From: Larry Klaes (email@example.com)
Date: Mon May 21 2001 - 11:05:03 PDT
From: firstname.lastname@example.org [mailto:email@example.com]On
Behalf Of John Wagoner
Sent: Monday, May 21, 2001 12:17 AM
Subject: [skyline] S&T's News Bulletin for May 18, 2001
SKY & TELESCOPE'S NEWS BULLETIN - MAY 18, 2001
For images and Web links for these items, visit http://www.skypub.com
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WHAT WE LEARNED FROM THE LAST YEAR'S COMET LINEAR
Last June, amateur and professional astronomers alike focused their
telescopes on what was thought to be a mediocre comet as best.
Discovered by the Lincoln Near Earth Asteroid Search, Comet LINEAR
(C/1999 S4) soon became a cosmic curiosity. In Hubble Space Telescope
observations taken in July, the comet appeared to have erupted, with a
large fragment having clearly broken off the nucleus. In the month
that followed, the comet crumbled apart completely, faded from view,
and in the process, exposed its interior to science.
In a series of papers published in today's Science, astronomers reveal
what they have learned thus far from the deceased comet. In addition
its chemical composition and probable origin, (see following story)
astronomers now think they may understand why the comet came apart in
the first place. Originally they assumed that Comet LINEAR's breakup
was due to massive outgassing -- perhaps the result of an outburst of
carbon monoxide (CO), the most probable suspect. However spectroscopic
analysis found the comet to be highly depleted of CO, thus making a
pressure build-up of gas within the nucleus an unlikely cause.
Instead, Hal Weaver (Johns Hopkins University), author on three of the
Science papers, believes the comet was most likely rotating quickly on
its approach to the Sun. If the rotation pole was pointed toward the
Sun, says Weaver, "there would be more efficient heat transfer
throughout the comet." But he adds that it isn't clear the comet was
rotating fast enough to break apart. The critical rotation speed
depends largely on the initial size of the comet's nucleus, a quantity
yet to be determined.
Another interesting facet of Comet LINEAR is what wasn't seen. The
assumed mass of the comet before it crumbled is 100 times larger than
the mass found afterward. Some of this is to be expected. Larger
blocks are directly detectable, as are the smallest of particles,
which refract light much the same way a thick fog would. But just as a
thick fog easily hides all but the largest of objects from view, so
too did LINEAR's dust cloud.
"Watching the unraveling of the comet allows us to look in reverse at
its formation," says Weaver. Thus determining the size of the
intermediate fragments directly relates to the size of cometary nuclei
in general. An abundance of intermediate-size building blocks imply
that, on the whole, cometary nuclei may be smaller than previously
Comet breakups are far from unusual. For example, another LINEAR
discovery, C/2001 A2 -- currently visible in Southern Hemisphere skies
-- recently broke in half on its approach to the Sun. On May 16th, the
8.2-meter Yepun reflector of the Very Large Telescope took an image
showing that a third piece has broken off. LINEAR C/2001 A2 reaches
its closest point to the Sun on May 24th. The comet also continues to
brighten. Observers report that it is a naked-eye 5th magnitude, where
the comet is about 20 deg. above the west-southwest horizon after
evening twilight. It will not be visible from the Northern Hemisphere
until late June. Here are coordinates for Comet LINEAR C/2001 A2 at 0
hours Universal Time for the coming week:
May 19 5h 35m -23.4 deg.
May 21 5 31 -24.1
May 23 5 27 -24.8
May 25 5 21 -25.5
A COMET'S LINEAR-AGE
Even before its slow-motion disintegration last year, Comet LINEAR
(C/1999 S4) was the crosshairs of many telescopes. Its brightness
allowed planetary scientists to conduct spectroscopic studies that
would be impossible on dimmer passers-by. Curiously, Comet LINEAR
proved to be unlike other bright comets, in that it contained
relatively little carbon monoxide (CO) relative to its water content.
