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A rogue planet three times as massive as Mars probably sideswiped Earth 4.5 billion years ago, vaporizing enough material from Earth's upper layers to form the moon, according to CfA's Alastair Cameron and colleagues at the University of Colorado at Boulder.

While many theories on the Moon's formation have been proposed, detailed analyses of lunar rocks obtained during NASA's Apollo missions were key in creating the "giant impact theory" in the 1970s that is now widely accepted, according to Robin Canup of CU's Laboratory for Atmospheric and Space Physics. But the Mars-sized "impactor" originally proposed by Cameron in the late 1980s is not large enough to account for the formation of our unusually large Moon, according to new calculations.

New modeling work by the CU-Boulder group, which is collaborating with Cameron, indicates the impactor must have been at least 2.5 to 3 times the mass of Mars to create the volume of debris required to eventually coalesce into the Moon. The "protoplanet" probably was orbiting the Sun somewhere between Earth and Mars when the collision occurred.

"This was a surprising result," said Canup. "Our calculations indicate a lot more impact energy than previously believed would have been required to produce enough material to form the Moon."

The CU research indicates an "oblique impact" between Earth and the ancient planet vaporized the upper portions Earth's crust and mantle, spraying the material into Earth's orbit. "The material appears to have spread into a gaseous disk around Earth, then formed a handful of small, extremely hot moonlets that eventually coalesced into the single, large moon we see today," said Canup.

"Large-scale impacts like this one probably played a crucial role in shaping the Solar System," said Canup. The puzzling size and composition of Mercury, the extreme tilt in Uranus' axis, and the peculiar, "double-planet" system of Pluto and its large moon, Charon, all suggest such impacts may have been relatively common.

"We believe this theory is a linchpin to understanding how planets formed in our Solar System and in solar systems that may exist around other stars," she said.

Cameron, Canup, and scores of other Solar System experts will come together later this year for a full-blown conference on the creation of Earth and Moon. Stay tuned for details on collision, catastrophes, and cataclysms in the early Solar System. "A very violent place," according to Cameron.


The first of a pair of new telescopes, funded primarily by NASA, has begun an ambitious three-and-a-half year near-infrared survey of the entire celestial sky, peering through the curtain of interstellar dust in the Milky Way galaxy.

The Two-Micron All-Sky Survey (2MASS), based at the University of Massachusetts, Amherst, features two 1.3-meter telescopes, one at SAO's Whipple Observatory on Mount Hopkins, AZ, and the other, now under construction, at a National Optical Astronomy Observatories site on Cerro Tololo, Chile.

The survey is designed to catalogue one million galaxies and 300 million stars in the "local" universe, along with quasars and galaxies thought to have black holes at their centers.

The 2MASS project will also observe many known asteroids and possibly some comets, and it is uniquely sensitive to exotic objects like brown dwarfs, which lack the mass needed to ignite and become full-fledged stars. The telescopes are equipped with near-infrared detector arrays that will provide the most complete census to date of cool stars in the Milky Way galaxy and provide new data for detailed studies of the galactic structure. Near-infrared emission is at wavelengths roughly two-to-four times longer than visible light and permits astronomers to "see through" the obscuring effects of interstellar dust in the Milky Way.


Peering into a massive swarm of ancient stars in our Milky Way galaxy, the orbiting Hubble Space Telescope has given astronomers the first chance to study rare cataclysmic events produced by colliding stars. New evidence reveals a previously unknown source of light near the heart of the million-star cluster, possibly a star stripped of its shroud of gas as another star races past it. The research provides a portrait of stars hurtling into catastrophic crashes while others become trapped in inescapable orbital duets, with one star sucking material from its companion.

"With a million stars concentrated in a space that would normally hold only one, these globular clusters force pairs of stars into interactions not seen anywhere else in the universe," says Adrienne Cool, a former CfA graduate student and now assistant professor of astronomy at San Francisco State University, who led an HST team that included the CfA's Josh Grindlay, her former advisor.

