Monday 26 September 2011

Eugene M. Shoemaker, 1928-1997 Although Gene didn't make it to the Moon during his lifetime, at least a small part of him is on the Moon.

(1) Cosmic Car Conker (2) The car Michelle Knapp, of Peekskill, NY, got her 15 minutes of fame when a football-sized rock from space hit her old junker Chevy Malibu while she watched TV. This rock is now known as the Peekskill Meteorite. The Chevy is known as the Peekskill Meteorite Car and has been exhibited around the world!

Hoba meteorite This is the famous Hoba meteorite, located in Namibia. It is about 2.5 x 2.5 x 1 meter in size, and is thought to have fallen to Earth about 80,000 years ago. This is an "iron" meteorite, with an elemental composition of about 84% iron and 16% nickel.

Chondritic meteorite This is a meteorite, a space rock that made it through the Earths atmosphere to land on the ground, here sliced open to reveal its internal makeup. This type of meteorite has small, BB-sized chunks of rock called chondrites. Radioactive dating shows that these are 4.56 billion years old. These are the oldest rock particles known, and date back to the beginning of the solar system.

Widmanstatten pattern Some meteorites are made almost entirely of iron and nickel. When these are cut open, and the surface polished and etched with acid, this pattern, called a Widmanstatten pattern, is seen. The pattern is formed by cooling of a iron- nickel alloy. It is essentially a crytallization effect. The formation of this pattern requires *VERY* slow cooling (1 to 100 degrees per MILLION YEARS!!) These meteorites are thought to have originated in the differentiated cores of large asteroids, which were then shattered by impacts. No terrestrial irons show this pattern

(1) World impact structures (2) North American meteorite impact structures These two images show known craters on Earth and on North America created by the impact of meteorites. (It appears that the US has gotten more than its "fair share" of hits, but this is due only to fact that US has been studied more thoroughly.) Many of these structures are buried and so are not visible from the surface. These buried craters were usually discovered when drilling brought up pieces of shocked quartz rock, a rare type of quartz that can only be made in the intense pressure of a giant impact. We talked specifically about: #1 - the Ames, Oklahoma buried crater; #3 - the fresh, easily visible (Barringer) Meteor Crater in Arizona; #10 - Chicxulub, the "dinosaur killer", and #30 - Manicouagan, the donut-shaped lake.

(1) Meteor Crater (2) Meteor Crater (3) Meteor Crater Three views of the almost mile-wide Meteor Crater in northern Arizona. This is the most famous impact crater on Earth, produced when a meteor about the size of the physics building hit the Earth about 50,000 years ago. If you ever get to northern AZ, its definitely worth a look! (Just go up to I40, hang a left, and drive about 840 miles).

(1) Roter Kamm (2) Roter Kamm This is an impact crater in Namibia. It is somewhat larger than the Arizona Crater, at 2.5 km diameter. The crater is thought be be about 4 million years old. Because of wind blown sand and erosion, the crater looks much "softer edged" than the Arizona Crater and is well along on the process of slowly disappearing by being filled in and the rim eroded away.

Geologic Time Geologists and paleontologists divide the Earths 4.5 billion year history into periods and subperiods. This is a simplified chart of geologic time. The time 65 million years ago when the dinosaurs met their end marks an important division, sometimes called the K/T boundary.

(1) Chicxulub (2) Chicxulub (3) Dinosaurs last supper? The enormous (over 100 miles across) buried crater called Chicxulub off the Yucatan Peninsula, Mexico. The second image shows a "gravity map" that reveals details of underground rocks. The "horseshoe shaped" structure is the remains of the crater. Thought by many people to be the scar left by the impact of a 10 mile wide rock or iceball that slammed into the Earth 65 million years ago, leading to catastrophic environmental change on our planet, which probably led to the demise of the dinosaurs.

(1) Manicouagan (winter) (2) Manicouagan (summer) This unusual donut-shaped lake about 60 miles across is the remains of a giant impact crater formed 200 million years ago. This structure is in eastern Canada. Both images are taken from the space shuttle- the first one in winter, the second in summer.

Tunguska aftereffects In 1908, in a remote part of Siberia, something hit the Earth, perhaps a small comet. The object appears to have broken up in the atmosphere, so it did not produce a crater, but the impact felled trees over an area about the size of a large city.

Near the center of the damaged area, some tree trunks were still standing, but had been stripped of their branches. Farther out, the trees were knocked down, with the trees pointing back to the central region. This is broadly consistent with the object exploding several kilometers above the ground (an airburst).

