• December 20, 2013

    BREAKING NEWS: Comet ISON Is Still Dead

    by Zolt Levay

    On December 18, 2013, the Hubble Space Telescope tried to observe Comet ISON one last time. As many people expected, there is no evidence of Comet ISON in these images.

    Comet ISON December 13, 2013

    The above image shows a composite of Hubble's observations. If there had been a feature that appeared in all of the images at the same place, that would have been very strong evidence of the comet. But all of the features in these images are not repeated at the same place from image to image. Nothing that appears in the images looks like a piece of a comet. Mostly what shows up are stars that are moving across the frame as the telescope tracks the comet's expected position, and a few galaxies, also trailed. 

    Other features are well-known artifacts produced within the camera, reflected and scattered light from brighter stars, and cosmic rays, which cause bright streaks across the images. Each of the four panels is a combination of two separate exposures. Had Comet ISON actually been present, it would have shown up in the same location in two or more of these frames.

    These two images are composites of several exposures each, with Hubble pointing at two different positions. The images have been combined so that features not at the same place in the various images are suppressed. Any comet fragments would show up more clearly in this composite, though stars still show up as faint streaks.

    There was some uncertainty in where Hubble should point to recover the comet because no observations showed the comet since soon after perihelion. According to astronomer Hal Weaver, who devised Hubble's strategy, there were two likely locations of the comet, predictions based on previous positions measured when the comet was still visible. Dr. Weaver also estimates that the faintest objects Hubble could see in these images would be about 25th magnitude. This means that Hubble could have seen comet fragments larger than about 500 feet (160 meters) in diameter.

    We can't completely rule out the possibility that something is left of the comet. After all, it was seen after its passage close to the Sun, but disappeared not long after. This material would still exist, but is likely very diffuse gas, dust, and very small pieces spread over an extremely large area.

  • December 6, 2013

    CometCon Liveblog: Ex Post Comet

    by Josh Sokol

    On December 6, Comet ISON observers are gathering at the Johns Hopkins University Applied Physics Laboratory in Laurel, MD. We'll be covering the meeting live as these experts share notes on the comet's perihelion passage, review what they've learned, and suggest steps to move forward.

    Rejected (mostly morbid) titles for this liveblog include:

    • ISON: It Just So Happens That Your Friend Here Is Only Mostly Dead
    • ISON's Warren Commission (too soon?)
    • Sitting Shiva for ISON
    • Please Don't Be Mad at Us: This Science is Even Better Than You Actually Getting to See ISON
    • Goodnight, Sweet ISON
    • All NASA's Telescopes and All NASA's Astronomers: After ISON, Picking Up the Pieces and Putting Them Back Together Again


    Afternoon Sessions: Dust, Nucleus, Spin, Active Areas, Gas

    5:59 PM I'm signing off now as the meeting winds down. It's clear from today that there are still mountains of data to be analyzed, and many things for ISON to teach us.

    5:46 PM Bad news from Kelly Fast as open discussion starts. A search for ISON remnants today at IRTF came up with nothing. This brings up a problem: where do we point Hubble to look for it? There's hope amateurs may come through.

    5:36 PM There's enough extra ISON telescope time to fill a donation bin. Most observing proposals had a post-perihelion component, but now that there's little-to-nothing left to see, those minutes or orbits may go to other astronomical subjects in need.

    5:01 PM Individual non-public presentations are wrapping up, and I can promise that very few of them concerned a botched plan to use ISON in a "false flag" attack on the Sun. Wait, no one was thinking that? Forget I said anything, again.

    3:56 PM Ron Vervack just displayed 20 frames from ~150 images that the Mercury orbiter MESSENGER took of ISON. There's a lot of processing to be done, but it looks like they'll make a cool movie.

    3:42 PM Casey Lisse is reviewing his Chandra observations of ISON. How can we learn about a dusty, icy comet through X-rays?  As he explains, comets emit X-rays after ionized particles from the solar wind tear electrons away from neutral atoms in the comet.

    2:45 PM One of the major unsettled questions discussed so far is ISON's outburst history. The mid-November formation of "wings" is still not fully understood. Another big issue is whether anything has survived of ISON. There isn't a lot of optimism here for detecting it again, although some folks are holding out hope.

    1:00 PM OK, here's the deal. The results presented this afternoon are very preliminary, so the livestream is going dark. To encourage open discussion, we have been asked not to publish the still-forming hypotheses that are being presented. 

