Solar System Debris

Solar System Debris © Sierra College Astronomy Department

Solar System DebrisSolar System Debris • Apart from the Sun (a large object) and the planets and larger moons (medium-sized objects), most of the other objects in the solar system can be classified as Solar System debris – a collection of ice and rock fragments. • Solar system debris comes in a number of forms, including asteroids, meteoroids, comets, dust, and Kuiper Belt Objects or Trans-Neptunian Objects). © Sierra College Astronomy Department

Solar System DebrisAsteroids • History: Bode’s Law and the “missing planet” • There are currently over 363,000 identified (and named) asteroids also known as minor planets. • Ceres, at 1,000 km (600 mi) in diameter, is the largest asteroid and makes up 30% of the mass of all asteroids. It large enough to be round and is therefore considered to be a dwarf planet (see later). • Pallas and Vesta have diameters greater than 500 km. • About 23 more asteroids have diameters between 200 and 500 km. • About 100 are larger than 100 km an all the rest are under 100 km in diameter. • There are probably more than a million asteroids with a diameter greater than 1 km. • If you put all the asteroids together they would produce an object barely over half the size of the Moon. © Sierra College Astronomy Department

Solar System DebrisAsteroids The Orbits of Asteroids • The asteroids revolve around the Sun in a counterclockwise direction like the planets. • Most asteroids orbit in or near the plane of the ecliptic. • Most asteroids orbit the Sun at distances from 2.2 to 3.3 AU (between Mars and Jupiter) in what is called the asteroid belt. © Sierra College Astronomy Department

Solar System DebrisAsteroids • Apollo asteroids are some 50 asteroids with diameters larger than 1 km that have eccentric orbits that cross the Earth’s orbit (e.g. Eros). • Asteroids are not evenly distributed across the asteroid belt. • At certain distances - 2.5 and 3.28 AU - gaps appear and are related, respectively, to 1/3 and 1/2 of Jupiter’s orbital period (resonances). • These Kirkwood gaps are due to synchronous tugs (orbital resonances) from Jupiter. © Sierra College Astronomy Department

Solar System DebrisAsteroids The Origin of the Asteroids • Astronomers originally thought the asteroids were due to an exploded planet, but there is no known mechanism for making a planet explode. • Most likely the asteroids are primordial material that never formed into a planet because of Jupiter’s gravitational influence. © Sierra College Astronomy Department

Solar System DebrisMeteors • Meteoroid is an interplanetary chunk of matter smaller than an asteroid. • Meteor is the phenomenon of a streak in the sky caused by the burning of a rock or dust particle as it falls into our atmosphere. • Meteorite is an interplanetary chunk of matter after it has hit a planet or moon. © Sierra College Astronomy Department

Solar System DebrisMeteors Meteors • A meteoroid’s typical speed is 50 km/s, so when it hits the Earth’s atmosphere, it heats up and begins to vaporize. • What actually lands on the Earth? • Micrometer sized objects float down to the Earth • millimeter-sized particles burn up in mesosphere (shooting star) • Centimeter-sized particles burn up as a fireball (rare) • Meter-sized particle strike ground (very rare) • It is estimated that 1,000 tons of meteoritic material hit the Earth every day. © Sierra College Astronomy Department

Solar System DebrisMeteors Meteoroids • It is estimated that only 1 in 1 million meteoroids that hit the atmosphere survives to reach the surface. • Unlike most asteroids, meteoroids may orbit the Sun in any orientation. • It is thought that many small meteoroids are debris from asteroid collisions. • Many other meteors come from material evaporated from a comet’s nucleus. © Sierra College Astronomy Department

Solar System DebrisMeteors Meteor Showers • Meteor shower is the phenomenon of a large group of meteors seeming to come from a particular area of the celestial sphere. The Earth actually passes through a swarm of small meteoroids. • Radiant of a meteor shower is the point in the sky from which the meteors of a shower appear to radiate. • Most of the major meteor showers are associated with comets. • Some showers change in intensity from year to year because the swarm of particles that cause the shower clump together in one region of the comet’s orbit. © Sierra College Astronomy Department

