ASTR 121, O'CONNELL. Study Guide 21 [Spring 2008]

ASTR 121 (O'Connell) Study Guide

21: INTERPLANETARY MATTER

Leonid Meteor Storm over Niagara Falls, 1833

There is a smattering of material lying between and around the 8 planets. This "interplanetary matter" (IPM) is mostly left over from the early protoplanetary phase of the Solar System---it consists of icy or rocky bodies that were never permanently incorporated into the planets or which were produced by the fragmentation of larger bodies.

This would seem to be a boring footnote to the planets themselves...except for two things:

Some of the most beautiful and spectacular astronomial events involve the IPM, e.g. the meteor storm shown above and the Hale-Bopp comet of 1997; and
This material poses the greatest natural threat to the survival of life on this planet.

A. GENERAL

The IPM contains material that ranges from satellite-sized objects through chunks the size of footballs through tiny dust grains and atoms of gas.

Total mass < Jupiter. Relatively small, but can have large "impact" on other objects

The gas is mostly the expanding outer atmosphere of the Sun (the "solar wind")

About 50 tons of IPM, mostly dust, rains down on the Earth's surface every day.

The solid components are mostly unaccreted leftovers from the protoplanetary nebula or fragments of collisions between larger bodies.

Although larger chunks of IPM (10-m or bigger) can be found everywhere in the Solar System, they are concentrated in two main regions:

The "asteroid belt": lying between the orbits of Mars and Jupiter (2-3.5 AU from the Sun). Dominated by rocky or metallic material like the terrestrial planets.
The "Trans-Neptunian" region: beyond the orbit of Neptune (mostly outside 30 AU). Dominated by icy material, like the satellites of the Jovian planets. Includes the "Kuiper Belt Objects" (see Study Guide 20) and the comets.

The nomenclature for the IPM is currently in a mess, with various new and traditional designations being used in overlapping ways. We focus here on the following three components of the IPM:

"Asteroids": rocky bodies ranging down to about 100-m diameter
"Comets": produced by icy bodies, mostly between 100-m and 10-km diameter, which enter the inner solar system
"Meteoroids": either rocky or icy bodies, less than 10-m diameter

B. ASTEROIDS

History

Bode's "Law" concerning the systematic spacing of planetary orbits suggests a "missing" planet between Mars & Jupiter.
The first asteroid, Ceres (975 km diameter), was discovered in this region in 1801. This inspired a number of systematic searches for asteroids, continuing to today. Ceres is large enough and round enough to now be classified as a "dwarf planet"; but none of the other asteroids are in this category. Traditionally, however, asteroids were regarded as "minor planets."
Now ~ 180,000(!) ID's with known orbits
Asteroids are detected by their motion with respect to the background stars.

Sizes: < 1 to ~ 1000 km diameter. The three largest asteroids are Ceres (975 km). Pallas and Vesta (both 570 km).

Compositions: rocky/metallic materials but several distinct types

Crude determination of composition is from reflectance spectra.
Most common are carbon-rich ("C") objects, with dark surfaces; others are stony ("S") or metallic ("M").

Shapes

Shapes are inferred from variations in brightness as asteriods spin
Mostly irregular (see image at right)
The more massive asteroids tend to be more spherical

Origin

Shattered rocky planetesimals from inner regions of solar nebula which never accreted into planets. Collisions cause continual "grinding down" to smaller pieces.
C-types were the least affected (processed) through interactions with each other or the pressure/heat of protoplanetary interiors. They yield important information on physical conditions in the primitive protoplanetary disk.
Metallic inclusions are from larger proto-planetary objects which partially melted & differentiated.

Orbits

Usually only modestly elliptical and lying close to the ecliptic plane.
Most fall in the "main asteroid belt" (2-3 AU, between Mars & Jupiter).
- The main belt probably represents a region of the protoplanetary nebula where Jupiter's gravity prevented accumulation of a single planetary-sized object.
There are over 900 known Earth-crossing asteroids, with orbits which cross that of Earth. The best known group are called "Apollos" after the prototype. These are potentially dangerous to us. More about this in Study Guide 22.
Here is a remarkable snapshot plot of the location of asteroids in the inner Solar System.

Images

First good pictures of asteroids: Viking images of Phobos and Deimos, the satellites of Mars. These are "domesticated" (i.e. captured by a planet).
A more recent image of the "wild" asteroid Ida by the Galileo mission during its traverse of the asteroid belt is shown above right. Ida is an elongated (35 x 13 miles) stony asteroid with a heavily cratered (old) surface and has its own satellite(!), Dactyl (starlike image below and to right of Ida).
Here is a video (451KB) of NEAR's encounter with Mathilde, a 33 mile-diameter, carbon-rich (primitive) asteroid. Closest approach: 740 miles.
NEAR (the Near Earth Asteroid Rendezvous) mission, completed a close-up study of the asteroid Eros in 2003. This was the first spacecraft to orbit and later to land on an asteroid.
- Eros is one of the largest "near-Earth" asteroids, coming within 0.15 AU (14 million miles) of Earth. Eros is potato-shaped (21x8x8 miles) and rotates once in 5 hours. It is an "S"-type asteroid.
- NEAR went into orbit around Eros on February 14, 2000 (get it?). It spent 12 months in orbit around the asteroid (distances 3-100 miles), and was then directed to soft-land on its surface.
- Here is a video of Eros in rotation taken from orbit by NEAR.
- Here is an animation of the last few images returned as NEAR moved to land on Eros.

