Comets Panstarrs And Lemmon Share A Kodak Moment

Two for the price of one! Comet L4 PANSTARRS is bright dot lower left with a short tail; Comet F6 Lemmon and its skinny tail are visible at right next to the Small Magellanic Cloud, a satellite galaxy of the Milky Way. Click to enlarge. Photo taken on Feb. 17, 2013 from Australia. Credit: Justin Tilbrook

Justin Tilbrook of Australia took a marvelous image earlier this week showing our two current comet celebs F6 Lemmon and L4 PANSTARRS together in the same picture.It’s not often you’ll see two tail-toting comets captured with a wide-angle lens at the same time.

To bring you up to date, Panstarrs is still visible very low above the horizon in morning twilight from far southern latitudes. This week it’s brightened to 4th magnitude and appears like a fuzzy pearl with a short tail. One observer noted a yellow color to the comet’s head caused by dust reflecting the ever-intensifying sunlight as PANSTARRS barrels sunward toward its March 10 perihelion.

Comet Lemmon photographed on Feb. 20, 2013 through a 19.6 inch telescope. Click to enlarge Credit: Martin Mobberley

Lemmon is higher up in the sky but fainter at magnitude 5.5. Right now it might be difficult to see with the naked eye because of moonlight. Binoculars show a bright head and a  1/2-degree-long tail.

You might be interested in a recent study on brightness predictions for comets L4 PANSTARRS and ISON by Ignacio Ferrin of the University of Antioquia in Medellin, Colombia. Here are the main points:

* C/2011 L4 Panstarrs will be less bright than Halley’s Comet was in 1986. It will show
a tail easily detectable with the naked eye.

* There’s a 75% chance that  C/2012 S1 ISON will continue to brighten and put on a great show late this fall. Ferrin predicts it could become as bright as the full moon (magnitude -12.6) when nearest the sun. But his prediction comes with a caution: ISON will pass within the Roche Limit when it swings around the sun in late November. This is the minimum distance a smaller body can hold together in one piece while orbiting a larger body without being torn to bits by the larger body’s overwhelming gravity.

At top, an object like a comet has crossed the Roche Limit and starts to disintegrate into pieces. Below, the individual pieces spread out according to distance. Particles closer to the sun (left) move more quickly (red arrows) than those farther away. Credit: Wikipedia

Ferrin writes: “Any object within this limit  has a large probability of disintegrating due to differential gravitational forces from the Sun. The combinations of Roche’s Limit, plus solar radiation plus very high temperature, suggest that the comet may not survive its encounter with the Sun, disintegrating into several pieces. Or it may survive, if its internal cohesion is
sufficient to endure those conditions.”

If you’d like to learn more, please check out the complete study.

27 Responses

  1. Edward M. Boll

    What about Comet Bress?i. It is either gone or has not been observed because of it’s small elongation from the Sun. I suppose that it could still be out there at magnitude 10.

    1. astrobob

      Probably both. PANSTARRS is only a couple degrees high in twilight right now; Bressi is even lower and considerably fainter.

  2. Bob Crozier

    If this question is off topic, please forgive me. But in speaking about these two comets, I went again and checked the JPL Small Body Database. I find the orbit diagrams that are provided there to be very helpful for me to try to understand what’s actually happening with these objects. Is there any way that I can get one single orbit diagram that will include not just one of the comets, but let’s say all three major players that are (or are coming) on scene: L4 Panstarrs, F6 Lemmon, and S1 Ison? Can these diagrams be customized like this?

    1. astrobob

      Unfortunately the JPL ones can’t be customized to include more than one object. You might be able to do this with certain planetarium-style programs but the four I use don’t have that feature. I’d love to have the ability to create an illustration showing all three.

    2. Sisko

      Bob, I had the same desire and managed to grasp the ‘big picture’ with SkySafari Pro. Although there is no option to view both orbits at the same time, you can ‘orbit’ the first object and then select the second one and track its movements plus you should select ‘show comets’. So you can really see them in 3D, pan and rotate, it’s much better than JPLs Orbit Diagram.