Solar System DebrisMeteors Types of Meteorites • In every case that someone has been able to track or film a meteor as it fell to the ground, the meteors have been discovered to originate from the asteroid belt • There are two basic types of meteorites: • Primitive: simple mixtures of rock and metal, sometimes also containing carbon compounds and small amounts of water • Processed: these appear to have undergone differentiation and have a core/mantle/crust structure. Some are made mostly of iron, suggesting they came from a core of a shattered asteroid. These are generally younger than the primitive meteorites. • Both types are informative about the early history of the solar system. • Some meteorites have come from the Moon and Mars. © Sierra College Astronomy Department

Solar System DebrisComets • Edmund Halley, a friend of Newton, used Newton’s methods, his own observations, and prior comet descriptions to calculate orbits for a number of comets. • He correctly surmised that these prior comets were in fact the same comet. He correctly predicted the next return of the comet that was then named in his honor. • Comet Halley is probably the most famous periodic comet. © Sierra College Astronomy Department

Solar System DebrisComets • The planes of revolution of comets are not limited to the ecliptic but are randomly oriented. • Consequently, comets sweep past the Sun from all directions. • Periods of revolution vary from a few years to millions of years. © Sierra College Astronomy Department

Solar System DebrisComet Composition • Head: • nucleus: relatively solid and stable, mostly ice and gas with a small amount of dust and other solid • coma: dense cloud of water, carbon dioxide and other neutral gases sublimed off of the nucleus • hydrogen cloud: huge (millions of km in diameter) but very sparse envelope of neutral hydrogen • Tail: • ion tail: as much as 1 AU long composed of plasma and laced with rays and streamers caused by interactions with the solar wind. • dust tail: up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases this is the most prominent part of a comet to the unaided eye and is caused by interaction from solar radiation pressure. © Sierra College Astronomy Department

Solar System DebrisComets • Fred Whipple proposed in 1950 that the nucleus of a comet is essentially a dirty snowball, as opposed to a “traveling gravel bank”. • The composition of the nucleus is water ice, frozen carbon dioxide, other ices, and small solid grains. • The nucleus also has a significant fraction of organic material. © Sierra College Astronomy Department

Solar System DebrisComets • Giotto, a European spacecraft, revealed that Halley’s coma is billions of times less dense than the atmosphere of the Earth at sea level. • The nucleus of Comet Hale-Bopp is about 15 km across and spins once every 12 hours (very similar to what Giotto found for Halley’s comet). • As Hale-Bopp’s nucleus spun, material was ejected from it in geysers and spiraled away from it. © Sierra College Astronomy Department

Solar System DebrisRecent Comets • Becomes bright/prominent when close to the Sun • Dozens of faint ones seen per year • A bright one seen once in ~ 10 years • A bright one seen once in ~ 10 years • West (1976) • Halley (1986) • Hale-Bopp (1997), Hyakutake (1996) • Semi-bright NEAT, LINEAR and Bradfield (2004) • Semi-bright Machholz (2004-05) • The Great Comet McNaught (January 2007) • Semi-bright Comet PanStarrs (Spring 2013) • Comet ISON (Fall/Winter 2013) © Sierra College Astronomy Department

Solar System Debris“Deep Impact” to Comet Tempel 1 • Objective: Determine the structure and composition of comet Tempel 1 • Significance of results • Comets formed in outer regions of the solar system • Early planetary material still frozen inside • We can learn much about the formation of the solar system by analyzing the composition of “pristine” comets • Tempel 1 is a “pristine” or “well-preserved” comet © Sierra College Astronomy Department

Solar System Debris“Deep Impact” Mission Results • Mission Results • July 3, 2005 • 820 lb “Hammer” Impacts Comet 9P/Tempel 1 • Ejected fluffy “powder” like talcum powder • Mostly dust (silicates), steam and carbon dioxide • Other compounds include carbonates, aromatic hydrocarbons • Some ice found on surface (surprise!) • Preliminary results indicate nucleus composition more like a “fluff ball” than an “ice cube” • Scientific analysis ongoing © Sierra College Astronomy Department