Comet Ikeya-Zhang (2002)

C. COMETS

Comets are the effluent of chunks of icy debris from the outer regions of the protoplanetary nebula that evaporate when they get within several Astronomical Units of the Sun, producing a gaseous coma and sometimes a tail.

History

Early interpretation: atmospheric exhalations. Evil omens.
Tycho (1577): demonstrates comets are astronomical objects, beyond Moon
Halley (1704): uses Newtonian theory of gravity to interpret 4 comets as the same object in an elliptical orbit with semimajor axis 18 AU, period 76 yr. Correctly predicts return (1759).

Orbits

Highly elliptical; at all angles to ecliptic. Here is a chart of the orbit of Halley's Comet.
Main reservoirs: the Oort Cloud (~spherical, enormous, ~ 50,000 AU) and the Kuiper Belt (more flattened, centered on ecliptic plane, ~ 50 AU size). These contain billions of comet nuclei (icy planetesimals).
Most have very long periods. Comets obey Kepler's Laws, as long as they are not disturbed by planets, so an orbit size of 10000 AU's implies a period of 1,000,000 years.
Only comets with small (< 20 AU) orbits have been observed on more than one solar passage; these are called "periodic" comets. Halley's Comet is the most famous of these.
Orbits can be modified by planets (esp. Jupiter) ===> throw into smaller orbits with shorter periods; returns

Structure/evolution

Nucleus: "dirty snowball"--mainly ices with embedded dust grains; typical size 1-10 km diameter.
The surface of the comet nucleus begins to evaporate when < 3 AU from Sun, produces a gaseous "coma" typically 10⁶ km in diameter. Here is a pictorial summary of comet evolution.
Tail(s) emerge from coma: cold, but reflect/fluoresce sunlight so look "flamelike"
- Tails point roughly away from sun, not away from comet motion
- Typical length 10⁸ km
- Gas (ion) tail: bluish, straight, complex structure. Dragged back by solar wind. Hale-Bopp (at right) had nicely separated ion and dust tails.
- Dust tail: dust grains, yellowish, smooth & broad. Driven back by radiation pressure of sunlight.
- Comet nuclei disintegrate if overheated by Sun; some collide with Sun.
- Here is a video (MPEG, 489K) of a comet-sun collision (12/96) taken by the SOHO solar satellite.

Famous comets: most comets are faint and only visible in telescopes. There are typically 20 of these observable each year. Brighter, naked-eye comets are less frequent---one every few years on average. The most spectacular comets, like Hale-Bopp are usually first-time visitors to the inner Solar System. Here are some well-known bright comets:

Halley's Comet (1910, 1986, 2062...). An "Armada" of spacecraft sent during 1986 passage confirms the dirty snowball model
Hyakutake 1996
Hale-Bopp, the "Great Comet of 1997"

The Deep Impact Mission

Spacecraft sent to rendezvous with the 6.5-km nucleus of comet Tempel-1, carrying an impactor unit that was sent to collide with the nucleus as a means of probing its structure. A perfect hit was accomplished at 23,000 mph on July 4, 2005. Complete information is at the Deep Impact Home Page.
Videos of the mission:

D. METEOROIDS

Smaller interplanetary bodies; not more than 10-m diameter; both icy & rocky/metallic types

Meteors (aka "shooting stars") are the incandescent trails of tiny meteoroids burning up at high altitudes in the Earth's atmosphere.

Meteoroids enter Earth's atmosphere at orbital velocities ~ 100,000 mph
Burn up, producing a fiery streak. A bright meteor has a typical mass of ~ 1 gram. There are typically about 10 meteors per hour visible from any location under good sky conditions.
"Meteor Showers" (concentrations) occur when Earth passes through the denser debris lying along the orbit of a comet. E.g.: Perseids (~ Aug 12; comet Swift-Tuttle); Orionids (~ Oct 22; comet Halley). Rates can be over 1000 per hour.
The Leonids have produced some of the best showers, starting in 1833. Unusually good Leonid showers in 1998 and 2001. (Shower maximum is Nov. 17-18 every year).

Meteorites

= meteoroids which survive to reach ground; rocky or metallic (icy types destroyed)
These are samples of asteroid material: highly valuable for insights into properties of otherwise mostly unreachable extraterrestrial objects
"Carbonaceous condrites": fragments of "C" asteroids, these yield important information on the primitive protoplanetary disk.
"SNC" meteorites: from Mars

Reading for this lecture:

Reading for next lecture:

Web Links:

Java Simulator of Comet & Asteroid Orbits

Asteroid Information & Links

NEAR Mission to Asteroid Eros

Deep Impact Mission to Comet Tempel-1

Comet Information

Meteor Showers

More information on meteorites

"The Colour Out of Space" by H. P. Lovecraft

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Last modified April 2008 by rwo

Asteroid 1997XF-11 animation courtesy of the University of Washington). Telescope video of Eros copyright © 1998 by Gordon Garradd. Ikeya-Zhang image copyright © 2002 by M. Jager. Text copyright © 1998-2008 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 121 at the University of Virginia.