This composite image of a series of 30-second exposures was made with the telescope tracking on the stellar background while asteroid 2005 YU55 passed through the constellation of Pegasus from 03:21:33 UT to 03:27:47 UT on November 9, 2011. The asteroid left a series of streaks as it moved during each time exposure taken with a refractor with 130mm of aperture and 1,040mm of focal length at f/8 with a Canon T2i (550D) at ISO 1600.

There is a lot of other stuff out there in the solar system orbiting the Sun along with the eight major planets. There are also dwarf planets, asteroids, meteoroids, and comets.

We have already discussed Pluto, which has been demoted to dwarf planet status. Likewise Ceres, Haumea, Makemake, and Eris are now classified as dwarf planets or Plutoids.

Ceres used to be considered an asteroid but now is a dwarf planet in the inner solar system. Ceres has been known since its discovery in 1801 by Giuseppe Piazzi. Haumea, Makemake and Eris are trans-Neptunian objects far out in the Kuiper belt. Undoubtedly other dwarf planets will also be discovered in the outer reaches of the solar system in the future.

Asteroids

Ceres was the largest asteroid with a diameter of about 974 kilometers (605 miles), but there are thousands of other smaller asteroids in the solar system with most located in the asteroid belt between Mars and Jupiter. Sometimes they are perturbed out of their orbits and flung into the inner solar system where they can pass close to Earth and even pose an impact hazard. Some are also flung out of the solar system altogether. This kind of thing happened much more frequently in the early solar system, but still occurs today.

The image at the top of this page shows asteroid 2005 YU55 passing the Earth at a distance of about 324,604 kilometers (201,700 miles), about 85 percent of the distance to the Moon. Astronomers knew there was no chance that 2005 YU55 would hit the Earth based on calculations of its exact orbit. But 2005 YU55 is a Near Earth Object (NEO), one of many that is classified as such because they do pass close to the Earth. There may be some out there that have not been detected and cataloged yet which could possibly impact the Earth. The Moon is a clear example of how many impacts and collisions there are in the solar system.

Asteroids are believed to have formed when the solar system formed. They are rocky or metallic objects that did not aggregate into larger planet-sized bodies, or that did, but broke apart into smaller pieces from collisions and impacts. There are millions of asteroids in the solar system. The larger asteroids are sometimes also called "minor planets."

A few larger asteroids even have their own moons, such as 243 IDA and its moon Dactyl. Asteroid 617 Patroclus is a binary asteroid with its companion Menoetius. Asteroid 45 Eugenia has two moons, Petit-Prince and S/2004 (45) 1.

Tips for Asteroid Photography

• Asteroid photography is similar to a normal long-exposure deep-sky shot. You will need a correctly polar aligned mount, and a telescope that will probably require guiding with a piggyback guidescope.

• The problem with shooting an asteroid is that it will usually move relative to the star background in a long exposure. If you track on the asteroid, the stars will trail. If you track on the stars, the asteroid will trail. You have to make a choice.

• It is easiest to track on the stars and let the asteroid trail. This will show the asteroids motion.

• For slow moving asteroids, you can take several images over a time span and blink the images to show the asteroids different location relative to the star background.

• If the asteroid is bright enough for your guider to lock on it, you can track on the asteroid. By shooting a sequence of frames, you can put them together into an animation showing the asteroid zipping through the sky.

Meteoroids, Meteors and Meteorites

Meteoroids are basically the same thing as asteroids, but much smaller in size. The dividing line is indistinct, but somewhere roughly between one and ten meters. Tiny meteoroids are called micrometeoroids.

When a meteoroid enters the Earth's atmosphere and starts to burn up, it becomes a meteor. Meteors are also called "falling stars" and "shooting stars."

Any part of a meteor that survives the fiery re-entry and impact with the ground is called a meteorite.

A Perseid meteor streaks through the sky displaying colors characteristic of its composition as it burns up in the Earth's upper atmosphere. Single 30 second exposure at ISO 1600, Canon 20Da, 24 mm lens working at f/2.8 on a fixed tripod.

Most meteors that we see are the size of grains of sand or small pebbles. They typically begin to burn up at a height of 75 to 100 kilometers (46 to 62 miles) above the Earth's surface, and disintegrate at altitudes of 50 to 90 kilometers (31 to 56 miles).

Larger meteors can become quite bright when they burn up. Any meteor brighter than magnitude -3 or brighter at the zenith is called a "fireball."

A "bolide" is an especially bright fireball that may even explode.

Small meteoroids are often dust ejected from comets. This material trails the comet in its orbit around the Sun. When the Earth passes through this debris, a meteor shower or meteor storm can occur.

The Eta-Aquarids meteor shower in May is believed to come from Halley's Comet (1P/Halley); the well-known Perseids in August from Comet 109P/Swift-Tuttle; and Leonids in November from Comet 55P/Tempel-Tuttle.

It is logical to think that some meteorites must also come from asteroids. Some achondrites may have come from the asteroid Vesta. Asteroids are constantly colliding, forming smaller and smaller pieces, some of which eventually are knocked out of their normal orbit and sent into the inner solar system where they may eventually run into the Earth. Some meteorites have even been identified with the same chemical composition as the Moon and Mars.

Meteorites are usually made either of stone, iron or a combination of both. Some, called carbonaceous chondrites, contain organic compounds such as amino acids which are the basic building blocks of life.

It is estimated that several tons of meteoric material may enter the Earth's atmosphere every day in the form of microscopic dust.

When the Earth passes through a debris stream from a comet that causes a meteor shower, all of the meteors will appear to radiate out of a single location in the sky called the radiant. The shower is usually named after the constellation they appear to originate in. The Perseids, for example, all appear to radiate out of a point in the constellation of Perseus.

