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The Minor Planet, Gaspra

The Minor Planets

Properties of selected minor planets

Introduction

There are eight Major Planets in orbit around the Sun. These worlds are accompanied by their satellites (also called moons). The four planets closest to the Sun are small and made of rock and metal. These are the Terrstrial Planets (because they are similar to the Earth). The are also called the Inner Plaents. The next four planets are large and mainly gaseous. They are called the Jovian Planets (because they are similar to Jupiter), the Gas Giants, or the Outer Planets (because of their great distances from each other and the Sun).

In addition to these major planets, there are thousands of Minor Planets. The first to be discovered (beginning in 1801) are the Asteroids (also called Planetoids). These orbit in the inner solar system mainly between the orbits of Mars and Jupiter. Examples are Ceres and Vesta.

Ceres
The Minor Planet, Ceres.

Some are found close to the Sun among the inner planets (eg Eros). A few actually go around the Sun in Jupiter's orbit. These are called Trojans. Several orbit among the outer planets and these are called Centaurs (eg Chiron). A few of the various types of asteroid are large enough to be spherical but most are irregular in shape. They are mainly rocky, metalic or carbon based.

Eros
The Minor Planet, Eros.

Kuiper Belt Objects (KBOs) are icy worlds orbiting in the outer solar system beyond the major planets. The larger ones are spherical. Pluto is the most famous KBO and it possesses satellites. Pluto was considered a planet when first found in 1930. The status of Pluto as a planet was in doubt after the discovery of several similar worlds in that part of the Solar System. A whole group of smaller bodies travel around the Sun in orbits roughly the same size as Pluto's orbit - these are called Plutinos.

The KBO Pluto and its satellite Charon
The KBO Pluto and its satellite Charon

Many of the KBOs are in the distant reaches of the Solar System. Many have markedly elliptical orbits, often not in the plane of the orbits of the major planets. Examples are Quaoar, Sedna and Eris.

The tables below describe various properties of selected minor planets. Below each table are explanations of the terms used. The Earth's Moon is shown for comparison.

Orbital Properties

Name
Type
Mean Distance
From
The Sun
(×106 km)
Mean Distance
From
The Sun
(Earth = 1)
Period To
Revolve Around
The Sun
(years)
Mean
Orbital
Velocity
(km s-1)
Orbital
Inclination
(Earth = 0)
Orbital
Eccentricity
The Moon n/a 149.6 1.0 1.00 year n/a n/a n/a
Eros NE Asteroid 218.1 1.458 1.76 years 24.36 10.83° 0.2229
Vesta MB Asteroid 353.4 2.362 3.63 years 19.34 7.14° 0.0895
Ceres MB Asteroid 413.9 2.767 4.6 years 17.882 10.58° 0.0789
Pallas MB Asteroid 415.0 2.774 4.61 years 17.65 34.84° 0.2299
Achilles Trojan 776.7 5.193 11.83 13.00 10.32° 0.147
Chiron Centaur 2,039.50 13.633 50.7 7.75 6.94° 0.3831
Pluto KBO (Plutino) 5,869.66 39.236 248 4.72 17.16° 0.2444
Orcus KBO (Plutino) 5,896.95 39.419 247.49 4.68 20.5523° 0.2255
Ixion KBO (Plutino) 5,916.39 39.548 248.63 4.6? 19.6134° 0.2412
Varuna KBO 6,467.66 43.129 283.20 4.53 17.2° 0.051
Quaoar KBO 6,470.05 43.249 285? 4.52 7.98° 0.034
Haumea KBO 6,484 43.335 285.4 4.48 28.19° 0.18874
Makemake KBO 6,850.2 45.791 309.87 4.42 28.96° 0.159
2002 AW197 KBO 7,108.99 47.520 327.25   24.35 0.131
Eris KBO 10,129 67.668 557 3.436 44.187° 0.442
Sedna KBO ? 75,100 525 11,249 1.04 11.932° 0.849

Type

Asteroids are minor planets made of stone, metal or carbon. The majority revolve around the Sun between the orbits of Mars and Jupiter. This is called the Asteroid Belt and these are labelled Main Belt (or MB) Asteroids in the table. The name asteroid means "star-like" because of their appearance in early telescopes. All are smaller than the Earth's Moon.

Some asteroids orbit close in to the Sun among the inner planets. Some even approach the orbit of the Earth. These are called Near Earth Asteroids (NE Asteroids).

