NEAR-EARTH OBSERVATIONS BY SPREAD TELESCOPES 3 0 0 2 February 2, 2008 n a J 8 I.A.Maslov,1 S.A.Aust,1 V.V.Eremin,1 G.I.Zubenko,1 A.V.Kondabarov,1 O.S.Ougolnikov,2,1 1 1Space Research Institute of RAS, Moscow, Russia, v 2Astro-Space Center, Lebedev’s Physical Institute of RAS, Moscow, Russia 8 2 Abstract 1 1 We suggest the all-sky survey at the International Space Station by 0 four little wide-angle telescopes with polarization filters and CCD-arrays 3 spread by several meters one from another. The video information pro- 0 / cessingwillbecarriedoutbyreal-timemultiprocessorsystemontheboard h of the station. This experiment would allow to observe the sunlit space p debrisandmeteoroidsofcentimetresizewiththeirdistancesandvelocities - o estimations at the distances up to 20km from station and to investigate r theinterplanetaryand interstellarmedium bythemakingof polarization t s sky mapsand detecting theweak-contrast features on it. a : v 1 INTRODUCTION i X The investigations of sky background that basically consists of zodiacal r a lightaredifficulttoconductonthegroundbecauseofsufficientcontribu- tion ofthelight ofothersources andzodiacal light itself (Bernstein et al, 2002)scatteredintheatmosphere. Thetranslucenthigh-latitudecirruses foundbyIRASwereobservedinthevisualbyCawson et al(1986). Since the scattered light is sufficiently polarized, these features are better to search for on the polarization sky maps. The space experiment with po- larization sky survey using wide-angle telescopes and CCD-arrays would allowtoinvestigatethedistributionandpropertiesofinterplanetarydust, thesize, shape and orientation of dust particles. Anotherproblemrelatedwithprolongedpolarizationbackgroundmap- ping is the possibility of supernova echoes discovering and investigation. The polarized spots with several years variability should be observable (Maslov, 2000) around the locations of supernovas that were observed 1 several centuries ago. Observations of these spots would give the infor- mationaboutinterstellarmediumofourGalaxy. Searchfortheseobjects fornonpolarized light was conductedbyVandenBergh (1966) withpho- tographic plates and brought the negative result, but using the modern technique and image processing methods allows to move forward in this question. Radarstationsofspacecontrolareregularlywatchingforseveralthou- sands spacecrafts and their fragments with sizes more than 20cm. The smallest size particles were registrated by the collisions with film screens (LDEF satellite) and spacecratfs surfaces. But experimental data about mostdangerousforspacecraftsmedium-sized(about1cm)particlesprac- tically are absent. 2 EXPERIMENT DESCRIPTION The choose of International Space Station (ISS) for this experiment is madeby three reasons: • Near-Earth medium watching near the ISS orbit allows to estimate the statistical parameters and to watch for the bodies dangerous for the station without additional assumptions about their space distribution; • ThesizeoftheISSallowstospreadthetelescopesfortriangulational; • Installationofthedevicesonoften-visitedstationsimplifiesthechanges of thedata processing program. The main threegoals of theexperiment are: • Investigations of distribution and properties of dust in the Solar System and Galaxy; • Obtaining the data about space debris and meteoroid particles of centimetre size nearthe ISSorbit; • Discoveringandobservationsofasteroids,cometsandvariablestars. The idea of the experiment follows. Four small telescopes with ∼ 8◦ fieldofviewinstalledonISS,watchingatthesamedirectionandobserved theskybyusingthecontinuousstationrotationwiththeangularvelocity about ∼ 4◦/min. The information processing of CCD-images is being doneby on-board computers in real-time mode. The light sources coordinates comparison with stars catalogue allows to determine the exact telescopes orientation, to find unknown sources andtomeasuretheirposition. Thedifferenceofthesepositionsmeasured on different telescopes allows to determine thedistance to theobject. The reasons touse four telescopes are following: • using of two or more telescopes spread by several meters give the possibility to determinethe distance to theparticle; 2 • theregistration of thetrackoffast-moving object bytwotelescopes with opposite CCD regimes (”exposition” on the first and ”read- ing” on the second and vice versa) allows to determine the angular velocity of the object; • usingthedifferentaxesdirectionof thepolarizing filtersof thetele- scopes, we can measure the linear polarization both point and ex- tended sources; • theworkfailureofonetelescopedoesnotbringsufficientchangefor