Viewing Other Solar System Objects
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To view meteors, you need patience, although clear skies, luck, and a calendar can also be of great help. Although meteors have been noticed since prehistoric times, it has only been relatively recently that we have discovered their true nature. Most meteors are caused by cometary debris, although some of the larger ones originate from asteroids. Figure 5.1 shows an example against the stellar background.
KeywordsGlobular Cluster Meteor Shower Meteoroid Stream Kuiper Belt Main Asteroid Belt
Many of the meteors people have seen were discovered by pure chance, such as during observing sessions of the night sky or while walking after dark. Indeed, should you happen to stare at the night sky for about an hour, on average you should see three meteors. The average background, or sporadic, rate for meteors is ten per hour, but, as we can normally see only about a third of the sky at a time, an individual observer will pick up three. Even though there is no guarantee of success, on some nights it is possible to see ten in a given hour. It is just down to luck and patience.
Sporadic meteors are caused by small debris spread around the Solar System. The general term for the objects that cause meteors is meteoroids. Most of this debris was left by long extinct comets, but some of it is caused by collisions between asteroids and even planets. Such debris is much larger in size than cometary debris and can, in extreme circumstances, be large enough to survive being burnt up in Earth’s atmosphere and land on the ground. Objects that reach Earth’s surface are called meteorites, and their collection is a sub-branch of astronomy. Rocks originating from the Moon, Mercury, Mars, and Venus have been identified.
Visually, the meteors resulting from the entry of large objects can be spectacular and have even been seen in daylight. Objects as large as small asteroids have been known to reach the Earth’s surface, causing large crater formation and massive damage to the environment. Indeed, the potential danger to life was the main driving force behind many asteroid search programs.
Very bright meteors caused by asteroidal debris are often seen by the non-astronomical general public. The chances of an individual astronomer seeing one, though, are remote, even if that astronomer can maintain a lifelong 24 hour watch. That is because the location will vary. The best chance of detecting a rare fireball is by looking at the “fish eye” all sky cameras and playing the replay at a fast rate. This method can also detect ordinary sporadic meteors as well. Most sporadic meteors are caused by cometary debris, and, though not exceptional, they are often bright enough to be seen clearly.
So far, it seems that meteor observing is a branch of astronomy that is best left alone. Although it is certainly true that lunar and solar observing require a lot less patience, if you wish to cross off seeing a meteor as something you want to do before you die or lose your eyesight, there are easier ways of seeing one than just waiting around for sporadic meteors.
Fortunately, there are several major meteor showers each year and a few minor ones. They occur when a meteoroid stream meets Earth. Meteoroid streams usually follow the orbit of a periodic comet, or indeed one that has broken up or expired. One meteoroid stream follows the orbit of the asteroid Phaethon, which may well be an expired comet, with its volatile materials having been exhausted and left a rocky core. Like the comets themselves, the orbits of the meteoroid streams can be perturbed by the gravity of planets. This can bring the streams to an orbit that intersects Earth or moves them away.
So if we wish to see a meteor or multiple meteors, do we just wait for the predicted time for the meteoroid stream to reach us and sit back and watch? Unfortunately, no! Apart from the possibility of cloud, especially if you live somewhere that gets lots of rain, there is the possibility that the peak of the shower will occur during daylight in whatever part of the world that you live. Indeed, according to Murphy’s Law, if you move to another country, the best show will occur in the country that you have just emigrated from! Then there is the sheer unpredictability of the meteor showers themselves.
Conventional wisdom tells us that meteoroid streams spread out over a few hundred miles in width and the meteoroids are spread around the orbit. There should be a few meteors at the start of the shower, a lot more at mid-shower, and it then should tail off. Showers can last about 2–3 weeks. What happens in reality is that the meteoroid streams are usually most dense around the time that the comet crosses Earth’s orbit. In fact, as most comets have orbits somewhat inclined to the ecliptic, the chance of a collision with us is reduced. Also meteoroid streams are depleted during close encounters with Earth and other planets, and their orbits may vary slightly. The end result is that you can look up the time of the peak activity and stare at a quiet patch of sky for hours on end!
