Section 3.3 (page 2) - Resources: Inventory

Astronomy, and space exploration by governments, is a large field of research, and there is a lot of accumulated data about objects beyond the Earth. We will not attempt to reproduce all of that data here, but rather summarize and link to more detailed information. Many of the headings are links to Wikipedia or other detailed articles. Historically objects have been sorted as a hierarchy of Star > Planets > Moons, which is based on gravitational binding. Here we take the approach of sorting by available resources, mainly mass, composition, and energy, instead of gravitational dependency. The point of view is what is there that could be used, rather than the pure science approach of current condition and how did it form.

The inventory is organized by Solar System matter and energy, followed by our Milky Way Galaxy matter and energy, generally in order of decreasing quantities, and without regard to the practicality of using the resources. Resources outside our galaxy are too far away to be of near-term interest. Most of our Galaxy also falls into far-term interest, but some of the near portions may become useful given advances in technology, so we include it for completeness. Descriptions of large scale engineering projects may also involve extra-Solar resources.

Solar System Matter


History: The Solar System, which includes the Sun and all the objects gravitationally bound to it, formed approximately 4.57 billion years ago from a giant molecular cloud, likely along with other stars. Since it formed by gravitational collapse, the original composition of the cloud primarily determined the composition of the Solar System. Some material has since been lost by strong solar winds during early formation, and weaker solar wind to date, and by gravitational ejection of bodies. Additionally some Hydrogen has been converted to Helium by the Sun. The resulting composition found today is noted in the list of major elemental composition by object below. Aside from the Sun, most of the elements are combined into molecules and minerals which represent local minimal chemical energy.

The NASA Jet Propulsion Laboratory (JPL) Solar System Dynamics website provides information on most known natural bodies in the Solar System. Wikipedia also has an extensive List by Size of the larger objects.

  • Mass: 1.981 x 1030 kg, or 333,000 x Earth's mass. This is all but 0.14% of the total mass of the Solar System.
  • Mass Balance: The Sun loses 4.28 million tons/second by virtue of the mass-energy of it's light output, and roughly 1.4 million tons/second due to the solar wind. It gains an (unknown amount) from comet and other object impacts.
  • Composition: The visible surface of the Sun, known as the photoshpere consists of 74.9% H, 23.8 % He, and 1.3% heavier elements. Due to the relatively high gravity of the Sun, denser elements have tended to sink to the core. The bulk composition is estimated at 71.1% H, 27.4% He, and 1.5% heavier elements. From fusion of H to He, the process by which the Sun generates energy, the core is now roughly 60% He. Although the heavier elements are a small fraction of the Sun's composition, they still represent about ten times the mass of the rest of the Solar System besides the Sun.

The Gas Giants


Gas Giants are sufficiently massive to have retained a substantial fraction of Hydrogen and Helium, the lightest two elements. The Solar System has four, which between them have 444.6 x Earth's mass, or nearly all of the mass not included in the Sun.


  • Mass: 1.899 x 1027 kg, or 317.8 x Earth's mass.
  • Mass Balance: Jupiter's mass balance is not available, it would come from atmospheric loss and comet and asteroid impact gain.
  • Composition: The atmosphere is about 75% Hydrogen, 24% Helium, and 1% heavier elements. The total composition is estimated as 71% Hydrogen, 24% Helium, and 5% heavier elements.


  • Mass: 5.685 x 1026 kg, or 95.15 x Earth's mass.
  • Mass Balance: Not available.
  • Composition: Saturn's atmosphere is 96.3% Hydrogen, 3.25% Helium, and 0.45% heavier elements. The total composition is estimated to be 20-32% heavier elements, and the remainder H and He.


  • Mass: 8.681 x 1025 kg, or 14.54 x Earth's mass.
  • Mass Balance: Not available.
  • Composition: Uranus' atmosphere is about 83% Hydrogen, 15% Helium, and 2% Methane. The total composition is estimated at 0.5-3.7 Earth masses rocky materials, 9.3-13.5 Earth masses ices (water, ammonia, and methane), and 0.5-1.5 Earth Masses H and He. The ices would be solids at this distance from the Sun, but in the interior of the planet they are actually hot dense fluids.


