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 that 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 Earth.
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). The belt is dominated by rocky or
metallic material like the terrestrial planets.
- The "Trans-Neptunian" region: beyond the orbit of Neptune
(30 AU). Dominated by icy material, like the satellites of the
Jovian planets. This includes the "Kuiper Belt Objects"
(discussed in 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. A
beautiful image of one of these is shown
is a montage of the visitees through 2010.
- "Asteroids": rocky/metallic bodies ranging down to about 50-m diameter
- "Comet nuclei": icy bodies, mostly between 50-m and 10-km
diameter, which can become "comets" if they enter the inner
- "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 2018, there are over 780,000 identified asteroids with known orbits, but
there are estimated to be millions of others.
- Asteroids are most easily identified by
their motion with respect to the background stars over
periods of hours. They produce streaks on long-exposure images or
shifts in position between exposures taken at different times.
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 16,000
Near Earth Asteroids, with orbits that approach Earth's
orbit or actually cross it. Over 800 have diameters of 1 km or
larger. The best known group are called "Apollos" after the prototype.
Because these could collide with Earth, they 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
- Although diagrams like this one and dramatic depictions of
asteroid belts in movies
like Star Wars give
the impression of impossibly crowded regions, the actual typical
separation between asteroids is over 1 million km --- it would be
difficult without telescopes to see other asteroids from the location
of a given asteroid.
- < 1 km to almost 1000 km diameter. The three largest asteroids
are Ceres (975 km). Pallas and Vesta (both 570 km). There are many
more small asteroids than large ones.
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
- The "C-type" asteroids were the least affected ("processed")
by interactions with each other or the pressure/heat of protoplanetary
interiors. They have chemical compositions most like the Sun's and
yield important information on physical conditions in the
early protoplanetary disk before large bodies had accreted.
- 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 Dawn imaging on its approach.
These appear to
be layers of reflective salts, deposited by liquid extruded from
- The first good pictures of asteroids were Viking Mission (1976)
images of Phobos and
Deimos, the satellites of Mars. These
are "domesticated" asteroids (i.e. captured by a planet).
- 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)
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.
The orbit of Halley's Comet
- Early interpretations of comets were as atmospheric exhalations
or supernatural manifestations, which were often thought to be
Perhaps the most famous instance of a comet as omen is found in the
which shows Halley's Comet portending the defeat of King Harold
of England by William the Conquerer at the Battle of Hastings in 1066 AD.
(Of course, if Harold had prevailed then the comet would have
been assumed to portend the end of William.)
Another famous, but happier, manifestation shows Halley's Comet (from
its passage in 1301) as the Star
of Bethlehem in Italian painter Giotto's 1304 version of the
Adoration of the Magi.
- 1577: Tycho demonstrates that comets are not in the Earth's
atmosphere. Rather, they are astronomical objects, lying beyond the
- 1704: Halley uses Newton's theory of gravity to
interpret 4 historical comets as the same object viewed during
4 separate passages as it moves in a highly
elliptical orbit with a semimajor axis of 18 AU and a 76 year
period. Halley correctly predicted that the comet would
return in 1759.
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. See the diagram above.
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.
- 19th century forward: Thousands of
amateur astronomers become avid comet hunters using small
telescopes. Many of the important comets were first identified by
amateurs. One of the first serious hunters
Messier (d. 1817), who compiled one of the first catalogs of
diffuse objects (nebulae, star clusters, and galaxies) so that these
wouldn't be mistaken for comets. One of the attractions of comet
hunting is that comets are the only kind of astronomical object
to be named for their discoverer.
- Main reservoirs: The
location of the largest concentrations of comets can be determined
from tracing their orbits into the outer Solar System. The primary
reservoirs are the Oort Cloud (~spherical, enormous, roughly
50,000 AU in radius) and the
Kuiper Belt (more flattened, centered on the ecliptic plane, ~ 50
AU size). These contain trillions of icy comet nuclei.
Most of these have orbits which never reach the inner Solar System.
The total mass in Kuiper Belt and inner Oort Cloud icy objects is
probably many times the combined mass of the main belt asteroids.
- Comet orbits are highly elliptical (see the diagram of
Halley's Comet above), but, unlike planets and asteroids, they lie at
all angles to the ecliptic plane.
Comets obey Kepler's laws, as long as they are not disturbed by
planets. This means that comets which enter the inner Solar System
will move much faster when near the Sun than when they are far from
it. By inspecting the dates on the Halley diagram above, you
will get an impression of the difference.
Most comets have very long periods. An orbital semi-major
axis 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
- 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. The blank dot at
the center of the field is an "occulting disk" that prevents
light from the nucleus itself from overwhelming the images.)
- 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.
- Dust tail: dust grains reflecting sunlight, so they are
yellowish-whiteish in color, smooth & broad. Driven back by the
radiation pressure of sunlight. Comet Hale-Bopp (at right) had nicely
separated ion and dust tails.
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
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 was first recorded by the Chinese in 240 BC and has
been seen somewhere in the world on every subsequent passage
(76 year intervals). Although the most recent passage in 1986
was not as close to Earth as some, space technology had
developed to the point where flyby spacecraft could be
deployed to get close-up images of its nucleus:
"Armada" of 5 spacecraft was sent to rendezvous with the comet. The
data they returned confirmed the dirty snowball model.
The image of the nucleus of Halley's Comet at right, taken by the
European Space Agency's Giotto spacecraft, 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 5 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, and it never
became a good naked-eye comet.
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 be 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 launched 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 (19 mile radius) 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
by the European Space Agency and used a complicated
gravity-assist flybys of the Earth and Mars in order to achieve
the velocity needed to rendezvous with the comet. It orbited the
comet, observing it through perihelion and the warm-up/cool-down
phases, and has delivered a massive amount of high-resolution imaging
and physical measurements of its surface and environment. 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 was that the 4-mile wide comet
nucleus has a remarkable double-lobed shape, more extreme than
any other yet studied. See the image at the right (click for
enlargement). To end the mission, the orbiter was crashed at low
speed into the comet's surface on 30 September 2016, sending back much
close-up data in the interim.
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")
- Meteors 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 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 only ~ 1 gram. There are typically about 10 meteors per hour
visible from any location under good night sky viewing 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.
(from Comet Tempel-Tuttle) have produced some of the best showers,
1833 with a meteor
"storm" (probably over 10000/hour for a short period). The picture
above right shows the 1833 storm over Niagara Falls. There were
unusually good Leonid showers in 1998 and 2001. (The date of
maximum is Nov. 17-18 every year).
- "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 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 the surfaces of
both the Moon and Mars (the "SNC" meteorites are from Mars;
see Guide 17).
- Although there was much early scientific controversy over the
nature of meteorites (including the skepticism of our own Thomas
Jefferson, who proclaimed "It is easier to believe that Yankee
professors would lie than that stones would fall from heaven"),
their extraterrestrial origin was unambiguously demonstrated in 1803
by the French astronomer Jean-Baptiste Biot. The notion that giant
rocks hitting the Earth at speeds up to 100,000 mph might present a
danger dawned only slowly on scientists, however, so that it was 150
years before this threat was clearly realized. We now understand that
meteoroid/asteroid impacts pose the most serious cosmic hazard to
human beings, and this is the subject of the
next study guide.
Reading for this lecture:
Study Guide 21
Bennett textbook, Chapter 12
Reading for next lecture:
Study Guide 22
Bennett textbook, Chapter 12
October 2018 by rwo
Asteroid 1997XF-11 animation courtesy of Eric Deutsch (University
of Washington). Telescope video of Eros copyright © 1998 by
Text copyright © 1998-2018 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.