To start with, be prepared that meteor observing is potentially a great challenge, but its unpredictability is part of the interest. For a first attempt to observe a shower, it is best to watch one of the more reliable ones. There have been many occasions where astronomers have stared at a quiet space of sky for half an hour. Yes, according to Murphy’s Law, there would be a storm of 1,000 meteors per hour after the viewer goes indoors.
One way around the boredom caused by staring at an area of sky is to observe in groups. Many astronomical societies organize meteor storm watches, the most popular meteor storm being the Perseids during August, when the warm weather in the Northern Hemisphere makes the whole experience rather pleasant. Just in case of zero or low activity, societies usually bring telescopes and binoculars as well. To see meteors, though, you do not need anything apart from your own eyes, and using any optical aid will more likely harm than enhance your view.
Apart from relying on other members to organize events, the monthly magazines will give details of upcoming meteor showers and the time of the predicted peak. It is no use relying on data from the previous year, as leap year adjustments and variations in the meteoroid stream can render such data inaccurate. In theory, you can do a Web search. It is optimistic to start looking for occasional meteors at the start of a predicted shower, but glancing in that general direction while out observing other things can often bring rewards. If you notice an early meteor, dedicate a quarter of an hour for a second one. It is not usually until 2 or 3 days before the peak that it is worth dedicating half-hour slots, unless you see more than one in a quarter-hour period.
The most prominent meteor showers are the Perseids, which peak between August 11 and 13 and the Geminids around December 14. Although the highest known rates are for the Leonids, around November 17, these are not expected to reach storm levels again until the 2030s. Outside of exceptional activity every 33 years, normal annual activity is quite low.
The part of the sky where the meteors seem to originate is called the radiant. In practice, meteors actually start about 5 or 10° from the radiant, and then continue in a direction straight away from it. This is why conventional wisdom suggests that you should not look straight at the radiant but away from it. This technique is OK when observing in groups, since a group member can shout the name of their sector when a meteor appears. Fortunately, some meteors leave persistent trails, so the other members have time to glance in the right direction. Although watching for sporadic meteors with people of limited conversational skills is not recommended, a common method is to have 4 members of a team facing each point of the compass. For team observing, there is often a nominated recorder who writes down the details of the meteors observed, and he or she will swap with another team member after a certain period, such as a half hour. When observing alone, try to focus on the relevant part of the sky, as it is easy to get distracted.
The Scientific Bit
Although most people who observe meteors are recreational astronomers, rather than wannabee research scientists, there is always the chance that a casual observer or even someone who has set out to observe a known or suspected shower may turn up with something unexpected. If you do, you should send an e-mail to the BAA Meteor Section, or post your findings on an amateur message board. This will help researchers refine their models of the orbits and distribution of meteoroid streams.
There are several ways of recording activity. Using a camcorder to record a shower is a good idea that many have used successfully. Some observers have used long exposure photography with a wide-angle camera lens, and other use a tape recorder to shout out the direction and brightness of meteors. It is through accurate recording that we can gain better understanding of the orbital mechanics of meteoroid streams and avoid those long nights of staring at quiet parts of the sky and going to bed tired and disappointed.
One way those of us who regularly watch the night sky can contribute is to watch for new showers that can be caused by perturbation of existing meteoroid streams or the visit of a new comet to the inner Solar System.
For Further Reading
Observing Meteors, Comets, Supernovae and other transient Phenomena by Neil Bone
Simply using an Internet search engine on the word “meteor” will reveal a wealth of information. The International Meteor Organization has a Web site that details all of the showers, major and minor, in an annual calendar. Most monthly magazines also give details of upcoming showers.
When you mention comets to the general public, they normally think of Hyukutake or Hale-Bopp. When Hyukutake first appeared, many of us thought it was very unlikely that we would see another comet like it in our lifetime. The next year, Hale-Bopp came.
When comets are visible to the unaided eye, they are easy objects. In the case of Hyukutake and Hale-Bopp, the comet tails were over 10° long, so the best view of the whole comet was to be had without telescope or binoculars. Optical instruments could be used to view the coma in a bit more detail. For more modest comets, such as 2006 Swan M4, some sort of optical aid is essential to see anything at all. Indeed, even in 15 × 70 binoculars (considered large by many amateur astronomers), it was very difficult to detect a tail. For observing comets of between 4th and 7th magnitude, binoculars are the instrument of choice. Comets are usually large objects, so the viewing techniques and restrictions make them similar to “faint fuzzies” such as galaxies and globular clusters.
