ASTR 330: The Solar System
Lecture 9:
Asteroids!
Image © Lucasfilm
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Hypothesis And Discovery
• Astronomers in the 18th century noticed the large gap between the
orbits of Mars and Jupiter, and guessed that there might be an
unseen planet remaining to be discovered.
• On January 1st 1801, Guiseppe Piazzi at Palermo discovered the
asteroid Ceres in the middle of the gap, at 2.8 AU. The missing planet
had been found at last!
• However, three more ‘asteroids’ (meaning ‘star-like’) were discovered
in the gap soon thereafter: Pallas, Juno and Vesta.
• The combined mass of all four much less than the Moon, so the
mystery of the gap remained.
• Some astronomers hypothesized that the asteroids were the remains
of a former, exploded planet.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Accelerating Rate Of Discovery…
• From 1801-1844 only 4 were known.
• By 1890, 300 known, photographic patrols begin..
• By 1923, 1000 known (700 in 33 years).
• 1984, 3000 known (2000 in 61 years).
• 1990, 5000 known (2000 in 6 years).
• 1997, 10,000 known (5000 in 7 years).
• 2000, 20,000 known (10000 in 3 years!)
• What happened in the 1990s?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Main Belt
• The so-called Main
Belt of asteroids lie
between the orbits of
Mars and Jupiter, with
semi-major axes 2.2 to
3.3 AU.
Picture credit: NASA GSFC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
By the Numbers
• Ceres, the largest asteroid is just less than 1000 km in
diameter.
• Total mass of all asteroids is 3x1021 kg:
= 1/2000
mass of Earth
= 1/20
mass of Moon
• We probably now know all asteroids larger than 25 km
across, and 50% of the ones down to 10 km in size.
• There are an estimated 100,000 asteroids larger than 1
km in size.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Expected Population
• What do we expect in terms of numbers of asteroids of different
sizes?
 More small ones?
 More large ones?
 Equal numbers in each size range?
• Scientists predict that fragmentation processes would produce equal
masses of material in each size range.
• But, a 10 km diameter object has 1000 times the volume (mass) of a
1 km diameter object.
• So, if there is equal mass in each range, then we expect 1000 times
as many objects of 1 km diameter as 10 km diameter.
• Does this match our observations?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Asteroid Size Distribution
• In mathematical notation, we
expect the number of objects of a
given diameter D to be inversely
proportional to the volume (cube
of diameter):
Expect:
1
N 3
D
• In fact we find that:
1
N  2.3
D
• Therefore proportionally more of
the mass in the larger objects.
Picture: Tom Quinn and Zeljko Ivezic, SDSS Collaboration
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Sizes and Masses
• Because most of the total mass is contained in the larger bodies, we
can be fairly sure we know the overall mass of the main asteroid belt
quite well.
• How do we describe average size in the distribution of this type? Most
asteroids are still small, but most of the mass is in the larger ones.
• Now think about how we measure the size of an asteroid. Do you
think it is practical to measure size directly using a telescope?
• Until about 1975, asteroids were mostly unresolved, star-like points in
the sky. We were largely restricted to:
1. charting their orbits, and
2. measure their rotation rates, by observing periodic changes in
brightness (think of a police light).
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Observing From Earth
• Two of the most interesting challenges for asteroid scientists were to
measure:
1. the actual sizes, and
2. the reflectivity.
• One method we can use to determine the size is to watch as asteroid
passing in front of (‘occulting’) a bright background star.
• If we observe the shadow of the asteroid simultaneously from various
points on the Earth, we can deduce the size and possibly the shape.
• This technique was first used to measure the size of asteroid 3 Juno
on Feb 19th 1958 in Malmo, Sweden (P. Bjorklund and S. Muller).
• Is this likely to work for very many asteroids?
(about 350 have actually been observed, most in the last 5 years, since Hipparcos).
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Picture: David Dunham/IOTA
Movie: Rick Baldridge/IOTA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Spectroscopy: Size
• We have mentioned spectroscopy several times in previous lectures.
• Spectroscopy can even be used to measure the size of an asteroid!
How?
• If we can measure both the visible and the infrared emission, we can
figure out whether the body is small and bright, or large and dark.
• E.g., consider two asteroids, one small but highly reflective, and one
larger and less reflective, which both have a similar visible brightness.
• But, the larger asteroid should have a much higher infrared
brightness, being both larger and hotter.
