Observing Deep Impactby Robert Bruce Thompson, Barbara Fritchman Thompson
Independence Day—July 4, 2005—brings with it a chance to observe with your own eyes a milestone in the human exploration of space. At about 0600 UT (0200 EDT; 0100 CDT; 0000 MDT; 2300 3 July PDT), NASA's Deep Impact mission culminates when the coffee-table size penetrator section of the Deep Impact spacecraft crashes into Comet 9P/Tempel 1 at about 37,000 kilometers (or 23,000 miles) per hour. The collision is expected to blast a large crater in the comet and expel massive amounts of dust and gases. By studying this ejecta, NASA hopes to learn a great deal about the composition of comets. It may also, incidentally, provide a great show here on Earth for anyone who takes the trouble to watch.
The exact results of the impact are uncertain. No one knows the composition of the comet (determining that is one of the primary goals of the mission) and the composition of the comet in large part determines how impressive the show will be for earthbound observers. At the time of impact, 9P/Tempel will be tenth magnitude, which means it may be just barely visible to a skilled observer using a standard 7X50 binocular from a very dark site. When the penetrator strikes 9P/Tempel, the gas and dust ejected from the comet body will form a reflective cloud that will cause the apparent brightness of the comet to flare. The question is, by how much?
At worst, although it is unlikely, the penetrator body may simply be absorbed by the comet, ejecting little dust and gas. If that occurs, there will be little or no visible change in the comet. We think it more likely that the impact will eject sufficient dust and gas to cause the brightness of the comet to flare by at least two to three magnitudes, making it visible in standard binoculars. It is possible that the brightness of the comet may flare by four or five magnitudes, putting it on the edge of naked-eye visibility from a dark site.
Choosing an Observing Site
The best way to observe Comet 9P/Tempel 1 is to find the darkest site within reasonable driving distance, one without local lights and as far as possible from the general light pollution produced by nearby cities and towns. The comet is in the constellation Virgo, which is setting in the west, so choosing a site to the west of any major source of light pollution lets you avoid the light domes that would interfere with viewing the comet. It's also important to have a reasonably clear western horizon, because hills, tree lines, or other obstructions may block your view of the comet.
You're in luck if your local astronomy club has scheduled a public observation event for Deep Impact. The club will have scouted out a good observing site, and club members will have telescopes set up for public viewing. To locate your nearest astronomy club, visit the Sky & Telescope website.
Locating Comet 9P/Tempel 1
Not all astronomy clubs have scheduled public viewing sessions, and not everyone is within easy driving distance of an astronomy club. Fortunately, it's possible to observe Deep Impact on your own, using as basic a viewing implement as a standard binocular. If the comet flares as expected, seeing it will not be the problem. Locating it, however, requires a bit of preparation.
Locating a tenth magnitude object can be challenging even for an experienced astronomer. Fortunately, at the time of impact, Comet 9P/Tempel 1 is located quite near the bright star Spica, and not far from Jupiter, both of which are very easy to find.
Figure 1 shows a 90° view of the western horizon on July 3 at 2300 local time, shortly after full dark for observers at mid-northern latitudes. Jupiter, at magnitude -2.1 in the WSW, is the brightest object in the sky and is unmistakable. The zeroth-magnitude star Arcturus blazes away high above Jupiter, and first-magnitude Spica is the bright star just above and to the left of Jupiter. Comet 9P/Tempel 1, our target, is located less than 3.5° from Spica, or about half a binocular field.
For those on the U.S. East Coast, Comet 9P/Tempel 1 sets before impact. From our home in Winston-Salem, North Carolina, for example, the comet sets at 0149 on July 4, about 11 minutes before impact. Not to worry, though. The brightening caused by the impact will persist for at least a few days, probably for several weeks, and possibly for a few months. Those of us who are prevented by geography or clouds from viewing the actual impact will have many opportunities to view the comet over the following days and weeks as it makes its way south from Virgo into Libra. By late August, Comet 9P/Tempel 1 will be in Scorpius, where its increased distance from Earth will have dimmed it by two to three magnitudes, after which it will gradually fade from view.
Those in the western continental U.S. may be able to view the comet as the impact occurs. In the CDT zone, shown in Figure 2, impact occurs at 0100 local time, when the comet is still a few degrees above the horizon, and just as Jupiter sets. While it is barely possible that some CDT viewers may be able to observe the impact, the comet will be so low that atmospheric haze will probably make the comet impossible to see, particularly if you're using only a binocular.
