Small Bodies

According to the nebular model of the formation of the solar system, the Sun formed from a cloud of gas & dust that had collapsed down to a disk, with the inner parts warmed by the proto-Sun more than the outer parts:

It was from this disk that dust grains stuck together, growing to kilometer-sized planetesimals that eventually coalesced by gravity to form the planets.

Inside a distance of about 5 AU, only dust grains composed of refractory materials such as silicates and metals could survive, so the resulting planets were terrestrial planets. Beyond 5 AU water ice and other volatiles were stable, so the resulting mass going into planet-formation was much higher: Ice Giants such as Uranus & Neptune could form, while the masses that eventually formed the Gas Giants Jupiter & Saturn would also trap light gases such as hydrogen & helium in large quantities.

Just inside the orbit of the proto-Jupiter the planetesimals were in unstable orbits due to the mass of this giant world, so they were never able to coalesce gravitationally. Today they remain that way in the form of asteroids (also called minor planets).

Minor Planet Eros	Comet Hyakutake

Those unincorporated small icy bodies – the cometesimals – because what we now refer to as comets.

Because the comets are the most pristine & “primitive” objects in the solar system, understanding their origin, evolution, and composition are key to unraveling the early history of our solar system. Nevertheless, the asteroids give us samples of the material from which the Earth formed, and they also provide clues about the evolutional history of the inner soar system

Asteroids

Nomenclature

Asteroids receive both names and numerical designations. Generally, the person who discovers them gets to name them, subject to the approval of the International Astronomical Union (IAU). At first these were given names consistent with the tradition used for the planets: people from Roman mythology. Thus the first 4 were named Ceres, Pallas (as in Pallas Athena), Juno, and Vesta. As the numbers of asteroids reached first hundreds, and then thousands, astronomers began running out of names. So began the naming of asteroids for scientists, spouses, pets, computers, and TV characters (there is an asteroid Mr. Spock, for example).

Eventually, the Minor Planet Center (MPC), the official IAU “clearing house” for this sort of activity, provides a numerical designation. So the “official” names of the first 4 asteroids discovered are now 1 Ceres, 2 Pallas, 3 Juno, and 4 Vesta.

It was in 1999 that the 10,000^th asteroid was to be named. Due to its diminutive size, Brian Marsden of the MPC suggested giving that “special” number to Pluto, which at that time would have had a “joint” designation as a planet and as an asteroid. The reaction on the part of astronomers was mixed. Some thought it a demotion of Pluto, and a terrible idea. Others liked the joint status. And others wanted only the asteroid designation, and drop Pluto as a Planet. (Personally, I have been in the last category, but was willing to accept a joint designation). Anyway, nothing happened, until the subject was raised again in 2006 – and that is when it really came to the attention of the public.

Now that little world has the undistinguished number of 134340 Pluto.

Orbits

All solar system objects move in elliptical orbits around the Sun, to a first approximation. Important orbital parameters:

T or P– period in years

a – semi-major axis in AU

e- orbital eccentricity

q – perihelion distance = a(1-e)

i – inclination of the orbital plane to the ecliptic in degrees

W - ecliptic longitude of the ascending node in degrees

w - argument of perihelion (angle between ascending node and perihelion) in degrees

The majority of asteroids have orbits between those of Mars and Jupiter.	But many have orbits which cross those of the terrestrial planets (including Earth!) See http://neo.jpl.nasa.gov/orbits/

The orbits are characterized by the size of the semi-major axis, orbital eccentricity, perihelion distance, aphelion distance, orbital inclination, etc. This web site has a tool for playing around with these: http://www.xs4all.nl/~dmsweb/video/orbit.html

The semi-major axes of the asteroids are not evenly distributed, but contain groupings separated by gaps (Note, because the orbits have non-zero eccentricity, asteroids are found at the locations of the gaps, because their orbits can cross them). The various groupings are caused by gravitational resonances with Jupiter.

Note: while some resonances tend to move the semi-major axis out of a particular value, others favor having objects at the resonance with Jupiter. Among these are the 3:2, 4:3, and 1:1. The 1:1 resonance is the location of the Trojan asteroids.

The Trojans meander around the L4 and L5 points of the Sun-Jupiter.