    I'll still take notes, and of course these results will soon be made public after they're more established. Rest assured that this material is technical and boring; these are minute details that might be wrong, not groundshaking revelations. When appropriate, I'll post general summaries of what's going on.

    Again, we are emphatically NOT going dark as part of an elaborate coverup, or to give the astronomers time to scrub away artifacts that look awfully like spaceships from their images, or to address a "mineshaft gap" between the United States and other nations in preparation for ISON debris hitting Earth.

    I may have said too much.


    Professional-Amateur Collaboration

    11:22 AM Everyone seems to agree about the pre-perihelion breakup seen by SOHO, at least. Casey Lisse argues that the combination of quick brightening and smearing out are clear evidence of disintegration.

    11:21 AM There's an open discussion, and Matthew Knight is asking if a consenus exists about when ISON fragmented -- not just at perhelion, but in possible prior outbursts. Hal Weaver questions whether there was disruption earlier; even after dusty "wings" were observed, for example, a well-defined nucleus still existed.

    11:10 AM Steve McCandliss, who used a FORTIS rocket to study ISON, is expressing gratitude to amateurs on social media for showing ISON was active enough to make the launch worthwhile.

    11:05 AM Matthew Knight, on behalf of Nalin Samarasinha, is showing how amateur observers helped uncover features in ISON's coma.

    11:00 AM Karl Battams is back with a review of the resources on the CIOC website, which received over one million pageviews. "It's been useful and I think people have liked it," he says. Classy understatement.

    10:46 AM Pro-Am collaboration has four phases, says Padma Yanamandra-Fisher. 1) Goals are defined by professional astromers, who 2) embrace the latest communications tech, 3) integrate science, knowledge and outreach, and 4) keep the story in the limelight. She's giving examples of the ISON campaign's efforts in each area.

    10:36 AM Elizabeth Warner is reporting on the huge online presence of amateur ISON images.


    Introduction: Welcome, Thanks

    10:00 AM Dean Pesnell is breaking down the harsh environment ISON encountered near the Sun, a corrosive combination of UV radiation and free electrons from the solar corona.

    9:54 AM Discussion has begun, and Hal Weaver is asking Karl Battams about the fateful moment of ISON's disruption. Does the bright, arrowlike smear in the last pre-perihelion LASCO image mark the breakup? Battams agrees: "as soon as we saw how pointlike the head of the comet was that got alarm bells going with us."

    9:48 AM Geraint Jones, presenting remotely from the UK, is describing plans to use the tail's behavior near the Sun to better understand the solar wind.

    9:38 AM Dean Pesnell shows his first slide to describe SDO's ISON observations. It's titled "We Saw Nothing." They looked in the right place, he promises. ISON just wasn't bright enough, perhaps because the comet didn't evaporate the way it was expected to.

    9:29 AM Referring to ISON's pre-perihelion streak of tail brightness, Battams says "Mathew (Knight) and I have seen a lot of comets --thousands of comets -- in this field of view, and we've never seen anything like this".

    9:24 AM The SOHO LASCO video, just shown, shows the post-perihelion remnant of the comet fading into nothingness. Doesn't look good for future detection.

    9:20 AM ISON Thansgiving-day celebrity Karl Battams is showing pretty movies! In solar observatory STEREO A, "we see the comet going in, and the object formerly known as ISON emerging from the other side."

    9:16 AM The CIOC, says Johnson, has worked so well that it'll be continued next year for more comets like Siding Spring. It's even keeping the same acronym under its new name: the Campaign for Integrated Observing of Comets.

    9:07 AM Lindley Johnson from JPL thinks the imaging and outreach campagins for ISON have been "a wild success for getting the public interested in comets and comet science." Kelly Fast from NASA HQ shares this enthusiam, adding that "the way it was highlighted to the public was a way of giving the public ownership of the science." There's lots of deserved praise for the Comet Ison Observing Campaign; they're all being given certificates, individually.

    9:01 AM As Lisse continues his overview, he's explaining what we can learn from the behavior of ISON's tail (the comet's dust composition), and from the way it changed brightness as it neared the Sun.

    8:56 AM Paul Feldman is speaking up from the audience to cite data from December 3 that found no Lyman-alpha emission from hydrogen atoms around ISON's remnant. That suggests no water -- and no comet -- remains.

    8:53 AM Lisse's current slide summarizes ISON's journey, ending with "the best studied comet breakup ever."

    8:48 AM We're beginning with a broad overview. Casey Lisse, presenting a graphic: "It's fair to say this comet has been observed by more spacecraft than any other comet."