Frozen CO vaporizes even at very cold temperatures, so when seen
abundantly in the comas of Halley, Hale-Bopp, and Hyakutake,
astronomers concluded that these iceballs formed very cold, in the
vicinity of Uranus or Neptune, before being flung out into the distant
cometary reservoir known as the Oort Cloud. This trio of comets also
exhibited a high ratio of deuterium to hydrogen, typical of that found
in interstellar clouds of gas. Because the comets' D-to-H ratio was at
least double that found here on Earth, cosmic chemists began to think
that only a fraction of our planet's water could have been delivered
However, Comet LINEAR's low CO value implies that it formed somewhat
closer to the Sun's warmth, perhaps near Jupiter, before taking up
residence in the Oort Cloud. As Michael C. Mumma (NASA/Goddard) and
his colleagues explain in today's issue of Science, such comets should
have D:H ratios much closer to that of seawater, which could mean that
incoming comets delivered the bulk of Earth's water after all. "The
idea that comets seeded life on Earth with water and essential
molecular building blocks is hotly debated," Mumma notes, "and for the
first time, we have seen a comet with the right composition to do the
job." But his team will have to wait for another bright comet to test
their hypothesis further, because Comet LINEAR's D:H ratio could not
Of course, comets weren't the only small bodies that formed near
Jupiter. Asteroids did too, and the most distant ones contain up to 20
percent water. In 1997, Harold Levison (Southwest Research Institute)
and Martin Duncan (Queen's University) estimated that roughly 8
percent of all the objects initially present in the outer asteroid
belt (3.3. to 5.0 astronomical units from the Sun) were ejected by
Jupiter into the Oort Cloud. Over time, some of these must boomerang
their way back to the inner solar system. Even if LINEAR wasn't one of
them, it's definitely caused a stir among planetary scientists.
INFRARED SPACE TELESCOPE MAKES PROGRESS
It's taken two decades, but the last of NASA's four "Great
Observatories" is finally taking shape. This week engineers at Ball
Aerospace & Technologies finished integrating the telescope assembly
for the Space Infrared Telescope Facility (SIRTF), and over the next
three months it will undergo environmental tests to ensure its
spaceworthiness. The telescope's beryllium primary mirror, 85
centimeters (33« inches) across, is designed to be cryogenically
chilled using superfluid helium to below 6 deg. K (-449 deg. F). It's
the cold heart of an f/12 optical system that will feed light to three
experiments, whose deep-frozen detectors will study cosmic sources
over a wavelength range of 2 to 200 microns.
When proposed in the early 1980s, the "S" in SIRTF stood for
"Shuttle," and plans called for the infrared observatory to be dropped
off in an orbit near Earth so that visiting astronauts could give it
periodic checkups. However, astronomers soon came to appreciate the
advantages of having their infrared eye far from Earth's glare, even
if it meant servicing would no longer be possible. At one point, SIRTF
was to occupy a circular orbit 100,000 kilometers high. Now plans call
for the spacecraft to be in a heliocentric orbit and gradually drift
farther from Earth during its 2.5- to 5-year mission. The tentative
launch date is July 2002.
STELLAFANE AND STATE PRISON REACH AGREEMENT
Since the 1920s, the Springfield Telescope Makers club has made Breezy
Hill in Vermont its base of operations. Amateur astronomers throughout
the Northeast participate in monthly club meetings and other events at
the site. Of particular note is Stellafane, the granddaddy of all
telescope conventions. The annual event brings some 2,000 astronomers
from around the world to the dark skies of central Vermont for a
weekend of telescope making, astronomical observing, and camaraderie.
The darkness of the skies above Stellafane was severely threatened two
years ago when the citizens of Springfield voted in favor of building
the state's largest prison just four miles from the astronomers' happy
haunt. The club members worried that the prison would fill the region
with light pollution.
That potential disaster was averted this week with the announcement
from the Springfield Telescope Makers, that the prison planners and
astronomers have reached a compromise concerning the facility's
lighting design. Under the agreement, the prison will employ only
full-cutoff lighting. The design will also minimize light pollution,
yet maximizing visibility for guards.
Despite the lighting design, it is estimated that some 150,000 lumens
of light will still shine into the night sky through reflection off
the ground. To combat this, the state of Vermont has agreed to take
light-reduction measures off-site with the Springfield Telescope
Makers acting as consultants.
As former club president Maryann Arrien explains, "This is really a
win-win. Not only will this agreement reduce electric costs [for the
prison], it will help to preserve the view of the stars for all the
citizens in the area."
DELTA SCORPII STILL BRIGHT
Now that Scorpius is rising into good view as early as 11 p.m. local
daylight time (look low in the southeast), more and more skywatchers
are noticing the unusual brightness of the star Delta Scorpii near
Antares. Normally magnitude 2.3, Delta slowly flared up last July and
has generally remained bright, with some fluctuations, ever since. For
the last three months it has hovered around magnitude 1.8, changing
the look of the familiar row of three stars known as the head of
Scorpius. Compare it with Beta, magnitude 2.6, and Antares, 1.1.
