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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