Solar System DebrisComets Comet Tails • A comet’s tail always points away from the Sun (and thus does not always follow the comet’s head). • After passing the Sun, a comet’s tail actually leads the head. • The comet’s “straight” (ion) tail consists of charged molecules (ions) which are dynamically influenced by the Solar Wind. • The curved diffuse (gas) tail is caused by dust in the coma being pushed away by solar radiation pressure. © Sierra College Astronomy Department

Solar System DebrisComet Origins • Öort cloud: aphelia of billions of comets lie about 10,000 – 100,000 AU from the Sun, • Proposed by Jan Öortin 1950 • Icy chunks of material ejected by Jovian planets in early solar system formation • Today, a few are Perturbed by nearby stars and brought in • Kuiper belt: comets which lie just outside Neptune • Includes Trans-Neptunian objects/Plutinos • Pluto may be king of these objects © Sierra College Astronomy Department

Solar System DebrisComet Origins What is the fate of a comet? • It can impact a planet or Sun • Like Shoemaker-Levy 9 into Jupiter • It can get ejected out of the solar system • It can get put into a shorter orbit • Eventually “burns-out” from repeated close encounters with the solar wind near perihelion which cause evaporation of nucleus and/or volatile material © Sierra College Astronomy Department

Solar System DebrisThe Discovery of Pluto • Clyde Tombaugh used a blink comparator to compare two photos of the sky taken a few days apart. • In 1989 Pluto was as close to the Earth as it had been for 248 years. (From 1979 to 1999 Pluto was inside Neptune’s orbit.) • Pluto’s average distance from the Sun is 40 AU, but its eccentric orbit causes it to vary in distance from 30 AU to 50 AU. © Sierra College Astronomy Department

Solar System DebrisPluto and Charon • Stellar occultations indicate that Pluto has a thin nitrogen, carbon monoxide and methane atmosphere. At aphelion, it is probably too cold for Pluto to maintain an atmosphere. • Pluto’s atmosphere limits an accurate determination of its size, which probably ranges from 2,362 to 2,412 km. • In 1978, J. Christy discovered that Pluto has a moon, now named Charon (KAIR en or SHAHR en). © Sierra College Astronomy Department

Solar System DebrisPluto and Charon • New Horizons Mission to Pluto • Launched: January, 2006 • Jupiter Flyby: 28 Feb 2007 • First Flyby Opportunity: July 2015 • Other Targets: Centaurs and KBOs • The two newest moons of Pluto (Nix and Hydra) were named not only because of there mythological connection to the underworld, but because the their initials are the same as those of the spacecraft mission to Pluto. © Sierra College Astronomy Department

Solar System DebrisPluto and Charon • Charon’s diameter (1,200 km) is about half that of Pluto • Pluto’s mass is about 12 times that of Charon but only 1/5 that of the moon • Charon density 1.2-1.3 g/cm3; Pluto 1.8-2.1 g/cm3 • Charon is less than 9 Pluto diameters away from Pluto (compare: Moon ¼ diameter of Earth, and 30 Earth diameters away from Earth) • Charon orbit is tilted 119o to Pluto’s orbit around the Sun (i.e. it is in the equatorial plane of Pluto) • Despite their small size, they are tidally looked in a 1:1 resonance with Charon orbiting Pluto every 6.4 days, the same as Pluto’s rotation • But wait there’s more! © Sierra College Astronomy Department

Solar System DebrisOrigins of Pluto A Former Moon of Neptune? • Because Pluto is small and has an eccentric orbit, some theorize that it a former moon of Neptune that was somehow ejected. • The discovery of Charon (and now 2 other moons) made it seem less likely that Pluto was once Neptune’s moon. • Also, the large difference in density between Charon and Pluto points to Charon’s capture by Pluto. © Sierra College Astronomy Department

Solar System DebrisOrigins of Pluto Planet, Asteroid, or ??? • Pluto doesn’t fit the asteroid classification since its density and composition is more consistent with a satellite of Jovian planet • In the 1990s Pluto was proposed to be the largest member of the “plutino” class objects found in the Kuiper belt. © Sierra College Astronomy Department