(1) Discovery images of 2008 TC3 (4 frame "movie") (2) 2008 TC3 final trajectory (3) Explosion of 2008 TC3 seen from above 2008 TC3 was an SUV- sized rock (2 - 5 meters across) that hit the Earth's atmosphere and exploded over Sudan on October 7, 2008. Rocks this size hit the Earth several times a year- what was special about this rock is that it the *FIRST* (and so for ONLY) natural space object that was predicted to hit the Earth and was observed to hit the Earth. The explosion had an energy of about 2 kilotons (or 0.002 MT) of TNT (about 1/10 the yield of the WWII A-bombs). The airburst explosion was imaged from above by a Europen weather satellite (3rd picture).

From intensive observations over the time from discovery until collision (only about 20 hours!) we know that 2008 TC3 had been orbiting the sun every 1.50 years, with a semimajor axis of 1.31 AU, an orbital eccentricity of 0.31 and an inclination of 2.5 degrees. The speed in its orbit was not too different from the Earth's speed and it hit the Earth at a relatively low speed (about 12 km/sec), which is not much above Earths escape speed.

NEA discoveries last 15 years Histogram of discovery observatories of NEAs (Near Earth asteroids) since 1994. (This graph uses a pretty loose definition of NEAs- see next slide). How things have changed since the pioneering days of the Shoemakers! Note that, prior to about 1998, only a handful of NEAs were found each year. In 1998 the LINEAR survey (blue bars) (see below) started up and found the majority of the NEAs until about 2005. Around 2004, the Catalina Sky Survey (purple) ( see below) started and it has found the most NEAs for the past few years.

A quick glance shows that several thousand NEOs have been found in the last 15 years. Many of these are smaller than the 1 km size usually taken as objects with "global" impact.

NEO orbit types These are names given to subtypes of asteroid orbits that are considered at "Near Earth Objects" (NEOs). Note that not all these NEOs are ECAs (Earth Crossing Asteroids). Objects in the Amor and IEO classes do not cross the Earth's orbit. However, orbits of asteroids do change over time, as they gravitationally interact with other bodies, so individual Amors or IEOs might become ECAs in the future.

LINEAR site The LINEAR (LIncoln labs Near Earth Asteroid Research) operates several meter-class telescopes near White Sands New Mexico. It is an Air Force / NASA /MIT project to find NEAs and, as shown in previous histogram, dominated the search from about 1998 to 2004.

Catalina Sky Survey 1.5meter (Arizona) Catalina Sky Survey 0.5meter (Australia) There are several telescopes that together have found the bulk of the NEAs so far known. One survey, known as the Catalina Sky Survey (CSS) associated with the U. of Arizona (on of the powerhouse universities in the astronomy world, and the place where drbill got his PhD) operates a 1.5 meter telescope near Tucson (first image- the streak of light behind dome is the International Space Station) and an associated 0.5 meter telescope in Australia (second image). By modern research telescope standards, these are modest, if not downright TINY, telescopes. The success of these modest telescopes in finding NEAs comes partially from their relatively wide fields, but mostly from the sophisticated software that is used to process the images, usually in near real time, to find moving objects. Telescopes of this size can image about a billion different stars- finding a few moving dots of light amongst these is a real tour de force of programming and image processing!

Pan-STARRS -- the Panoramic Survey Telescope & Rapid Response System The next "big thing" in NEA work will be Pan-STARRS. This is a set of 4 1.8 meter telescopes located on Haleakala, Maui HI. Each scope will have a 1.4 billion pixel camera. The first telescope, PS1, and camera are now being tested.

Large Synoptic Survey Telescope (LSST) Perhaps by the middle of the next decade, the LSST will be working. The proposed LSST will have a mirror roughly equal in size (8.4 meters) to the mirrors in the largest telescopes now operating, but will operate quite differently from other large telescopes. Current large telescopes can only look at a very small part of the sky at once. The LSST will be able to look at a much larger piece of sky. The LSST will take very detailed pictures covering the ENTIRE SKY every few nights. Current big telescopes would require years or decades to take pictures covering the entire sky. The LSST will allow astronomers to do many types of projects, including searching for asteroids that might someday hit the Earth. If the $300 million can be found to build the telescope, it could be operating around 2016. The data will be made public immediately. This will allow astronomers from anywhere to do their own research using the data. Although there are current astronomical data archives that allow public access to vast amounts of data, such as the HST archives, this telescope should mark a new way to obtain and deal with optical images from large groundbased telescopes. There is, of course, a website: www.lsst.org.

Large Synoptic Survey Telescope optics To make a large telescope that can cover a 3 degree wide field of view, LSST designers invented a radical optical system. The main mirror has a unique "mirror within a mirror" configuration.

Large Synoptic Survey Telescope optical path The light comes from the top, hits mirror M1, bounces up to mirror M2, then down to M3 (which is physicaly part of M1), then up through several lenses to come to a focus above the set of 3 lenses. The bottom of the 3 lenses will be 1.5 meter across, the largest precision lens ever made.

