Sky Map: The positions of comet SW-3 at one-week intervals as of 1 a.m. local time from mid-northern latitudes.
Coming close on the heels of the recent passage of Comet Pojmanski in March, sky watchers will have an unusually favorable opportunity during the next several weeks to view the passage of another comet in small telescopes, binoculars and even the unaided eye.
The comet bears the names of two German astronomers-Arnold Schwassmann and Arno Arthur Wachmann-who together discovered three periodic comets during the early 20th century. This was their third and final discovery hence it was christened comet Schwassmann-Wachmann-3.
Discovered 76-years ago, in May 1930, orbital calculations quickly revealed that this comet was moving in a small elliptical orbit around the Sun. In fact, it's actually a member of Jupiter's family of comets, since at aphelion (its farthest point from the Sun) the comet lies near Jupiter's orbit and sometimes its orbit can be perturbed by that giant planet's gravitational pull. Shortly after its 1930 discovery, Comet SW-3 passed a mere 5.7 million miles from the Earth at the beginning of June, one of the closest comet approaches ever recorded.
If SW-3 were a reasonably sized comet, it would have created quite an amazing sight in the sky back then. But careful observations made at the Meudon Observatory in France suggested that its nucleus was probably no more than about two-tenths of a mile in diameter. By cometary standards, tiny to say the least! It is for this reason that the comet never got very bright in 1930, and was best seen in telescopes or good binoculars.
Even though SW-3 orbits the Sun in only 5.3 years, 1930 was the last time anyone saw it for quite a long time. In some ways, it's like a near-Earth asteroid in that very occasionally it can come tantalizingly close to us, but when it doesn't (more often than not) it's usually much too faint to be seen. In fact, between 1935 and 1974, SW-3 came and went by Earth eight times without being recovered. It finally was caught on photographs taken in Australia in 1979, was missed in 1985 and recovered again in 1989-90.
Going to pieces
The comet was supposed to make another uneventful return in the fall of 1995. During October of that year, word spread rapidly of a new comet that numerous observers had sighted, low in the twilight evening sky. Some reported the object as bright as magnitude 5½, and plainly visible in binoculars as a slightly diffused star.
But this wasn't a "new" comet at all. It was comet Schwassmann-Wachmann-3!
This was a huge surprise, because the comet didn't get any closer to Earth than 122 million miles and was not expected to get much brighter than twelfth magnitude. Yet, here it was, somehow having increased its brightness by many hundreds of times!
As for what caused this tremendous brightness outburst, the answer came a couple of months later, when astronomers at the European Southern Observatory in Chile reported that SW-3's tiny nucleus had fractured into several parts. No one can say for sure what caused the comet to break apart, though the most likely reason is thermal stress, similar to what would happen if you poured hot tea into a cold glass: the comet cracked apart after as it approached the Sun after spending a long time in the frigid depths of space some half a billion miles from the Sun.
During the fall of 2000, although poorly placed for observation, the comet was widely seen because it was again brighter than expected. And while two of the nuclei seen in 1995 (known as B and C) were back, a new fragment (E) was also discovered. Astronomers now count at least eight and according to Donald Yeomans at NASA's Jet Propulsion Laboratory in Pasadena, California, "Some of the fragments are themselves forming their own sub-fragments, which means the number could multiply further as the comet approaches."
And indeed, the well-known comet observer, John Bortle, recently noted that on the night of April 2-3, Fragment B looked " . . . like a bright little star instead of the faint diffuse patch it had been just a few days before and implying that it has fragmented further (the much fainter fragment G has also reportedly split recently)."
What to expect
Comet Schwassmann-Wachmann-3 is in our sky once again. And the circumstances for viewing it will be the most favorable since the year of its discovery back in 1930.
How bright will the comet get?
Nobody can quite say for sure. While it was much brighter than normal at its return in 2000, it was only about 1/6 as bright as it appeared after its 1995 breakup. This fading trend is likely to continue for 2006, but counteracting this is the fact that the comet will approach Earth very closely during mid May. So it still should get at least as bright as fifth magnitude, making it an easy binocular object, as well as making it dimly visible to the unaided eye.
And it could possibly even become as bright as fourth magnitude around the time when its largest fragment (C) passes closest to Earth: 7.3 million miles on the morning of May 12. Not since Comet IRAS-Araki-Alcock passed just 2.9 million miles from Earth in May 1983 has a comet passed so close to our planet. In fact, this will be the twelfth closest approach of any comet in known history.
