Zzzzzz … Rosetta’s Comet Takes A Nap

Unlike the images of comet 67P/Churyumov-Gerasimenko obtained on April 30 (right), on June 4th no signs of an extended dust coma are evident. At that time,  267,000 miles separated Rosetta from its destination. Credit: ESA/Rosetta/MPS for OSIRIS Team

Comets are always full of surprises. That’s why we love them so. Take 67P/Churyumov-Gerasimenko, better known as Rosetta’s comet. Back on April 30, the Rosetta spacecraft saw a very active comet with a coma of dust and gas measuring 800 miles across. Even at 4 times the Earth-sun distance, solar heating had begun to vaporize comet ice.

But by June 4, when 67P was 18.6 million miles closer to the sun, activity appears to have shut down. Without its fuzzy coma, the comet looks like one of the many stars in the photo above.

Artist’s impression of the Philae lander expected to touch down on 67P/Churyumov-Gerasimenko this November. A comet’s core or nucleus is typically as dark as charcoal and only a few miles across. Sunlight reflecting off the dust and gas in their much larger comas and tails makes them appear bright . Credit: ESA

“67P is now almost within our reach – and teaching us to expect the unexpected,” said OSIRIS Principal Investigator Holger Sierks. “After 67P’s onset of activity, our images are currently showing a comet at rest,” he added. OSIRIS is the imaging system Rosetta uses to take photos of its target.

Jets flare in this illustration of 81P/Wild. Material from the jets goes into forming the comet’s dust envelope or coma. Sunlight pushes back coma gases and dust to form the comet’s tail. Credit: NASA

As the sun heats the icy nucleus, cracks develop and expose fresh ice and pockets of trapped gas to the vacuum of outer space. These rapidly vaporize and spew dust and gas like rocket thrusters. Astronomers call them ‘jets’. Jets can turn on and off and new jets can develop as the comet approaches the sun and then returns to deep space. That’s why it’s no surprise to see 67P take a temporary nap.

Usually, once a comet is within Mars distance of the sun, it remains active with an expanding coma and ever growing tail. We have much to look forward to. Currently, Rosetta’s comet is only a pixel across, but in just a few weeks, the OSIRIS camera will discern the shape of the nucleus. Things are heating up – both for the comet and for us watching the show.

20 Responses

    1. astrobob

      Hi Mahdi,
      I see now that your times are all UT. To convert to your time zone I will need to know what the offset from UT is and also when (if) you are on Daylight Saving Time. Did you consider my question about “central Iran” instead of setting it up for your exact latitude and longitude?

    1. astrobob

      Hi Edward,
      I observed it first back in late summer 2011 and then again in spring 2013. I’ve not seen it this year – it’s very faint around mag. 14 right now.

  1. Les

    Hi bob! I was curious if u knew what these lights in the California sky are that people are claiming to see. Of course people are saying aliens but that just scares me. Los Angeles channel 7 news has an article on it too? any suggestions?

    1. astrobob

      Most of the descriptions would indicate it was a bright meteor that people saw. I also watched a different video showing a couple orange dots hovering – these look exactly like the popular sky lanterns people have been launching the past few years.

  2. Mahdi

    Oh, I didn’t convert times from UT to my local time! So, time errors may be solved.
    My time settings are visible in the screenshot of my location settings.
    I don’t understand what do you mean about central Iran and my exact location.
    I think it should works for any location on earth.at least in Iran.
    Do you mean errors occurred because my location isn’t in skymap’s database?

    1. astrobob

      As long as your latitude and longitude are correct then it should work. I thought you chose central Iran as a default. Now that I look at your time zone information I see you’re 3.5 hours ahead of UT (sorry, I missed that the first time). On the Venus-Mercury event, the times shown in the conjunction box are UT with the conjunction occurring on Oct. 17 at 5:56 p.m. UT, but your chart showing the two planets is set for 1:45 a.m. Oct. 18. It should be set for Oct. 17 at 9:26 p.m. to match the UT times in the conjunction box.

  3. Mahdi

    Yes, but the starry night seems to be more accurate in this case. Because it’s similar to the magazine’s (which I talked about before) prediction.
    So, how should I know which tells the truth?