Large Synoptic Survey Telescope mirror blank The LSST main mirror is 8.4 meters across - thats about 9.2 yards, so, as I tell undergrads in intro astronomy "its almost a first down".

The mirror is not a solid disk of glass (it would be VERY heavy and impossible to cool downto night time temperature, inducing optical turbulence). Instead it is a solid thin disk on top supported by a honycomb glass structure, which can be seen through the top sheet of glass and along the sides. These mirrors are made by first making a mold of "cores" which define where the glass isn't, filling up the mold with chunks of glass, then melting the glass in a giant rotating oven (located under the football stadium at the University of Arizona). The melted rotating glass mass forms a concave shaped surface (meaning there is much less glass to figure away when the mirror is being ground and polished) and is cooled slowly to preserve that shape. This process, invented by UA professor Roger Angel (in red shirt at far side of mirror) has been used for many of the largest telescope mirrors.

Model of detector for LSST camera The camera for the LSST will use a large number of individual CCDs (electronic "film") arranged in a big array. This is an "actual size" model of the array. Each little square is a CCD bigger and more sensitive than the best digital camera chip. All together, the camera will have about 3.2 Billion pixels, or individual "dots" in the image. In terms of digital camera advertising, this is a "3200 megapixel" camera!! The camera will produce a staggering amount of digital data each clear nite, and the guys from Google are planning to help deal with the tremendous data flow. Each clear *nite*, the camera is expected to generate about 10 TeraBytes of digital images!!

Comparison of existing and proposed sky surveys One quick figure of merit for sky surveys is the "etendue", which is derived by multiplying the collecting area of a telescope (in square meters) by its field of view (in square degrees). Current productive NEA surveys (CSS and LINEAR) have etendue around 1. The LSST will have an etendue of 300, so that, roughly speaking, the LSST will be able to do in a single night what LINEAR does in a year!!

How often do big impacts happen on Earth? (See next slide also.) This graph shows the typical time between impacts of different energy. The downward sloping line shows that bigger impacts are rarer than smaller impacts (which is a Good Thing!). For example an impact with the energy of the 1908 Tunguska impact in Siberia (equivalent to the energy of an asteroid about 50 meters (150 feet) in size) -or larger energy- is expected to happen every few hundred years. (If you find the point labeled Tunguska and go over horizontally to the "Typical Impact Interval", you see that such events happen every few centuries.) Such impacts might cause great local damage and loss of life, IF the impact took place in a densely populated area. The impact of a 1 to 3 kilometer sized asteroid (roughly 0.5 to 1.5 miles across)(this is about where the line changes from solid to dotted) will happen every few hundred thousand years. Such an event would cause immediate and total devastation over an area the size of a large state. It could also cause large loss of human life through indirect effects- such an impact might fill our atmosphere with dust and acid rain which could cause widespread crop failure, and subsequent mass starvation. If we find such an object on a collison course with Earth, our response will depend on the amount of time we have until impact. If we get only days or weeks of warning, there is little that we can do. With months or a few years of warning, we might at least save some select group of humans by hiding them in caves with ample food supply and air filtering systems (but think about being left on the outside!). With a number of years or decades of warning, we can hope to take direct action to avoid the impact- this would probably involve some sort of rocket mission to the object, perhaps to nudge it into a different orbit so that it misses the Earth.

NEO number vs size Essentially an updated version of plot as that shown the other day from Shoemaker (1982). This is from a 2007 NASA report to Congress on the NEO situation. Congress has asked NASA to study how we can find 90% of NEOs bigger than 140 meters by the year 2025. Objects 140 meters (about size of OU football stadium) hit the Earth every few millennia, with the impact energy of hundreds of megatons of TNT (that is, energies multiple times the largest H-bombs ever detonated). As you can see from this plot, the current estimate is that there are 100,000 such objects!!

OU Stadium and 140 meter asteroid Congress has asked NASA to study how we can find 90% of NEOs bigger than 140 meters by the year 2025. Objects 140 meters (about size of OU football stadium) or larger hit the Earth every few millennia, with the impact energy of hundreds of megatons of TNT (that is, energies multiple times the largest H-bombs ever detonated). As you can see from a plot shown last lecture, the current estimate is that there are 100,000 such NEOs!! (and many, many more in the main asteroid belt)

Bright meteor over OkieTex starparty Every fall the Oklahoma City Astronomy club (www.okcastroclub.com) holds the OkieTex star party, one of the "Top 10" star parties in the US. For the past few years, OkieTex has been held at the western end of the Oklahoma Panhandle- a fantastically DARK site. I had the honor of giving several talks at the 2008 star party. At the 2008 OkieTex, this spectacular meteor was seen. The red lights are the camps of the astronomers, who fill up a large field with campers, tents, and of course all manner of telescopes. (Unfortunately, I was asleep when the meteor came by!)