1Astro 202Spring 2008COMETS and ASTEROIDSSmall bodies in the solar systemImpacts on Earth and other planetsThe NEO threat to EarthLecture 4Don CampbellHow do telescopes work?Simple refracting telescope like Fuertes- uses lensesTypical telescope used by a serious amateur –uses a mirrorPublic viewing nights on clear Fridays, from 9pm to midnight. Call 255-3557 to find out if Fuertes is currently open. Public viewing nights are run by the Cornell Astronomical Society.2One way big modern telescopes workCameraBigger => more area to collect light so more sensitiveBigger => Better resolution – can distinguish between two objectsclose together or better images of the Moon. Resolution given by Diameter / wavelength of light10 m Keck TelescopeField of View - FOV:Most telescopes can focus light from only a very small area of sky – small FOVFor asteroid searches want large FOV so can cover a lot of sky fastSchmidt telescopes have a big field of view – complicated opticsCameras: If telescope resolution is 1 second of arc (3,600 sec of arc in a degree)Moon is ½ degree in diameter, ~¼ square degreesOne degree squared ~ 3,600 x 3,600 arc seconds squaredNeed camera with this number of pixels - ~ 16 mega pixels - some digital cameras have 10 mega pixels3Three 4-minute exposures with a 16 inch, 0.25 degree field of view, telescope made by an amateur, Dennis di CiccoThis 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. 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 telescopeHow fast can the whole sky be surveyed - determined by the “field of view” (FOV) of the telescope and/orby the number of pixels in the CCD array detector (same as your digital camera)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 SystemTwo 1 m class telescopesLimiting magnitude of ~19, over a 2-square degree field-of-view, with less than 100 seconds of integration. 2560X1960 pixel CCD cameraPixel spacing 2.25 arc seconds – not great1 meter GTS-2 telescope and camera http://www.ll.mit.edu/4Siding Springs Schmidt telescope, Warrumbunglemountains, AustraliaTelescope in Catalina mountains, ArizonaCATALINA SKY SURVEYSearch for 1 km sized objects about done – what next5NEW OBJECTIVE TO STUDY:Discover 90% of asteroids > 140 m by 2020Determine orbits to find hazardous onesCharacterize them – i.e. find their propertiesDesign mitigation schemesReport to Congress, March 2007NEW SEARCH CAPABILITIESPan STARRS - Panoramic Survey Telescope & Rapid Response System- First prototype just started workingLSST – Large Synoptic Survey TelescopeDetailed design stage – 2012 completion (?)Dark EnergyDark matterNEO searchesETCPanSTARRS - the Panoramic Survey Telescope & Rapid Response System 1.8 -meter telescopeFOV of 7 deg2Plans to build 4 telescopesCost – 10s of millionsThe 1.4 gigapixel Pan-STARRS camera~ 40,000 x 40,0006How sensitive will Pan-STARRS be?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 skyvisible 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 disksMirror size is 8.4 m (~29 ft) [Model]LSSTLSST – Large Synoptic Survey TelescopeComplex optics to get 10 sq deg FOVCost – hundreds of millionsLSST CameraSize of a small car 3.2 Giga pixelsPixel angular size on sky 0.2 arc secondsField of View 10 square degrees Time on each location ~ 15 secMagnitude limit in 15 sec = 24Time 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 SEARCHORALTERNATIVELY – PUT INFRARED SENSITIVE SPACECRAFT IN VENUS LIKE ORBITNew, expensive NASA missionLook away from the Sun so can detect more NEAs than telescopes on EarthNEAs emit heat – far infrared wavelengths7Shared ground based - Pan STARRS and LSSTDedicated ground based – NASA builds its own LSST8HOW CAN WE MITIGATE THE THREAT (WITHOUT BRUCE WILLIS)Protecting the EarthMITIGATION• Destruction: Can’t control fragments easily• Deflection: Relatively Safe (?) but complex=> DANGEROUSCohesive (solid rock) asteroid or rubble pile?Energy SourcesHigh Explosive (TNT)Kinetic (10 km/sec)Nuclear110800,0009This painting by Pat Rawlings depicts a lunar mass driver(courtesy Lunar and Planetary Institute) Mass drivers and mining the MoonSlow Push Asteroid Tugboat• 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 needed71 365 24 60=×××~ 10-5Schweikart et al. (2003)Scientific American (November)10AVERTING A COLLISIONA 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 Picture credit: Dan DurdaSlow 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