In the MDT zone, shown in Figure 3, the comet is several degrees higher still at midnight, when impact occurs. It may be possible for those on the western side of that time zone to view the impact, particularly if they use a telescope.
Observers in the southwestern U.S. are ideally placed to view the impact. In the PDT zone, shown in Figure 4, impact occurs at 2300 local time (on July 3), with the comet still at high elevation, as shown in Figure 4.
As previously mentioned, before impact, Comet 9P/Tempel 1 is about magnitude ten, which means it would be so dim as to be on the edge of visibility with averted vision for an experienced observer using a standard 7X50 or 10X50 binocular from a very dark site (assuming the comet is high enough to be out of the muck and haze near the horizon). With a larger instrument, such as a 70mm or 80mm "giant" binocular or an 80mm short-tube refractor, Comet 9P/Tempel 1 may be visible from a dark site prior to impact as what looks like a very dim and possibly slightly fuzzy star.
Figure 5 shows a binocular view of the region around Comet 9P/Tempel 1 one hour before impact. The red circle covers 7°, which is a typical field width for a standard binocular. (Most 7X and 10X binoculars have fields of view ranging from 5.5° to 8.5°, so take into account the actual field width of the binocular you use.) Figure 5 shows stars down to magnitude 8.4, which is 0.5 to 1 magnitude brighter than the dimmest stars visible with direct vision in a 50mm binocular from a dark site.
To locate Comet 9P/Tempel with your binocular, begin by centering Spica in the field of view. It's easy to verify that you have Spica in the field. With the binocular, 1st-magnitude Spica is extremely bright, brighter than Jupiter seen with the naked eye. Once you've acquired Spica, pivot the binocular up and slightly left, putting Spica at about five o'clock near the edge of the field. You will see two fifth-magnitude stars, 76 Virginis and 82 Virginis, forming a vertical line with Spica. With the binocular, these two stars are quite prominent, about as bright as the brightest stars seen with the naked eye, and much brighter than anything else in the field of view. If you draw an imaginary line from 76 Vir to 82 Vir, Comet 9P/Tempel 1 is at about the midpoint of that line and slightly to the left of it. (Although comets move fast for celestial objects, they move slowly in real terms. Comet 9P/Tempel 1 changes its position against the background stars noticeably from one day to the next, but an hour or two either way makes little difference in its position.)
If you can't see the comet in your binocular, don't worry. You'll have plenty of company. What you will be able to see is the arc of eighth-magnitude stars that curves up and to the left of 76 Vir. These stars are dimly visible in Figure 5, just below the comet symbol. Note the positions of these stars. You might even try making a sketch of them for comparison after impact.
An hour or so later, when impact occurs, we can hope to see a dramatic change. Comet 9P/Tempel 1 should brighten significantly, possibly even becoming bright enough to be dimly visible to the naked eye under good conditions. If the comet brightens by four magnitudes, as we hope it will, the line of three stars from Spica to 82 Vir will become a line of four "stars," with Comet 9P/Tempel 1 appearing almost as bright as 76 Vir and 82 Vir, and located about midway between them. If the comet brightens by five magnitudes, which is unlikely but not out of the question, its brightness will be nearly the same as 76 Vir and 82 Vir.
In one sense, even the best-case display is unimpressive. You may be left wondering if that's all there is. But remember this: humans accomplished this feat. For the first time, the human race has by its own efforts changed the visible appearance of a far-distant celestial object. And that is impressive indeed.
If you have even a small telescope, or can use one at a public observation, the view will be more impressive. At low magnification, the comet will appear star-like, with perhaps some fuzziness. The tail may or may not be visible, depending on the aperture of your instrument, the magnification you use, and the darkness of your site. But, unless the penetrator body is simply absorbed by the comet, the increased brightness after impact should be immediately obvious even in an instrument of modest size.
Relative to a binocular, the narrower field of view of a telescope makes it harder to get the comet in your eyepiece. If you have a Telrad or similar unit power finder, you can use it to acquire the comet quickly. Figure 6 shows a Telrad view of the comet's location just before and at the time of impact. (Although this graphic shows stars down to magnitude 8.4 for context, in reality the two stars 82 Vir and 76 Vir are about the dimmest you can expect to see in the Telrad from a dark site.) To acquire the comet, put the two stars about equidistant from the middle circle, with 82 Vir at about twelve o'clock and 76 Vir at about five thirty.
