S&T's Weekly News Bulletin
March 20, 1998
(33K JPEG) When deployed as intended, an Iridium communications satellite's axis points to the center of Earth, while three silvered Main Mission Antennas (MMAs) lean outward, peppering the Earth with data-bearing radio transmissions -- and the occasional glint of sunlight. (The third MMA lies behind the triangular spacecraft in this depiction.) Courtesy Donald Baker, Iridium LLC.
Preserving Radio Astronomy
After five years of negotiations, astronomers who use the Arecibo Radio Telescope in Puerto Rico finally have assurances that modern communications satellites won't seriously disrupt their work. The major concern for astronomers was a "constellation" of satellites called Iridium, which by the end of 1998 will consist of 66 satellites in 6 orbital planes. The satellites, dozens of which are already in orbit, operate at a frequency range of 1616 to 1626.5 megahertz. This is unsettlingly close to 1612 megahertz, the emission frequency of hydroxyl, a molecule of one atom of oxygen and one atom of hydrogen. Observations of hydroxyl are important for studying star-forming clouds and cool stars. The agreement announced on March 18th between Motorola, Inc., operators of Iridium, and the National Astronomy and Ionosphere Center, which runs Arecibo, guarantees that radio astronomers will have eight hours of clear observing time each day between 10:00 p.m. to 6:00 a.m. Eastern Time. Additional time slots can be granted if required for special observing sessions.The Iridium satellites have another feature of interest to amateur astronomers. Their large, flat antennas act like giant mirrors and can reflect a narrow beam of sunlight to the Earth. From the ground, such a "glint" in the early evening or predawn skies can be 100 times brighter than Venus! You can find out more about how to observe Iridium satellites and the concerns of radio astronomy in the May 1998 issue of Sky & Telescope, page 36.
(45K JPEG) Left: A near-infrared view of planetary nebula NGC 7027 in Cygnus, made with the Hubble Space Telescope's Near Infrared Camera and Multi-Object Spectrometer (NICMOS). Right: A composite of the NICMOS image and a visible-light image from the Wide Field and Planetary Camera 2. In visible light alone the nebula's central star is completely hidden by dust. Courtesy William Latter (SIRTF Science Center/IPAC/Caltech), STScI, and NASA.
Focus on Planetary Nebulae
New images from the Hubble Space Telescope are allowing astronomers to probe deep into planetary nebulae. Hubble's infrared image of NGC 7027 in Cygnus clearly reveals its central star and regions of hot and cool gas. Pictures of two other proto-planetary nebulae -- dubbed the Cotton Candy and Silkworm nebulae -- reveal outflowing gas "breaking out" of surrounding cocoons of previously ejected material, resulting in butterfly-shaped patterns.
Right: (32K JPEG) Hubble's Wide Field and Planetary Camera 2 has compiled quite a beautiful gallery of planetary nebulae, objects created when Sun-like stars shed material near the end of their lives. The two butterfly-shaped nebulae in the top row are very young protoplanetaries, while the two roughly spherical nebulae in the bottom row are more mature. Courtesy Sun Kwok (University of Calgary), Robert Rubin (NASA/Ames), H. Bond (STScI), and NASA.
(28K JPEG) The meteorite known as EET96008, about half of which is covered by a black, glassy fusion crust from its fall through Earth's atmosphere, has been identified as having come from the Moon. Courtesy NASA/Johnson Space Center.
New Meteorite from the Moon
A new meteorite that originated from the surface of the Moon has been brought back from the Antarctic. The 53-gram meteorite, identified as EET96008, was discovered in the Elephant Moraine region in Antarctica, which earlier yielded a Martian meteorite. Analysis of the sample reveals that the composition is like that of lunar basaltic breccia, which consists of broken rock fragments fused back together by meteoritic impact.
Right: (67K JPEG) In this section of the meteorite EET96008, the largest grains are about 1.2 millimeters across. Analysis of the chemical makeup of the various grains confirms that the rock is lunar, not terrestrial. Courtesy NASA/Johnson Space Center.
(18K GIF) The path that Jupiter will take behind the Moon as seen from various cities on March 26th. Celestial north is up. Sky & Telescope illustration.
Challenging Jupiter Occultation
There's a tough lunar occultation on tap for the morning of Thursday, March 25th. The thin crescent Moon will pass in front of Jupiter for eastern North America. But it will be a tricky sighting low in the east-southeastern sky. See the March Sky & Telescope, page 98, for details and a map or visit SKY Online's Occultation Page.
(15K JPEG) A giant sunspot complex prepares to rotate off the visible face of the Sun in this white-light image recorded on March 19th. Courtesy Big Bear Solar Observatory/New Jersey Institute of Technology.
Sun Moves North
As seen from our vantage on Earth, the Sun appeared to cross the celestial equator at 19:55 Universal Time (2:55 p.m. Eastern Standard Time) on March 20th, heading north. This equinox begins spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
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