• November 12, 2013

    Comets and Jets

    by Tracy Vogel

    One of the big differences between Hubble’s first and latest image of Comet ISON is its jet – or lack thereof. In Hubble’s April image, computer modeling showed a jet streaming away from the comet’s nucleus. In the October image, that jet seems to have vanished.

    Where did it go? Well, first let’s talk about why it was there to begin with.

    The surface of a comet’s nucleus isn’t smooth. It’s porous and uneven. Because of this structure, the surface area doesn’t warm up equally, and the comet’s ice vaporizes unevenly. The Sun heats the frozen carbon dioxide beneath the surface of the nucleus, changing it directly into a gas, a process known as sublimation. The heated gas wants to expand and escape, and it picks a weak spot in the surface of the comet to break forth.

    Gas and dust bursts forth like a geyser – in fact, even the dynamics are somewhat similar: An area below the surface warms up and punches through the surface with explosive force due to its gas content. In comets, the emerging gas carries a stream of dust along with it, instead of the water you would see with a geyser.


    Jets stream away from Comet Hartley 2. Credit: NASA/JPL-Caltech/UMD


    So here you have this ball of ice and dust drifting through space, and suddenly a stream of gas and dust erupts from it. Unsurprisingly, this affects the movement of the comet. If the jet is strong enough, it causes the comet to begin to spin. If the comet spins fast enough, its structure can become compromised, and it could break apart.

    We see this with asteroids, which are less structurally sound than comets and more like piles of rubble held together by gravity. When asteroids spin, stuff flies away. Comets are made of ice, and therefore a little more strongly held together, so they can spin faster than an asteroid – but they’re still capable of cracking. As jets spin a comet, extra material is injected into the tail, causing striations.

    Jets can tell us about the composition of a comet, and how active it is – jets won’t occur unless a comet has a lot of volatiles and is heating up. ISON’s jet was unusual in that it appeared quite far away from the Sun, just inside Jupiter’s orbit. The jet was thought to be near one of the comet’s poles, because it was quite steady. A non-polar jet would have had more of a wobble.

    A team at the Planetary Science Institute (PSI) is trying to get to the bottom of some of the questions about comets and jets with the help of amateur astronomers. The group is asking amateurs with access to professional-quality telescopes around the world to share pictures of Comet ISON to help answer questions about the evolution of jets over time, the rotation rate of the nucleus, and more.

    “For some comets, we observe rotation changes, and they speed up or slow down,” said Dr. Beatrice Mueller of PSI. “Because this one is going to get so close to the Sun, the activity will be off the charts.”

    Since ISON is not well positioned for observation all night long from one location, the team is counting on observers around the globe to create a record that provides long-term coverage of the comet. Both professional and amateur astronomers have already begun submitting observations.

    The team is fairly certain that the vanished jet has turned off or significantly decreased in brightness, rather than simply being hidden by the position of the comet during observations, said Dr. Nalin Samarasinha of PSI. “Comets are known to throw surprises at us. Having said that, there are multiple reasons why a particular feature may turn off or decrease in brightness, and we have seen such scenarios in other comets.”

    The team was hoping to use the jet to help get a handle on the comet’s rotation rate and the orientation of the spin axis, so the jet’s absence is a little disappointing, Samarasinha added. But the project is still monitoring the overall behavior of the comet and possible outbursts. The comet’s closest approach to the Sun is the time when the team expects to get the best and most useful observations.

    “What we can do is plan for the comet," Samarasinha said. "What it decides to reveal is up to it.”

    Stay tuned for Hubble’s upcoming observations of ISON, which will shed more light on whether ISON’s jet is still around.

    To find out more and participate in the PSI ISON observations, visit this link. Participants will need access to a powerful telescope.