  • Mass: 1.024 x 1026 kg or 17.15 x Earth's mass.
  • Mass Balance: Not available.
  • Composition: The atmosphere is about 80% Hydrogen, 19% Helium, and 1.5% Methane. The total composition is estimated at 1.2 Earth masses rocky materials, 10-15 Earth masses ices, and 1-2 Earth masses H and He.

Objects With Atmospheres


Every object of sufficient mass and temperature will have some trapped molecules. For the purpose of this page, we define Atmosphere to be sufficiently dense to flow, rather than free molecule interactions, and is mostly non-ionized.


  • Mass: 4.87 x 1024 kg, or 0.815 x Earth's mass. Atmosphere = 4.8 x 1020 kg.
  • Mass Balance: Venus appears to be losing about 16 grams/sec x atomic weight of atmosphere due to solar wind stripping. Average accretion from comet and asteroid impacts is not available.
  • Composition: The atmosphere is about 96.5% CO2, 3.5% Nitrogen, with trace compounds. Because of similarity in density and total mass to the Earth, Venus is expected to have a similar total composition and structure, consisting of a rocky outer portion, and a metallic core.


  • Mass: 5.974 x 1024 kg, or exactly 1.000 x Earth's mass. Atmosphere = 5.28 x 1018 kg.
  • Mass Balance: Earth loses about 3 kg/s of Hydrogen and 0.05 kg/s of Helium from the upper atmosphere. A small amount of man-made objects depart the Earth per year. On the order of 1.25 kg/s of extraterrestrial material of all sizes (dust to dinosaur killing asteroids) accretes. Therefore as a whole the Earth is losing mass, but the current rate amounts to 40 parts per billion of the total mass over the remaining life of the Earth.
  • Composition: The atmosphere contains 78% Nitrogen, 21% Oxygen, 1% Argon, variable amounts of water, and trace compounds. The total composition consists of the following major elements: Iron (32.1%), Oxygen (30.1%), Silicon (15.1%), Magnesium (13.9%), Sulfur (2.9%), Nickel (1.8%), Calcium (1.5%), and Aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to density segregation, the core is 95% Iron and Nickel, while the uppermost layer, the Crust, is nearly all metal oxides. By weight these are oxides of Si, Al, Fe, Ca, Mg, Na, K, H, C, Ti, and P.


  • Mass: 6.42 x 1023 kg, or 0.107 x Earth's mass total. Atmosphere = 24.8 x 1015 kg.
  • Mass Balance: Loss of Oxygen ions due to the solar wind is estimated at 16-35 grams/sec in a 2010 article by Fang et. al., Effect of Martian Crustal Magnetic Field on Atmospheric Erosion. At this rate the atmosphere would be eroded in 22.5 billion years. Accumulation from incoming asteroids and meteorites is not available.
  • Composition: The atmosphere contains 95.3% CO2, 2.7% Nitrogen, 1.6% Argon, and trace components. The total composition is generally a silicate outer layer with a metallic core, with elements by mass roughly in the order oxygen, silicon, iron, magnesium, aluminum, calcium, and potassium.


  • Mass: 1.345 x 1023 kg, or 2.25% of Earth's mass in total. Atmosphere = 9.05 x 1018 kg.
  • Mass Balance: Not available, but considerations are stripping by Saturn's magnetosphere, accretion from small objects, and stripping from larger impacts.
  • Composition: The atmosphere is variable by altitude. Higher levels are 98.4% Nitrogen, 1.4% Methane, and 0.2% trace compounds. Lower levels are 95% Nitrogen, 4.9% Methane, and 0.1% trace compounds. Total composition is roughly half silicate core and half ices and likely liquid water at some depth. The surface appears to be mostly water ice, with variable amounts of Carbon Dioxide, Methane, Methane Hydrate, Ammonia, and Methanol by location.