As a comet’s brightness is usually spread out over an area, in the same way that a galaxy’s or globular cluster’s light is, it needs to be about 5th magnitude to be visible to the unaided eye from a dark site and clear conditions, whereas a star’s light is concentrated in a single point, so that it can be seen to about 6th magnitude from a suitable location. Unlike galaxies and globular clusters, a comet’s brightness varies. When out beyond the orbit of Mars, a comet is faint. Even the brighter comets, such as Halley’s, are only a few miles across, and they are not very reflective. When near the Sun, the material sublimates and forms a reflective shroud of dust and gas known as the coma, which causes the comet to brighten several times. The other factor that causes a comet to brighten is its distance from Earth. In fact, statistically, there are many more comets that are not visible to amateur equipment (even advanced amateur equipment) than actually are, and many experts would claim that this is a gross understatement.
So the question becomes, “How do you find a comet that is not visible to the unaided eye?” Or even, “How do you know the comet is there in the first place?”
What Comets Are There?
Many people of a certain age will bemoan the “good old days” when you could run without getting short of breath, but there are some advantages to modern life, apart from solar hydrogen alpha telescopes and digital photography. In times gone by, the only way of knowing what was there was the printed word, which was very good at telling you which comets might be visible during a given month, especially the periodic ones that returned at regular intervals, such as Halley’s Comet and Encke’s Comet. Unfortunately, most periodic comets are faint. Encke’s Comet can reach 6th magnitude every few years, and it is visible through binoculars under favorable conditions. Halley’s Comet is the brightest but will not return to the inner Solar System until 2064. The problem is that a comet can be discovered and fade from the range of amateur instruments before the next month’s magazine hits the stands.
The best place to find what comets are visible is the British Astronomical Association (BAA)’s Comet Web site. This tells you what is there, where it is, and how bright it is. For some of the brighter comets, the Web site provides finder charts or links to other sites. It is best to check the site at least once per week. Although most of the new discoveries are too faint for most amateurs to detect with their instruments, it is best to follow their progress until they reach 8th magnitude or brighter and then observe at any opportunity.
To be quite frank, an 8th magnitude comet seen through 70 mm binoculars is not too impressive. Neither are most 8th magnitude galaxies or globular clusters either. Most of the time, you will see a faint blur where you think the comet should be. It is a bit “seen that, got the T-shirt,” and on to the next one. Using smaller aperture instruments is even more difficult and needs clear skies from a dark site.
Comets are capable of throwing up the odd surprise. In late 2007, Comet Holmes was beyond the reach of all but the largest amateur telescopes, but it shed a lot of material and brightened by about a million times. It was even possible to take photographs using a domestic digital camera. It is always worth taking a look, just in case.
Discovering New Comets
Most of us are content to look at the night and/or daytime sky without making any great scientific breakthrough – or are we? Just as there is a chance that someone might catch a record fish by pure chance, there is the possibility that a new amateur with his or her first pair of binoculars might just discover a new comet. Having said that, just as record or large fish are now more likely to be caught by people specializing in that branch of fishing, it is now more likely that new comets will be discovered by design rather than accident, as was the case many years ago.
Hereby is a cautionary tale, and, indeed, demonstrates why Charles Messier is more famous for his catalog (or list) of noncomets than he is for the cometary discoveries he made. As is a habit of mine, I was scanning the sky around Lyra and Draco with my binoculars one day and saw a bright comet-like object. Fortunately, I did not make an idiot of myself by contacting the BAA Comet Section but checked my star atlas and found that my “comet” was none other than the globular star cluster M92! It was not the first time I had ever seen M92, but that part of the sky was unusually clear that night, so it appeared almost a magnitude brighter than I had seen it before.
Now had I not found it in the star atlas, I would have checked the BAA Comet Section Web site to see if its position and description matched any known comets. The next step would be to contact other astronomers to independently confirm its presence and then to stake a claim.