•
The important principle here that we obtained two different pieces of
information from two different spectral regions.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Spectroscopy: Composition
• Of course, spectroscopy is also useful in determining composition,
although the spectral features of minerals are much less sharp than
the spectral lines seen in gases (atmospheres).
• This figure shows spectral
data of bright and dark
terrain on asteroid 433
Eros, as measured by the
NEAR spacecraft.
• The spectra are similar in
some respects to primitive
meteorites, but
differences in composition
remain to be explained.
Figure: from Clark et al 2001
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Orbits and Collisions
• Main-belt asteroids orbit the Sun between 2.2 and 3.3 AU, with
corresponding periods of 3.3 to 6 years.
• They occupy a donut-shaped volume, 100 million km thick and 200
million km across.
• Typically, they are separated from each other by millions of km, and
pose no danger to passing spacecraft, unless we decide to go close.
• Most have stable orbits with eccentricities less than 0.3 and
inclinations less than 20 degrees.
• Collisions would have been much more frequent in the past.
• Even so, with 100,000 objects there should be collisions every few
10,000 years.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Orbits: Gaps
• In the main belt, orbital distances are not distributed evenly.
Picture: JPL/SSD Alan B. Chamberlain
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Orbits and Resonances
• The gaps in the orbital distances are known as resonance, or
Kirkwood gaps.
• A resonance effect is essentially the principle that a small push or
perturbation applied repeatedly in the same way can add up to a
large effect: think of a pushing a child’s swing.
• In this case, the resonance effect is the gravity of Jupiter: if the
asteroid keeps passing Jupiter at the same places in its orbit, then the
tugs from the giant planet’s gravity will eventually alter the orbit.
• For example, the outer edge of the asteroid belt is defined by the 2:1
resonance with Jupiter. An asteroid at 3.3 AU would take exactly half
as long to orbit the Sun as Jupiter, and get a repeated push at the
same points in its orbit. The 4:1 resonance defines the inner edge of
the main belt.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
More Resonances
• What do you think
the 3:1, 5:2 and 7:3
resonances are?
• Do you think the
resonance gaps are
entirely unpopulated?
(clue: think of
eccentricities)
• Saturn’s rings also
have resonance
gaps, but they are
completely empty:
why?
Figure: Nanjing University Astronomy
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Family Values
• An asteroid family is a group which has similar orbits; e.g.
the Koronos, Eos and Themis families.
• Although the family members are not now in the same
place, they apparently were in the past.
• In fact, members of a family tend to have similar surface
reflectivities and spectra.
• We therefore conclude that all the objects in each family
are fragments of the same shattered asteroid, still
following similar orbital paths.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Asteroid Albedo Classes
• Many asteroids have albedos in one of two ranges: 3-5% or 15-25%.
Num
Name
1 Ceres
511 Davida
Radius Distance*
Albedo
Discoverer
Date
1801
1903
1910
1885
1802
1852
1866
1807
(km)
466
168
(10^6km)
413.9
475.4
0.1
0.05
433
15
52
951
10
243
Eros
Eunomia
Europa
Gaspra
Hygiea
Ida
17.5 x 6.5
136
156
17x10
215
58x23
218
395.5
463.3
330
470.3
428
?
0.19
0.06
0.2
0.08
?
G. Piazzi
R. Dugan
G. Witt, A.
Charlois
De Gasparis
Goldschmidt
Neujmin
De Gasparis
J. Palisa
704
253
2
16
87
4
Interamnia
167
Mathilde
28.5 x 25
Pallas
261
Psyche
132
Sylvia
136
Vesta
262.5
458.1
396
414.5
437.1
521.5
353.4
0.06
0.03
0.14
0.1
0.04
0.38
V. Cerulli
J. Palisa
H. Olbers
De Gasparis
N. Pogson
H. Olbers
Table: Calvin J Hamilton, Solarviews.com
1893
1851
1858
1916
1849
1884
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Compositional Classes
• The two albedo classes also have spectral differences:
• The darker, low-albedo asteroids have no visible absorption
features, but a signature of water in the infrared.
• The bright, high-albedo asteroids show the signature of common
silicates: olivine, pyroxene.
• We thereby divide asteroids into three classes:
1. C-TYPE: carbonaceous; dark, with water; primitive (e.g. Ceres)
2. S-TYPE: stony, with silicates; primitive (e.g. Eros)
3. M-TYPE: (rare) metallic; radar-bright (e.g. Psyche)
• How do these correspond to meteorite classes?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Albedo vs Orbital Distance
• The brighter asteroids (stony and irons) tend to be on the inner
edge of the main belt (25% of total), while the darker
(carbonaceous) asteroids are nearer the outer edge (75%).