theworseofthequalityoftheinformationremainingtheexperiment conduction possible; • the measurements exactness is better, the probability of space de- brisormeteoroiddiscoveringorunusualevent(short-timeburst,for example) observation increases; • the extension of observable sky area is possible. The telescopes are installing in pairs on two platforms with vertical (relatively Earth) axis of rotation. The telescopes are watching at one directionbytheangleabout30−60◦ tothezenith. Thedistancebetween platforms should be not less than 5m. The platforms are turning the telescopes to therequired sky region not emitted bythe Sun. 3 APPARATUS DESCRIPTION The apparatus complex consists of two same devices. The mass of each one is not more than 40kg, the size is 940×550×550mm3, that makes theirtransferandinstallationontheInternationalSpaceStationpossible. The telescopes are developed in Space Research Institute basing on StarSensor(Ziman,1994)thatissuccessfullyworkingnowongeostation- ary communication satellite ”Yamal100”. The basic parameters of the telescopes are shown in the Table1. The telescopes are able to work at angles down to30◦ from theSunand from theEarth horizon. Addingto thevideoinformation, theysupplytheparametersofeach imageorienta- tion, that is simplify thefurther information processing. Table 1: Telescopes characteristics Lens diameter 26mm Focal distance 58mm Visible area 8◦×8◦ Spectral range 0.5 – 1.0mkm CCD format 512×512 ′ Angular resolution 1 Information reading period 1sec Noise of reading 100e− 3 The Information Processing and Saving Device is the special board computerbeingdeveloped in Space Research Institute. It is consisted of: • two processors of Intel-486 typewith thefrequency 66MHz; • energy-independentflash memory not less than 8GBit; • specialmodulesbasedonprogrammedlogicalmatrixesforfastimage processing. One such device can process information from two telescopes. If we useseconddevicefortwotelescopes, itwillbereserveoneorwewillhave the possibility of processing programs debugging and comparison. The basic way to pass theinformation to Earth and programs edition is their copyingusing theISS server and changeable information holders. 4 ALGORYTHMS AND PRINCIPLES OF INFORMATION PROCESSING The input information flux is the sky images made by four telescopes eachsecond. Thisfluxfulfilsthememoryofcomputersatseveralminutes, that’swhytheinformationcompressionisnecessary. Itisbettertodoitin order to havethe complete astronomical data (such as maps, catalogues, lightcurvesetc.) attheoutput. Thesourcesintheimagescanbeclassified: • by theextension: – point sources: thestarsandstar-likeobjects(for1′-resolution); – tracks: thetrace as a straight line made by movingobject; – extended sources: thesourcewithangularsizefrom1′toseveral degrees; – background: thesourcewithangularsizemorethanvisiblearea. • by thetime averaging: – model: given initially, with parameters correction while theex- periment if necessary; – momentary: present just in one image; – current: present on the map obtained by the images addition at the single sky survey near thesource; – seasonal: present on the map obtained by the images addition duringseveralmonthsofwork(untilthedatapassingtoEarth). Thetracksandpointsourcesinformationwillbesavedinthecatalogs andtheextendedsourcesandbackground-inthe2′-resolutionskymaps. Finally, we will obtain thefollowing information: • last 200 skyimages; • point momentary sources catalogue; • point current sources catalogue with variability data; 4 • current sky map; • tracks catalogue; • seasonal sky map; • current maps of some sky regions; • some sky images. Theapparatusmodelisgivenby”darkimage”, ”flat fieldimage” and point spread function (PSF) of point source depending on the orbital declination where thesurvey was made. The computer memory is holding the background sky map and stars catalogue that are the sky model for given spectral region. The deflec- tions from this model are recording during the survey, that makes the search for new and variable sources easier. The model image is the sum of background map and point sources with account of PSF. Wesuggest thefollowing processing sequence: • correction of outputskyimages by”dark image” and ”flat field im- age”; • subtractionofmodelimagewiththemodelpointsourcesbrightness correction; • photometric calibration by themodel stars in thevisible area; • thesearchfortracksandnewpointsourcesinimageswithsubtracted model andtheirincludetothetracksandpoint momentary sources catalogues; • thecreation of currentskymap with thesize about10◦×10◦ using the images with subtracted tracks and point momentary sources; • thesearchforweaktracks,starsandextendedsourcesinthecurrent map; • the information about brightness of sky regions out of current map is including to theseasonal map. ′ The seasonal map of the whole sky with 2-resolution requires about 200-300MBytesofmemoryforeachtelescope,ifnottotakethecompres- sion into account. The apparatus parameters and sky model are being corrected during the time of experiment and changing after passing the data toEarth. Polarimetric and parallactic measurements are made bas- ing on the mapsand catalogues obtained by different telescopes. 