1445 Introductory Astronomy I

1. 1445 Introductory Astronomy I Chapter 9 Asteroids, Comets & Objects Beyond Neptune R. S. Rubins Fall, 2010

2. Asteroids 1 • Asteroids are rocky objects (planetesimals) orbiting the Sun, typically over about 100 meters across. They are thought to be the building blocks of planets, which were formed about 4.6 billion years ago at the same time as the Sun.

3. Asteroids 2 • There are probably billions of asteroids less than 1 km (1000 m) across, millions larger than 1 km, with about 200 larger than 100 km (60 miles) across. • The main asteroid beltlies between Mars and Jupiter, at 2.2 AU and 3.3 AU from the Sun. • Asteroids have distinctly elliptical orbits about the Sun, moving in the same sense as the planets, and inclined to the ecliptic by as much as 30o. • As of 2005, more than 3300 asteroids, with orbits intersecting the Earth’s orbit, have been found. • Together, all the material in the asteroid belt would form a single planet of diameter of about half that of the Moon. • Within the asteroid belt, the average distance between asteroids is about ten million km, so that the possibility of a collision with a passing spacecraft is negligible.

4. Asteroid Orbits

5. Asteroids 3 Asteroids are thought to be the building blocks of planets, which were formed with the Sun about 4.6 billion years ago. No major planet was formed in the asteroid belt because of Jupiter’s immense gravity, which also created gaps in the asteroid belt, and forced the Trojan asteroidsto orbit in two clusters in roughly the same orbit as Jupiter. The largest asteroid, Ceres , now classified as a dwarf planet, is almost spherical, with a diameter of 930 km (580 miles), making it a little smaller than Pluto’s satellite, Charon. Smaller asteroids, Pallas and Vesta, have diameters of roughly 600 km and 530 km (330 miles) respectively.

6. The Asteroid Belt The Trojan asteroids lie in the range 5.0 to 5.5 AU, level with Jupiter’s orbital path.

7. Dawn Asteroid Mission 2007-2015 1 • The Dawn asteroid mission was launched in July 2007, and is scheduled to use Mars for a slingshot in 2009, to reach the asteroid Vesta in 2011, and the dwarf planet Ceres in 2015. • Vesta, the 4th largest asteroid, was hot and volcanic early in its life, so that its original surface has probably been replaced by ancient lava flows to give it a smooth surface. • Vesta’s surface is dominated by a giant crater at its south pole, about 285 miles across and 8 miles deep. • Ceres is the more interesting object of study, since it is should be close to its original state of 4.6 billion years ago. • Ceres is thought to contain a layer of water ice (and possibly liquid water) 40 to 80 miles thick, which has kept its surface cool, and eliminated volcanic activity.

8. Dawn Asteroid Mission 2007-2015 2 • Unlike the chemical rockets of past use, which provide bursts of rapid acceleration, Dawn uses the gentle, continuous and energy-efficient propulsion, provided by xenon ions in an electric field, known as ion drive propulsion. • Taking about 4 days to reach 60 mph, and 1 year to reach 5500 mph, it will ultimately escape into space at 89,000 mph. • It will test its instruments during the Mars flyby in 2009, and orbit Vestafor 9 months, beginning in 2011. • On hopefully reaching Ceres in 2015, it will orbit the dwarf planet for at least 5 months. • Dawn carries an optical camera, gamma ray and neutron detectors, and a mapping spectrometer.

9. Ceres, the Largest Asteroid Ceres and the Moon have diameters of 930 km and 3480 km respectively.

10. Asteroid Mathilde

11. Ida (53 km across) and its Tiny Moon, Dactyl Several asteroids have been found to have orbiting moons.

12. Five Reasons to Care about Asteroids 1 • 1. They should tell us about the origins of the Solar System Asteroids are the building blocks of the planets, so that the water ice found recently on 24 Themis (width 200 km), suggests that asteroids could have provided the early Earth with water. • 2. They will help us understand about the origin of life Asteroids such as 2 Pallas, 10 Hygiea and 24 Themis, appear to contain organic compounds, indicating that early asteroids falling on the Earth could have deposited some of the building blocks of life. • 3. Nearby asteroids could in the future be mined for metals Besides being sources of valuable metals, the study of these objects complements studies of the planets.

13. Five Reasons to Care about Asteroids 2 • Some asteroids will be threats to collide with the Earth In January 2010, asteroid 2010 AL30 (just 11 m wide) passed within 130,000 km (80,000 miles) of the Earth. More seriously, Apophis (2 football fields across) should pass only 18,500 miles from the Earth on April 13, 2036. • President Obama has announced the goal of visiting an asteroid by 2025 • It has been suggested that astronauts might attempt to deflect an asteroid from its orbit, which, if successful, would be a landmark in human history.