  • Mass: 2.14 x 1022 kg, or 0.36% of Earth's mass in total. Atmosphere ~5 x1013 kg.
  • Mass Balance: Not available.
  • Composition: The thin atmosphere is mostly Nitrogen. The solid composition based on density is likely 30-45% water ice, and the remainder rocky core. Surface is 55% solid Nitrogen, 15-35% water ice, and 10-20% carbon dioxide ice.

Objects Without Atmospheres


This group includes those without significant atmospheres. This happens whenever the combination of low mass and temperature allows gas molecules to escape or to be stripped off by Solar wind interaction. They are grouped by the International Astronomical Union into one planet (Mercury), several dwarf planets, the remaining satellites of the major planets, a very large number of Small Solar System Bodies which are further divided into types, and a possible class of Rogue Objects. These divisions into groups are more for convenience by orbit location than mass, composition, shape, and internal arrangement. There is a more or less continuous distribution of all of these properties, including orbits. Because most orbits are elliptical, and sometimes highly so, a single object can cross several nominal orbit locations. Wikipedia has a table of Solar system rounded objects which includes the dwarf planets, larger satellites, and dwarf planet candidates.

  • Mass: 3.3 x 1023 kg, or 5.5% of Earth's mass.
  • Mass Balance: Not available.
  • Composition: Approximately 70% metallic core and 30% silicate outer layers.

Dwarf Planets


Dwarf planets are defined as large enough to have been shaped by gravity (into an ellipsoid), but not so massive as to clear its neighborhood of other objects. The category was created in 2006 when it was obvious that Pluto was part of the Kuiper Belt region, which contains some objects larger than the former planet. Currently 5 dwarf planets are officially recognized (1 Ceres, Pluto, Haumea, Makemake, and Eris), and it is expected several hundred to 2000 others will be found as observations of the region beyond Neptune improve. Objects larger than 838 km in diameter are provisionally classed as dwarf planets, but final status requires they be rounded, and most such objects have not been observed that well yet.

  • Mass: Five official dwarfs: 3.8 x 1022 kg, or 0.63% of Earth's mass. Perhaps 10% Earth mass once all outer system objects are discovered.

Planetary Satellites


This group of objects is distinguished by being attached to a major planet and not large enough to maintain a significant atmosphere. The Wikipedia List of Natural Satellites shows the known ones to date, from which Titan (Saturn VI) and Triton (Neptune I) are excluded since they have atmospheres and were listed above. It is very likely there are additional small satellites orbiting the major planets which have not been discovered yet. Total mass is around 475 x 1021 kg, mostly in the 4 largest moons of Jupiter and the Earth's moon. Composition varies by distance from the Sun from rocky (the Moon) to icy. We group these objects into two classes by size: Large and Small. The large group are shaped by gravity, and possibly internal heating from radioactive decay or tidal friction from the planet it orbits. This group are approximately spherical, and would be considered dwarf planets if not orbiting a planet themselves. The more numerous Small group are irregular in shape and likely not differentiated by internal melting.

Outer System Objects


The outer system includes objects outside the orbit of Jupiter, but not the outer planets themselves or their associated satellites. In approximate order of semi-major axis (symbol a, in AU) this includes the Centaurs, Kuiper Belt, Scattered Disk', Hills Cloud, Oort Cloud, and Rogue Objects. There is overlap between these groups and with asteroids. The distinctions are based on convenience in describing their orbits and composition rather than a real difference in type. They are all objects not attached to a major planet with orbits beyond Jupiter and too small to maintain a permanent atmosphere. All the outer system objects are presumed to form from the same solar disk which formed the planets, but were then scattered by the gas giants once they grew massive enough. Comets II and The Solar System Beyond Neptune are recent reviews of the state of knowledge for some of these objects.

The total mass of all the outer system objects is poorly known and is an active area of study. Density and spectra indicate the outer system objects as a whole consists mostly of Hydrogen-bearing ices: Methane (CH4), Ammonia (NH3), and Water (OH2), which are all solids at large distances from the Sun. Comets are former outer system objects whose orbit now comes closer than Jupiter to the Sun, so that gas evaporates off their surface, thus they are listed below. The remainder stay cold enough to be solid.

