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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 Four th
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 know Tempel 1 before it changes forever. A comet is described best as a "dirty snowball," a concept first proposed by American astronomer Fred Whipple more than 50 years ago. It is a chunk of primordial matter left over from the solar system's creation. A typical comet is the size of a small mountain, just a couple of miles across. A comet's surface is not much more reflective than a blacktop highway. Crater walls and fissures expose ice to sunlight. Solar energy evaporates the ice directly (liquid water cannot exist in these conditions), releasing any dust trapped within the comet. Ejected dust enshrouds the nucleus in a sunlit cloud called the "inner coma." It blocks our view of surface events. Beyond Jupiter, sunlight produces little dust. As a comet approaches the Sun, dust production increases. More dust and sunlight quickly increase the comet's brightness. When Ernst Tempel first saw Tempel 1 in 1873, he was searching deliberately. In 1866, Tempel had codiscovered Comet 55P/Tempel-Tuttle, the parent comet of the Leonid meteor shower. Astronomers computed the 1873 comet's orbit and recognized it was the ninth comet known to return periodically, leading to its modernday designation of 9P/Tempel 1. Jupiter perturbs Tempel 1's 6-year orbit significantly. In fact, the comet was lost for almost a century due to a close passage to Jupiter in 1881 that changed its orbit. Astronomers recovered Tempel 1 in 1972.
KECK OBSERVATORY

COMET TEMPEL 1 was visible but not bright when imaged August 21, 2000. This infrared image shows how the comet is radiating heat.

This is no Hale-Bopp
You'll be able to find and follow Tempel 1 with a 4-inch telescope from a dark site. However, it won't be bright like Comet Hale-Bopp was a decade ago. Know when and where to look -- it will be worth it. And, hopefully, you'll see the comet change after it's been smacked. From late May through early June, Tempel 1 glows between 10th and 11th magnitude, similar to the toughest galaxies on the Messier list. Fortunately, it will be relatively easy to locate near Delta () Virginis, to the upper left of brilliant Jupiter as viewed from the Northern Hemisphere. Also, during this time, the Moon is out of the evening sky. Wait until twilight ends to begin your search. If you're trying to see Tempel 1 from suburbia, a 10-inch scope probably is the required minimum. On June 4/5, Tempel 1 passes less than 1 apparent Moon-width from NGC 4845. Compare the two objects. The elongated spiral galaxy has a partially obscured
Alister Ling is a meteorologist for Environment Canada and a long-time comet watcher.

/// THE TALE OF THE TAIL
Comets typically produce two types of tails: gas and dust. The gas (ion) tail forms as chemical elements break apart because of the sunlight and glow as their electrons recombine with protons. By studying the spectrum, scientists can determine the comet's makeup. Typically, a comet's gas tail is a faint slate blue, but it can be green sometimes. Pressure from the solar wind makes it straight and carries it away from the Sun. Dust is heavier than gas, so cometary dust particles tend to orbit the Sun with the comet. The bigger the specks and clumps of dust, the less effect the solar wind has, producing a gradually cur ved tail. Just as dust in Earth's atmosphere scatters sunlight, the comet's dust tail tends to be pale-yellow or white. Take a close look on a few evenings either side of July 9. Earth will look right along the plane of the comet's orbit then, so we may see an "anti-tail" appearing to point toward the Sun. Imagine a hand fan, spread it out, and look at it edge-wise, with the hinge closest to your eyes. You'll see material on both sides of the hinge, which, in this case, represents the comet's nucleus. ­ A.L.

Gas tail Nucleus
ASTRONOMY: ROEN KELLY

Dust tail

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

DEEP IMPACT was launched toward Comet Tempel 1 January 12 aboard a Delta II rocket.

NASA

Bang!
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) Energy released: Equivalent to 4.8 tons of TNT
NASA

THE "IMPACTOR" SPACECRAF T launched from Deep Impact weighs 820 pounds (370 kilograms). It will strike Comet Tempel 1 at 23,000 mph (36,700 km/h).

center; the rounder comet has a halo that brightens quickly to a sharp central point. Look carefully at the comet's periphery. One flank should be sharper. This edge is the "bow shock," where the solar wind pushes the dust outward. From June 8­23, the evening sky is awash with moonlight. This natural form of light pollution hides the outer coma and makes low-contrast features difficult to make out. These 2 weeks are prime time for CCD imagers because the extra light can be electronically subtracted from the image.

