Small asteriods like Ida (above) have insufficient gravity to draw them down into spherical shapes.  (1.6.x)

The "dwarf planet" Haumea is one of the fastest rotating objects in the Solar System!  It spins on its axis once every four hours!  (1.6.x)

Some asteroids like Ida and 1999 KW4 (above) have their own moons! 

In CELESTIA you can view Pluto, Charon,
Hydra and Nix orbiting
their own system's barycenter!  (1.6.x)

Note: after running a link like the one above at top for the asteroid Ida, Right Drag with your mouse to get a sense of the 3-dimensional aspects of its structure and orbit.

Are you unfamiliar with our 1.6.x and 1.4.1 links?  For an explanation  click here.


Humankind's exploration of our Solar System's smaller worlds is still in its infancy.  So few spacecraft have yet sent back close-up pictures.  Here are links to artists' depictions of varied worlds we may explore in the near future and beyond.

Dawn Orbiting Vesta

Dawn Near Vesta's Surface

Exploring an Asteroid Close-up

Astronaut on NEO Asteroid Surface

Future Asteroid Exploration

Close-up View of Comet Tempel

Conjectural Close-up View of a Comet

Robot Probe on a Comet


The following will help you enjoy this page's 1.6.x and 1.4.1 links that run events directly in CELESTIA.  If you're new to the program, these tips will also help you learn to use it.

You'll find more information about many of CELESTIA's controls on our  Learning Center  page.


Here we see the
most famous of the dwarf planets and
its largest moon,
Charon, orbiting
their own system's




From NASA / JPL, here's a link to  Keeping an Eye on Space Rocks, a nice introduction to some of the smaller worlds of our Solar System and how they may affect us here on Earth.  Here's an excellent NASA "Fact Sheet" that examines  Comets vs. Asteroids.  And here's a nice NASA graphic that compares the  Sizes of Several of our Solar System's Smaller Worlds.


© SkyMarvels, enhancement of NASA, STScI image

A dwarf planet is a body a.) in orbit directly around the Sun, b.) with enough mass so its gravity draws it into a roughly spherical shape, and 3.) whose gravity has not cleared the "neighborhood" around its orbit.  The five bodies currently recognized as dwarf planets (Pluto, Eris, Makemake, Haumea and Ceres) are shown above compared in size to Earth and our Moon.  Ceres (1.6.x)  orbits in the Asteroid Belt, while  the other four (1.6.x)  (with their primary moons labeled in italics above) orbit in the Kuiper Belt.

Some astronomers speculate that the total number of dwarf planets may be in the thousands.  In fact, some maintain that certain already-discovered larger  Kuiper Belt & Trans-Neptunian Objects  (KBO's and TNO's) like Sedna and Orcus should be classified as dwarf planets.

On July 14, after a flight of 9 years and 3 billion miles, NASA's  NEW HORIZONS spacecraft  made its closest approach to dwarf planet Pluto, that mysterious body at the outer reaches of the Solar System.  Where NEW HORIZONS is now.   NASA's NEW HORIZONS page.

Some of the best NEW HORIZONS pictures of Pluto and Charon are available on the from on the  JPL-NASA Photojournal page for Pluto.

Here's the NEW HORIZON'S:  Flying Past Pluto.

And here's an IAU Image:  Pluto and Its 5 Moons.


Asteroids are rocky objects which orbit the Sun but are too small to be classified as planets.  These form the Asteroid Belt, actually more of a donut-shaped region, lying mainly between the orbits of Mars and Jupiter.

Most asteroids are thought to be fragments "left over" from the formation of our Solar System!  Although they range in size from minor worlds hundreds of kilometers in diameter to chunks smaller than an office building, most are small and irregularly shaped.  Larger asteroids are relatively rare, but more than 150 are nonetheless large enough to have companion moons!  Already over 500,000 asteroids have been identified, though millions more are suspected to exist!

© SkyMarvels, enhancement of NASA, JPL Caltech image

Immediately above, we see how a view from within the Asteroid Belt is most often depicted, like a continuous cloud of rubble with numerous asteroids very near and visible to each other.  However, this is a misconception about the belt.  In reality, space is incredibly empty.  So, despite their great numbers, asteroids on average are separated by such great distances that spacecraft have had no problem flying through the Asteroid Belt.  It is estimated that, on average, even very tiny asteroids are separated by tens of thousands of kilometers, while those of appreciable size are separated by 1 to 3 million kilometers!

Asteroids are generally classified into three main types.  C-type (carbonaceous) asteroids are the most common, are dark in appearance and are probably made up of of silicate rocks and clay.  These are thought to be among the most ancient objects in our Solar System.  S-type (silicaceous) asteroids are generally composed of silicates and nickel-iron, and are thus often called the "stony" asteroids.  M-type (metallic) asteroids are made up largely of nickel-iron.