Sweeping the sky with binoculars in likely places
Photographing parts of the sky at close intervals to check for anything that has moved
Checking the SOHO Web site for signs of sun-grazing comets
When it comes to hunting for comets, it is good news and bad news. The good news is that in the last 20–30 years, there have been a great number of advances in equipment and techniques. The bad news is that there are a lot more people hunting for comets, and a lot are found not by comet hunters but by sky surveys designed to find near-Earth asteroids. The late, great George Alcock discovered several comets visually, using nothing more than a pair of binoculars. Although it is possible to emulate this feat, it is becoming statistically less likely.
Your first instinct when hunting for a new comet would be to scan along the ecliptic in a dark sky. Wrong! This is precisely where the various near-Earth asteroid programs hunt. Anything brighter than about 18th magnitude (and a good deal of fainter objects) will be swept up in no time. Although it is possible that the odd object might slip through the net, the odds are not in your favor.
Apart from the inherent difficulty in finding anything along the plane of the Milky Way, it is also pretty much covered by nova patrollers, who scan this part of the sky.
The ecliptic (or let us say most of it) is ruled out because of the number of professional and amateur asteroid hunters who are already hunting there. Also most comets in that region are short-period, ones whose orbits are in a similar plane to the planets, and most of them will already be known. This leaves the polar regions, which are not covered that well, except by other comet hunters. For those of you who enjoy or have enjoyed more competitive pursuits, astronomy (in the main), is not competitive, but when it comes to finding new objects, you have to view other hunters as competition. Indeed, if you are really determined to get your name in lights, one option is to move to the Southern Hemisphere, where the skies are generally clearer and there is a lot less people looking in the same place.
Long period comets can approach Earth from any direction, although it is more likely that they are within 30° of the ecliptic. In a dark sky, most comets are around 15th magnitude or fainter at the time of discovery, so a 60 mm refractor will just not do the job. Visually, you will need to be looking at least at a 300 mm telescope, but if you have a good mount you can photograph an area of sky. Modern digital photographic techniques allow you to take long exposure photographs to 15th or 16th magnitude. You just photograph the same area of sky a day or two later and check for anything that has moved or was not there before. There are devices called blink comparators that can help you do this automatically. It was one of the forerunners of this technique that was used by Clyde Tombaugh to discover Pluto in 1930. At the time of discovery, most comets will not show any coma or tail and will appear as an asteroid or nova. It is only their orbit that is likely to reveal their true nature.
Fortunately, there is one part of the sky neglected by professional asteroid hunters, and that is the area of sky within 15° of the Sun. Those of you who like to see Mercury will be well-acquainted with the problems of viewing this part of sky, especially from temperate latitudes, such as England and Canada. Apart from moving to a tropical location with little or no cloud, we can rely on some seasonal help. In spring, the ecliptic is well-inclined to the horizon in the evening, and in fall (autumn) it is well-inclined in the morning. These are good times to look for comets (and asteroids) by sweeping with binoculars or taking photographs.
Many comets, especially smaller ones, can slip through the net of professional and amateur search programs because they do not brighten until they are near the Sun. This method of comet hunting has proved productive, even in recent years. The best recent discovery was Comet Bradfield in 2004. Many missed it visually (due to bad weather) but saw it on the SOHO images, during its closest approach to the Sun, which brings us onto the next topic on cometary viewing and discovery.
Now, it could be argued that this topic is against the spirit of the book, which is about using amateur equipment, but it is included because of its relevance to cometary viewing. Judging from the BAA Comet Section Web site, there must be people who are watching for comets on it 24/7, as the same names seem to crop up every time.
The home page of the SOHO Web site is http://sohowww.nascom.nasa.gov/. If you explore the site, you will see the solar image through various instruments, but the ones relevant to cometary viewing are LASCO C2 and LASCO C3. LASCO stands for Large Angle Spectrometric Coronograph. C2 shows the inner corona up to 8.4 million kilometers (5.25 million miles) from the Sun or about 5° field of view. C3 shows 45 million kilometers (29 million miles), or about 16° field of view. Comets have been discovered in both the C2 and C3 images. Comets that reach C2 and avoid detection in C3 are usually very small ones that only become visible when very near the Sun.