These are objects which have closest approach to the Sun (perihelion) between Jupiter and Neptune, and no limit on the farthest point. Approximately 200 are known as of 2012, and are being discovered at about 15-20 per year. The discovered ones range in size from 200 down to 2 km in diameter. Estimated number larger than 1 km in size is 44,000. Because they cross the orbits of the gas giants, their orbits are unstable on a scale of a few million years.

The Kuiper Belt includes objects in stable orbits outside the orbit of Neptune (30 AU) up to 50 AU, having eccentricity generally from 0 to 0.2, except for resonant objects which can go to about 0.4, and with inclinations up to about 35 degrees. Pluto is now considered the largest such object, and 1241 such Trans-Neptune Objects (TNOs), as they are also known, have been found since 1992. Estimated number of TNOs larger than 100 km diameter is 0.1 million, >10 km possibly as high as 10 million. Estimated mass is 0.04-0.1 x Earth's mass, but models of planet formation predict there were 30 Earth masses of such objects originally. Where the rest went in that case is unknown at present. Composition in general is poorly known, but a combination of CH4 (methans), NH3 (ammonia), and H2O (water) ices are expected from the small number of density and spectral observations.

Scattered Disk Objects (SDOs) have perihelion (closest approach to the Sun) beyond Neptune (ie 30 AU) with higher eccentricity (0.20 to 0.94) at a given distance, and extending to larger distances than the Kuiper Belt. Their inclinations range up to about 40 degrees. The rise in eccentricity with distance may be a selection effect, since we can only find objects that come closer to the Sun. Outer system objects observed from Earth dim as the 4th power of distance. This is the product of solar intensity, which falls as the square of distance, and angular area, which also falls as the square of distance. The name comes from their orbits being gravitationally scattered by the gas giant planets at some point in their history from the lower inclination and eccentricity of the Solar nebula from which they formed. Their orbits are subject to further changes from planet interactions, so are not stable over the long term. Since 1995, a total of 167 SDOs have been found (in that table, the ones with q >30 AU). Their total mass is estimated at 0.01-0.1 Earth Mass.

The Hills Cloud, also known as the Inner Oort Cloud, are objects with semi-major axes between 1,000 and 10,000 AU. The Sun is expected to have formed in a star cluster embedded in a gas cloud. Computer simulations (2011) of such clusters indicates objects forming closer to the Sun would be scattered out to these distances and into the outer Oort Cloud with an efficiency of ~1.5%, leading to an estimate of ~3 Earth masses currently. The Hills Cloud is bound strongly enough to the Sun that perturbations from outside sources and the gas giants rarely put them in orbits where we can detect them. Their large distance makes them too dim to see except when they approach closest to the Sun. Thus we have only discovered a few objects on orbits which enter this region.

The Oort Cloud is made up of objects with semi-major axis > 10,000 AU. At such great distances from the Sun, their orbits are affected by close star passes and galactic tides. Their origin may partially be capture of loose objects in the cluster where the Sun formed. The remainder would be highly scattered from the solar disk during the Sun's formation. Estimated mass may be 0.1-7 Earth masses, consisting of about 6 x 1010 to 1012 objects. This estimate comes from observing the frequency of long period comets, but this does not place strong limits on their number.

Rogue objects, also called nomads or rogue planets, are not bound to the Sun and merely passing nearby. None have been definitely identified in our vicinity, but in 2011 some candidates in our galaxy were discovered through gravitational microlensing. Since Solar System objects have been ejected by the gas giants, by symmetry there should be objects ejected from other stars which are currently near the Sun. In addition to ejected objects, objects that formed separate from stars by the same mechanism are expected. Total numbers are very uncertain, with estimates from 2 to 100,000 such objects per star, including our own.

