OUT THERE 



Sizing Up Pluto 



The runt of the solar system 

 turns out to have three moons. 



By Charles Liu 



If you believed half the spam you 

 got in your e-mail, you'd think 

 size is everything. So pity poor 

 Pluto. When the American astron- 

 omer Clyde W.Tombaugh discovered 

 it back in 1930, it was immediately 

 dubbed the ninth planet of our solar 

 system. Most astronomers thought it 

 was as big as the gas-giant planets 

 Uranus and Neptune — maybe even 

 bigger. But almost immediately after 

 its discovery, astronomers agreed that 

 Pluto would have to shrink; then, as 

 telescopes got progressively more 

 powerful, Pluto got even smaller.To- 

 day, we astronomers know it's not 

 even as big as Earth's moon. Total 

 shrinkage: more than twenty times 

 the original estimate. 



But Pluto lovers were in for even 

 more distress. In 1978, when as- 

 tronomers discovered Pluto's moon 

 Charon, they realized that both bod- 

 ies revolve about a center of mass that 

 does not lie inside either one of them. 

 Pluto was not a single planet, but half 

 of the Pluto-Charon double-planet 

 system (though Charon continues to 

 be called a "moon"). Worst of all was 

 the recent discovery of 2003 UB313, 

 an object larger than Pluto yet farther 

 from the Sun [see "Number Ten?" by 

 Charles Liu, October 2005]. With that, 

 the case became even stronger that 

 Pluto was misclassified in the first 

 place — it's not even a "real" planet! 



Before you get all teary-eyed over 

 Pluto's waning status, though, con- 

 sider this: if astronomers are really 

 so down on Pluto, why did we work 

 so hard to launch a new spacecraft 



recently to probe it? Just as you 

 should trash all that silly "ENlaarge 

 it" spam as hype, you might also 

 delete the notion that size is all that 

 matters in a heavenly body. In fact, 

 in the past decade astronomers have 

 sent several spacecraft to "minor" tar- 

 gets: NEAR-Shoemaker to the aster- 

 oid 433 Eros, Stardust to Comet Wild 

 2, and Deep Impact to Comet P/Tem- 

 pel 1. The largest of those targets, 

 Eros, is barely twenty miles across.Yet 

 each of those missions has yielded 

 important insights about the compo- 

 sition and history of our solar system. 



As for the current mission to Plu- 

 to, Neu> Horizons is perhaps the most 

 sophisticated space probe ever 

 launched — certainly not just a bone 

 tossed toward a B-list planetary body. 

 It left Earth's vicinity at nearly 36,000 

 miles an hour (the fastest ever launch 

 speed) and passed the Moon's orbit 

 in approximately nine hours. (Apolio 

 11 took three days to do the same 

 thing.) If all goes well, New Horizons 

 will reach Jupiter in thirteen months, 

 then take advantage of the giant plan- 

 et's gravity to slingshot on to Pluto 

 sometime in July 2015. 



As it turns out, an unexpected 

 bonus — two, actually — will greet 

 New Horizons when it reaches its 

 primary target: a pair of tiny moons 



whose existence was confirmed 

 barely a month after launch. • * 



Since Tombaugh's time, Pluto may 

 have given up its claim to be a 

 gas giant, but as of this year its new 

 status as a many-mooned planet fi- 

 nally enables it to join the ranks of 

 Uranus and Neptune. In May 2005 

 Harold A. Weaver of Johns Hopkins 

 University's Applied Physics Labora- 

 tory in Laurel, Maryland, and S.Alan 

 Stern of the Southwest Research In- 

 stitute in Boulder, Colorado, trained 

 the Hubble Space Telescope on Plu- 

 to for some of its most detailed im- 

 ages ever. The astronomers saw two 

 dots of light, each about 35,000 miles 

 from Pluto — much farther from the 

 planet than Charon is, but only about 

 one-seventh the distance between 

 the Moon and Earth. To verify that 

 the dots were moons, the team re- 

 viewed past data and found evidence 

 of them, in different positions with 

 respect to Pluto, on images dating 

 back to 2002. They calculated orbital 

 paths for the two objects and took 

 (Continued on page 66) 



Enhanced-color images of Pluto made by 

 the Hubble Space Telescope on February 15, 

 2006 (top of page), and March 2, 2006 

 (above), reveal the faint light of two newly 

 discovered moons (small white dots). Their 

 movement relative to Pluto (brightest spot at 

 center of images) over time led investigators 

 to conclude that they orbit the planet. 

 Charon, smaller than Pluto but substantially 

 larger than the two new moons, appears to 

 the upper right (in the top image) and to the 

 upper left (in the above image) of the planet. 



May 2006 NATURAI HIS ! OK. Y 63 