Solar System DebrisThe Kuiper Belt Kuiper Belt ??? • In addition to the Asteroid Belt, the Solar System appears to have a second belt, now called the Kuiper belt: • Support for this comes from the detection of about 600 small, presumably icy, bodies orbiting near and beyond Pluto (first object discovered was 1992QB1). • Extent of belt is unknown, but statistical analysis indicates that the Kuiper belt may have an total mass far greater than that found in the asteroid belt. • Objects in the belt are sometimes referred to as KBOs or plutinos, and Quaoar (found in 2002 with a diameter of 1250 km), Sedna(discovered in 2003 with a diameter of 1600 km), and2004DWare among its largest members. © Sierra College Astronomy Department

Solar System DebrisA New Planet? • July 2005: Mike Brown and associates announced the discovery of 2003UB313 as the “tenth planet” • 2003UB313 is just a bit larger than Pluto • How can they know that? • Discovered some 97 AU from the Sun, near its aphelion • It’s the furthest object detected in our solar system • Huge 557-year eccentric orbit takes it within 38 AU of Sun • It was discovered to have a moon! But what to name these objects …. © Sierra College Astronomy Department

Solar System DebrisThe Kuiper Belt Associations • Triton’s orbit is “backwards” and is highly tilted with respect to Neptune’s equator – Triton is perhaps a captured planetesimal from the Kuiper belt • Short-period comets are now believed to be icy nuclei from the Kuiper belt • The centaurs, of which Chiron is a well-known example, appear to originate from the Kuiper belt. • 2003 UB313 (Eris) is a KBO larger than Pluto, in an orbit that crosses that of Pluto, and has a moon (Gabrielle?) • Should Pluto still be considered a planet or a member of the Kuiper belt? © Sierra College Astronomy Department

Solar System DebrisGood-bye Planet Pluto • Recently, Pluto was “demoted” from the ranks of planethood at the International Astronomical Union Meeting in August, 2006, to the status of a dwarf planet (Next Meeting Summer 2009) • A planet is officially defined as an object • that is in orbit about the sun. • has sufficient mass for its self gravity to overcome rigid-body forces so that it assumes [a nearly round] shape. • has cleared the neighborhood around its orbit. • A dwarf planet is officially defined as an object • that meets the first two criteria of a planet, but not the third. • Pluto, Eris, and Ceres are now dwarf planets – others will most certainly follow. © Sierra College Astronomy Department

Solar System DebrisCollisions What happens when something strikes Earth? • We have evidence of things hitting the Earth • Craters, meteorites • As a general rule the craters made by meteors are 10 times bigger than the impactor • The most prominent impact crater on Earth is Meteor Crater near Winslow, Arizona. • There may have been impacts which affected life significantly • Chicxulub meteor which landed off the Yucatán Peninsula may have wiped out the dinosaurs © Sierra College Astronomy Department

Solar System DebrisCollisions Have we ever witnessed a major impact? • We have not witnessed a major impact on a solid body, but in 1994 Comet Shoemaker-Levy 9 (SL9) impacted into Jupiter. • This event had 2 effects: • It was one of the best examples of international cooperation • It made the public awareness of current nature of giant collisions in our solar system © Sierra College Astronomy Department

Solar System DebrisCollisions Did an impact kill the dinosaurs? • We have identified more than 150 impact craters on the Earth • One impact, off the coast of the Yucatan Peninsula, may have wiped out the dinosaurs 65 million years ago • Clues such as the deposit of iridium sediment (coming from an asteroid) at the right geological depth in the soil helps verify such a claim • This meteor impact lead to a mass extinctionwhere 99% (and 75% of the species) were extinguished © Sierra College Astronomy Department

Solar System DebrisCollisions Is the impact threat real? • There are certainly many objects that could hit us: we have detected over 800 asteroids over 1 km in size which pass near the Earth’s orbit. • The threat is real, but the chances of something big hitting us in our lifetime is small • Nevertheless, we were hit by a comet or asteroid in the region of Tunguska, Siberia in 1908 resulting in a tremendous explosion. A hit like this over a major city would be devastating. • If a big asteroid were headed for us, could we prevent the impact? © Sierra College Astronomy Department

Solar System Debris