Inner System Objects


This group includes objects from the orbit of Jupiter inwards. It is listed after the outer system objects because this is a resource-based inventory, and the total mass is smaller. In decreasing distance from the Sun it includes:

  • Jupiter Trojans

There are stable Lagrange Points located 60 degrees ahead and behind a smaller body orbiting a more massive one. The most massive such pair in the Solar System, Jupiter and the Sun, has the strongest such stable regions, extending about 30 degrees of Jupiter's orbit each. Objects trapped there are called Trojans because the first few found were named after mythical characters from the Trojan War. As recently as 1961, only 14 Jupiter Trojans had been discovered, but by 2012 the Minor Planet Center lists over 5300. There are estimated to be 0.1-0.3 million Trojans larger than 2 km diameter. Total mass and composition are poorly known at present, but can very roughly be estimated at 2 x 1019 kg. A relatively small number of similar Trojan objects exist around other mass pairs.

  • Main Belt Asteroids

About 600,000 total asteroids are known in the Main Belt which ranges from 1.3 AU minimum distance to Jupiter's orbit. The dense region of the Main Belt has a doughnut-shaped volume of about 70 AU^3, and so an separation of 0.05 AU, or 7,300,000 km. The number of known asteroids has increased by 60 times since 1980, and is expected to continue growing, but newly found ones are generally small. The total mass in this region is estimated at 3 x 1021 kg (0.05% of Earth). Total number larger than 1 km is roughly 1 million, and >100 meters ~25 million.

These are defined as all objects which have a closest approach to the Sun of less than 1.3 times the Earth's distance (1.3 AU), with the exception of the Sun itself, Mercury, and Venus. Mars has an average distance of 1.52 AU, but is lower mass than the Earth. The 1.3 AU definition is thus approximately closer to the Earth than outer planets from a gravity standpoint, or close enough to be interesting. This is an artificial definition from a human interest standpoint - there is no distinct physical grouping for this class of bodies, such as there is for the Main Belt asteroids. NASA has a Near Earth Object Program to discover and characterize them, and as of mid-2012 there are about 9000 known. Estimated number >1 km size is around 1000, and >100 m is about 200,000.

The NEO category includes all object types, including asteroids, comets, extinct comets (which can be hard to distinguish from asteroids), and manufactured spacecraft. There is an undefined lower bound to size, which we will assume to be 1 meter diameter, below which we refer to them as meteoroids, dust, or particles. The population of NEOs is not permanent. Over periods of approximately 1-10 million years either gravity effects from planets and larger bodies, or collisions with them, will remove them from the NEO orbit range. Objects smaller than approximately 1 cm are also affected by light pressure or other effects more than gravity interactions, and have even shorter lifetimes in those regions.



Comets differ from the solid objects listed previously in that they show periodic vaporization and dust emission. This is caused by heating of their surfaces when they get close to the Sun. Water sublimation becomes strong at less than 2.5-3 AU, and other volatiles at other distances. The lost material can create spectacular though not very massive tails. Since comets are in effect boiling gases from their surface, the built up pressure can cause them to fragment. Their lives are limited by the amount of volatiles they start with. Defunct comets resemble asteroids, and distinct trails of debris in their orbits are observed as periodic meteor showers. Comets ultimately originate in the Oort Cloud, and migrate to closer orbits by gravitational perturbations.

Comets are divided by their orbits into Short Period, Jupiter Family, Halley Family, and Long Period classes. Typical orbit changes in terms of semi-major axis are about 0.001 per orbit. If there were no planets around the Sun, the comet orbits would tend to stay fixed. Since Jupiter is roughly 1/1000 the mass of the Sun, it can be thought of as a blender blade mixing up the orbits by that amount per pass. Therefore comet orbits tend to randomly migrate among orbit classes.

Jupiter Family

Since Jupiter is by far the most massive planet in our Solar System, it has the most influence on comets which cross it's orbit. There is a noticeable cluster of comets whose maximum (aphelion) distance (Q) from the Sun is close to that of Jupiter (5.2 AU). The cluster roughly ranges from 4.2 AU < Q < 11 AU, with minimum (perihelion) distance (q) ranging from 0.5 AU < q < 5.5 AU. This range is generally just outside that of the main Asteroid Belt. Jupiter family comets generally have inclinations less than 35 degrees. Approximately 200 Jupiter Family comets are known.