5 EXPECTED RESULTS The exactness of single position measurement of an object relatively the stars is 1′. Since the stationary and slowly moving objects are being recorded about 100 times at one crossing of visible area, the average- ′′ squared exactness can reach 6 . The same exactness can be reached for 5 trackpositionmeasurementperpendicularitsdirection,sincethisestima- tion is madeby about 500 pixels. The sensitivity of the telescopes (by S/N level equal to 1) will be about10m forpointobjects. Theexactnessofphotometricmeasurements for bright star-like objects will be about 10 percents. The magnitude of the object present in all images obtained during 2-minute survey, and magnitudeof thetrack can be estimated with exactness 0.01−0.02m. The magnitude of space debris or meteoroid with albedo about 0.1 andthesize D,flyingatthedistancer withtangential velocityvt can be estimated by using Bagrov and Vygon’s (1998) formula: m∗ =−31.1+2.5lg(cid:18)Dr22vrtβ1(cid:19), (1) where β is the angular size of one image pixel which is equal to about 3×10−4. Corresponding to this formula, fragment with size equal to 1cm, flying at 20km from ISS with velocity 40km/s will be recorded by m the telescopes as a 10 -track, i.e. with S/N ratio equal to 1. With the velocityorthedistancedecreasetheS/Nratiowillrisebackproportional tothese parameters. Ifweincrease thedistance betweenthetelescopes to5-6meters, than the parallax of the fragment at the distance equal to 20km will reach 1′ and it will bepossible to measure it with 10-percent uncertainty. ◦ Theangular velocityof slowmoving(from 0.001 to1 /sec) fragments can be measured by the displacement of the object in different images. HavingmeasuredtheanglebetweenthetracksrecordedbyCCD-matrixes of two telescopes (in ”exposition” and ”reading” regimes) we can deter- mine the angular velocity of fast moving (from 0.1 to 8000◦/sec) frag- ments. Thevelocityofmeteoroid equalto40km/scanbemeasuredfrom thedistance 300 meters! Thus, the apparatus will be able to find and measure the brightness, angular velocity and distance and estimate the size of all space debris and meteoroids larger than 1cm, flyingat thedistances from 1 to20km. If the flux of such fragments is dangerous by possible collisions with the stationonetimeper10years,thantheywillberevealedbythisapparatus complex several times per day. Polarimetric observations will be conducted by thecomparison of ob- ject brightness at four telescopes with different polaroid axes. For ex- ◦ tendedobjects with size more than 1 it is possible to measure polarized light with the intensity equalto 10−4 from thebackground (Sholomitskii et al, 1999), using the large number of pixels. Sky mapping prolonged for theseveral years would decrease this value for one more order and to investigatethedetailedfeaturesofGalacticbackgroundandzodiacallight variations. 6 CONCLUSION Theexperiment would allow to obtain: 6 • distribution and scattering parameters of interplanetary and inter- stellar dust by theprolonged regular polarimetric sky mapping; • statistical estimations of concentration, velocities and sizes of space debris and meteoroids near to International Space Station orbit; • data about Novasat early stages before theregistration byground- based observatories (especially at low angular distances from the Sun) and statistical characteristics of burstingand variable stars. Acknowledgements The work is supported by Russian Foundation for Basic Research, grant 00-02-16396. References [1] BagrovA.V., VygonV.G. (1998) in ”Near-Earth Astronomy (space debris)”, ed. MassevitchA.G.,Moscow, 193 (in Russian). [2] BernsteinR.A., FreedmanW.L., MadoreB.F. (2002) Astrophysical Journal 571, 85 [3] Cawson M.G.M., McGraw J.T., Keane M.J. (1986) Preprint of the Steward Observatory No691 [4] MaslovI.A.(2000) Astronomy Letters 26, 428 [5] SholomitskiiG.B., MaslovI.A., VitrichenkoE.A. (1999) Astronomy Letters 25, 697 [6] Vanden BerghS. (1966) Pub.A.S.P.78, No460, 74 [7] ZimanY.(1994) Space Bulletin1, No4 7