The type of comet that is discovered close to the Sun is usually referred to as a sungrazer. Indeed, many of them meet a fiery end by falling into the Sun. Most of these comets belong to two groups of comets known as the Kreutz Group and the Meyer Group and are believed to be fragments of much larger comets that have broken up by being pulled apart by the gravity of the Sun or a planet. Indeed, there is much speculation about how small comets can actually get, and some scientists believe that Earth is being bombarded constantly by house-sized comets. Small comets are only likely to be discovered away from the near-solar neighborhood if they pass very close to Earth and then will probably be classified (wrongly) as asteroids due to the lack of obvious cometary features. Could a comet be discovered by transiting the Sun? It is possible but statistically extremely unlikely. A tail would not show up at all, and only a very large comet would have a silhouette that would show up and such a comet would be discovered long before it transited the Sun.
For Further Reading
The New Amateur Astronomer by Martin Mobberley
Observing Meteors, Comets, Supernovae and other transient Phenomena by Neil Bone
Simply using an Internet search engine on the word “comet” will reveal a wealth of detail that will take more than a single human lifetime to read. The one site that is worth checking at least twice per week is the BAA Comet Section Web site.
Although the new category of dwarf planet is confusing to amateur astronomers, and speculation continues about undiscovered objects in the Kuiper Belt that may render Mercury, Mars, and even Venus and Earth as ex-planets, observationally there is little doubt as whether astronomers should go about viewing them.
The first dwarf planet and easiest to view is Ceres. It lies in the main Asteroid Belt between Mars and Jupiter and can reach a maximum magnitude of 6.9 at opposition. This place it on the edge of unaided eye visibility by a sharp-eyed observer from a very dark sky, preferably from a high altitude. Ordinary human beings have to resort to binoculars. Although some people normally use quite powerful binoculars by amateur standards, it is bright enough to be seen through quite small ones, such as a 8 × 25, from suburban skies.
Ceres has been known since 1801, and its orbit is well known; thus, its position can be predicted accurately. As a result, the monthly magazines publish finder charts when it is near opposition, which is the best time to look for it.
Visually, however, Ceres is incapable of stirring the imagination. It never shows more than a point of light, hence William Herschel’s description as “asteroid” or star-like. Unlike Jupiter’s Galilean moons or Titan, it never shows a disc in amateur instruments and does not show surface features particularly well even in advanced professional ones. It is an object to tick off the list of things to do and get the T-shirt, rather than something to be enjoyed for its aesthetic beauty.
For the record, Ceres is about 600 miles in diameter and is thought to be composed of a rocky core, covered by ice.
The problem with both Pluto and Eris is their extreme distance from us. Were either of them at the same distance as Mars, they would be visible to the unaided eye and would show a disc in small amateur instruments, possibly showing some surface features in larger instruments. However, their extreme distance means that Pluto is about 14th magnitude and Eris is about 19th. Pluto has been seen in high quality refractors of 150 mm aperture, but Eris will require something right at the top of the amateur range to see visually. Most amateurs prefer to image them using long-exposure photography instead. As well as needing large instruments and/or sophisticated photographic techniques to detect them, there are several background stars that can be confused with Pluto and Eris, so very accurate finder charts are essential and not regularly published in monthly magazines. If you wish to see either of them, you should take the chance soon, as they will recede from us and get even fainter.
Ceres has been recorded as it occulted stars, although this is not a frequent occurrence. Both Pluto and Eris are capable of it, but their apparent angular size makes it much more unlikely.
The chance of even the professional discovery of a new dwarf planet in the main Asteroid Belt is extremely remote. Amateur discoveries in the Kuiper Belt are certainly possible, but with the number and accuracy of the professional search programs discovery of a new comet is far more likely.
Main Belt Asteroids
The other main belt asteroids are smaller and fainter and can be very irregularly shaped – not that their shape can be determined by amateur instruments. All of those visible in mid-range amateur instruments have well-defined orbits, although they can be perturbed by the gravity of Mars or Jupiter, or even other asteroids. Also in the main asteroid belt are comets. This is really where the difference between dwarf planet, asteroid, and comet gets hard. Any object that has a surface made of volatile material, such as ice and frozen hydrocarbons, will form a coma and tail when close enough to the Sun. An object that is made entirely of these materials will eventually disintegrate completely, whereas one with a rocky core will continue to exist but be regarded as an expired comet. The rocky core will simply continue to appear and behave like an asteroid.