This does not mean that we will see all of the meteors in that constellation. We can see them anywhere in the sky. But if you trace their path back it will originate in the location of the radiant in that constellation.

All of the meteors in a shower actually enter the Earth's atmosphere parallel to each other. That we see them in different parts of the sky is an effect of perspective, like railroad tracks which appear to reseed to a point in the distance where they meet, yet are, in fact, parallel.

The best time to shoot a meteor shower is usually just before dawn, when the Earth is rotating directly into the meteor stream. But meteors can be seen from a shower when the radiant is above our local horizon. Some very interesting and beautiful "Earth Skimmers" can be seen when the radiant is just coming above the horizon. These meteors, which can be long and colorful, are entering and skimming along the very top of the Earth's atmosphere.

Tips for Meteor Photography

• Use a wide-angle zoom lens on a fixed tripod.
• Focus the lens on infinity, then turn off the autofocus.
• Use a wide aperture.
• Point the camera at the area of sky about 45 degrees away from the radiant.
• Use a high ISO and an exposure of 30 seconds to several minutes long from a dark-sky observing site.
• Open the shutter with a remote release. A remote interval timer will help greatly.
• Take lots and lots of images and hope to get lucky and capture a bright meteor in one of them.
• Use a heated anti-dewer strip if you plan to shoot for an extended period of time in an environment where dew is likely.
• Go someplace dark, far away from light pollution. You will see more meteors!

Comets

Comet Lulin (C/2007 N3) displays an ion tail pointing to the upper right, and an "anti-tail" dust tail pointing to the lower left. This image was shot with a refractor with 130mm of aperture, 1040 mm of focal length at f/8 with a Canon 20Da DSLR and a stack of 19 x 2-minute exposures at ISO 1600.

Comets are small bodies in orbit around the Sun that are made mostly of ice, dust and small rocks. They are believed to have formed in the cold outer reaches of the solar system at the same time it formed 4 to 5 billion years ago.

A comet's nucleus, which contains the ice, dust, rocky material and frozen gases such as carbon dioxide, carbon monoxide, methane and ammonia, can be anywhere from a few hundred meters in diameter to tens of kilometers wide. Comet Hale-Bopp's nucleus may be as large as 40 to 60 kilometers (25 to 37 miles) in diameter. This may seem small compared to a planet, but it is larger than a mountain and if an object this size ever impacted the Earth, it would be considered an extinction-level event.

When a comet comes close to the Sun, the ice in its nucleus begins to heat up and the ice starts to sublimate, going directly from a solid to a gas. Dust and gas are released and they form an atmosphere around the nucleus called the "coma." The dust and gas can separate. The gas is pushed by the Sun's radiation pressure and the solar wind causes the gas to form an ion tail that points away from the Sun. The dust trails behind the comet in its orbit, sometimes forming a dust tail. The ion tail glows from ionization, and the dust tail is visible by reflected sunlight.

Comet Holmes shows a faint outer coma as a green halo of ionized molecular C2 gas, and a gray inner coma of dust reflecting sunlight. Shot as a single frame with a refractor with 130mm of aperture at f/8 with a Canon 20Da DSLR and a composite of 3 x 600-second exposures and 15 x 45-second exposures, all at ISO 800.

Some comets, such as Comet Hyakutake (C/1996 B2) in 1996 passed so close to the Earth that its ion tail stretched across the night sky. Comet Hale-Bopp (C/1995 O1) had two spectacularly visible tails, one ion and one dust seen in 1997. Comet Holmes (17P/Holmes), although located at the time in 2007 between the orbits of Mars and Jupiter, underwent an outburst where the comet brightened more than a million times from magnitude 17 to magnitude 2.5, and its coma grew larger than the Sun. Comet Lulin (C/2007 N3) in 2009 displayed an anti-tail for a long period. An anti-tail appears to point ahead of the comet, but is really just the comet's dust tail seen in perspective stretched out behind the comet.

It is estimated that as many as a trillion comets may exist in a giant cloud around the solar system with each orbiting slowly around the Sun at a great distance. Occasionally, a passing star or planet may change the orbit of one of these comets causing it to come into the inner solar system near the Sun.

Most comets have highly eccentric orbits which take them far beyond the orbit of Pluto. These long-period comets only return near the Sun on a time scale of hundreds or thousands of years. Hale-Bopp will not return again for another 2,380 years. Short-period comets, caught in the inner solar system, can return as often as every couple of years.

Tips for Comet Photography

• For really bright large comets, you can use the same technique of a wide-angle lens on a fixed tripod as for meteor photography.

• For the majority of comets which are fainter, you can use the same techniques you would use for normal long-exposure deep-sky astrophotography of objects such as nebulae and galaxies.

• This requires accurate polar alignment and guiding at long focal lengths.

• The problem, as with asteroid photography, is that many comets will move relative to the star background. If you track or guide on the stars, the comet will be blurred. If the comet is bright enough you may be able to guide on the comet itself with a sensitive guider and piggyback guidescope. You won't be able to guide on a comet with an off-axis guider.

• You can also shoot a lot of shorter exposures and align them on the comet. This will produced stars that are trailed, which is not that big of a deal if the comet is sharp. You can also use different methods of compositing the image frames, such as median and sigma combine to remove the stars in the comet image, and then composite that image into a longer exposure that was tracked on the stars.

• It can also be cool to shoot a lot of frames and put together an animation showing the comet's movement through a star field.

Find out more information about comets, meteors, and asteroids at The Transient Sky.