How do telescopes work? Simple refracting telescope like Fuertes- uses lenses. Typical telescope used by a serious amateur uses a mirror

Transcription

1 Astro 202 Spring 2008 COMETS and ASTEROIDS Small bodies in the solar system Impacts on Earth and other planets The NEO threat to Earth Lecture 4 Don Campbell How do telescopes work? Typical telescope used by a serious amateur uses a mirror Simple refracting telescope like Fuertes- uses lenses Public viewing nights on clear Fridays, from 9pm to midnight. Call to find out if Fuertes is currently open. Public viewing nights are run by the Cornell Astronomical Society. 1

2 One way big modern telescopes work 10 m Keck Telescope Camera Bigger => more area to collect light so more sensitive Bigger => Better resolution can distinguish between two objects close together or better images of the Moon. Resolution given by Diameter / wavelength of light Field of View - FOV: Cameras: Most telescopes can focus light from only a very small area of sky small FOV For asteroid searches want large FOV so can cover a lot of sky fast If telescope resolution is 1 second of arc (3,600 sec of arc in a degree) Moon is ½ degree in diameter, ~¼ square degrees One degree squared ~ 3,600 x 3,600 arc seconds squared Need camera with this number of pixels - ~ 16 mega pixels - some digital cameras have 10 mega pixels Schmidt telescopes have a big field of view complicated optics 2

3 Search system considerations: How sensitive is the telescope i.e. how small an asteroid can be detected at what distance from the Sun and Earth and in what exposure time determined by diameter of telescope How fast can the whole sky be surveyed - determined by the field of view (FOV) of the telescope and/or by the number of pixels in the CCD array detector (same as your digital camera) Three 4-minute exposures with a 16 inch, 0.25 degree field of view, telescope made by an amateur, Dennis di Cicco This is how asteroids are found three exposures within a few minutes shows asteroid moving against background stars. Can estimate orbit by speed of motion relative to stars and direction. NOTE: Only the dark sky can be observed about 90 deg from the Sun and not on bright Moon nights so takes 12 months to observe whole sky. LINEAR NEO Search System Two 1 m class telescopes Limiting magnitude of ~19, over a 2-square degree field-of-view, with less than 100 seconds of integration. 2560X1960 pixel CCD camera Pixel spacing 2.25 arc seconds not great 1 meter GTS-2 telescope and camera 3

4 CATALINA SKY SURVEY Telescope in Catalina mountains, Arizona Siding Springs Schmidt telescope, Warrumbungle mountains, Australia Search for 1 km sized objects about done what next 4

5 NEW SEARCH CAPABILITIES NEW OBJECTIVE TO STUDY: Discover 90% of asteroids > 140 m by 2020 Determine orbits to find hazardous ones Characterize them i.e. find their properties Design mitigation schemes Report to Congress, March 2007 Pan STARRS - Panoramic Survey Telescope & Rapid Response System - First prototype just started working LSST Large Synoptic Survey Telescope Detailed design stage 2012 completion (?) Dark Energy Dark matter NEO searches ETC PanSTARRS - the Panoramic Survey Telescope & Rapid Response System 1.8 -meter telescope FOV of 7 deg 2 Plans to build 4 telescopes The 1.4 gigapixel Pan-STARRS camera ~ 40,000 x 40,000 Cost 10s of millions 5

6 How sensitive will Pan-STARRS be? LSST A single 30 sec observation will reach 24th magnitude. How long will it take to survey the sky? About three quarters of the total sky can be observed from Hawaii, or about 30,000 square degrees. Pan-Starrs will look at about 7 square degrees in each 30 seconds exposure, so in an eight-hour night it will be able to map about 6,000 square degrees. Given that the weather is not always perfect, it will therefore take about a week to survey the whole sky visible at night once. Asteroid searches require that each spot be imaged 3 to 4 times to detect motion of an asteroid and obtain a rough orbit. DATA rate: 1.4 GBytes every 30 sec or less ~1.4 terra bytes per night - 1 or 2 large disks Mirror size is 8.4 m (~29 ft) [Model] LSST Large Synoptic Survey Telescope Complex optics to get 10 sq deg FOV Cost hundreds of millions LSST Camera Size of a small car 3.2 Giga pixels Pixel angular size on sky 0.2 arc seconds Field of View 10 square degrees Time on each location ~ 15 sec Magnitude limit in 15 sec = 24 Time to survey whole (night) sky ~ 3 days (slower for NEA searches as need to observe each location 3 or 4 times to see the asteroid move on sky) PLAN USE Pan-STARRS and LSST for NEO SEARCH OR ALTERNATIVELY PUT INFRARED SENSITIVE SPACECRAFT IN VENUS LIKE ORBIT New, expensive NASA mission Look away from the Sun so can detect more NEAs than telescopes on Earth NEAs emit heat far infrared wavelengths 6

7 Shared ground based - Pan STARRS and LSST Dedicated ground based NASA builds its own LSST 7

8 Protecting the Earth HOW CAN WE MITIGATE THE THREAT (WITHOUT BRUCE WILLIS) MITIGATION Destruction: Can t control fragments easily => DANGEROUS Deflection: Relatively Safe (?) but complex Energy Sources High Explosive (TNT) Kinetic (10 km/sec) Nuclear ,000 Cohesive (solid rock) asteroid or rubble pile? 8

9 Mass drivers and mining the Moon Slow Push Asteroid Tugboat This painting by Pat Rawlings depicts a lunar mass driver (courtesy Lunar and Planetary Institute) Schweikart et al. (2003) Scientific American (November) Unmanned tug attaches to asteroid Plasma (ions) generated by a nuclear power source provides a PUSH moves one diameter in ~7 min Change in period needed 7 = ~

10 AVERTING A COLLISION A space tug can alter an asteroid's orbit by pushing in the direction of its orbital motion. This diagram Assume that the tug beings pushing 12 years before the projected impact and that the asteroid has an orbital period of 1.15 years. The space tug pushes the asteroid for three months, boosting its orbital velocity by one centimeter per second and slightly expanding its orbit. After about 12 years of traveling in the expanded orbit, the asteroid is 6,720 kilometers behind where it would have been if it had not been deflected. By the time the deflected body reaches the impact point, Earth has moved out of harm's way Slow Deflection: Yarkovsky Effect Analogous to non-gravitational forces in comets Push supplied by extra radiation from warm side of asteroid rather than by sublimation of ice Produces a very weak force which changes the orbit VERY SLOWLY Of great academic interest Known to work over long time scales in the asteroid belt Of little practical use in deflecting potentially dangerous asteroids Too weak, too slow! Picture credit: Dan Durda Do it with mirrors - enhanced Yarkovsky effect and surface vaporization using spacecraft born mirrors. Can also coat asteroid in reflective material. Attach giant solar sail (Melosh) 10

11 ORBIT DETERMINATION AND CHARACTERIZATION Policy Issues Who makes the decisions? Schweichart et al, Vaile NEO meeting 11