COMETARY ASTRONOMY On July 4, Deep Impact will blow a hole in Comet Tempel 1. Don’t miss it. /// BY ALISTER LING Fireworks on the Fourth This summer could provide the most exciting comet observations ever. Scientists are slamming a projectile into Comet 9P/Tempel 1, and — just maybe — the effects will be visible through a backyard telescope. To best appreciate the impact, get to Beyond Jupiter, sunlight produces little know Tempel 1 before it changes forever. dust. As a comet approaches the , dust A comet is described best as a “dirty snow- production increases. More dust and sun- ball,” a concept first proposed by American light quickly increase the comet’s brightness. astronomer Fred Whipple more than 50 When Ernst Tempel first saw Tempel 1 ago. It is a chunk of primordial mat- in 1873, he was searching deliberately. In ter left over from the solar system’s cre- 1866, Tempel had codiscovered Comet KECK OBSERVATORY KECK ation. A typical comet is the size of a small 55P/Tempel-Tuttle, the parent comet of the mountain, just a couple of miles across. Leonid meteor shower. Astronomers com- COMET TEMPEL 1 was visible but not bright when imaged August 21, 2000. This infrared A comet’s surface is not much more puted the 1873 comet’s and recog- image shows how the comet is radiating heat. reflective than a blacktop highway. Crater nized it was the ninth comet known to walls and fissures expose ice to sunlight. return periodically, leading to its modern- Solar energy evaporates the ice directly (liq- day designation of 9P/Tempel 1. This is no Hale-Bopp uid water cannot exist in these conditions), Jupiter perturbs Tempel 1’s 6- orbit You’ll be able to find and follow Tempel 1 releasing any dust trapped within the comet. significantly. In fact, the comet was lost for with a 4-inch telescope from a dark site. Ejected dust enshrouds the nucleus in a almost a century due to a close passage to However, it won’t be bright like Comet sunlit cloud called the “inner coma.” It Jupiter in 1881 that changed its orbit. Hale-Bopp was a decade ago. Know when blocks our view of surface events. Astronomers recovered Tempel 1 in 1972. and where to look — it will be worth it. And, hopefully, you’ll see the comet /// THE TALE OF THE TAIL change after it’s been smacked. From late May through early June, Comets typically produce two types of tails: from the solar wind makes it straight and car- Tempel 1 glows between 10th and 11th gas and dust. The gas (ion) tail forms as ries it away from the Sun. , similar to the toughest chemical elements break apart because of the Dust is heavier than gas, so cometary dust on the Messier list. Fortunately, it will be sunlight and glow as their electrons recom- particles tend to orbit the Sun with the comet. relatively easy to locate near Delta (δ) bine with protons. By studying the spectrum, The bigger the specks and clumps of dust, the Virginis, to the upper left of brilliant Jupiter scientists can determine the comet’s makeup. less effect the solar wind has, producing a as viewed from the Northern Hemisphere. Typically, a comet’s gas tail is a faint slate gradually curved tail. Just as dust in ’s Also, during this time, the is out of blue, but it can be green sometimes. Pressure atmosphere scatters sunlight, the comet’s the evening sky. Wait until twilight ends to dust tail tends to be pale-yellow or white. begin your search. If you’re trying to see Take a close look on a few evenings either Tempel 1 from suburbia, a 10-inch scope Gas tail side of July 9. Earth will look right along the plane of the comet’s orbit then, so we may see probably is the required minimum. Nucleus an “anti-tail” appearing to point toward the On June 4/5, Tempel 1 passes less than Sun. Imagine a hand fan, spread it out, and 1 apparent Moon-width from NGC 4845. look at it edge-wise, with the hinge closest to Compare the two objects. The elongated ROEN KELLY your eyes. You’ll see material on both sides of spiral has a partially obscured Dust tail the hinge, which, in this case, represents the

ASTRONOMY: comet’s nucleus. – A.L. Alister Ling is a meteorologist for Environment Canada and a long-time comet watcher.

68 astronomy /// june 05 /// FLY-BY SHOOTING

The Deep Impact spacecraft, launched January chemical composition of the spray. Unfortu- 12, 2005, will study the pristine interior of comet nately, the data transmission rate isn’t high DEEP IMPACT was 9P/Tempel 1. Literally executing a “fly-by” shoot- enough to run live video. But that won’t stop sci- launched toward ing, Deep Impact will release a large copper entists from learning how a comet’s interior is Comet Tempel 1 “bullet” 24 hours before striking ground zero. different from its surface. NASA plans to broad- January 12 aboard a The bullet’s onboard guidance system cast the event on television and on the Internet, Delta II rocket. homes in on the comet. To minimize the so we’ll see images arrive in near-real time. chance of a miss, the main vehicle keeps a Will this fastball pitch alter the comet’s close watch, swinging past at a tight but safe course? No. As impressive as it sounds, it’s like distance of 300 miles (500 km). Because of firing a peashooter at a moving snowman. The the speed of the interception, there’s only one long-term gas and dust production of the comet chance to get everything right. The result will caused by the Sun’s radiation has a far greater be a crater 80 feet deep and more than 300 (but still small) effect on the orbit. feet wide (25 by 100 meters). Deep Impact is one of NASA’s Discovery The science instruments on the primary missions, a low-cost but focused approach to vehicle will capture images during the solar-system science. The probe was launched encounter. Also, spectroscopy will analyze the on a simple arc to reach the comet in the short- est time at the highest speed. Other missions to land on a comet and return a sample need to match a comet’s speed and trajectory more accurately. – A.L.