• Metallic asteroids tend to be towards the middle (rare).
Figures: Wm Robert Johnson
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Asteroid Densities: Useful or Not?
• We saw in Lecture 2 that densities provide a good way to
characterize planets into groups: might we do the same with
asteroids?
• We need to know the mass (Kepler’s third law) and volume (from
size). For a certain few asteroids we have enough information to do
this.
• When we calculate densities, we do not find a strong correlation with
presumed composition: metal asteroids are not necessarily denser
than stony ones, why?
• The answer lies in the internal structure: many asteroids are ‘rubble
piles’ of loosely agglomerated rocks, or ‘Swiss cheese’ metallic types,
rather than compact solids.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Density Examples
Asteroid
Density
gm cm -3
Volume Reference
1 Ceres
2.05 ± 0.05
Merline et al. 1996
2 Pallas
4.2 ± 0.3
Drummond & Cocke 1988
4 Vesta
4.3 ± 0.3
Thomas et al. 1997
16 Psyche
1.8 ± 0.6
Viateau 1999
20 Massalia
2.7 ± 1.1
Bange 1998
45 Eugenia
1.2 (+0.6,-0.3)
Merline, et al. 1999
121 Hermione
1.8 ± 0.4
Viateau 1999
243 Ida
2.7 ± 0.4*
Petit et al. 1997
253 Mathilde
1.3 ± 0.2
Veverka et al. 1997
433 Eros
2.67 ± 0.03
Yeomans, et al. 2000
Table: J Hilton, USNO
• Ceres, Pallas,
Vesta, Ida,
Hermione, Eros
are probably 035% porous,
somewhat
fractured, but
still coherent.
• On the other
hand, Mathilde,
Eugenia and
Psyche are
>35% porous;
probably
loosely-bound
‘rubble piles’.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Vesta
• We will conclude our discussion of the general properties
of main belt asteroids by considering the bright asteroid
Vesta.
• In Lecture 7 we discussed the eucrite group of meteorites,
which form a distinct category of basalts.
• In fact, the spectra of these meteorites closely match the
spectrum of certain regions of the asteroid Vesta: believed
to be large lava flows.
• Are the eucrites then from Vesta, or could they have come
from a similar asteroid to Vesta, but now broken up? We
can test this second hypothesis…
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Origins Of Eucrites
• If the eucrites are surface ‘crustal’ rocks, we can predict quite well
what the interior ‘mantle’ rocks should be like from the same parent.
• The mantle is thicker than the crust, so there should be a lot of
meteorites of this type.
• The fact that we have found no example of these hypothetical mantle
meteorites shows that the break-up never took place.
• As Vesta is the only large asteroid with the right surface properties,
we conclude that the eucrites are from Vesta.
• This gives us our fourth definitive sample of a known solar system
object. What are the other three?
• Eucrites have a solidification age of 4.5 Gyr and a gas retention age
of 3.0 Gyr: what does this tell us about Vesta?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Outside The Main Belt
• Outside the main belt, the gravity of Jupiter makes most nearby orbits
unstable.
• The exceptions are the
Lagrangian points:
regions of gravitational
stability for small bodies in
the fields of two larger
bodies, predicted in 1772.
• There are five Lagrangian
points, but in terms of
asteroids, the L4 and L5
points equidistant from
Jupiter and the Sun are
most important.
Figure: Nanjing University Astronomy
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Trojan Asteroids
• The L4 and L5 points of Jupiter are occupied by hundreds of asteroids.
• The first was named
Hektor in 1907, and all
subsequent finds have
been named after the
heroes of the Trojan
War. Hence, these
asteroids are named
‘Trojans’.
• Their distinct spectra
indicates that they are
primitive bodies,
trapped there since the
birth of Jupiter.
Figure: Nanjing University Astronomy
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
More On Trojans…
• The Lagrangian L4 and L5 points exist for all planets (paired with the
Sun), but Jupiter has the most stable L4 and L5 orbits.
• Several small asteroids have been discovered in the Lagrangian
regions for Mars and Neptune, but none for the Earth or the other
planets.
• Although they are dark and apparently carbonaceous, the spectra of
the Trojans is different, redder, than the main belt C-types.
• We do not appear to have examples of the Trojans in our meteorite
collections. How do we know?
• They are probably composed of primitive carbonaceous chondrite
material, although a different type and composition from the mainbelt.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Centaurs
• Centaurs are another class of objects which followed a mythological
naming convention, taking after Chiron, the second one discovered.
