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 18 orbits of Mars th century noticed the large gap between the and Jupiter , and guessed that there might be an unseen planet remaining to be discovered.

• On January 1 st 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.

Dr Conor Nixon Fall 2006

Picture credit: NASA GSFC

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

3x10

21

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 More Equal

small

large

ones? ones? 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:

N

 1

D

3 • In fact we find that:

N

 1

D

2 .

3 • 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 19 th 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 How?

of an asteroid! • 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.

Dr Conor Nixon Fall 2006 Figure: from Clark et al 2001

ASTR 330: The Solar System

Orbits and Collisions

• Main-belt asteroids orbit the Sun between 2.2

corresponding periods of 3.3

to 6 years.

and 3.3

AU, with • 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 large effect : think of a pushing a child’s swing.

can add up to a • 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?

Dr Conor Nixon Fall 2006 Figure: Nanjing University Astronomy

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 433 Eros 15 Eunomia 52 Europa 951 Gaspra 10 Hygiea 243 Ida 704 Interamnia 253 Mathilde 2 Pallas 16 Psyche 87 Sylvia 4 Vesta

Radius Distance*

Albedo

(km)

466 168

(10^6km)

413.9

475.4

0.1

0.05

17.5 x 6.5

136 156 17x10 215 58x23 167 28.5 x 25 261 132 136 262.5

218 395.5

463.3

330 470.3

428 458.1

396 414.5

437.1

521.5

353.4

?

0.19

0.06

0.2

0.08

?

0.06

0.03

0.14

0.1

0.04

0.38

Discoverer

G. Piazzi R. Dugan G. Witt, A. Charlois De Gasparis Goldschmidt Neujmin De Gasparis J. Palisa V. Cerulli J. Palisa H. Olbers De Gasparis N. Pogson H. Olbers

Date

1801 1903 1893 1851 1858 1916 1849 1884 1910 1885 1802 1852 1866 1807

Dr Conor Nixon Fall 2006 Table: Calvin J Hamilton, Solarviews.com

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 Asteroid

Density Examples

1 Ceres 2 Pallas 4 Vesta 16 Psyche 20 Massalia 45 Eugenia 121 Hermione 243 Ida 253 Mathilde 433 Eros

Density gm cm -3

2.05 ± 0.05

4.2 ± 0.3

4.3 ± 0.3

1.8 ± 0.6

2.7 ± 1.1

1.2 (+0.6,-0.3) 1.8 ± 0.4

2.7 ± 0.4

* 1.3 ± 0.2

2.67 ± 0.03

Volume Reference Merline et al. 1996 Drummond & Cocke 1988 Thomas et al. 1997 Viateau 1999 Bange 1998 Merline, et al. 1999 Viateau 1999 Petit et al. 1997 Veverka et al. 1997 Yeomans, et al. 2000 • Ceres, Pallas, Vesta, Ida, Hermione, Eros are probably 0 35% 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 Table: J Hilton, USNO

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 of 3.0 Gyr : what does this tell us about Vesta?

gas retention age

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.

Dr Conor Nixon Fall 2006 Figure: Nanjing University Astronomy

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.

Dr Conor Nixon Fall 2006 Figure: Nanjing University Astronomy

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 main belt.

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 Earth crossing 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… Dust, and small rocks A Car

Continually Twice a month

A blue whale The Titanic Half mile One mile Three miles

Every few centuries Every few hundred centuries A few per million years Every million years Every ten million years

Potentially Causing…

Shooting stars Explosions high in the atmosphere with the force of a small atomic bomb A powerful shockwave traveling 100 miles A tsunami, if it hit an ocean A regional calamity A world-wide calamity 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!

Dr Conor Nixon Fall 2006 Sources: US House of Rep, Hearing 10/03/02

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 • Ida in 1991, an S-type in the Flora family, 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 high resolution 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.

Dr Conor Nixon Fall 2006 Image: NASA/USGS

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

• FLYBY MOVIE • 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 S types was stony, including Fe, Mg, Si and O composition.

Dr Conor Nixon Fall 2006 Image: JHU/APL/NASA

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.

Dr Conor Nixon Fall 2006 Image: JHU/APL/NASA

ASTR 330: The Solar System

Satellites Of Mars

• In the 1700s, astronomers knew that Earth had one moon had 4 , so Mars should have 2 , right?

and Jupiter • In this case, numerology proved correct and Phobos Deimos (Fear) and (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. represent a window of opportunity for Mars.

Phobos and Deimos

Dr Conor Nixon Fall 2006

ASTR 330: The Solar System

Asteroids or Moons?

Phobos Ida Gaspra Deimos

Dr Conor Nixon Spring 2004 Picture: Bill Arnett, LPL

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/cm 3 . 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 a violent past !

are remnants of

Dr Conor Nixon Fall 2006 Image: NASA/JPL/Malin SSS

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