With the Telrad oriented properly, use your low-power wide-field eyepiece to acquire the comet. (If you don't have a Telrad, you can star-hop to the comet using your charts or planetarium software.) The circle in Figure 7 describes a 1° true field of view, about average for a low-power "finder" eyepiece in a typical amateur scope. Stars are shown down to 12th magnitude, or about the dimmest stars that are clearly visible with direct vision in a small telescope. Note that Figure 7 shows a correct-image view of the star fields; in your eyepiece, the star fields may be reversed, inverted, or both. The brightest stars in the field of view, shown here at 11:30, 1:30, and 8:45, are of eighth and ninth magnitude, bright enough to be prominent even in a small scope. By orienting those stars properly in the eyepiece field of view, you can center Comet 9P/Tempel in the field of view.
At low magnification, Comet 9P/Tempel before impact will appear to be an unexceptional 10th magnitude star, with perhaps a bit of fuzziness, coma, or other non-stellar characteristics. Once you believe you have the comet centered in the low-power eyepiece, swap out that eyepiece for one that provides at least 100X to 150X. At that magnification, the comet should be clearly visible as a non-stellar object. If you're fortunate enough to be viewing from a location where impact occurs before the comet sets, keep watching the comet at medium to high magnification to view the changes that occur immediately following the impact.
The Day(s) After
Not everyone is located where the comet will be high enough, or the sky dark enough, to view the actual impact. Even those who are may be clouded or rained out during the event. Fortunately, viewing the results of impact is not a one-time opportunity. Whatever brightening occurs as a result of impact is likely to remain visible for at least several days after impact.
For example, because Comet 9P/Tempel sets for us a few minutes before impact, we plan to observe it just after full dark on the evening of July 4—21 hours after impact. The comet will be setting, but still at an elevation of about 30°, high enough to be clear of the muck on the horizon and above any light domes at our observing site. We hope to see Comet 9P/Tempel as a "new star" in our binocular, but we know that regardless of how much brightening occurs we'll be able to view it in our telescopes.
The problem will be locating it on the fourth—or a subsequent evening, if we're clouded out on the fourth—because the comet moves noticeably relative to the background stars in the course of a day. Figure 8 shows a binocular view at 2300 local time on July 4. Comparing Figure 8 to Figure 5 shows noticeable movement in the position of the comet, in only 21 hours.
As the days pass, the comet's motion takes it farther and farther from Spica, toward the constellation Libra. Figure 9 shows the position of Comet 9P/Tempel at 2300 on 7 July. In only three days, the comet has moved enough to almost take it out of the original binocular field, although it still easy to locate because of its proximity to the sixth-magnitude star 86 Vir. A week or so later, the comet will enter a region that is sparsely populated with bright guide stars—near where the tail ends in Figure 9—and will be considerably harder to locate.
If you attempt to locate Comet 9P/Tempel 1 after the first week in July, it's important to have detailed star maps to show the actual current location of the comet. You can obtain these online from the sites listed in the "Resources" section below, but for the most accurate and up-to-date position data you can generate your own star maps using Cartes du Ciel or a similar planetarium program. If you do that, make sure to download and install the latest ephemerides to ensure that the displayed comet position is accurate.
But the week or two after impact is only the beginning of the show. Although it will gradually dim as the effects of impact wear off and as it gets farther from Earth, Comet 9P/Tempel should remain visible long after impact for northerly observers, and longer still for those in the southern hemisphere. Although Luna will interfere with observing the comet at times, midnorthern observers should be able to track Comet 9P/Tempel at least through early September, when it approaches the bright star Antares in the southerly constellation Scorpius.
We plan to continue tracking Comet 9P/Tempel until we can no longer do so. And there's a special reason for that. This is the end of an era for Comet 9P/Tempel. Close encounters with Jupiter's gravity field in 2024 and 2036 will change the orbit of the comet, moving its perihelion distance from current 1.5 astronomical units to 2.0 AU. Not until the year 2122 will Comet 9P/Tempel ever again become as bright as 11th magnitude. See it while you can.
For more information about locating and observing Comet 9P/Tempel 1, as well as historical background about it, visit the following sites:
Robert Bruce Thompson and Barbara Fritchman Thompson are experienced amateur astronomers, co-founders of the Winston-Salem Astronomical League astronomy club, and the co-authors of several O'Reilly books, including Astronomy Hacks, Building the Perfect PC, PC Hardware Buyer's Guide, and PC Hardware in a Nutshell.
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