on the thin hairy edge of being a perfect parabola, so a small change can have a big ramification. In this case, it makes all the difference in the solar system: Over the weekend, the MPC determined the eccentricity of ISON to be 1.0000019, while JPL found an eccentricity of .9999977109551715!

In other words, MPC says the comet will escape the Sun forever and never come back. JPL says its orbit is closed, and it will come back (though not for another 400,000 years). Which is it?

Right now there’s no way to know. The measurements are too close to call, and the uncertainties still a bit too big to know which one may be more accurate. The comet is moving so close to escape velocity that it could go either way. Worse, there are non-gravitational forces affecting it: the comet is blowing out gas, which acts like a rocket, changing its velocity. That might get amplified as it passes the Sun on the 28^th. Also, the exact distance it passes the Sun makes a big difference in its orbit after, so a small variation in the orbit now may get ramped up in a few days.

The only way to know is — and stop me if you’ve heard this before — to wait and see. That should be a mantra for comets. After ISON passes the Sun more observation  will probably be able to nail down the orbit better, and we’ll know if this is a one-off event, or if ISON will once again head into our neck of the woods… even if we might have to wait a few hundred millennia or so.

Tip o' the dew shield to Kristoffer Åberg and Karl Battams about the eccentricity measurements.