Halley Family

This group of comets have orbits with periods of 20-200 years which are oriented so that Jupiter does not affect them strongly. Their inclinations range from 0 to 180 degrees, with a slight excess below 60 degrees.

Long Period

These are defined by having an orbital period > 200 years, and thus a semimajor axis (a) > 34 AU. Long period comets strongly cluster in semi-major axes near 10,000 AU, which makes them hard to distinguish from parabolic. This cluster of orbits led to the assumption of the Oort cloud as their source. Their inclinations span the full range from 0 to 180 degrees.



Dividing line between distinct objects and regions or masses of particles better tracked as a whole.

Particle Belts


Interplanetary dust Gas and solar wind

Solar System Energy


The Sun


Current Energy Output From fusion of hydrogen to helium

Estimate Energy Reserves

  • Hydrogen Fusion
  • Additional Fusion Reactions
  • Stored Thermal Energy
  • Gravitational Collapse Energy
  • Minor Energy Reserves - Spin, stratification, magnetic field

Everything Else


Everything else is lumped under one heading because magnitude of the Sun's energy reserves is so much larger than everything else combined.

Latent Heat of Formation - When massive objects like planets formed within the Solar System, the collision and gravity well energies, and later the stratification of the interiors by density released a lot of energy, part of which went to heating the interior of the object. For the larger objects, some of that heat is still stored in their interiors.

Nuclear Fission - Objects in the Solar System incorporated elements with radioactive isotopes which decay naturally, heating their interiors, or can be made to fission on purpose.

Nuclear Fusion - Just like the Sun, but on a smaller scale, there is potential energy in light elements that can be fused together. Since it takes at least 75 Jupiter masses for this to happen naturally, then it must be made to happen artificially.

Chemical Reactions - Materials such as fossil fuels and undecayed plant matter can burn with oxygen in the atmosphere to release energy

Orbital Kinetic Energy - The bulk of the kinetic energy in the Solar System resides in the motion of the planets and smaller bodies. When spacecraft use a flyby to alter their motion, they extract a little of this energy.

Minor Energy Reserves -

Galactic Matter


The Galaxy or Milky Way in capitalized form refers to the gravitationally bound object which the Sun and Earth orbit within. In lower case, galaxy refers to the general class of such objects.

Summary description of the Galaxy as a whole: total mass of the Galaxy, and baryonic vs Dark Matter.)

Components sorted by Mass:

Dark Matter




Satellite Galaxies






Central Black Hole


Substellar Objects

Brown Dwarfs -

These are objects too small to count as stars, but above the limit for planetary bodies. The upper limit is about 80 times Jupiter's Mass, above which hydrogen fusion can happen and the object is considered a star. The lower bound is about 13 times Jupiter's mass, below which no fusion will occur. Above this lower limit, Deuterium and Lithium fusion can happen, but since these are much rarer than Hydrogen, it limits their life and brightness.

Planetary Systems

In recent years a large number of planetary systems around other stars have been detected. The Extrasolar Planets Encyclopedia catalogs them with references to original papers. As of 2012 there were 660 such systems with 837 planets detected. Methods of detection vary. In addition to planets which have formed, Circumstellar Disks represent early stages of formation or incomplete condensation to larger objects.

Rogue Objects

These are objects not tied to a star, which can range from planets below the brown dwarf limit down to ejected comets. They are currently not well understood, as only a few candidates have been detected by gravitational lensing in 2011. Estimates of their number range from two per main sequence star up to potentially 100,000 per star. The larger number depends on optimistic assumptions for the mass function (number of objects vs their mass).

Interstellar Medium

Dust Particles
Interstellar Gas

Galactic Energy


Total Power output

Total Energy reserves



Gravitational energy


Dark Energy


Angular Momentum


Thermal Energy


Magnetic Fields


High Energy Sources


This includes cosmic ray flux and X-Ray sources.