As you can see on our map, SW-3 will spend much of the rest of April slowly cruising through the constellation of Corona Borealis, the Northern Crown and visible throughout the night, passing almost directly overhead between 1 and 3 a.m. local daylight time. On the nights of April 20 and 21, it will be passing just to the north of the second-magnitude star Gemma (the "gem" of the Crown). The comet will be about 19 million miles from Earth, yet perhaps no brighter than seventh magnitude.
At the start of May, SW-3 will be picking up speed as it continues to approach the Earth. From May 1 through 3 it will be passing across the lower half of the famous "Keystone" of Hercules.
Hurrying eastward, it will pass south of the brilliant blue-white star, Vega in the constellation of Lyra on the morning of May 8 and probably shining as bright as magnitude 4½. In the nights that follow, the comet will turn southeast and will race across the Milky Way in Cygnus (May 10-12), then gradually slow its pace as it begins to move into western Pegasus (May 15), and graze the "Circlet" of Pisces (May 21-22).
If you plan to look for the comet, you should also be aware during early May of the presence of the waxing Moon, whose increasingly bright light will make dim, fuzzy objects like the comet harder to find. Certainly, the best time to look for SW-3 will be after the Moon has set and before the onset of morning twilight.
Generally speaking, from May 1 through 9, the comet will appear highest in the sky (almost overhead) between 3 and 4 a.m. local daylight time. The Moon will have set earlier in the night, leaving the sky nice and dark. However, the interval between moonset and the first light of dawn will be shrinking each day. On the morning of May 1, for instance, the Moon will set around midnight and twilight won't begin until after 4 a.m., so you'll have at least four hours of a dark sky.
The very best view of SW-3 might come for just a scant several minutes around 4 a.m. (local daylight time) on May 9, with the comet standing nearly overhead in a dark sky. The bright Moon will have just set, but morning twilight will just about be ready to begin.
Thereafter, unfortunately, the Moon will always be above the horizon, right on through the growing light of dawn, becoming full on May 13, less than a day after SW-3's closest approach. (Talk about bad timing!).
The comet will arrive at perihelion (its closest approach to the Sun) on June 7 at a distance of 87.3 million miles. By then, low altitude and closeness to the bright dawn twilight will make it difficult to see in the Northern Hemisphere, but those in the Southern Hemisphere will have it well placed in the east-southeastern predawn sky as it passes through the constellation Cetus, the Whale. It will still be only about 21 million miles from Earth and might still be glowing at around magnitude 6.
Comet Schwassmann-Wachmann-3 is crumbling before our very eyes. Bortle comments that if big fragment C were to undergo another big disruption around the time of its closest approach to Earth (only a very small possibility), it could suddenly appear very bright in our sky and really put on a show for us!
"Enough new fragments of 73P have been discovered inbound during this return of the comet to use up the entire alphabet (!). There are now up to 40 reported fragments and conjecture is that as the procession passes by us perhaps 100 or more will be briefly in sight of very large telescopes! Right now it looks like half a dozen will come within visual range of amateur telescopes."
As for those other tiny comet pieces, they're all lagging well behind fragment C and most will come even closer to Earth than C will. If, for example, the tiny Fragment E hasn't already completely disintegrated, it is (or was) predicted to pass a mere 4.8 million miles from Earth on May 17.
No need to be alarmed, however. That's still twenty times farther than the Moon!
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- Night Sky Main Page: More Skywatching News & Features
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.
1 AU, or astronomical unit, is the distance from the Sun to Earth, or about 93 million miles.
Magnitude is the standard by which astronomers measure the apparent brightness of objects that appear in the sky. The lower the number, the brighter the object. The brightest stars in the sky are categorized as zero or first magnitude. Negative magnitudes are reserved for the most brilliant objects: the brightest star is Sirius (-1.4); the full Moon is -12.7; the Sun is -26.7. The faintest stars visible under dark skies are around +6.
Degrees measure apparent sizes of objects or distances in the sky, as seen from our vantage point. The Moon is one-half degree in width. The width of your fist held at arm's length is about 10 degrees. The distance from the horizon to the overhead point (called the zenith) is equal to 90 degrees.
Declination is the angular distance measured in degrees, of a celestial body north or south of the celestial equator. If, for an example, a certain star is said to have a declination of +20 degrees, it is located 20 degrees north of the celestial equator. Declination is to a celestial globe as latitude is to a terrestrial globe.
Arc seconds are sometimes used to define the measurement of a sky object's angular diameter. One degree is equal to 60 arc minutes. One arc minute is equal to 60 arc seconds. The Moon appears (on average), one half-degree across, or 30 arc minutes, or 1800 arc seconds. If the disk of Mars is 20 arc seconds across, we can also say that it is 1/90 the apparent width of the Moon (since 1800 divided by 20 equals 90).