      Cheers 🙂

          1. Bob Crozier

            That’s pretty impressive! But I wonder why they would not have they ability to select at least a few small bodies at the same time? I understand if you had too many selected at one time that the number crunching required by the computer might become onerous. But even if a person could select 3 or 4 small bodies to chart at the same time, that should be do-able thing, I would think.

  3. Bob Crozier

    OK Bob, so you have introduced me to something new again – this concept of a Roche Limit.

    First, how far inside that limit is ISON going to pass? I assume that the further it passes inside that limit, the less likelihood that it will survive intact?

    Second, I read somewhere that the Roche Limit has a negligible effect on small bodies of less than 50 kilometers in diameter. I assume that means when it is at or near that limit and that the further inside that limit it goes, the greater the effect. What are the estimates for the size of ISON?

    Third, I read that it isn’t just the /gravity/ of the small body that holds it together as it approaches the Roche Limit of a larger body, but also (and significantly!) the /cohesion/ of the material with which that body is made: a hard rocky or metalic body will survive the Roche Limit whereas a soft or gaseous body would not. Do we have any way of knowing or estimating with some degree of certainty how cohesive ISON is?

    Fourth (ok, I’ll stop with the questions here!), I read that stars (orbiting other stars) are very susceptible to the Roche Limit (because they are not cohesive). I recently saw what was reported to be an animation created from pictures of stars oribting very close around the super-massive blackhole at the center of our galaxy. Wouldn’t these be within the Roche Limit of gravity source that massive and so how do they survive that? The same question can be asked of some of the massive, super-Jupiter exoplanets that have been discovered recently orbiting /very/ closely to their host planets.

    Thanks Bob!!

    1. astrobob

      I don’t know how far – the information I have is based only on Ferrin’s paper. I’ve seen estimates of 3-4 km for ISON’s size, but remember, we’re talking a comet, which has a lot of ice. They’re also friable or fragile and subject to breakup. I don’t think anyone knows exactly how cohesive ISON is, but I’ll bet it’s less rather than more due to the nature of comets. Comet Lovejoy passed very close to the sun a year ago or so and was predicted by many to fall apart but didn’t. At least not at first. It went on to become a beautiful southern comet with a long tail and began disintegrating some time later. Some things can survive falling in to each other at least for a time because they’re revolving at incredible rates of speed when so close.

      1. Bob Crozier

        Thanks Bob.

        Do you know, off the top of your head, if Lovejoy passed closer to or further from the Sun than where ISON is calculated to pass? I saw some pictures of Lovejoy and if ISON turns out to be anything like that, I’ll be one happy camper!

  4. Edward M. Boll

    I just saw a diagram for Mar. 12. This will be an interesting night, first to have Mars set right after sunset. Uranus sets and following right on it’s tail, sort of speak is Panstaars and then the Moon sets about 7 minutes after Uranus.

    1. Bob Crozier

      You got me playing with Stellarium and I discovered that 10 days later, on the 22, there might be three time zones in the world (and I am not in one of them!) where they could maybe witness Mars passing completely in front of Uranus. What’s that called? Now I gotta scoot because I am going to be late! Grrrr…..

      1. astrobob

        It IS fun. Pity about the conjunction though – I doubt anyone will see it since the planets are only about 5 degrees from the sun, much too close for viewing.

  5. Giorgio Rizzarelli

    Great photo. Spaceweather just shared this link

    Marvelous time lapse of the two comets together with aurora, midnight sun and falling stars, and closeups of each comet showing them moving respect stars, in hires color. Not to lose!

    1. Bob Crozier

      That is pretty cool: southern lights and all. Thanks Giorgio!

      Man! How many lifetimes will it be before people can again see two fairly easily visible comets in the sky at the same time? Would that be something like a once-in-a-millenia kind of thing?

      Here’s hoping we’ll get a show – and some clear skies with which to see it! – here in the northern half of the world. Surely it must be our turn!

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