A group of about a hundred asteroids revolve around the Sun in the same orbit as Jupiter keeping either 60 degrees ahead or 60 degrees behind the giant planet. These positions in a planet's orbit are called Lagrange Positions. They can hold small asteroids in stable orbits as long as the object, Jupiter and the Sun form an equalateral triangle and the object is small compared to the other two bodies in the triangle. These asteroids are called Trojans. Achilles occupies the position 60 degrees preceeding Jupiter. Recently, Mars and Neptune have been found to have a small number of Trojan asteroids.

Another group orbit amongst the outer planets. These are called Centaurs.

Beyond Neptune orbit a large number of icy bodies called Kuiper Belt Objects (KBOs). All are smaller than the Earth's Moon but the larger ones are bigger than the asteroids. Beyond the Kuiper Belt is a region called the Oort Cloud which contains comets. Sedna lies at the inner region of the Oort Cloud.

Plutinos are KBOs that have orbits similar in size to Pluto's and have a 3:2 resonance with Neptune. That means that every time Neptune goes around the Sun three times, Pluto and the Plutinos revolve around the Sun twice.

Mean Distance From The Sun

Minor Planets travel around the Sun in elliptical orbits. This means that the distance between the object and the Sun varies slightly during its orbit. The Mean Distance is an average. This distance is given in two ways. The first is in millions of kilometres. This is a metric unit.

The second compares a planet's distance to the Sun to the Earth's distance. This unit is called the Astronomical Unit (written AU).

The NE Asteroids have distances close to the Earth's distance from the Sun (around 1 AU). The MB Asteroids orbit between 2 and 4 AUs as they lie between Mars (distance 1.5 AU) and Jupiter (5.2 AU). The Trojan asteroids orbit at roughly the same distance from the Sun as Jupiter (5.2 AU).

Pluto and the Plutinos have distances around 39 AU. The other KPOs are beyond this distance (between 40 and 70 AU). Sedna is so far away (over 500 AU) that some astronomers consider it a new type of object (an Inner Oort Cloud Object).

Period to Revolve Around The Sun

This is a planet's Sidereal Period. How long it takes to complete a single orbit around the Sun relative to the stars. The square of the period of a planet is proportional to the cube of its mean distance from the Sun. This means that the closer a planet is to the Sun, the less time it needs to orbit the Sun. The full relationship is given in the equation below and is called Kepler's Third Law.

Kepler's Third Law

Where

As seen from above the north pole of the Earth, most objects orbit in an anticlockwise direction. This is called Direct Orbital Motion.

Mean Orbital Velocity

This is the average velocity in orbit. A body will change its velocity as it travels in an elliptical orbit. It moves faster when it is closer to the Sun in accordance with Kepler's Second Law.

Orbital Inclination

The Major Planets generally revolve around the Sun in almost the same plane. Many Asteroids also lie in this plain. Kuiper Belt Objects like Pluto and a few peculiar Asteroids tend to have highly inclined orbits.

Orbital Eccentricity

The orbits of all the Major Planets are ellipses. This curve resembles a flattened circle. The eccentricity describes how much the ellipse differs from a circle. An orbit with an eccentricity of 0 is a circle. An orbit with an eccentricity of 1 would be an open curve called a parabola. No object would stay in orbit with that kind of path. The Major Planets have orbits with very low eccentricity, close to zero. Their orbits are very nearly circular.

Kuiper Belt Objects (like Pluto) have orbits that are more elliptical. Pluto's orbit is so eccentric that it sometimes moves closer to the Sun than Neptune. There is no danger of a collision because Pluto's orbit is so highly inclined. Other Kuiper Belt objects have similar orbits. Sedna has an extremely elliptical orbit.

Physical Properties 1

Name
Diameter
(km)
Diameter
(Moon = 1)
Rotational
Period
Oblateness
Axial Tilt
The Moon 3,474.8 1.0 27.32166 days 1.5424°
Eros 33 × 13 × 13 0.009 × 0.003 × 0.003 5.270 hours
Vesta 578 × 560 × 458 0.166 × 0.161 × 0.132 5.342 hours 29°
Ceres 960 × 932 0.276 × 0.268 9.075 hours
Pallas 570 × 525 × 482 0.164 × 0.151 × 0.139 7.811 hours ~60°
Achilles 135.5 0.039 ~12 hours
Chiron 180 0.052 5.9 hours
Pluto 2,390 0.688 -6.38 days 0.0 119.6°
Orcus 840 × 1880 0.242 × 0.541 ?
Ixion 820 0.236
Varuna 936 0.269 3.17 hours
Quaoar 1,200 0.345
Haumea 1,960 × 1,518 × 996 0.564 × 0.437 × 0.287 3.9154 hours
Makemake 1,600 × 2,000 0.460 × 0.576 ? hours
2002 AW197 890 0.256 ?
Eris 2,400 0.691 ~ 8 hours
Sedna 1,800 0.518 10 hours

Diameter

This is the average diameter of each body.