Figure: CAPS, Kent Univ Canterbury
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Centaurs contd.
• The first discovered was Hidalgo, a dark object with a highly eccentric
and inclined orbit which reaches out to Saturn.
• Chiron was discovered in 1977, with an eccentric orbit ranging from
8.5 AU (near Saturn) to 19 AU, near Uranus.
• In 1992, Pholus, discovered in 1992 is named after another good
centaur from Greek myth. Pholus is the reddest object in the solar
system, whose surface is still a mystery.
• These orbits are similar in many respects to comets.
• Speculation as to whether these objects were really comets
(developing atmospheres) rather than asteroids (no atmospheres)
was confirmed in 1988 when Chiron ventured close enough to the
Sun to out-gas volatiles, brightening considerably.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Near-Earth Objects
• Only about 1% of asteroids cross the Earth’s object, but we are very
interested in them! Why?
• The first one discovered was Apollo in 1948: for this reason Earthcrossing asteroids are called Apollo asteroids.
• The terms Near-Earth Asteroid (NEA) or Near-Earth Object (NEO,
which includes some comets as well) are used collectively for
potentially Earth-crossing bodies.
• The largest Earth-crosser is Eros (30 km).
• About 1000 larger than 1 km are expected, and 250,00 down to 100
m in size.
• Most are S-type, but some are C-type.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Threat Of NEOs
• Even a small NEO has a lot of energy, and can cause a lot of
damage…
An Asteroid the
size of…
Enters Earth’s
atmosphere…
Potentially Causing…
Dust, and small
rocks
Continually
Shooting stars
A Car
Twice a month
Explosions high in the atmosphere with the
force of a small atomic bomb
A blue whale
Every few centuries
A powerful shockwave traveling 100 miles
The Titanic
Every few hundred
centuries
A tsunami, if it hit an ocean
Half mile
A few per million years
A regional calamity
One mile
Every million years
A world-wide calamity
Three miles
Every ten million years
Human extinction; an impact this size is
believed to have killed off the dinosaurs
Sources: NASA, Eric Asphaug, Univ of CA,
Santa Cruz, and Wall Street Journal, 9-20-2002
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Spaceguard
• To protect against NEOs, Congress mandated a search in 1994, to be
carried out by NASA, to find 90% of 1 km or larger NEOs by 2008.
• Since 1998 the effort
has been carried out
by computerized Air
Force telescopes,
finding about 10
NEAs per month.
• Close approaches
include a 100 m
object (2002MN)
which passed less
than 1/3 the distance
to the Moon!
Sources: US House of Rep, Hearing 10/03/02
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Meeting Asteroids Up-Close
• Spacecraft have made close-flybys of 4 asteroids, and even landed
on one!
• The first two significant encounters were due to the Galileo
spacecraft, en route to Jupiter, which made flybys of:
• Gaspra in 1991, an S-type in the Flora family,
• Ida in 1993, an S-type in the Koronos family.
• Several years later, the NEAR-Shoemaker spacecraft made two
encounters:
• A flyby of Mathilde in 1997, a C-type.
• Orbited and finally landed on Eros, an S-type, in 2000.
• Let’s look at these encounters.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
951 Gaspra
• Galileo encountered Gaspra on October 29, 1991. The highresolution image below was taken just before closest approach, at a
distance of 5300 km.
• Gaspra measures 19x12x11 km. More than 600 small craters are
visible here, from 100-500m in size.
• The highly irregular
shape indicates that
Gaspra suffered a
massive collision(s) in
the past which nearly
destroyed it.
• Gaspra movie
by A. Tayfun Oner.
Image: NASA/USGS
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
243 Ida and Dactyl
• Galileo encountered Ida on August 28, 1993, finding an irregular body
58x24x21 km in size.
• The main discovery was that Ida is accompanied by a small moon,
Dactyl, the first natural satellite of an asteroid ever discovered.
• This image was taken
from a distance of 11000
km near closest approach,
and shows that Ida is
even more heavily
cratered than Gaspra.
• Dactyl is just over a km in
diameter, and has a
different spectrum from
Ida, indicating a capture
origin.
Image: NASA/JPL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
NEAR Shoemaker
• The Near-Earth Asteroid Rendezvous (NEAR) spacecraft, was
launched in 1996, to encounter, orbit and land on asteroid 433 Eros.