Bang! NASA Impact date: July 4, around 2 A.M.EDT Mass of impactor: 820 pounds (370 kilograms) Speed of impact: 23,000 mph (36,700 km/h) THE “IMPACTOR” SPACECRAFT launched from Deep Impact weighs 820 pounds (370 Energy released: Equivalent to 4.8 tons kilograms). It will strike Comet Tempel 1 at of TNT

23,000 mph (36,700 km/h). NASA center; the rounder comet has a halo that character. Zoom in for a more detailed view brightens quickly to a sharp central point. of the central area before starting to look at /// COMET WATCH Look carefully at the comet’s periphery. the tails. Straight features are evidence of One flank should be sharper. This edge is either jet activity or that you’re looking Learn how amateurs worldwide are helping the “bow shock,” where the solar wind through a thicker part of the tail. the mission with round-the-clock coverage pushes the dust outward. Practice your observing by increasing of Tempel 1’s behavior before, during, and From June 8–23, the evening sky is the magnification. Although the field of after impact. Information about the Deep Impact Mission’s Small Telescope Science awash with moonlight. This natural form view darkens, your eye better detects faint Program and a registration form can be of light pollution hides the outer coma and structure when that structure occupies found at deepimpact.umd.edu/stsp. makes low-contrast features difficult to more of the field. Be patient — you can’t Observers with telescopes as small as 8 make out. These 2 weeks are prime time for train to expert level in a night. The more inches are participating. – A.L. CCD imagers because the extra light can be you observe, the more detail you’ll see. electronically subtracted from the image. Slowly move your eye around, and try to pick out high-contrast zones. Use avert- If all goes as planned, impact will occur Best viewing ed vision — look to the side of the comet at 2 A.M. EDT. East of the Mississippi River, Dark skies return after June 23, when the but note what’s in the middle. the comet will have already set. West-coast Moon rises in late evening and, finally, observers may see the comet brighten dur- after midnight. Tempel 1 is now sliding Ker-splat! ing the following minutes and hours. Check just north of bright Spica (Alpha [α] In a solar system first, humans will slam a the Deep Impact web site, deepimpact.jpl. Virginis) and has brightened to about large copper weight into Tempel 1 July 4. nasa.gov, for an up-to-the-minute schedule. magnitude 9.5. Scientists hope to excavate a crater and see No fireworks in the usual sense will Start with low power to locate the comet if new jet activity develops from the freshly occur because the projectile isn’t a bomb. and to familiarize yourself with its overall exposed material. However, the impact speed of 23,000 mph

www.astronomy.com 69 /// FINDING TEMPEL 1

Use this chart to zero in on the area. Remember, Newtonian telescopes hide the comet’s faint, fuzzy glow. Nudge the power up to about 100x. Try rotate the image 180°. Refractors and Schmidt-Cassegrain telescopes with using a dark cloth or towel to cover your head and eyepiece. By eliminat- star diagonals leave the image upright but flip it left to right. Even from the ing as much stray light as possible, you can see faint objects and details countryside, the background sky can be bright enough at low power to more easily. Also, try not to fog the eyepiece with your breath.

N Apr 1 16 Dec 2 How to spot Tempel 1 17 ε May 1 Jan 1 17 Mar 2 Feb 15 31 16 SERPENS 16 CAPUT 31 δ

ζ β γ June 15 May 1 June 1 Apr 1 July 1 Mar 1 Aug 1 Path of Jupiter Feb 1 E Jan 1 β 30 Impact Spica

γ LIBRA July 15 α SCORPIUS β 30

δ Path of Comet Tempel 1 Aug 14 π ο

29 5° KELLY : ROEN ASTRONOMY

COMET TEMPEL 1 will be in Virgo the Maiden before and after its encounter with Deep Impact. Note the positions of Jupiter and Spica, two bright objects that will help you locate the field of Tempel 1. This chart presents a normal view, with north up and east to the left.

Tempel 1 at launch (36,700 km/h) packs quite a punch. There Winding down December 2004 should be lots of stuff blown off the comet. By the end of July, Tempel 1 will be cruis- Mission leaders hope the effects will be ing slowly about halfway between Spica visible through 8-inch scopes. Most likely, and (Alpha [α] Scorpii). For much observers simply will see a brighter comet of the Northern Hemisphere, its position Mars from one night to the next. Scientists sus- low in the sky will begin to affect observa- Earth at launch pect the spray from the collision itself will tions. Take advantage of a late-summer star be too small to see. party to get a good look at it from a dark Spacecraft Maybe Tempel 1 will brighten and fade site with a large scope. Sun dramatically as the new zone rotates in and As Tempel 1 speeds away from the Sun, out of the Sun’s rays. Because astronomers it will become less active. Keen observers Earth at encounter are unsure of the comet’s orientation and can follow it through August as it slides Tempel 1 at encounter July 2005 spin or the exact location of the impact, it’s beneath Scorpius. Astrophotographers will impossible to know what will be seen. grab farewell shots as it poses with star Moonlight returns to evening skies by cluster M6 on the evening of October 1. KEY POSITIONS of Tempel 1, Deep Impact, and Earth, as well as the of the second week of July. Wait until moon- After that, the comet will slowly limp out to Jupiter, appear in this diagram. set, and you can observe in darkness up to off into the sunset, shot through the heart Deep Impact’s travel time to Tempel 1 is July 10. After that, it’s a good idea to keep a with a boulder-size copper bullet by the approximately 6 months. ASTRONOMY: ROEN KELLY close watch, just in case. gunslingers at NASA. X

70 astronomy /// june 05