One column gives diameters in kilometres, the other relative to the Moon. All Minor Planets have diameters smaller than the Moon (which is smaller than any of the major planets).

Rotational Period

This is how long a body takes to rotate once on its axis. As seen from above the north pole of the Earth, most of the bodies rotate in an anticlockwise direction. This is called Direct Rotation. A few objects (like Pluto) rotate in a clockwise sense. This is called Retrograde Rotation and is shown by the presence of a minus sign.

Minor planets typically rotate quickly in a few hours. Pluto has a slow rotation period because of the presence of a satellite which slows the planet's rotation over time. The Moon's rotation has been slowed by the Earth.

Oblateness

This is how much the planet is flattened because of its rotation. A value of zero denotes a perfectly spherical planet. The smaller minor planets are not large enough to be spherical.

Axial Tilt

The line joining the two poles through which the body rotates is called its axis. If a body rotated with its axis perpendicular to its orbital plane then its axial tilt would be zero.

An axial tilt of more than 90° implies that the planet rotates in a retrograde direction.

Physical Properties 2

Name
Mass
(Moon = 1)
Density
(×103 kg m-3)
Surface
Gravity
(Moon = 1)
Escape
Velocity
(km s-1)
Escape
Velocity
(Moon = 1)
The Moon 1.000 3.34 1.000 2.38 1.00
Eros 9.797 × 10-8 2.40 0.0036 0.01 0.004
Vesta 3.67 × 10-3 3.40 0.136 0.35 0.147
Ceres 0.0129 2.08 0.167 0.51 0.214
Pallas 2.99 × 10-3 2.80 0.111 0.32 0.134
Achilles 3.54 × 10-5 2.0 0.023 0.072 0.030
Chiron 3.40 × 10-5 2.0? 0.025 0.07 0.029
Pluto 0.163 2.03 0.357 1.1 0.462
Orcus 8.84 × 10-3 2.0? 0.216 0.5? 0.2?
Ixion ? ? ? ? ?
Varuna 8.03 × 10-3 1? 0.093 0.39 0.163
Quaoar 0.027 2? 0.19? 0.6? 0.25?
Haumea 0.057 3? 0.271 0.84 0.353
Makemake ? ? ? ? ?
2002 AW197 ? ? ? ? ?
Eris ? ? ? ? ?
Sedna 0.05? 2.0? 0.25? 0.75? 0.3?

Mass

Mass is the amount of matter that an object contains. On the Earth mass can be measured by weight.

The mass of bodies with satellites can be measured by observing the motions of the satellites and applying Kepler's Law.

The mass of Pluto is so low that many astronomers do not consider it a major planet. Recently other Pluto-sized bodies have been found in the distant part of the Solar System. These are the Kuiper Belt Objects.

All minor planets have a smaller mass than the Moon.

Density

Density tells how concentrated the matter in a planet is. Asteroids are the most dense. They are made mainly of metals and rocks. The Kuiper Belt Objects are the least dense, being made up of lighter ices.

The density of a body is its mass divided by its volume. The units are kilograms per cubic meter.

Surface Gravity

On the Earth, the acceleration due to gravity is 9.80665 meters per second per second. The figure for the Moon is 1.619 meters per second per second (roughly one sixth of the Earth's). This column compares the acceleration of gravity of each body to that of the Moon.

The Surface Gravity of a planet is proportional to the planet's mass and inversely proportional to the square of the planet's radius.

Acceleration of Gravity

Where

Escape Velocity

This is the speed that an object must attain in order to escape from a minor planet's gravitational field. For the Earth this speed is about 11 kilometres per second (7 miles per second). One column expresses this speed for each body in kilometres per second; the other relative to the Moon (2.38 kilometres per second). A body's Escape Velocity depends on its mass and radius as shown below.