Delta Scorpii is a hot, luminous B0 star shedding gas and may be a
spectroscopic binary. So far astronomers don't know why it is acting
up or what it will do next. There's no telling when Delta may fade
back down -- or brighten further! Don't wait until Scorpius is placed
at its evening best in July; take a look now to be there for a
naked-eye event that's becoming part of variable-star history.
THIS WEEK'S "SKY AT A GLANCE"
Some daily events in the changing sky, by the editors of Sky &
MAY 20 -- SUNDAY
* Some doorstep astronomy: The brightest star shining in the
east-northeast after dark this month is Vega. Dangling from it to its
lower right are the little triangle and parallelogram of the
constellation Lyra, made of 3rd- and 4th-magnitude stars.
MAY 21 -- MONDAY
* Mercury is at greatest elongation, 22 degrees east of the Sun.
Look for it low in the west-northwest about 60 minutes after sunset.
MAY 22 -- TUESDAY
* New Moon (exact at 10:46 p.m. EDT).
* The 9.2-magnitude asteroid 2 Pallas can be spotted with a
telescope 4 arcminutes south of the 6.1-magnitude yellow star 56
Herculis this evening. Check back tomorrow evening to see that it has
MAY 23 -- WEDNESDAY
* More doorstep astronomy: The brightest star very high overhead
toward the south these evenings is Arcturus, shining pale
yellow-orange. Far to its lower right, by about three fist-widths at
arm's length, is blue-white Spica, less bright. To Spica's lower right
look for the four-star pattern of Corvus, the Crow.
To identify constellations all around your sky, use the printable
evening star map and instructions at
http://www.skypub.com/sights/northern/0105skyn.shtml (if you're in the
mid-latitudes of the Northern Hemisphere. Southern Hemisphere
skywatchers: use the map at
MAY 24 -- THURSDAY
* Look low in the west-northwest about 40 minutes after sunset for
the thin crescent Moon. Look to its right (by about three
fingers'-breadths at arm's length) for faint little Mercury. Farther
to their lower right is Jupiter. Binoculars will help.
MAY 25 -- FRIDAY
* The waxing crescent Moon shines in the west as twilight ends.
Above it are Pollux and Castor, lined up about horizontally. To the
Moon's left is Procyon. Farther to the Moon's right is Capella.
* Saturn is in conjunction with the Sun.
MAY 26 -- SATURDAY
* The Moon this evening is nearly in line with Pollux and Castor,
which are twinkling to its right. To the lower left of the Moon is
THIS WEEK'S PLANET ROUNDUP
MERCURY, fading fast, is low in the west-northwest after sunset, above
brighter but much lower JUPITER. Use binoculars to look for them about
30 minutes after sundown early in the week. They're far to the lower
left of Capella.
VENUS (magnitude -4.4) blazes low in the east during dawn.
MARS (magnitude -1.8) rises in the southeast shortly after the end of
twilight. By midnight it dominates the low southeast, shining bright
yellow-orange. Mars is highest in the south around 2 or 3 a.m. In a
telescope, Mars is already an unusually large 18 arcseconds in
diameter and growing as it approaches Earth. It will reach 21
arcseconds when nearest Earth in mid- to late June. See the observing
guide to Mars in the May Sky & Telescope, page 102.
SATURN is hidden in the glare of the Sun.
URANUS and NEPTUNE (6th and 8th magnitude, respectively) are in
Capricornus in the southeast before dawn.
PLUTO (magnitude 14) is in Ophiuchus in the south in the early-morning
(All descriptions that relate to the horizon or zenith -- including
the words up, down, right, and left -- are written for the world's
midnorthern latitudes. Descriptions that also depend on longitude are
for North America. Eastern Daylight Time, EDT, equals Universal Time
[GMT] minus 4 hours.)
More celestial events, sky maps, observing projects, and news of the
world's astronomy research appear each month in SKY & TELESCOPE, the
essential magazine of astronomy. See our enormous Web site and
astronomy bookstore at http://www.skypub.com/ . Clear skies!
SKY & TELESCOPE, 49 Bay State Rd., Cambridge, MA 02138 *
Copyright 2001 Sky Publishing Corporation. S&T's Weekly News Bulletin
and Sky at a Glance stargazing calendar are provided as a service to
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