1445 Introductory Astronomy 9.ppt

1、1,1445 Introductory Astronomy I,Chapter 9 Asteroids, Comets i.e. the same age as the solar system.,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 o

2、f 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,

3、 such as iron and nickel, graphite, and “nanodiamonds”.,15,16,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 met

4、eorites 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

5、 (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 whi

6、ch it is locked in a 3:2 orbital resonance.,21,Plutos Orbit,There is an angle of 17o between the orbits of Pluto and Neptune.,22,Pluto and Neptunes Orbits,23,About Pluto and Charon 1,Plutos orbit is significantly elliptical, and very tilted with respect to the ecliptic. Recent studies showed that de

7、spite an almost non-existent atmosphere, Pluto has wind, seasons, and appears to have gone through a global warming. Plutos satellite Charon has 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

8、 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 image

9、s were the first to show two tiny moons orbiting Pluto and Charon.,“New Horizons” Mission,In April 2010, NASAs 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

10、 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.,26,27,Dwarf Planets,In a conference, held in Prague in 2006, the smaller planet-like objects, Pluto, Eris and Ceres were termed Dwarf Planets. The mo

11、st massive dwarf planet is the Kuiper belt object Eris, although 2010 observations indicate that it is about the same size as Pluto. Eriss 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

12、Sun, when it will approach Neptunes 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. Haumea has the elongated shape of a rugby football, possibly due to its rapid rotation, with a period of about 4 ho

13、urs.,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 2

14、006, 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. Ab

15、out 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,

16、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 wo

17、rk 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 pl

18、anets 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

19、,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 familys 2 inch telescope. She received a medal from the King of Denmark for her discovery, which was known as “Miss Mit

20、chells 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 womens rights. Mitchell Crater on the Moon is named after her.,34,St

21、ructure of a Comet,35,Comets 1,A comet 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 d

22、uring 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 sur

23、rounding 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) tail and a dust tail (each about 100 million km long). Because of p

24、ressure 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 Suns radiation, re

25、gardless of the direction of the comets 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 comets motion.,Comets 3,In August 2009, extraterrestrial amino acid glycene was reported in material obtained fr

26、om a comet. The material was collected in 2004, when the spacecraft Stardust passed through 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 prot

27、eins. 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.,37,38,Comet Orbit and Tails,39,Structure of a Com

28、et,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,Comet Wild 2 Nucleus 1,46,Comet Wild 2 Nucleus 2,47,Comet Flies Into the Sun,48,Comet,49,Kuiper Belt,The solar system appears

29、 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

30、 to very red. The dwarf planets, Pluto, Eris, Makemake and Haumea are 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 Sedna and Quaoar.,Largest Known Kuiper Belt

31、Objects,50,2010 measurements indicate that Eris, although more massive, may be slightly smaller than Pluto.,51,Oort Cloud,The Oort cloud is a spherical distribution of icy bodies orbiting the Sun, which extends from about 10,000 AU to about 100,000 AU. 100,000 AU is about 9.3 trillion miles (or 1.6

32、ly), which is almost 40% of the way to the nearest star, Proxima Centauri. The periods of most comets from the Oort cloud may be millions of years, compared to about 100 years for the Kuiper belt comets. The Oort cloud is estimated to contain trillions of comets, only very few of which get gravitati

33、onal pushes into the inner solar system, where there become short-period comets with altered orbits, such as Comet Halley.,52,Kuiper Belt,Pluto is the second largest known object in the Kuiper belt.,53,The Oort Cloud,54,Oort Cloud and Kuiper Belt,55,Changing a Comets Orbit,The Heliosphere 1,The heli

34、osphere is the region in which the solar wind exists, which extends from the Sun to the heliospause.,Termination Shock and Voyager Spacecraft 1,The speed of the solar wind in the vicinity of the Earth is about a million mph, but slows down as it moves away from the Sun. The first boundary of the hel

35、iosphere occurs when the solar wind drops from supersonic to subsonic speeds, producing a shock wave, known as the termination shock. Voyager 1, launched in 1977, traveling north from the Sun, passed through the termination shock in 2004, at a distance of about 94 AU (almost 9 billion miles) from th

36、e Sun, as indicated by changes in its magnetic readings. Voyager 2, also launched in 1977, traveling south, reached the termination shock in 2006, at a distance of about 76 AU (7 billion miles) from the Sun. Voyager 2 made about five shock crossings in a couple of days. Voyager 2, unlike Voyager 1,

37、has the advantage of having a plasma measuring instrument, which is still functioning.,Termination Shock and Voyager Spacecraft 3,The Heliosheath, Heliopause and Bow Shock,The Heliosheath is the region of the heliosphere beyond the termination shock. Here, the solar wind interacts with the interstel

38、lar medium. The Heliosheath bulges out in the direction opposite the Sun. Its closest distance to the Sun is roughly 80-100 AU. The heliopause is the solar winds final boundary, at which it is finally stopped by winds in the interstellar medium. The bow shock, just beyond the heliopause, at about 23

39、0 AU from the Sun (22 billion miles), is so-called because it resembles the wake left by a ships prow (or front).,The IBEX Mission,Launched in 2008, the Interstellar Boundary Explorer (IBEX) is finding results which do not fit the current theoretical models. IBEX measures energetic neutral atoms (ENAs) which are created, when the solar wind and the interstellar medium interact in the heliosheath, and