Escape Velocity

Where

Thermal Properties

Name
Solar
Irradiance
(W m-2)
Solar
Irradiance
(Earth = 1)
Albedo
(%)
Surface
Temperature
(° C)
The Moon 1367.6 1.00 12 -153° to 107°
Eros 16 -150° to 100°
Vesta 38 -130° to -60°
Ceres 10 -130° to -38°
Pallas 16
Achilles 3 -150°?
Chiron 10 -200°?
Pluto 0.89 0.0007 30 -223°
Orcus 9
Ixion 15
Varuna 4 -230°
Quaoar 7
Haumea 70 -?°
Makemake 80 -?°
2002 AW197 10
Eris 86 -248° to -232°
Sedna ~20 -261°

Solar Irradiance

This is the amount of solar energy (in watts) that passes through a square meter of the body's surface. The closer an object is to the Sun, the more energy it receives. The Major Planet, Mercury, receives over six times more energy than the Earth. Kuiper Belt Object, Pluto receives less than a thousandth of the energy that the Earth does. The second column shows the figure relative to the Earth.

Albedo

This is the percentage of sunlight that is reflected by the body.

Surface Temperature

The Surface Temperature of a body depends on several factors. The distance from the Sun determines how much energy is received. The rotation determines how long the surface is heated or remains in the dark: the slower the rotation, the more extreme the temperature changes.

Observational Properties

Name
Synodic
Period
(days)
Apparent
Diameter
(seconds
of arc)
Maximum
Apparent
Magnitude
Colour
The Moon n/a 31' 5.16'' -12.74 Grey
Eros
Vesta
Ceres
Pallas
Achilles
Chiron
Pluto 366.73 0.06 - 0.11 +13.7 Yellow
Orcus
Ixion
Varuna
Quaoar +18.6
Haumea +17.4
Makemake +17
2002 AW197
Eris
Sedna +20.5 Red

Synodic Period

This period is relative to the Earth.

If the body is at its closest to the Earth, the Synodic Period describes how long the body will take to get back to the same position relative to the Earth. The closer the object is to the Earth, the longer its Synodic Period.

Pluto travels around the Sun so slowly that it takes the Earth a little over a year to orbit the Sun and catch up to it.

Apparent Diameter

This is how big the minor planet appears in the sky as seen from Earth. Most of these objects are too small or too distant to appear as anything more than a point of light. There is a minimum and maximum value because a planet changes its distance from the Earth. The units are seconds of arc. A degree is divided into 60 minutes. A minute is divided into 60 seconds. The Moon and Sun have an apparent diameter of about half a degree (30 minutes).

Apparent Magnitude

Apparent Magnitude tells how bright a planet (or star) is as seen from the Earth. The magnitude scale was devised by the Ancient Greeks. The brightest stars were called First Magnitude, the next brightest were called Second Magnitude, etc.

In modern times, the scale has been defined mathematically. A star of magnitude 1 is about 2.5 times brighter than a star of magnitude 2 which in turn is 2.54 times brighter than a star of magnitude 3. The brighter a star, the smaller its magnitude. Many stars are brighter than first magnitude. Some stars are so bright they have negative magnitudes. Most of the naked eye planets also have negative magnitudes. The faintest stars visible to the naked eye are sixth magnitude. The asteroid Vesta is occasionally on the limit of naked eye visiblity but other objects cannot be seen without optical aid.

The brightness of an object as seen from Earth depends on the closeness of a planet to the Sun (more light to reflect), the albedo (how much light is reflected) and the distance between the body and the Earth.

Colour

Minor Planets have different colours depending on the type of surface they possess.

Miscellaneous Properties

Name Composition of Atmosphere Discovery
Number
Of
Moons
Eros None Germany 1898 0
The Moon Trace Prehistoric 0
Vesta None Germany 1807 0
Ceres Frost? Italy 1801 0
Pallas None Germany 1802 0 ?
Achilles None Germany 1906 0
Chiron Cometary coma USA 1977 0
Pluto CH4 N2 (trace amounts) USA 1930 3
Orcus None USA 2004 0
Ixion None USA 2001 0
Varuna None USA 2000 0
Quaoar None USA 2002 0
Haumea None USA 2004 2
Makemake CH4 (trace) USA 2005 0
2002 AW197 None USA 2002 0
Eris Possibly USA 2005 1
Sedna USA 2003 0

Composition of Atmosphere

Most minor planets lack an atmosphere. Pluto has a trace atmosphere - essentially none. Chiron occasionally has a coma (a type of mist of outgassing materials) like a comet.

Discovery

The first minor planet was discovered in 1801. The first Kuiper Belt Object (Pluto) was discovered in 1930.

Number of Moons

Pluto has three satellites. Haumea has two.

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KryssTal Related Pages

The Major Planets

Tabulated details about the eight major planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune) with explanations of the terms used. There are links to tables about the satellites of Mars, Jupiter, Saturn, Uranus and Neptune.