ASTR 1210 (O'Connell) Study Guide


Comet McNaught 2007 at Sunset

Comet McNaught at Sunset (2007; Akira Fujii)

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 that were produced by the fragmentation of larger bodies.

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

  1. Some of the most beautiful and spectacular astronomial events involve the IPM, for example Comet McNaught (shown above) or the 1833 Leonid meteor storm shown below; and...

  2. 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 skyscrapers to tiny dust grains and atoms of gas. The number of objects of a given mass decreases dramatically as the mass increases.

The total mass in the IPM is smaller than Jupiter's. But even relatively small bodies traveling at typical orbital speeds of over 50,000 miles per hour can have "large impacts" on other objects, like the Earth.

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

The dust is composed of tiny (mostly smaller than 0.01 cm diameter) grains of material distributed throughout the ecliptic plane. Much of this has been released from comets as they are heated by the Sun. Although larger chunks of IPM material (10-m or bigger) can be found everywhere in the Solar System, they are concentrated in two main regions:

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:

Spacecraft have been sent (either as a prime or secondary mission) to rendezvous with a number of asteroids and comets since 1986. A beautiful image of one of these is shown below. Here is a montage of the visitees through 2010.

Lutetia Closeup

Closeup image of asteroid Lutetia, about 75 miles on its longest diameter (Rosetta Mission, 2010)

B. Asteroids


Asteroid Belt Diag

Planetary orbits showing location
of Asteroid and Kuiper belts (Addison-Wesley)



Vesta Topo

Topographic map of asteroid Vesta showing two large impact basins.
The altitude range (coded blue to red) spans 25 miles At 570 km (353 miles) diameter,
Vesta is the second largest asteroid. Map from the Dawn Mission (2012).




Ceres Mosaic

Mosaic image of the surface of asteroid Ceres, the largest asteroid and the first to be discovered.
The images were taken by the Dawn spacecraft, which entered orbit around Ceres on 6 March 2015.
The mosaic shows the heavily cratered surface of Ceres and, on the upper right,
two mysterious "bright spots" first revealed by Dawn imaging on its approach.
These appear to be layers of reflective salts, deposited by liquid extruded from the interior.


Comet West

Comet West 1975, showing dust (white) and ion (blue) tails (J. Laborde)

C. Comets

The flamelike apparitions in the sky that we call "comets" are the effluent of icy planetesimals from the cold, outer regions of the solar system that begin to evaporate when they get within several Astronomical Units of the Sun, producing a gaseous coma and sometimes tails.

Halley's Orbit

The orbit of Halley's Comet



Comet ISON

Comet ISON (2013), about 2 weeks before its destruction at perihelion.
Image by Damian Peach. (Click for enlargement.)


Famous comets

The Deep Impact Mission

The Rosetta Mission

Comet CG Nuc

D. Meteoroids

1833 Leonid Storm,
Niagara Meteoroids are smaller interplanetary bodies, of both icy and rocky/metallic types. The boundary in size between these and the larger types of IPM bodies is arbitrary, but here we will take it to be 50-m in diameter.

Meteors (aka "shooting stars")



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Last modified January 2024 by rwo

Asteroid 1997XF-11 animation courtesy of Eric Deutsch (University of Washington). Telescope video of Eros copyright © 1998 by Gordon Garradd. Text copyright © 1998-2024 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.