Many astronomers surmise that the asteroids represent the material of a planet which never formed, due to the interference of Jupiter's gravity.  Even so, the combined mass of all asteroids found in the Asteroid Belt is but a small fraction of the mass of our Moon!

Here's a link to a Chrome Experiment 3-D animation that beautifully depicts the swarms of worldlets circling the Sun in the  Asteroid Belt.  (Requires browser supporting  WebGL )


A subset of Near-Earth Objects (NEO's), which include both asteroids and comets, near-Earth asteroids are ones whose orbits bring them into proximity of Earth's orbit.  As of October 2016, the  Catalog of Near-Earth Asteroids had identified over 15,000 objects,  over 1,400 having been designated potentially hazardous asteroids (PHA's).  NEA's are classified according to their orbits as shown below:

      Types of NEA's and Their Percentages:
            Apollo:    62% of all NEA's
            Aten:    6% of all NEA's
            Amor:    32% of all NEA's
            IEO:    6 known objects

Due to findings of NASA's Wide-field Infrared Survey Explorer (or WISE) spacecraft, estimates of the number of Near-Earth Asteroids have been revised downward.  This video from NASA's Photojournal pages shows why.  Changing Views of Our Solar System     And here is a nice chart that summarizes the WISE spacecraft's discovery:  A Near-Earth Asteroid Census  

Nonetheless, the inner Solar System is still a very busy place.  Moreover, as the recent video below soberingly shows, asteroids apparently collide with Earth far more frequently than once suspected.

From NASA's excellent new CNEOS Near-Earth Object site, here are links to its  Latest NEO News  and  NEO Close Aproaches  pages.

And here is a link to an excellent animation that shows  the dynamics of the Near-Earth Asteroids.

From NASA/JPL here are animations that show   Asteroid 1950 DA's Predicted Close Encounter with Earth in 2880     and what has been called  Asteroid 2002 AA29's "Cat & Mouse" Orbit     .


Trojans and Hildas are two families of asteroids which are classified according to the dynamics of their orbits.

Trojans asteroids share an orbit with a larger body and cluster near its L4 and L5 Lagrange points.  Therefore they orbit the Sun about 60° ahead of and behind their parent planet or moon.  Typically those near L4 (ahead) are called "Greeks", while those near L5 (behind) are called "Trojans".  Nonetheless all are trojan asteroids in the more general sense.  Jupiter's trojans are the most famous, numbering in excess of five thousand known objects, though Mars is also known to have trojans.

© SkyMarvels, enhancement of NASA image

The intriguing Hildas (or Hildian asteroids) all orbit the Sun in stable 3:2 resonances with Jupiter.  This means that the family's individual asteroids each orbit the Sun three times in the same period that Jupiter orbits twice.  Nevertheless, as a whole, the Hildas continually appear to cluster into a convex-sided equilateral triangle, with their greatest numbers always found at this triangle's vertices.  As these vertices are opposite Jupiter and at the L4 and L5 Lagrange points, the giant triangle thus rotates, synchronized to Jupiter's orbit around the Sun!  Over 1,100 Hildas are known to exist.

And here is a link to an excellent animation that shows  the dynamics of the Trojans and the Hildas.

Here is is a link to a similar animation which highlights the  orbits of a few example Hildas.  This gives a better sense of the paths traced by the individual members of this family as they orbit the Sun.


View some of the most well-known asteroids with  NASA's Eyes on the Solar System.  (Requires JAVA.)

NASA's  Touring the Giant Asteroid Vesta   .

For shape-models of many known asteroids, try  Interactive Service For Asteroid Models.

For the latest orbital elements of asteroids and comets, try the  JPL Small-Body Database Browser  webpage.

IAU's List of Potentially Hazardous Asteroids (PHA's)


Generally speaking, a centaur is a smaller object whose orbit around the Sun is of a size (on average) between that of Jupiter and that of Neptune.  (However, the Minor Planet Center (MPC), the Jet Propulsion Laboratory (JPL), and the Deep Ecliptic Survey (DES) all define centaurs slightly differently.)

Centaurs interest astronomers because they exhibit characteristics not only of asteroids but also of comets.  In addition, as they cross the orbits of one or more of the gas giants, most have unstable orbits.  As a family, they also display an unexplained variation of surface colors.  Here are good Wikipedia graphics showing the  orbital distribution of the Centaurs  and the  colors of the Centaurs.  And here is   Wikipedia's Centaur page.