Our most detailed information on these objects come from spacecraft. In some cases, these are flyby trajectories, but I the case of Eros, the Near Earth Asteroid Rendezvous (NEAR) actually landed on it (even thought it was not designed to do so).

Ida and its moon Dactyl	Mathilde

Using the angular size and distance, the size of the asteroid can be calculated (not that they are often not spherical). By measuring the change in the spacecraft trajectory as it passes by (or orbits), the mass can also be calculated. From these the average density can be determined. In many cases, the density is much less that that of solid rock, leading to the idea that these objects are porous, and may in many cases be just rubble piles.

The Japanese spacecraft Hayabusa attempted to land on Itokawa and return a sample of material to Earth. That effort may have failed, but the images it obtained of Itokawa truly make it look like it is composed of a pile of material loosely held together.

NEAR approached Eros with care, since it (like most asteroids) actually rotate:

After closing in, NEAR was put into an orbit around Eros: http://www.physics.uc.edu/~sitko/AdvancedAstro/25-SmallBodies/263.mov

A 3-D model of Eros: http://www.physics.uc.edu/~sitko/AdvancedAstro/25-SmallBodies/erostrue.mov

NAS animation of the landing: http://www.physics.uc.edu/~sitko/AdvancedAstro/25-SmallBodies/NEAR-lands.mov

Most meteorites are fragments of asteroids that have found their way to Earth. Because of this, there is a rough correspondence between the reflectance spectra of meteorites and asteroids.

Getting a match of the spectra of the most common meteorites, the ordinary chondrites, has not been easy. It may be that the samples we have, which have been collected in an instant of time (astronomically speaking) came from a very small sample of NEAs whose spectra we have not yet measured. Asteroids belong to spectral classes as well. E – enstatite (Mg-rich pyroxene) S – silicate – related to stony meteorites M – metallic – related to iron meteorites C – related to carbonaceous chondrites P & D – very dark V – Vesta

Vesta

Vesta is an unusual asteroid. Although it is not the largest (that title belongs to Ceres) it is the brightest, and can even be seen with the unaided eye. Its surface markings and reflectance spectrum suggest a body that is not only differentiated, but covered partly by basalt!

One class of relatively rare meteorite, the howardites, may be pieces of Vesta, or a similar asteroid

Asteroid Families & Sources of Meteorites

A number of groups of asteroids share a common set of orbital elements, almost as if they were “born” together. The truth is that more likely they came from a single object that fragmented, probably due to a collision some time in the past.

It seems likely that the impacts that produce such families may also be responsible for the production of the meteorites that we have collected. The collision process can occur in undifferentiated bodies, to produce mineralogically similar fragments and chondritic meteorites. If one has a differentiated body with an iron core and melt-processed mantle, one can get asteroid families consisting of an M-type object, and a number of S-types. This process us also the likely source for iron meteorites, achondritic stony meteorites, as stony-iron ones such as the pallasites.

Centaurs

There are a number of small bodies that orbit within the zone where the jovian planets are. These are the Centaurs, named after the first one of these that was recognized as a new class of object: Chiron. Although Chiron itself was discovered in 1977 (by Charles Kowal of the Lowell Observatory), it is now apparent that other “asteroids” that were discovered earlier, such as Hidalgo (in 1920) are also members of this class,

One of the Centaurs, Pholus, is amongst the reddest objects in the solar system.

The orbits of the Centaurs are not very stable, and can be chaotic, due to the gravitational tug of the jovian planets. For so many to be known requires that they be replenished. The most likely source was the Kuiper Belt, beyond the orbit of Neptune.

The Kuiper Belt was also implicated as the source of the short-period comets, and so it was only a little surprising when Chiron was observed to develop a coma (or “coma-like atmosphere”), just as comets do! The activity for most comets results when they get close enough to the Sun for their ice to sublime. The Centaurs are far too cold for this to happen, so that the sublimation must be from some much more volatile molecules. But because of this activity, many Centaurs, which once had asteroid designations, now (also) have comet designations. So 2060 Chiron is also 95P/Chiron.

To confuse things even more, a few main belt asteroids have recently been observed to undergo weak activity! These main belt comets may be yet another interesting class of object