Although the main Asteroid Belt appears to consist mainly of objects that were created between the orbits of Mars and Jupiter, it has also been joined by Kuiper Belt objects that have drifted into the inner Solar System.
An amateur discovery of a faint asteroid is possible, but the professional search programs render this very improbable. Amateur discovery of a change in the orbit of an asteroid is more likely.
Near Earth Asteroids
Some would say that the study of these objects is the most important branch of science, as it may determine the fate of the human race. Large objects have been known to strike Earth during its history, and, as Earth becomes more heavily populated, the chance of major loss of life increases. A major impact at sea is probably worse, as the tsunami it would create and could wipe out many millions of people.
The official definition of a near-Earth asteroid is one that comes within 0.3 astronomical units (AU) of Earth’s orbit. There are some large asteroids that are known to cross our path, but their orbital mechanics are well-known, and any danger would have been apparent by now.
Viewing an asteroid that passes close to us is quite different to viewing a main belt asteroid or Kuiper Belt object. There are some known ones that have their details published in the monthly magazines whenever they get close enough to be visible in amateur telescopes, but many can be discovered and be out of range before they get to press. The exception was NY40, which reached 9th magnitude and moved very fast (over a degree per hour) across the night sky around Cygnus.
It is the rapid motion across the sky that differentiates these objects from any others and what gives them their unique appeal. To get the best from this type of object, you should use a small tripod-mounted instrument (such as binoculars) with a wide field of view. Since many of these objects are rather faint, you will need good photographic equipment to record one. Probably the best result would be a guided exposure where the asteroid appears as a trail. It would certainly be possible for an amateur to discover one, but it is more likely to be by accident than design, either while viewing another object over a period or hunting for comets, novae, or other objects. Although there is no definite record of a near-Earth asteroid crossing the Sun, there have been apparent sightings of unknown objects transiting the solar disc. These led to the myths about Vulcan, a planet inside the orbit of Mercury, but, despite many searches and mechanical modeling, nothing has ever been found. The passage of a near-Earth asteroid (unknown in the days that these sightings were reported) would certainly be a plausible explanation.
Kuiper Belt Objects
Like the main Asteroid Belt, the Kuiper Belt is believed to be home to many objects from very small up to over 2,000 km. Indeed, there is even speculation that even larger objects may exist. As even the larger objects usually require a telescope with an aperture of at least 250 mm, the chances of an amateur discovery are somewhat remote. It is safe to say that anything smaller than a dwarf planet in that region (around the orbit of Neptune and beyond) would need a professional instrument to detect.
Having exhausted comets, dwarf planets, asteroids, in the main Asteroid Belt, and near-Earth asteroids, we should have covered everything. For most amateur observing, we just about have, but there are various pieces of debris from around the orbit of Mercury right out to where nobody knows. There have certainly been asteroids or comets (although at that distance it would be hard to know which) discovered between the orbits of Jupiter and Neptune. Some of the moons of the planets are thought to be captured asteroids and comets, rather than created along with the planet, as we believe our Moon to be. Indeed, a moon has been discovered recently orbiting Saturn at a distance of 13 million miles. The Kuiper Belt is thought to extend to about 50 astronomical units (AU) from the Sun, and the Oort Cloud (if it really exists) can be anything up to a light year away. However, anything that far away in the right direction would certainly be influenced by nearby stars, and even something as large as a brown dwarf would be difficult to detect in amateur instruments, assuming you knew where to look. There is certainly speculation that there is something out there in an eccentric orbit that disrupts the inner Solar System every 26–30 million years, which causes mass extinctions of life on Earth. Some think it is some sort of invisible “nemesis” object, such as a brown dwarf or even a stellar mass black hole. Alternative theories are related to the Solar System passing through spiral arms of our galaxy during its orbit and interactions with dark matter of an (as yet) unknown form. What we do know is that there are lots of discoveries still to be made about our Solar System.