***** Asteroids- How Many? Where are they? Some interesting points on their motion***********
Asteroids and comets in inner solar system / How many asteriods? "Snapshot" of the positions of several thousand asteroids at some particular time. The two groups of asteroids 60 degrees on either side of Jupiter are the Trojan asteroids. These are located around the L4 and L5 Lagrange points of the Sun- Jupiter system. The Trojans need not be located exactly at the L4 or L5 points- they can wander around the L points quite a bit, but still be in 1:1 resonance with Jupiter (that is, have same period around Sun as Jupiter.) This explains the extended nature of the two Trojan "clouds" of objects.
How many asteroids are there? The "official tracker of the asteroids" is the Minor Planet Center http://www.cfa.harvard.edu/iau/mpc.html. Astronomers around the world report observations (position on sky and time of observation) of moving objects to the MPC. The MPC first checks if the object is already known. If not, it is given a provisional designation (e.g. 2008 TC3), where the letters are related to when the object was discovered. After enough observations have been made that an object has a good orbit (typically dozens of observations over several years) the object is given a number. As of September 2011, there are almost 285,000 numbered asteroids. About 290,000 asteroids have provisional designations but not yet a number. About 16,000 of the numbered asteroids also have a name.
The latest estimate is that there are about 2 million main belt asteroids larger than 1 km in size. New surveys should be able to see all these 2 million, plus millions of somewhat smaller rocks, so that the number of known asteroids will almost certainly exceed a number of millions in the next decade as the new surveys (CSS, PanSTARRS, LSST) get cranked up.
Kirkwood gaps.. A histogram of the semi-major axes of asteroids show gaps at periods which are a small integer ratio to the period of Jupiter. These are called Kirkwood gaps. They are caused by resonant effects of Jupiter preferentially removing asteroids at these periods. Note that the gaps do NOT show up in a snapshot of the positions of asteroids at any time (see asteroid position image above). This is because the eccentricity of asteroid orbits means that the distance from the Sun of an asteroid at any one time is not equal to its semimajor axis. IF all asteroids had circular orbits with the a distribution as given in the diagram, then we WOULD see "gaps" in asteroid position "snapshot" (like we see gaps in Saturns rings due to resonant effects).
Lagrange points of Sun-Earth system.. In a system consisting of a very massive object and a moderately massive object (like a star and an orbiting planet) there are 5 points where a low mass body can orbit the massive body with the same period as the moderate mass body. These are called Lagrange points. The L4 and L5 points are stable- the Jovian Trojan asteroids are located around the L4 and L5 points of the Sun-Jupiter system. There was a famous space society (well, famous to space geeks!) called the L5 Society which was formed in 1975 to promote space colony ideas. The L1 and L2 points of Earth-Sun system have been used as places to "park" satellites for special purposes (e.g. SOHO, a satellite to study the Sun, is at L1, and WMAP, a satellite to study the cosmic microwave background, is at L2). The L1 and L2 (and L3) points are unstable, but a satellite can be made to stay near one of these points by occasional corrections using small thrusting jets on the satellite. Satellites at L1 and L2 stay in the same place relative to Earth, making communication easier, but are far enough from Earth that the Earth doesn't block much of the sky (and L1 of course offers a continuously unobstructed view of the Sun). As far as we know, there are no natural objects at the Earth's L3 point, but it always behind the Sun as seen from Earth (also the point is unstable, so we would not expect objects to stay there if they got there). Because it would be hard to observe anything that is located at L3, the L3 point is a place where science fiction authors (and occasionally other forms of wacky folks!!) like to place "alien planets" etc.
In this diagram, the sizes of the Earth, Sun and Moon, size of Moons orbit around the Earth and the distance from Earth to the L2 point are NOT to scale with the Sun-Earth distance. The diameter of the Moons orbit around Earth is about 1/200 of the Earth-Sun distance, and the Earth-L2 distance is about 1/60 the Earth-Sun distance. (And the size of the spacecraft is *SERIOUSLY* NOT TO SCALE with Earth-Sun distance- or the size of Earth!!)
