ASTR 1210 (O'Connell) Study Guide
21: INTERPLANETARY MATTER
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 which were produced by the fragmentation of larger
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, for example Comet McNaught (shown above)
or the 1833 Leonid meteor storm shown below; and...
- This material poses the greatest natural threat to the survival
of life on this planet.
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
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
The gas is mostly the expanding outer atmosphere of the Sun (the
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.
Scattering of sunlight by interplanetary dust is responsible for
Light," which can be easily seen from dark locations at certain
times of the year.
About 50 tons of interplanetary dust rain down on the Earth's
surface every day.
Although larger chunks of IPM material (10-m or bigger) can be found
everywhere in the Solar System, they are concentrated in two main
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:
- 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
- The "Trans-Neptunian" region: beyond the orbit of Neptune
(30 AU). Dominated by icy material, like the satellites of the
Jovian planets. Includes the "Kuiper Belt Objects"
(see Study Guide 20) and
Spacecraft have been sent (either as a prime or secondary mission) to
rendezvous with a number of asteroids and comets since 1986.
Here is a montage
of the visitees through 2010.
- "Asteroids": rocky/metallic bodies ranging down to about 50-m diameter
- "Comets": produced by icy bodies, mostly between 50-m and 10-km diameter, which
enter the inner solar system
- "Meteoroids": either rocky or icy bodies, less than 50-m diameter
Closeup image of asteroid Lutetia, about 75 miles
on its longest diameter (Rosetta Mission, 2010)
- In the 18th century, Bode's "Law" concerning the systematic
spacing of planetary orbits suggests a "missing" planet between Mars &
- 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 always regarded as "minor
planets." (Well, I said the nomenclature was messy.)
- As of 2013, there are over 625,000 identified asteroids with known orbits, but
there are millions of others.
- Asteroids are most easily identified by their motion with respect to
the background stars over periods of hours.
Here is a sample animation of
asteroid detection with an electronic camera [University of Washington].
Here is a video
(266KB) of asteroid Eros' motion as seen in a small, Earth-based
Planetary orbits showing location
of Asteroid and Kuiper belts (Addison-Wesley)
- Asteroid orbits are usually only modestly elliptical and lie
close to the ecliptic plane.
- Most fall in the "main
asteroid belt," lying 2-3 AU from the Sun, between Mars & Jupiter.
(See drawing above).
- The main belt probably represents a region of the protoplanetary
nebula where Jupiter's gravity prevented accumulation of a single
- There are over 10,300
Near Earth Asteroids, with orbits that approach Earth's
orbit or actually cross it. Over 900 have diameters of 1 km or
larger. The best known group are called "Apollos" after the prototype.
These are potentially
dangerous to us. More about this in Study
a remarkable snapshot plot of the current location of known asteroids in the inner Solar
- < 1 km to almost 1000 km diameter. The three largest asteroids
are Ceres (975 km). Pallas and Vesta (both 570 km).
Topographic map of asteroid Vesta showing two large
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).
- Rocky/metallic materials but several distinct types
- Crude determination of composition is possible
from the reflectance spectrum of an asteroid's solid surface.
- Most common are carbon-rich ("C")
objects, with dark surfaces; others are stony ("S") or metallic
- A handful of asteroids have been imaged by spacecraft or radar,
but most shapes are inferred from variations in brightness as
- Asteroids are mostly irregular in shape (see
and covered by impact craters
- The more massive asteroids tend to be more spherical
- Most are fragments from shattered rocky planetesimals from those
inner regions of the solar nebula that never accreted into planets.
Collisions cause continual
"grinding down" to
- C-types were the least affected (processed) by interactions with
each other or the pressure/heat of protoplanetary interiors. They
yield important information on physical conditions in the primitive
- Asteroids with substantial metallic inclusions are from larger
proto-planetary objects which partially melted &
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 Ceres imaging on its approach.
They will be studied
by Dawn over the next year.
- The first good pictures of asteroids were Viking Mission images
of Phobos and Deimos, the satellites of Mars. These are "domesticated" asteroids (i.e. captured by a
- An image of the "wild" asteroid Ida taken by the Galileo
mission during its 1993 traverse of the asteroid belt is shown
here. Ida is a large, elongated (35 x
13 miles) stony asteroid with a heavily cratered (old) surface and has
its own satellite(!), Dactyl (the starlike speck to the right of
- 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
- Eros is one of the largest "near-Earth" asteroids, coming within 0.15
AU (14 million miles) of Earth. It is potato-shaped (21x8x8 miles)
"S"-type asteroid and rotates once in 5 hours.