    1. astrobob

      So you made the time change from UT to your local time and SkyMap is still inaccurate? As for the magazine, do you know what source they use for their predictions? How far off are SkyMap’s predictions compared to Starry Night – seconds, minutes, hours??

  4. Mahdi

    As you can see in screenshots, the difference may be some hours.
    In the case of venus-mercury conjunction, the starry night says the minimum separation occur on 1:45 am Oct. 18 and skymap predict it on 9:26 pm Oct. 17 (both on local time). It’s about 4 hours.

    I don’t know which sources they use, but I think one of most important sources they use is the skymap.
    Do you know a way to test predictions and find out which is true?

    1. astrobob

      I can’t fully answer your question about how to test predications of planetary positions that differ between programs by several hours. Jean Meeus comes to mind and his book “Astronomical Algorithms”. The new edition has coordinates for the outer planets through 2025. You might also e-mail Chris Marriott, the creator of SkyMap, at chris@chrism.demon.co.uk
      One thing I noticed about your comparisons will clearly introduce inaccuracy. SkyMap shows the UT times for these three events: Jupiter-moon conj., Moon-Spica conjunction and the time of new moon. You did not convert those times to your local time to create the diagrams using Starry Night. For example: the Jupiter-moon conj. in SkyMap occurs at 1:02 UT. Your Starry Night map shows the position at 1:02 local time.

  5. Mahdi

    Thank you for your answers.
    I will check events in some other sources to find a solution.
    Thanks a lot.

  6. Richard Keen

    Bob, the Rosetta spacecraft itself put on a nice show nine years ago, back when it made its first Earth flyby a year after launch. Rosetta was 0.0015076613 AU, or 225543 km, from the observer (me) at the time. Here’s what I had to say about that on the Seesat list (I’ve seen you there, too)….
    I just observed my first interplanetary spacecraft, Rosetta, from 0840 to 0925 UT March 4 (2005). It was magnitude 11.4 in the 12.5-inch reflector, about 1 magnitude brighter than the predictions posted on the ESA web site. It’s motion was easy to perceive, especially when the spacecraft made a rapidly changing equilateral triangle with two similarly bright stars.

    1. astrobob

      What a great observation. I wish I had tried to tackle it at the time. I imagine its magnitude has dropped into the 20s by now.

      1. Richard Keen

        Yeah, that was real thrill. Sort of like seeing my first satellite way back when – Sputnik 4 (which turned out to be a dry run of the Vostok for Gagarin’s flight the following year).
        The NASA Horizons ephemeris say Comet C-G is about magnitude 20 now, and it’s a lot larger than the Rosetta that’s supposed to land on it (otherwise the comet would land on Rosetta, no?). And both are 2.8 AU from the Earth, which is about 2000 times farther than Rosetta was when I saw it. Square that and you get 4 million times fainter, or 17 magnitudes. And Rosetta is 2.8 times farther from the sun, adding another 2 magnitudes. So it’s about magnitude 30 now.
        That’s out of my range!

        1. astrobob

          Guess I’ll stick with the space station. Our discussion reminds me that last night I discovered another “nova”. A new star of 3rd magnitude stood out below Aquila’s tail. For a couple minutes it didn’t move and then I saw it very slowly move a smidge east and fade. I once saw a nearly identical satellite and got it ID’d as the Galaxy 11 geosync. My guess is I saw it again or another geosynchronous satellite.

          1. Richard Keen

            Your “discovery” reminds me of the “Aries Flasher” (actually in Perseus) that made the rounds in Sky & Tel in the 1980s (Sky and Telescope, Vol. 73, NO. 6/JUN, P.604, 1987). It turned out to be Cosmos 1400.
            But then there was the warm August evening in 1975 when I saw a 2nd-magnitude object in Cygnus that I presumed was the high, giant, and slow-moving balloon satellite Pageos, launched 9 years before.
            I didn’t know that Pageos had broken apart a month earlier, and the object I saw never moved. It was Nova Cygni.
            So the satellite-nova confusion works both ways!

          2. astrobob

            I remember the Aries Flasher. I’ve never caught a nova unawares on the rise, but I’ve been tricked by asteroids in variable star fields on several occasions.

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