14. Meteoroids, Meteors and Meteorites • A meteoroid is a small asteroid, which becomes a meteor or shooting staras it penetrates the Earth’s atmosphere. • While there is no precise definition for the size of an meteoroid, objects smaller than about 100 m across are typically termed meteoroids . • Most vaporize completely before striking the ground. • A meteorite is a portion of a meteor reaching the ground intact, possibly producing an impact crater. • A meteor shower occurs when the Earth moves through the debris left behind by a comet. • Carbon dating shows that the oldest meteorites are about 4½ billion years old; i.e. the same age as the solar system.

15. 80-Ton Asteroid Hits the Nubian Desert • In October 2008, an asteroid (which could also be called a meteorite) about 12 ft across with a mass of about 80,000 kg (80 metric tons) exploded at an altitude of about 37 km, the parts scattering in the Nubian desert of Sudan. • By a lucky accident, asteroid 2008 TC3 became the first such object to be observed before impact. • Fifteen fragments were recovered were later recovered over a region of length 29 km. • The black jagged rocks collected contained metals, such as iron and nickel, graphite, and “nanodiamonds”.

16. Stony Meteorite Found in Texas Stony meteorites look like ordinary rocks, often covered with a dark crust, caused by the melting of the outer surface during its descent through the atmosphere.

17. Iron Meteorite Found in Australia Iron meteorites contain iron-nickel compounds.

18. Impact Crater Formed by a 12 kg Meteorite

19. Meteor Crater, Arizona • Caused some 50,000 years ago by a meteoroid about 50 m in diameter. • Meteor crater is about 1.2 km across and 200 m deep.

20. Pluto Data • Average distance from Sun: 39.5 AU. • Mass: 0.2% of Earth (0.002 ME). • Diameter: 2370 km (19% of Earth, 49% of Mercury). • Average density: 36% Earth density. • Orbital eccentricity 0.25. • Siderial revolution period: 248 Earth years. • Rotation period: 6.4 Earth days (retrograde). • For 20 years of its 248 year orbit, Pluto is closer to the Sun than Neptune, to which it is locked in a 3:2 orbital resonance.

21. Pluto’s Orbit There is an angle of 17o between the orbits of Pluto and Neptune.

22. Pluto and Neptune’s Orbits

23. About Pluto and Charon 1 • Pluto’s orbit is significantly elliptical, and very tilted with respect to the ecliptic. • Recent studies showed that despite an almost non-existent atmosphere, Pluto has wind, seasons, and appears to have gone through a global warming. • Pluto’s satellite Charonhas a diameter of 1190 km (just half that of Pluto), and its distance from Pluto is less than 5% of the Moon-Earth distance, making Pluto/Charon appear to be a double planet. • Pluto and Charon move in a unique form of synchronous rotation, in which each has a face locked to the other body. • Thus, observed from Pluto, Charon, would appear to hover in the sky, and vice-versa.

24. Discovery of Charon, 1978

25. Images of Pluto and Charon 1 These 2006 Hubble images were the first to show two tiny moons orbiting Pluto and Charon.

26. “New Horizons” Mission • In April 2010, NASA’s New Horizons space-probe passed the half-way point of its almost 3 billion mile journey to Pluto. • Launched in 2006, and traveling away at about 36,000 mph, it should reach Pluto in 2015. • In a single flyby, it should make the first detailed observations of Pluto/Charon and the tiny moons Nix and Hydra, before heading into the Kuiper Belt.

27. Dwarf Planets • In a conference, held in Prague in 2006, the smaller planet-like objects, Pluto, Eris and Ceres were termed Dwarf Planets. • The most massive dwarf planet is the Kuiper belt object Eris, although 2010 observations indicate that it is about the same size as Pluto. • Eris’s orbit is very elliptical, and it is currently near its aphelion, about 100 AU from the Sun. • In 280 years, Eris will reach its perihelion at about 38 AU from the Sun, when it will approach Neptune’s distance of 30 AU. • The most recent objects to be identified as dwarf planets are Makemake and Haumea, the latter having been first observed in 2003. • Haumeahas the elongated shape of a rugby football, possibly due to its rapid rotation, with a period of about 4 hours.

28. Dwarf Planet Haumea • Haumea has about 1/3 the mass of Pluto and two moons. • It is a trans-Neptunian object (TNO), with a very elliptical orbit, varying between 35 AU to 50 AU from the Sun.

29. Plutoids, Plutinos and SSSBs • The International Astronomical Union (IAU) adopted other definitions in 2006, which are somewhat simplified below. • Plutoids are dwarf planets with mean orbits around the Sun, further than that of Neptune, so that all the dwarf planets except Ceres are plutoids. • Plutinos are those objects of the Kuiper Belt, which (like Pluto) have a 2:3 orbital resonance with Neptune. • About 25% of the known Kuiper Belt objects, including Charon, are plutinos. • Small solar-system bodies (SSSBs) refer to all objects in the solar system that are not planets, dwarf planets, or moons. • Charon is an example of an SSSB.