• It was later re-named ‘NEAR-Shoemaker’ in honor of the pioneering
solar system astronomer, Gene Shoemaker.
• NEAR missed its original meeting with Eros in 1998 due to a
malfunction, but was able to recover and finally arrived in Feb 2000,
going into orbit (a first).
• After 1 year in orbit, studying and mapping the asteroid in detail,
NEAR lowered its orbit and landed on Feb 12, 2001, another first.
• The spacecraft continued operations for more than a week on the
surface.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
253 Mathilde
• The NEAR spacecraft flew past Mathilde en route to a rendezvous
with Eros. Mathilde is extremely black: 3% albedo, twice as dark as
coal! What type do you think it is? Is it primitive?
• Mathilde is twice as large as Ida and 4 times the size of Gaspra, at
50x53x57 km. It also rotates extremely slowly: 415 hrs = ? Earth days?
• Mathilde has giant craters such as the one in the image. The implication
is that Mathilde is a very soft, porous dusty ball: est. 50% porosity.
Image: JHU/APL/NASA
• FLYBY
MOVIE
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Dr Conor Nixon Spring 2004
3 Asteroid Close-Up
• A composite of images from NEAR and Galileo
ASTR 330: The Solar System
433 Eros
• Asteroid 433 Eros was
the prime target of
NEAR.
• This image shows the
eastern (top) and western
hemispheres in detail.
• The large crater in the
western hemi is Psyche,
5 km in diameter.
• Eros is stony and
primitive; and about 25%
porosity. Landing proved
the composition of Stypes was stony,
including Fe, Mg, Si and
O composition.
Image: JHU/APL/NASA
• FLYBY
MOVIE
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Eros Surface
• Long ridges seen on the surface indicate that Eros is a solid
collisional fragment of a larger parent body, with a heavily fractured
interior.
• The surface is cratered, with a deficiency of small craters and an
excess of boulders.
• The bottoms of craters seem to be
flattened, filled with fine dust, and
hence were named ‘ponds’ by
scientists.
• On crater walls, dust had flowed
downhill, exposing brighter
underlying terrain, protected from
space weathering.
Image: JHU/APL/NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Satellites Of Mars
• In the 1700s, astronomers knew that Earth had one moon and Jupiter
had 4, so Mars should have 2, right?
• In this case, numerology proved correct and Phobos (Fear) and
Deimos (Panic), named after the horses of Ares were found in 1877.
• These satellites seem to have little to do with Mars, and we suspect
that they are captured asteroids. How could that happen?
• A passing asteroid may have been slowed by friction with an early,
dense atmosphere of Mars (sometimes called ‘aerobraking’), falling
into orbit.
• Too much atmosphere and the satellites would have crashed into
Mars, too little and they would not be captured. Phobos and Deimos
represent a window of opportunity for Mars.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Asteroids or Moons?
Phobos
Ida
Gaspra
Deimos
Picture: Bill Arnett, LPL
Dr Conor Nixon Spring 2004
ASTR 330: The Solar System
Phobos and Deimos
• Phobos (22km) and Deimos (13 km),
photographed on the previous slide
by Viking in 1977, have a lower
density than rock, 2.0 g/cm3. What
does this tell us about their interior?
• Phobos (left, MGS/MOC image 1998),
like Eros, has long scars on the
surface, apparently fractured which
occurred in an early massive impact.
• The impact was probably the large
crater Stickney (upper left), 10 km
diameter, which must have nearly
destroyed the satellite.
• Phobos and Deimos are remnants of
a violent past!
Image: NASA/JPL/Malin SSS
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
1. What do we mean by the main belt of asteroids?
2. Is most of the mass in small or large asteroids?
3. What are the three main asteroid types?
4. Which is the most common type, and where are its members
concentrated, relative to the Sun?
5. Name 2 of the 10 largest asteroids, and say what type each one is.
6. How do asteroid types relate to meteorite types?
7. Describe one of the two methods used to derive asteroid size.
8. Are asteroid densities a good guide to composition? Give your
reasoning.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
9. Is the main belt likely to be the remains of an exploded planet? Give
you reasoning.
10. What are resonance gaps, and why do they occur?
11. Name an asteroid family. In what respects do family members
resemble each other?
12. What is a eucrite and where does it come from? How do we know?
13. What is a Trojan asteroid, and where are they found?
14. What is a centaur, and where are they found?
15. What is a NEO/NEA and why are we interested in them?
16. Name one asteroid visited by a spacecraft, and say what we found
there.
Dr Conor Nixon Fall 2006