© SkyMarvels, enhancement of NASA, STSci image

Bodies composed mainly of frozen water, methane and ammonia, Kuiper Belt Objects (KBO's) orbit the Sun in a considerably larger and more distant counterpart to the Asteroid Belt.  The Kuiper Belt stretches roughly from Neptune's orbit out to 50 AU from the Sun.

In addition to constituting their own family of objects, KBO's also belong to another more general family, the Trans-Neptunian Objects (TNO's).  A TNO is simply any object whose orbit around the Sun is larger on average than the orbit of Neptune.  By this definition, TNO's therefore technically include all objects in the Oort Cloud.  Here is a good Wikipedia graphic that clarifies  the distinction between KBO's and TNO's  while at the same time showing many smaller worlds of our Solar System.

It should be noted that, as the accepted KBO and TNO classifications depend on location instead of structure, the 4 outer dwarf planets (Pluto, Eris, Haumea and Makemake) are both KBO's and TNO's. 

Theoretical KBO Structure:  © SkyMarvels, enhancement
of NASA/Conceptual Image Lab/Tyler Chase image

By 2013, over a thousand KBO's had been discovered.  However, because they are so distant from the Sun, and therefore difficult to detect, it is believed that there may be hundreds of thousands of KBO's that are larger than 100 kilometers in diameter!  Smaller KBO's also almost certainly exist, and these are likely to be more irregular in shape.

Almost all KBO's are believed to have originated in our Solar System's protoplanetary disc, but they were too thinly scattered and too far from the developing Sun to coalesce into full-sized planets.  Despite the substantial numbers that have been theorized, and the relatively large numbers of sizeable ones, KBO's are believed to have a total combined mass of only about three or four times the mass of our Moon.


IAU List of Trans-Neptunian Objects

Wikipedia's Kuiper Belt page

Wikipedia's Trans-Neptunian Object page


Comets are the objects that undergo by far the greatest transformation in appearance as they move around the Sun or through the Solar System.  When far from the Sun, comets are simply solid, irregularly shaped bodies (most smaller than 20 km along their long axes), made up of ices, rock and dust.  But, when their elliptical (and sometimes hyperbolic) orbits take them inside the orbit of Jupiter, the Sun's heat (and possibly electrical effects from the solar wind) cause their ices to vaporize and form a halo-like coma and one or more impressive tails.  Reflecting light from the Sun, the coma and tails often become visible from Earth, and some can be awesome!  The largest comas can be larger than the Sun, and tails, which usually point away from the Sun due to radiation pressure, may grow to over 500 million kilometers long!

© SkyMarvels, enhancement of
original image courtesy: ESO / E. Slawik

Unmanned missions to various comets have confirmed that the gases and dust escaping their nuclei "jet forth" like immense fountains, rather than uniformly as had once been believed.  The escaping material then often differentiates into separate gas and dust tails.  The gas tail (also called the "ion" tail) points directly away from the Sun, while the dust tail can bend noticeably away from it and give the comet the appearance of an angle sign (<).

From NASA Goddard's Scientific Visualization Studio, here's a great animation showing  14 Years of Comets Seen by SOHO.   .  It gives us an appreciation for just how many comets whisk through our Solar System.

Comet Nucleus: © SkyMarvels, enhancement of NASA image

As comets lose a significant amount of mass during each visit to the inner Solar System, their lifetimes may only extend from 50 to a few thousand orbits.  Finally, having lost most of their volatile gases, they will more resemble asteroids than their former selves.

Over 4,000 comets have been identified, this relatively small number due to the difficulty in detecting ones far from the Sun.  Nonetheless, it is theorized that billions and possibly trillions of comets may exist as part of our Solar System.  Less numerous "short-period" comets are thought to originate in the Kuiper Belt, while "long-period" comets are thought to originate in that "great comet repository" the Oort Cloud.

From NASA / JPL, here's a link to  Keeping an Eye on Space Rocks, a nice introduction to some of the smaller worlds, including comets, of our Solar System and how they may affect us here on Earth. 


The Windows to the Universe site has a great page with an excellent  Interactive Comet Animation  Watch as your comets' tails grow inside the orbit of Jupiter!

Here's a cool NASA interactive on the  DEEP IMPACT Mission to Comet Tempel 1.

ISON's Journey in 3-D! (Requ.  WebGL  browser)

Here's a cool NASA interactive on the  STARDUST Mission to Comet Wild.

Link to a nice interactive from the New York Times:  Rosetta's Philae Lander Wakes Up From Hibernation.

A nice NASA animation of Rosetta's Comet Target:  67P/Churyumov-Gerasimenko.   .

