Spun by the Sun 



How does an asteroid pick up a satellite? 

 Well, a big asteroid can capture a small 

 passerby with its gravitational pull, but how 

 a small one — less than six miles in diameter — 

 gains any company has been a puzzle to astrono 

 mers. About 15 percent of the solar 

 system's known small asteroids have 

 satellites — they're "binary asteroids." 



Kevin J. Walsh, now at the Observatory 4 

 of the Cote d'Azur in Nice, France, and two 



nomenon named after the 

 initials of its four discoverers). As 

 irregularly shaped, so-called "rubble-pile" 

 asteroids absorb and reradiate infrared solar 

 energy, their rotation gradually speeds up or 

 slows down — that's YORP, in a nutshell. 



When YORP sufficiently accelerates a rubble- 

 pile asteroid's rotation — a process that can 

 take between tens of thousands and mil- 

 lions of years — rock moves from the 

 poles toward the equator, from where 

 it may launch into space. In some cases, 

 the rock coalesces into a solid body orbiting the 

 parent. Voila! A little binary asteroid is born. 



Walsh's team successfully tested its model 

 against several of the solar system's small 



SAMPLINGS 



He Came, He Saw, He Sailed On 



Caesar and his troops attack native Britons 

 within sight of the white cliffs of Dover. 



When Julius Caesar arrived 

 off the coast of Britain with 

 his hundred-ship force in Au- 

 gust, 55 B.C., he was greeted 

 by a host of defenders poised 

 to hurl spears down on his 

 invading army from the tow- 

 ering Dover cliffs. Seeking a 

 better landing site, he sailed 

 on a strong afternoon current 

 and landed his troops at a 

 beach seven miles away, ac- 

 cording to his own account. 



Caesar neglected to men- 

 tion, however, whether he 

 sailed southwest or northeast. 

 The only shoreline within seven 

 miles of Dover that matches 

 Caesar's description lies to the 



northeast, near present-day Deal. That would settle it, except that the current 

 flowed southwest from Dover on the afternoons of August 26 and 27 — four 

 days before the full Moon, as Caesar obliquely reported the landing date. (It's 

 unknown whether he counted the day of the full Moon itself.) For centuries, the 

 paradox has provoked debate among historians and astronomers. 



Enter forensic astronomer Donald W. Olson of Texas State University in San 

 Marcos. With a colleague and two honors students, Olson traveled to Brit- 

 ain in August 2007, when astronomical conditions almost exactly duplicated 

 those of 55 B.C. They confirmed that on August 26 and 27, the afternoon cur- 

 rent ran southwestward. But on the 22nd and 23rd, it flowed strongly north- 

 eastward, toward Deal. So that's where, and when, Caesar landed. 



Could the great warrior have erred by four days? Probably not, says Ol- 

 son, but his original manuscript is long vanished, and only copies of copies, 

 made centuries later, survive. At some point, Caesar's handwritten VII or 

 VIII — indicating August 22 or 23, seven or eight days before the full Moon — 

 was likely mistranscribed as Mil. {Sky & Telescope) — H.L. 



IV.IUUI I I 



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Immigrant Pathogen 



Lyme disease entered American con- 

 sciousness in 1975, when a cluster of 

 cases turned up around Old Lyme, Con- 

 necticut. Thirty-plus years later, it's still 

 on the rise in the United States. New re- 

 search shows that the pathogen respon- 

 sible for the ailment, which produces 

 arthritis and neurological symptoms, 

 originated overseas, millennia ago. 



Lyme disease is caused by the spi- 

 rochete bacteria Borrelia burgdorferi, 

 which live in small and medium-size 

 mammals and are transmitted to humans 



named 1999 KW 4 , which has almost all 

 the properties pre- Animatlon st///s show 

 dieted by the model. 1999 «W 4 —a binary 

 (Nature) 



—H.L. 



asteroid. 



via tick bites. Little known stateside is 

 that it's the most common tick-borne 

 disease in Europe, as it is in the U.S.; two 

 other species of Borrelia are implicated 

 there, though, along with B. burgdorferi. 



To probe the U.S. pathogen's origins, 

 Gabriele Margos of the University of 

 Bath in England and sixteen colleagues 

 compared DNA samples from B. burg- 

 dorferi collected across the U.S. and 

 Europe. Contrary to earlier studies that 

 located our B. burgdorferi's origins in 

 the New World, the research concluded 

 that the pathogen originally came from 

 Europe. The team studied eight "house- 



V'l 



keeping" genes, which are involved in 

 cell maintenance and evolve slowly. As 

 such, their variation is thought to yield 

 more reliable insights than that of less 

 conservative genes studied by others. 



The team thinks the pathogen has 

 resided quietly in North America for mil- 

 lions of years, only recently coming into 

 sufficient contact with humans to war- 

 rant the medical community's attention. 

 (PNAS) —S.R. 



Borrelia burgdorferi bacteria viewed 

 with fluorescence microscopy and 

 magnified 400x 



S 'A ' w • * . 



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mammais ana are transmitted to numans turope. 1 ne team stuaiea eignt nouse- magnified 4uux e 



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