Download april 2009 - Holt Planetarium

FRIENDS OF THE PLANETARIUM NEWSLETTER – APRIL 2009
The evening sky is a bit bereft of planets these days but the apparition of Saturn is giving astronomers a rare
view at the ringed planet from a different perspective. At a distance of 1.43 billion km, almost ten times the
Sun-Earth distance, Saturn takes 29.5 years to complete one orbit. Most of the time the magnificent ring
system is tipped towards us but once every 15 years the rings are presented edge-on. Despite its high
reflectively due to the icy nature of the particles, the ring system is so thin that it disappears in even the most
powerful Earth-based telescopes. What does happen, however, is that Saturn’s moons pass in front of the
planet and are more easily seen as they cast their shadows on Saturn’s cloud tops. These rare transits help
astronomers learn more about this complex system.
In February, the
Hubble Space
Telescope took this
amazing image of
Saturn and four of its
moons. Titan and its
shadow are at the top
of the image. Mimas
is visible close to the
ring plane while
farther to the left and
off Saturn’s disc, are
the bright moons
Dione and the fainter
Enceladus.
Credit: NASA, ESA, and the Hubble Heritage Team
(STScI/AURA)
Saturn’s rings will be perfectly edge-on to our line of
sight on Aug. 10 and Sept. 4 this year. Unfortunately,
Saturn will be visually too close to the Sun to be seen
by viewers on Earth during that time. Meanwhile, the
Cassini spacecraft has had its initial four-year
mission extended as it continues to send back
stunning images and a wealth of detail. Here is a
recent Cassini picture of the moon Rhea. Note the
huge ray crater just below centre that stretches across
half of the leading hemisphere. Rhea is 1528 km in
diameter, a little less than half the diameter of our
own Moon. Cassini was 820,000 km from Rhea
when it took this photo.
Credit: NASA/JPL/Space Science Institute
Meanwhile, a bit closer to Earth, NASA's twin STEREO probes are entering a mysterious region of space to
look for remains of an ancient planet which once orbited the Sun not far from Earth. If they find anything, it
could solve a major puzzle, the origin of the Moon. The name of the planet is Theia. Some researchers
believe it existed 4.5 billion years ago, and that it collided with Earth to form the Moon.
The "Theia hypothesis" starts with the popular Great Impact theory of the Moon's origin. Many astronomers
hold that in the formative years of the solar system, a Mars-sized protoplanet crashed into Earth. Debris from
the collision, a mixture of material from both bodies, spun out into Earth orbit and coalesced into the Moon.
It's a good theory, but it leaves one awkward question unanswered: Where did the enormous protoplanet
come from? Two Princeton theorists believe it came from a Sun-Earth Lagrange point.
Sun-Earth Lagrange points are regions of space where the pull of the Sun and Earth combine to form a
"gravitational well" where the flotsam of space tends to gather. 18th-century mathematician Josef Lagrange
proved that there are five such wells in the Sun-Earth system: L1, L2, L3, L4 and L5 located. When the solar
system was young, Lagrange points were populated mainly by planetesimals, the asteroid-sized building
blocks of planets. The hypothesis suggests that in one of the Lagrange points, L4 or L5, the planetesimals
assembled themselves into Theia, nicknamed after the mythological Greek Titan who gave birth to the Moon
goddess Selene. If this idea is correct, Theia itself is long gone, but some of the ancient planetesimals that
failed to join Theia may still be lingering at L4 or L5. The STEREO probes are entering these regions of
space now, which puts them in a good position to search for Theia's asteroid-sized leftovers. Just call them
"Theiasteroids." Hunting for Theiasteroids is not STEREO's primary mission, however. STEREO is a solar
observatory. The two probes are flanking the sun on opposite sides to gain a 3D view of solar activity. They
just happen to be passing through the L4 and L5 Lagrange points en route. This is purely bonus science.
With solar activity at its lowest in one
hundred years, the STEREO
spacecraft have had plenty of time to
train their sensors on other targets of
opportunity. While observing a
Coronal Mass Ejection, STEREO’s
COR1 camera caught Jupiter and its
retinue of Galilean moons passing by
in the distant background. Note the
size of the CME compared to the
white line showing the actual size of
the Sun. This image would be
impossible to take from Earth. Visit
the STEREO website to watch a
movie of this event showing the
orbital motion of the moons.
Credit: STEREO/COR1 (NASA)
This is the time of the year when we ask our supporters to renew their membership in Friends of the
Planetarium. The Planetarium receives no government or local body funding. We rely on donations and
patronage for our survival. A renewal form is enclosed for your convenience. Please feel free to photocopy
this and give one or two to your friends so that they can become our Friends. Remember that your
membership fee is tax deductible.
If you haven’t been in to see us recently, the foyer area displays have been completely changed with new
interactive computers and lots more posters. We are also clearing out the last of the 2009 New Zealand
Almanacs for just $10 (RRP $21) and the 2009 New Zealand Astronomical Yearbook for $18. Watch for our
new website which should be up and running soon.
Thanks again for your support.