30. Eris and its Moon, Dysnomia Artists rendition of Eris and its moon, which are roughly at 100 AU (9 billion miles) from the Sun.

31. New Horizons Pluto Mission This mission should reach Pluto-Charon in 2015.

32. What are the Planets? • Historically, the Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn were considered planets circling the Earth. Following Copernicus work in 1543, scientists placed the Sun at the center of our universe, so that the Sun and the Moon were dropped as planets and the Earth included. Uranus was added in 1781, and Neptune in 1846. Pluto was considered a “normal” planet from 1930 to 2006. • An answer to the question,”how does one define planets planets?”, was given in a 2007 Scientific American article. Not only must they be massive enough for gravity to give them an essentially spherical shape, but it must also sweep up or scatter most of their immediate neighbors, at the same time holding smaller bodies (moons) in stable orbits.

33. Maria Mitchell, Astronomer Maria Mitchell (1818-1889) was the first woman elected to the American Academy of Arts and Sciences, after she discovered a new comet in 1847, using her family’s 2 inch telescope. She received a medal from the King of Denmark for her discovery, which was known as “Miss Mitchell’s Comet”. In 1848, she became the first female astronomy prof. in the US, when she joined the faculty of Vassar College. In addition to her scientific work, she was outspoken in her opposition to slavery, and as an advocate of women’s rights. Mitchell Crater on the Moon is named after her.

34. Structure of a Comet

35. Comets 1 • Acomet consists of a solid nucleus, surrounded by a gaseous coma, distinguished by a long dust and ion tails, which are caused by the vaporization of the nucleus when it approaches the Sun. • Comets passing the Sun lose between 1% and 2% of their masses to evaporation during each passage by the Sun, so that they would be expected to make fewer than 100 orbits. • The nucleus (10-20 km across), is a fluffy amalgum of rock, dust and water ice, with a density about 1/5 th that of ice. • The coma (about 100,000 km in diameter) consists of very low pressure gas and dust surrounding the nucleus. • UV light breaks water molecules into hydrogen and oxygen, so that the coma is surrounded by an invisible H2 envelope.

36. Comets 2 • The low pressure gas and dust also form two diaphanous tails: an ion (or plasma) tailand a dust tail(each about 100 million km long). • Because of pressure of the solar wind, which consists of fast-moving charged particles from the Sun, comet tails point away from the Sun. • The less massive plasma tail, which often emits the blue light of carbon monoxide (CO) ions, always points away from the Sun because of the pressure of the Sun’s radiation, regardless of the direction of the comet’s motion. • The heavier dust tail also points away from the Sun, but because of the inertia of the dust particles, bends away from the direction of the comet’s motion.

37. Comets 3 • In August 2009, extraterrestrial amino acid glycenewas reported in material obtained from a comet. • The material was collected in 2004, when the spacecraft Stardust passedthrough the dust and gas tails of the comet Wild 2, and was analyzed later after the samples were parachuted to the ground. • Amino acids, of which glycene is the simplest, when strung together in chains, can form proteins. • The research group at The Goddard Flight Center in Maryland, were able to confirm that the carbon-isotope concentrations in the samples were typical of an extraterrestrial origin, and so could not have been contamination, coming from the Earth.

38. Comet Orbit and Tails

39. Structure of a Comet

40. Two Tails of Comet Mrkos

41. Comet Kohoutek by UV Showing H Cloud

42. Comet Ikeya-Seki, 1965

43. Comet West, 1976

44. Comet Hayakutake, 1996

45. Two Tails of Comet Hale-Bopp

46. Comet Wild 2 Nucleus 1

47. Comet Wild 2 Nucleus 2

48. Comet Flies Into the Sun Comet

49. Kuiper Belt • The solar system appears to contain two comet reservoirs. • The Kuiper Belt, estimated to contain over a hundred million comets, begins near Neptune in the plane of the ecliptic, extending from 30 AU to at least 100 AU from the Sun. • For reasons not yet known, Kuiper Belt objects show a wide range of color, from slightly blue to very red. • The dwarf planets, Pluto, Eris,Makemakeand Haumeaare Kuiper Belt objects. • Eris, which is about 50% larger than Pluto, is further from the Sun, with an elliptical orbit, of period about 650 years. • Other large objects in the Kuiper Belt are SednaandQuaoar.

50. Largest Known Kuiper Belt Objects 2010 measurements indicate that Eris, although more massive, may be slightly smaller than Pluto.