The paired stars locked in orbit act like egg-beaters, keeping things stirred up deep within the cluster's core and preventing the core from collapsing into a black hole. Evidence of the dynamic star interactions comes from unusual wavelengths of light shining from faint stars that Cool and her colleagues have been able to detect over the last three years. Most individual stars of this brightness appear red, but these are blue. The stars' brightness varies rapidly, and the stars emit an unusually large amount of ultraviolet light. All three traits are telltale signs that the source is not a typical star, but rather two stars trapped in an orbiting dance.

"The light we are detecting is powered by gravitational energy, not the nuclear fusion that normally lights stars," Cool explains.

The phenomenon, called a cataclysmic variable, has been known for some time in the galaxy, but those seen before involve stars paired since birth. The pairs inside the dense globular cluster, by contrast, become locked together when two mature stars pass too closely in the night.

Globular clusters are the only regions of the galaxy yet studied where such stellar collisions are likely to occur. The densely packed stars here create far more stellar traffic than in almost any other cosmic region, with the distance between neighboring stars in a cluster only 1/100 the typical distance between stars. Although stellar collisions may be spectacular, they are uncommon even here. Due to the vastness of space, collisions within a globular cluster occur once in every 10 to 100 million years. Only because clusters are more than 10 billion years old has there been time for hundreds of devastating collisions.


On November 17, SAO Senior Scientist Margaret Geller became "A Library Lion." Each year, the New York Public Library "celebrates four remarkable individuals whose achievements have enriched our culture and our lives.'' She is the first scientist to receive this recognition, named for the trademark twin lions flanking the Library's entrance on Fifth Avenue.

The other three "Lions" named this year were Johnnetta B. Cole (the President of Spelman College), Robert Hughes (the art critic of Time magazine who made the PBS documentaries "American Visions" and "The Shock of the New"), and George C. Wolfe (producer of The Joseph Papp Public Theatre/New York Shakespeare Festival and Tony Award winner for directing and writing "Jelly's Last Jam").

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Irwin Shapiro has been named Timken University Professor of Harvard University. Also a senior scientist at SAO, a professor of astronomy and physics at Harvard, and director of the CfA, Shapiro is the first recipient of the new Chair, established by the Timken family and Timken Foundation of Canton, Ohio. The Timken Company is the world's largest manufacturer of tapered roller bearings.

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The SAO web site, created in large part by Megan Watzke of Public Affairs and Julie Shaw of the Computation Facility, has been selected as a featured site in StudyWeb as "one of the best educational resources on the Web." StudyWeb is an Internet site for educational resources for students and teachers. Since 1996, StudyWeb reviewers have selected outstanding Internet sites to be included as educational links, with each site reviewed for editorial and visual merits.

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Bob Kirshner became CfA Associate Director of the Optical and Infrared (OIR) Division, effective 1 February 1998. In announcing the appointment, Director Irwin Shapiro noted that this "is a time of great challenges and even greater opportunities as the CfA is involved in bringing into operation new large optical telescopes--the converted MMT and the 'twin' Magellan telescopes--and the extraordinarily powerful new instruments being built for these telescopes. In addition the three new Major Scientific Instrumentation projects--VERITAS, SOAR, and OASIS--are all centered in the OIR division." Kirshner replaces John Huchra, who, as Shapiro noted, "has done a truly outstanding job...bringing CfA to the brink of this explosion in our instrumental capabilities..."

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On Wednesday, November 12, Bob Noyes presented the second Harvard University Science Center Research Lecture of 1997, with a talk entitled "Planets Orbiting Other Stars."

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John Kohl, principal investigator for the Ultraviolet Coronagraph Spectrometer (UVCS) experiment on SOHO and a 1963 graduate of Muskingum College in New Concord, Ohio, returned to his alma mater last fall to speak to some 200 students and faculty gathered for the college's second annual Alumni Convocation Series lecture. Kohl, who initially pursued football rather more vigorously than physics, told students how he was greatly influenced to pursue a career in space science by the 1960 orbital flight of fellow alumnus John Glenn.

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Alastair Cameron has been named the Donald H. Menzel Professor of Astrophysics, a chair established through gifts from the former director of HCO and his wife Florence, and which was previously held by David Layzer.

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