Further readable info on Lagrange points can be found here.
In this diagram, the sizes of the Earth, Sun and Moon, size of Moons orbit around the Earth and the distance from Earth to the L2 point are NOT to scale with the Sun-Earth distance. The diameter of the Moons orbit around Earth is about 1/200 of the Earth-Sun distance, and the Earth-L2 distance is about 1/60 the Earth-Sun distance. (And the size of the spacecraft is *SERIOUSLY* NOT TO SCALE with Earth-Sun distance- or the size of Earth!!)
Further readable info on Lagrange points can be found here.
Hildas and Trojans - The Movie!
Asteroid families When precise orbital parameters are plotted (here the eccentricty vs. orbital inclination) some "clumps" of asteroids with similar orbital properties are found. These are called "Hirayama families" in honor of the Japanese astronomer who first discovered these. (Family members are not clustered in space. Two asteroids with very close orbital properties that star off together will soon drift apart as they go on their very slightly different orbits.) Families are produced during asteroid collisions, when pieces of an astroid are broken off a larger asteroid and become separate asteroids.
***Asteroids- Discovery, designations and physical properties************
Asteroid designations When a new asteroid is discovered, it is given a provisional designation consisting of the year of discovery and a 2 letter (+ number sometimes) code indicating when it year it was found. If the asteroid is then observed long enough for a good orbit to be determined, the asteroid is given a number. Some numbered asteroids are also given names.
Asteroid from groundbased telescope Asteroids are just dots of light from the Earth. We can tell them apart from stars from their motion relative to the stars, as shown in this 4 frame "movie" of an asteroid.
Earth, Moon and Ceres to scale and sizes of asteroids The largest asteroid (now called a dwarf planet) is Ceres. It is about as big as the state of Texas. Asteroid sizes range down from there to rocks and pebbles.
First 10 asteroids vs Moon This illusterates the relative sizes of the first 10 numbered asteroids vs the Moon.
Most of the asteroids that have "closeups"
Asteroid closeups and some general properties (1) (253) Mathtilde (2) (21) Lutetia (3) (951) Gaspra (4) Vesta Most asteroids are still only unresolved (no detail visible) dots of light to our telescopes. A handful of asteroids have been imaged at close range by spacecraft. Here are 3 moderately large (10s to ~100 of km in size) such objects and one of the largest (4) Vesta, about 530 km in diameter.
Some general properties of asteroids:
The majority are of rocky composition. A small fraction appear to be mostly iron/nickel (the iron/nickel objects are over-represented in meteorites, as the iron masses are far stonger than rocks, so the irons tend to make it to the surface more often than the rocks.) The rocky asteroids are made of minerals that are common on the Earth. The vast majority of asteroids are made of silicate minerals- things like olivine, feldspars and pyroxenes. (Which may mean something to you if you took a geology class- otherwise, don't worry about it. The important thing is that they are same minerals as found on Earth.) Some meteorites (and so presumably asteroids) contain chondrules, very ancient small bits of rock, which as previoulsy discussed, are of exterme importance as "time capsules" that we can use to probe the very beginning of our Solar System.
All the asteroids imaged appear patato-shaped and to have been battered by collisions and covered with craters. This (and measured bulk densities- which are lower than the rocks they are made up of) have lead to the paradigm that the asteroids are "rubble piles". Instead of solid rock, the asteroids have been splintered and broken up (and sometimes gravitationally reassembled) so that there interiors have numerous cracks and voids.
**** Binary (double) asteroids- Why two rocks are MUCH better than one!***
Power Punt on Binary Asteroids The last 15 years has seen the opeining of the field of study of double or binary asteroids. These are multiple objects held together by their mutual gravity. Binaries are of extreme importance as they are the best way (besides spacecraft) of measuring the masses of asteroids. This is from a talk I gave at the Okie-Tex star party (at end of Oklahoma Panhandle) a while ago.
