1. ASTR 330: The Solar System Lecture 9: Asteroids! Dr Conor Nixon Fall 2006Image © Lucasfilm

2. ASTR 330: The Solar System Hypothesis And Discovery Dr Conor Nixon Fall 2006 Astronomers in the 18 th 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 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.

3. ASTR 330: The Solar System Accelerating Rate Of Discovery… Dr Conor Nixon Fall 2006 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?

4. ASTR 330: The Solar System Main Belt Dr Conor Nixon Fall 2006 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

5. ASTR 330: The Solar System By the Numbers Dr Conor Nixon Fall 2006 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.

6. ASTR 330: The Solar System Expected Population Dr Conor Nixon Fall 2006 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?

7. 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: In fact we find that: Therefore proportionally more of the mass in the larger objects. ASTR 330: The Solar System Asteroid Size Distribution Dr Conor Nixon Fall 2006 Picture: Tom Quinn and Zeljko Ivezic, SDSS Collaboration

8. ASTR 330: The Solar System Sizes and Masses Dr Conor Nixon Fall 2006 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).

9. ASTR 330: The Solar System Observing From Earth Dr Conor Nixon Fall 2006 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).

10. ASTR 330: The Solar System Dr Conor Nixon Fall 2006 Picture: David Dunham/IOTA Movie: Rick Baldridge/IOTA

11. ASTR 330: The Solar System Spectroscopy: Size Dr Conor Nixon Fall 2006 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.

12. ASTR 330: The Solar System Spectroscopy: Composition Dr Conor Nixon Fall 2006 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 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).

13. ASTR 330: The Solar System Orbits and Collisions Dr Conor Nixon Fall 2006 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.

14. ASTR 330: The Solar System Orbits: Gaps Dr Conor Nixon Fall 2006 In the main belt, orbital distances are not distributed evenly. Picture: JPL/SSD Alan B. Chamberlain

15. ASTR 330: The Solar System Orbits and Resonances Dr Conor Nixon Fall 2006 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.

16. ASTR 330: The Solar System More Resonances Dr Conor Nixon Fall 2006 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

17. ASTR 330: The Solar System Family Values Dr Conor Nixon Fall 2006 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.

18. ASTR 330: The Solar System Asteroid Albedo Classes Dr Conor Nixon Fall 2006 Many asteroids have albedos in one of two ranges: 3-5% or 15-25%. Table: Calvin J Hamilton, Solarviews.com

19. ASTR 330: The Solar System Compositional Classes Dr Conor Nixon Fall 2006 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?

20. ASTR 330: The Solar System Albedo vs Orbital Distance Dr Conor Nixon Fall 2006 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

21. ASTR 330: The Solar System Asteroid Densities: Useful or Not? Dr Conor Nixon Fall 2006 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.

22. ASTR 330: The Solar System Density Examples Dr Conor Nixon Fall 2006 Ceres, Pallas, Vesta, Ida, Hermione, Eros are probably 0- 35% porous, somewhat fractured, but still coherent. Table: J Hilton, USNO On the other hand, Mathilde, Eugenia and Psyche are >35% porous; probably loosely-bound ‘rubble piles’.

23. ASTR 330: The Solar System Compositions and Positions* Dr Conor Nixon Fall 2006 Most asteroids do seem to be composed of primitive material and we also see a variation in composition with distance from the Sun. Can we then use this distribution to infer something about the initial solar nebula? It is tempting to say that because the C-type asteroids are near the outer edge (as we’d expect) and the S-types at the inner edge of the main belt, that these accurately represent local conditions. But, there may be complications: asteroids may have changed positions (solar distance over time). C-type asteroids may have been ‘herded’ in from a formation region further out. S-types may have formed further in and been gravitationally scattered outward to present positions. We do not yet know to what extent positions changed over time.

24. ASTR 330: The Solar System Vesta Dr Conor Nixon Fall 2006 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. broken upAre 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…

25. ASTR 330: The Solar System Origins Of Eucrites Dr Conor Nixon Fall 2006 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?

26. ASTR 330: The Solar System Outside The Main Belt Dr Conor Nixon Fall 2006 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

27. ASTR 330: The Solar System Trojan Asteroids Dr Conor Nixon Fall 2006 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. The L4 and L5 points of Jupiter are occupied by hundreds of asteroids. Figure: Nanjing University Astronomy

28. ASTR 330: The Solar System More On Trojans… Dr Conor Nixon Fall 2006 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.

29. ASTR 330: The Solar System Centaurs Dr Conor Nixon Fall 2006 Centaurs are another class of objects which followed a mythological naming convention, taking after Chiron, the second one discovered. Figure: CAPS, Kent Univ Canterbury

30. ASTR 330: The Solar System Centaurs contd. Dr Conor Nixon Fall 2006 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.

31. ASTR 330: The Solar System Near-Earth Objects Dr Conor Nixon Fall 2006 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.

32. ASTR 330: The Solar System Threat Of NEOs Dr Conor Nixon Fall 2006 Even a small NEO has a lot of energy, and can cause a lot of damage… Sources: NASA, Eric Asphaug, Univ of CA, Santa Cruz, and Wall Street Journal, 9-20-2002

33. ASTR 330: The Solar System Spaceguard Dr Conor Nixon Fall 2006 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. Sources: US House of Rep, Hearing 10/03/02 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!

34. ASTR 330: The Solar System Meeting Asteroids Up-Close Dr Conor Nixon Fall 2006 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.

35. ASTR 330: The Solar System 951 Gaspra Dr Conor Nixon Fall 2006 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. Image: NASA/USGS The highly irregular shape indicates that Gaspra suffered a massive collision(s) in the past which nearly destroyed it. Gaspra moviemovie by A. Tayfun Oner.

36. ASTR 330: The Solar System 243 Ida and Dactyl Dr Conor Nixon Fall 2006 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

37. ASTR 330: The Solar System NEAR Shoemaker Dr Conor Nixon Fall 2006 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.

38. ASTR 330: The Solar System 253 Mathilde Dr Conor Nixon Fall 2006 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 FLYBY MOVIE

39. ASTR 330: The Solar System 3 Asteroid Close-Up Dr Conor Nixon Spring 2004 A composite of images from NEAR and Galileo

40. ASTR 330: The Solar System 433 Eros Dr Conor Nixon Fall 2006 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. Image: JHU/APL/NASA FLYBY MOVIE FLYBY MOVIE

41. ASTR 330: The Solar System Eros Surface Dr Conor Nixon Fall 2006 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. Image: JHU/APL/NASA 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.

42. ASTR 330: The Solar System Satellites Of Mars Dr Conor Nixon Fall 2006 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.

43. ASTR 330: The Solar System Asteroids or Moons? Dr Conor Nixon Spring 2004 Ida Deimos Gaspra Phobos Picture: Bill Arnett, LPL

44. ASTR 330: The Solar System Phobos and Deimos Dr Conor Nixon Fall 2006 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 are remnants of a violent past! Image: NASA/JPL/Malin SSS

45. ASTR 330: The Solar System Quiz-Summary Dr Conor Nixon Fall 2006 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.

46. ASTR 330: The Solar System Quiz-Summary Dr Conor Nixon Fall 2006 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.