Best viewing
Dark skies return after June 23, when the Moon rises in late evening and, finally, after midnight. Tempel 1 is now sliding just north of bright Spica (Alpha [] Virginis) and has brightened to about magnitude 9.5. Start with low power to locate the comet and to familiarize yourself with its overall

character. Zoom in for a more detailed view of the central area before starting to look at the tails. Straight features are evidence of either jet activity or that you're looking through a thicker part of the tail. Practice your observing by increasing the magnification. Although the field of view darkens, your eye better detects faint structure when that structure occupies more of the field. Be patient -- you can't train to expert level in a night. The more you observe, the more detail you'll see. Slowly move your eye around, and try to pick out high-contrast zones. Use averted vision -- look to the side of the comet but note what's in the middle.

/// COMET WATCH
Learn how amateurs worldwide are helping the mission with round-the-clock coverage of Tempel 1's behavior before, during, and after impact. Information about the Deep Impact Mission's Small Telescope Science Program and a registration form can be found at deepimpact.umd.edu/stsp. Observers with telescopes as small as 8 inches are participating. ­ A.L.

Ker-splat!
In a solar system first, humans will slam a large copper weight into Tempel 1 July 4. Scientists hope to excavate a crater and see if new jet activity develops from the freshly exposed material.

If all goes as planned, impact will occur at 2 A.M. EDT. East of the Mississippi River, the comet will have already set. West-coast observers may see the comet brighten during the following minutes and hours. Check the Deep Impact web site, deepimpact.jpl. nasa.gov, for an up-to-the-minute schedule. No fireworks in the usual sense will occur because the projectile isn't a bomb. However, the impact speed of 23,000 mph
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/// FINDING TEMPEL 1
Use this chart to zero in on the area. Remember, Newtonian telescopes rotate the image 180°. Refractors and Schmidt-Cassegrain telescopes with star diagonals leave the image upright but flip it left to right. Even from the countryside, the background sky can be bright enough at low power to hide the comet's faint, fuzzy glow. Nudge the power up to about 100x. Try using a dark cloth or towel to cover your head and eyepiece. By eliminating as much stray light as possible, you can see faint objects and details more easily. Also, try not to fog the eyepiece with your breath.
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How to spot Tempel 1
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Path of Comet Tempel 1
ASTRONOMY: ROEN KELLY

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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 December 2004

Mars

Ear th at launch Spacecraft
Sun

Ear th at encounter Tempel 1 at encounter July 2005

KE Y POSITIONS of Tempel 1, Deep Impact, and Earth, as well as the orbits of planets out to Jupiter, appear in this diagram. Deep Impact's travel time to Tempel 1 is approximately 6 months. ASTRONOMY: ROEN KELLY 70 a s tronomy
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(36,700 km/h) packs quite a punch. There should be lots of stuff blown off the comet. Mission leaders hope the effects will be visible through 8-inch scopes. Most likely, observers simply will see a brighter comet from one night to the next. Scientists suspect the spray from the collision itself will be too small to see. Maybe Tempel 1 will brighten and fade dramatically as the new zone rotates in and out of the Sun's rays. Because astronomers are unsure of the comet's orientation and spin or the exact location of the impact, it's impossible to know what will be seen. Moonlight returns to evening skies by the second week of July. Wait until moonset, and you can observe in darkness up to July 10. After that, it's a good idea to keep a close watch, just in case.

Winding down
By the end of July, Tempel 1 will be cruising slowly about halfway between Spica and Antares (Alpha [] Scorpii). For much of the Northern Hemisphere, its position low in the sky will begin to affect observations. Take advantage of a late-summer star party to get a good look at it from a dark site with a large scope. As Tempel 1 speeds away from the Sun, it will become less active. Keen observers can follow it through August as it slides beneath Scorpius. Astrophotographers will grab farewell shots as it poses with star cluster M6 on the evening of October 1. After that, the comet will slowly limp off into the sunset, shot through the heart with a boulder-size copper bullet by the gunslingers at NASA. X

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