- 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.
- The Dawn
spacecraft went into orbit around Vesta, the second largest asteroid,
in July 2011 and delivered thousands of observations before leaving in
August 2012 to rendezvous with Ceres. It reached orbit around Ceres
in March 2015. Visit the mission site for a large collection of
images and videos.
Comet West 1975, showing dust (white) and ion (blue) tails (J. Laborde)
Comets are the effluent of icy planetesimals from the cold,
outer regions of the solar system that evaporate when they get within
several Astronomical Units of the Sun, producing a
gaseous coma and sometimes tails.
- Early interpretation: atmospheric exhalations, supernatural manifestations,
often thought to be
- Tycho (1577) demonstrates that comets are not in Earth's
atmosphere. Rather, they are astronomical objects, lying beyond the
- Halley (1704) uses the Newtonian theory of gravity to interpret 4
historical comets as the same object in a highly elliptical orbit with
a semimajor axis of 18 AU and a 76 year period. Halley correctly
predicted the comet's return in 1759.
Here is a chart of the orbit of Halley's Comet.
Halley's work dramatically expanded the horizon of the Solar
System. The comet's aphelion (greatest distance from the Sun) is
35 AU's, over three times farther than Saturn. But remember that in
1704, neither Uranus nor Neptune had been discovered. So, the
elongated orbit of Halley's Comet plunged it into a completely
mysterious outer realm that offered the promise of many discoveries
yet to come.
- Highly elliptical, but (unlike planets and asteroids)
lying at all angles to the ecliptic plane.
- Main reservoirs:
the Oort Cloud (~spherical, enormous, ~ 50,000 AU) and the
Kuiper Belt (more flattened, centered on the ecliptic plane, ~ 50
AU size). These contain trillions of icy comet nuclei.
The total mass in Kuiper Belt and inner Oort Cloud icy objects is
probably many times the combined mass of the asteroids.
- Most comets have very long periods. Comets obey Kepler's
Laws, as long as they are not disturbed by planets, so an orbit
semi-major axis of 10000 AU's implies a period of 1,000,000
- 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
- Long period orbits can be shifted by the gravitational field of planets,
especially Jupiter, into shorter period ones.
- The brightness and size of a comet as seen from Earth depends on
both the comet's orbit around the Sun and
the location of Earth when the comet is near the
Sun. See this animation of
the passage of Hale-Bopp in 1997.
Comet ISON (2013), about 2 weeks
before its destruction at perihelion.
Image by Damian Peach. (Click
- The nucleus of a comet is best characterized as
a "dirty snowball"--mainly ices with embedded minerals and dust
grains; typical size 0.5-10 km diameter, and usually elongated or
irregular in shape.
- The surface of the comet nucleus begins to evaporate when < 3 AU
from Sun, producing a gaseous "coma" typically 106 km in
diameter. Here is a diagram showing
the basic anatomy and scale of a typical bright comet.
Here is a pictorial
summary of comet evolution.
Here is a
video showing gaseous and dust outbursts from the nucleus of comet
Tempel-1 (from the Deep Impact mission)
- Tail(s) emerge from the coma: these are cold, but they
reflect/fluoresce sunlight and so look "flamelike"
- Tails point roughly away from the sun, not away from the comet's motion.
On the outward leg of a comet's orbit, the tails will be leading, not following,
- The tails of bright comets can be up to 1 AU in length
- Gas (ion) tail: bluish, straight, complex structure. Dragged back
by solar wind. Comet Hale-Bopp (at right) had nicely separated ion
and dust tails.
- Dust tail: dust grains, yellowish-whiteish, smooth &
broad. Driven back by radiation pressure of sunlight.
Mission flew through the tail of comet Wild 2, trapping dust
particles in a gel, and returned the sample to Earth in 2006.
Analysis showed the presence of many organic molecules and evidence
for liquid water on the comet nucleus at some time in the past.
- Comet nuclei disintegrate if overheated by the Sun. Some collide
with the Sun. Satellite observatories that monitor the sun have picked
up hundreds of these, many not otherwise detected because they were too
faint until they approached the Sun. Here is a video compilation of comet-Sun passages or strikes.
