We are preparing to visit the North Pole on December 22, to see an unusual meteor shower called the Ursids. This meteor shower radiates from the constellation known as the Little Dipper: Ursa Minor. This minor shower is quite unremarkable in most years, but this year the parent comet 8P/Tuttle is approaching, and it has a strong Perseid-level shower in store for us. We think. We predict a peak time around 20 - 22.2 Universal Time and a width of 5 to 8.5 hours at half the peak rate.
Unfortunately, that means that North America is not a good place to be, for our purpose. The outburst will happen during our daytime. Best viewing will be in Asia and Europe. Even from those locations it won't be easy to see this shower, because of a bad moon that night, and frequent bad weather in December. There is a reason why people want to huddle indoors around a warm fire and a Christmas tree this time of year.
The Ursid outburst is too rare an opportunity to miss, though, because the comet returns only every 13.6 years. This time around, the comet comes closer to us than ever before, at least since it was discovered in 1790, passing at only 0.25 AU on January 5. This is still a big distance, though, making for no more than a nice binocular comet. The meteoroids bring the comet to us and permit studies of the comet's composition and activity history. The meteoroids will hit at a speed of 35 kilometers in a second and can damage satellites in orbit. It is a good thing that the stream is pretty diluted when it reaches Earth.
The Ursid outbursts are somewhat of a mystery. They have been heard, more than seen, in the years around past returns of the comet, by people listening for distant radio stations. Each time a meteor appeared in the cold winter sky, it would create a trail of ions, which reflected radio waves to the observer. They would hear a brief "ping". While counting those pings, it was clear to those listeners that something unusual was happening on December 22 in some years.
In collaboration with Esko Lyytinen of Helsinki, Finland, and Jeremie Vaubaillon of Caltech, I investigated these reports, and we discovered that the dust may have been ejected by comet Tuttle around AD 300 to 900, or perhaps earlier. Results will be published in the upcoming December issue of the Journal of the International Meteor Organization. It takes a long time for the dust to move from an orbit similar to that of the comet into an orbit that can hit Earth. Close encounters of Jupiter near the ascending node of the comet orbit seem to play an important role, especially when they occur shortly after the dust has been ejected and is still concentrated in space. The dust itself is most affected when its motion around the sun "rings," or resonates, with that of Jupiter.
The upcoming outburst is a great opportunity to test this model. When the Earth travels through the stream of dust, we may hope to see the dominance of some particular returns of the comet in the past, when all the dynamics worked in our favor. This could cause a particular peak time and rate profile of the shower.
To investigate this, we are hoping for an opportunity to observe the outburst from a Gulfstream V aircraft in a mission similar to our campaign to study the September 1 Aurigids. If approved, the Ursid flight will be a long 16-hour mission, involving one aircraft, departing from NASA Ames Research Center in the early morning of December 22nd. The plane will fly north-west towards Alaska, land in Anchorage for a refuel stop, and then continue on to follow the Earth's shadow, fly over the Arctic and turn towards Canada to return at Ames just after sunset. We don't need to fly all the way to the North Pole, just far enough north to stay in darkness all the time. The aircraft will follow the Earth's rotation, making the meteors fall from a radiant high in the sky throughout the mission. The moon will stay low on the horizon on one side of the plane. The mission is described at: http://ursid.seti.org.
The observations are also expected to yield data on the parent comet itself. We can measure the relative content of sodium, magnesium, iron and some other elements. We can measure the range and frequency of meteoroid sizes, and compare that to what astronomers observe to come off comet Tuttle today. Perhaps we will learn about how meteoroids survive the conditions of space.
I have already received great pictures of the approaching comet 8P/Tuttle. The best will be posted at: http://ursid.seti.org/index-8P.html. The comet is now in the constellation of Cepheus and moving south. This comet is within reach of binoculars and should reach naked-eye brightness, around magnitude +5.7, in early January. Observing programs to study this comet are scheduled for the Hubble, Spitzer and Chandra space telescopes, as well as from many ground-based observatories.
The comet is as big as comet Halley, but not quite as bright because it does not emit the same amount of tiny dust grains that scatter sunlight efficiently, and create a haze around the comet. Instead, the comet releases large dust grains that, many centuries later, evolve into orbits that hit Earth and create the mysterious Ursid meteor shower.
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