Here's a direct link to a NASA video of  STARDUST Flying Past Comet TEMPEL 1

The Amazing Space site of the The Space Telescope Science Insitute (STScI) has this nice interactive tool answering the question  How Fast Do Objects Move in the Solar System?.  Watch how Halley's Comet speeds up when it is near the Sun!


Though "meteors", "meteoroids" and "meteorites" are three terms that are often used interchangeably, there are subtle but significant differences to their meanings.

Smaller than asteroids, meteoroids are natural rocky or metallic objects that move through space in great numbers.  Most are believed to have once been parts of asteroids or comets, though some are the result of collisions between much larger bodies—even moons and planets.

Meteoroids are so numerous that they are constantly bombarding Earth.  Most are between the size of a grain of fine sand and the size of a small pea, though even objects as sizeable as a large beachball are considered meteoroids rather than asteroids.  On average, about 50 tons of meteoroids are "intercepted" by our planet each and every day!

When meteoroids first encounter Earth's atmosphere, they are instantly and violently heated by atmospheric friction.  This often causes them to radiate visible light and sometimes even glowing trails.  We call these visual spectacles "meteors", or just as commonly "shooting stars".

Meteoroids that survive their brief and buffeting "mad dashes" through the atmosphere and strike Earth are called "meteorites".  While most are inconsequential, the largest of meteorites have left sobering evidence of just how catastrophic their impacts can be.

And here is a nice interactive 2-D Map of the  Major Recorded Meteorite Impacts From 2300 BCE to Early 2013 CE.

Here's a nice poster from the AMS:
      Meteor Terminology   High-Res PDF.

And don't forget this sobering video:


What If An Asteroid or Comet Hits My Town?

From ESA, the Science and Technology Facs. Council, and Faulkes Telescopes:  Impact Calculator

Purdue University's  Impact Earth!

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      Visible in N Hemisphere
      Visible in S Hemisphere

Meteor Showers:

    IMO:   Calendar   PDF
    AMS:   Calendar

NEO Close Approaches


DAWN Mission Status


Near-Earth Object Info:

    CNEOS Site
    NEO Close Approaches


      Where Is Rosetta?


Though we now call Pluto a dwarf planet, together Pluto and Charon really constitute a binary dwarf planet!  It is the only one known to exist in our Solar System!

With a 76-year orbital period, Halley's Comet is classified as a "short-period" comet.  Some "long-period" comets are known to have orbital periods of millions of years!

Earth passed right through the tail of Halley's Comet in 1910!

The Kuiper Belt Object now called "Eris" was originally called "Xena", nicknamed after the TV series heroine, the warrior princess who screamed, "IY-YI-YI-YI-YI !"

Scientists have concluded that  much of Earth's water was brought here by comets  that collided with Earth! 



Note: some links are echoed elsewhere on this page and may include descriptive text.

NASA Keeping an Eye on Space Rocks

Asteroid Belt.  (3-D.  Requ.  WebGL  browser)

ISON's Journey in 3-D! (Requ.  WebGL  browser)

NASA's Eyes on the Solar System.  (Requires JAVA.)

JPL Small-Body Database Browser

NASA's  DAWN Mission to Vesta and Ceres

NASA's  DAWN's Framing Camera

Interactive Service For Asteroid Models

NASA's  DEEP IMPACT Mission to Comet Tempel 1

A Windows to the Universe  Interactive Comet Animation

From Amazing Space-STScI:  How Fast Do Objects Move in the Solar System?


Eclipse Finder

Eclipse Finder


Time Zones

Earth's Tides

Our Corner of the Cosmos

Anatomy of the Milky Way


Perseid Meteor Shower 2010

Geminid Meteor Shower 2010

Geminid Meteor Shower 2012

Perseid Meteor Shower 2013


The Speed of Light

The Solar System Barycenter

Stunning Fields of View 001

Stunning Fields of View 002

Have You Ever . . . ?

"celestia4all" Site Preview

The Sun's Signature

Earth's Structure (longer)

Solar Eclipses:

   Solar Eclipses thru 2012

   Solar Eclipse 2010 Jul 11

   Solar Eclipse 2012 Nov 13

   Solar Eclipse 2013 Nov 3

Lunar Eclipses:

   Lunar Eclipses thru 2012

   Lunar Eclipse 2010 Jun 26

   Lunar Eclipse 2010 Dec 21

   Lunar Eclipse 2011 Jun 15


Moon's Occultation of Venus 2010 May 16

Moon's Occultation of Venus 2010 Sep 11


How Big Is Our Moon?

The Same Side of the Moon Always Faces Earth

The Earth-Moon Barycenter

Phases of the Moon