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
- Halley's Comet
(1910, 1986, 2062...). An "Armada" of spacecraft sent to rendezvous
with Hally's during its 1986 passage confirms the dirty snowball
The image of the nucleus of Halley's Comet at right, taken by the
Giotto spacecraft (1986), shows an elongated, 7 mi long object, with a
dark crust (reflecting less light than does asphalt) and gas jets.
Click on the image for a labeled diagram.
This was the first resolved image of
a comet nucleus. Spacecraft have since obtained images of 4 more.
A montage is shown here.
- Hale-Bopp, the "Great
Comet of 1997"
Orbit: semimajor axis 260 AU; period about 2400 years
Closest approach to Earth: 1.3 AU (March 22, 1997)
Best Web-available pictures:
- Comet ISON, the "Not Quite
Great Comet of 2013"
Discovered before its first passage in September 2012 and predicted to
pass within 0.02 AU of the Sun in November 2013, Comet ISON had the
potential to be very bright. Although it was a
beautiful telescopic object at 5th magnitude before perihelion
(closest approach to the Sun), its nucleus apparently disintegrated
near the Sun, and only a few fragments survived.
Here is a remarkable time-lapse video of
ISON about a week before perihelion taken by the STEREO spacecraft.
It shows the rapid changes in the comet's tail induced by gas outflow
from the Sun as well as the behavior of a second comet (Encke) that,
by coincidence, happened to in the same field.
Here is a time-lapse
video taken by the SOHO spacecraft of ISON passing by the Sun.
The Deep Impact Mission
- The Deep Impact spacecraft was sent to rendezvous with the 6.5-km
nucleus of comet Tempel-1. It carried 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, blasting material off the comet nucleus and allowing analysis of
its composition. The main spacecraft was then redirected to a 2010
rendezvous with the nucleus of comet Hartley-2, for which it provided
images. Complete information is at
Impact Home Page.
Videos of the Tempel-1 impact mission:
The Rosetta Mission
- On 6 August 2014, the Rosetta
spacecraft was succesfully placed in orbit around the nucleus of
the (6.5-year) periodic comet Churyumov-Gerasimenko at a distance of
almost 4 AU's from Earth. Rosetta was launched in 2004 and used a
series of gravity-assist flybys of the Earth and Mars in order to
achieve the velocity needed to rendezvous with the comet. It will
orbit the comet for over a year as it is warmed during its approach to
the sun. It sent a probe down to the surface in November 2014, but it
landed in shadow, where it cannot readily recharge its batteries. An early
surprise is that close-up imaging shows the comet to have a remarkable
double-lobed shape, more extreme than any other yet studied.
See the image at the right (click for enlargement). Watch the
Rosetta site for updates as
the mission proceeds throughout 2015-16.
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
Meteors (aka "shooting stars") are the incandescent trails of
tiny meteoroids burning up at high altitudes in the Earth's
- Meteoroids enter Earth's atmosphere at orbital velocities of
~ 50,000-100,000 mph
- Smaller objects burn up, producing a fiery streak and occasionally
airborne explosions. 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.: the
Perseids (~ Aug 12; from Comet Swift-Tuttle); the Orionids (~ Oct 22;
from Comet Halley). Rates can be over 1000 per hour.
- The Leonids have produced some of the best showers, starting in
1833 with a meteor
"storm." The picture above right shows the 1833 storm over Niagara
Falls. There were unusually
showers in 1998 and 2001. (The date of maximum is Nov. 17-18 every
- "Meteorites" are meteoroids that survive to reach the ground; they are rocky
or metallic (icy types are destroyed). A slice through a metallic
meteorite is shown at right.
- Meteorites are predominantly samples of asteroidal
material. They are extremely valuable for the insights they
provide into the properties of otherwise mostly unreachable
- "Carbonaceous condrites" are fragments of "C" asteroids. These
are especially important because they yield information on
the primitive protoplanetary disk.
- Remarkably, we have found meteoritic samples of both the Moon and
Mars (the "SNC" meteorites, see Guide 17).
Reading for this lecture:
Study Guide 21
Bennett textbook, Chapter 12
Reading for next lecture:
Study Guide 22
Bennett textbook, Chapter 12
March 2015 by rwo
Asteroid 1997XF-11 animation courtesy of Eric Deutsch (University
of Washington). Telescope video of Eros copyright © 1998 by
Text copyright © 1998-2015 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.