0010-9525/03/4105- $25.00 © 2003

MAIK “Nauka

/Interperiodica”0521

Cosmic Research, Vol. 41, No. 5, 2003, pp. 521–525. Translated from Kosmicheskie Issledovaniya, Vol. 41, No. 5, 2003, pp. 553–557.Original Russian Text Copyright © 2003 by Avanesov, Artamonov, Aust, Eremin, Zubenko, Kondabarov, Maslov, Polyakov, Ougolnikov.

THE GOALS OF THE EXPERIMENT

The background radiation of the sky, the main partof which is zodiacal light (solar radiation scattered byinterplanetary dust) is very difficult to study from theEarth due to the comparable (at best) contribution of theEarth’s atmosphere illuminated by the Sun (under thehorizon), the Moon, and other sources of light (includ-ing zodiacal light itself! [1]). The

IRAS

observations [2]and ground-based survey experiments [3] have demon-strated the complex structure of the background, theexistence of “cirruses,” and other details that are relatedto the tracks of comets and asteroids. Since the lightduring scattering gains considerable polarization, it isappropriate to detect such details on the polarizationmaps of the sky; however, there is a small number ofpolarization measurements of the zodiacal light at dif-ferent angles to the ecliptic and the Sun. The spaceexperiment NORT (the Russian abbreviation for “near-Earth observations by spaced telescopes”) related to thepolarization sky survey using wide-angle telescopesand 2D array photodetectors could make it possible tostudy the distribution and properties of the interplane-tary medium, for example, the sizes, forms, and orien-tations of dust grains.

One more problem related to the multiannual car-tography of the polarization background is the possibil-ity of detection and investigation of the light echoesfrom supernovae that were observed from Earth in thehistorical epoch (hundreds of years ago). Around theareas in the sky where those supernovae were seen inthe Middle Ages, now polarized spots and spots varyingon intervals of several years should be observed, thedetection and study of which could present valuableinformation related to the interstellar medium of ourGalaxy [4]. Such investigations, with the use of photo-graphic plates, were conducted in the 1960s with nega-tive results for unpolarized light [5], but the employ-ment of modern instrumentation and image processingmethods allows one to make a significant advance inthis direction.

Optical observations of space debris fragments ofmedium sizes were thus far executed in only one spaceexperiment, onboard the Infrared Astronomical Satel-lite

IRAS

(USA, 1983). During 9 months of continuousobservations the

IRAS

telescope detected more than2500 fragments of space debris [6]. The placing ofsmall telescopes into space and the prolonged monitor-ing of the surrounding space would allow one to obtaininformation related to the fragments of space debris andto the meteoroids (FSDM) of centimeter range, andpolarization studies in some cases could refine the dataon their properties.

THE SCHEME OF EXPERIMENT AND INSTRUMENTATION

The experiment scenario is as follows. Four smalltelescopes located onboard the

ISS

are sighted in onedirection. They detect by CCD matrices signals fromthe stars and near-Earth objects with a field of view of

~8°

in the upper hemisphere under continuous rotationof the station with an angular velocity of

~4

deg/min.The information is processed by the onboard computersin a real time scale. Polarization filters with differentmutual orientations of the principal axes are mounted inthe telescopes, and this gives the possibility of measur-ing the linear polarization of both starlike and extendedsources. Four telescopes are used due to the followingcauses:

—the possibility of determination of the distance toa FSDM when it is observed by two telescopes spacedseveral meters apart;

—the possibility of determination of the angularvelocity of a FSDM with its simultaneous detection bytwo CCD matrices operating in opposite phases in the“exposure” and “reading” modes;

—the possibility of simultaneous detection ofobjects through different optical filters (polarizationand/or spectral ones);

Experiment NORT onboard the International Space Station (

ISS

)

G. A. Avanesov

1

, V. V. Artamonov

1

, S. A. Aust

1

, V. V. Eremin

1

, G. I. Zubenko

1

, A. V. Kondabarov

1

, I. A. Maslov

1

, E. V. Polyakov

2

, and O. S. Ougolnikov

1,

3

1

Space Research Institute, Russian Academy of Sciences, ul. Profsoyuznaya 84/32, Moscow, 117997 Russia

2

Main Astronomy Observatory, Russian Academy of Sciences, St. Petersburg, Russia

3

Astrospace Center, Lebedev Physical Institute, Russian Academy of Sciences, Profsoyuznaya ul. 84/32, Moscow, 117997 Russia

Received April 15, 2002

SHORTCOMMUNICATIONS

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et al

.

—increase of reliability, because in the proposedscheme of the space experiment with four telescopesthe breakdown of any of them does not lead to dramaticdeterioration of the information quality and allows oneto solve all problems in order to accomplish the goalsof the experiment;

—increase of the accuracy of measurements, of theprobability of detection of FSDM, and of the reliabilityof detection of uncommon phenomena, for example ofshort-term flares;

—the possibility (if necessary) to increase the skyarea under survey.

The complex of scientific instrumentation

NORT

consists of two similar instruments, each of which con-sists of two telescopes on a common single-axis auto-matic rotating platform (ARP) and of two data process-ing and storage units (DPSU). The instruments aremounted at a distance of no less than 5 m from eachother. The mass of each instrument is no more than40 kg, and its dimensions (

940

×

550

×

550

mm

3

) guar-antee their standard transportation and mountingonboard the

ISS

.

The

NORT

telescopes were designed at the SpaceResearch Institute of the Russian Academy of Scienceson the basis of the star sensor of orientation

BOKZ

(Fig. 1), now successfully operating in the standardsystem of the communication geosynchronous satellite

Yamal-100

[7]

1

. Each telescope consists of an objective

with a blend, a CCD matrix, spectral and polarizationlight filters, and a control unit with a computer. Thetelescopes of

NORT

have the following parameters:

—the diameter of the objective is 26 mm

—the focal distance is 58 mm

—the field of view is

8°

×

8°

—the admissible angle to the Sun and the Earth’shorizon is 30

°

—the spectral range is 0.5–1.0

µ

m

—the format of the matrix is 512

×

512

—the angular resolution is 1 angular min

—the period of reading the information is 1 s

—the noise of reading is 100 e.

The rotating platforms assure the possibility of therotation of telescopes to a sky area where the Sun doesnot prevent one from making the survey and also forobservation of the most interesting objects. The dataprocessing and storage unit (DPSU) is a specializedonboard computer now under design at the SpaceResearch Institute of the Russian Academy of Sciences.The DPSU contains two processors corresponding toIntel 486 with a frequency of 66 MHz, a nonvolatileflash-memory of no less than 8 Gbit, and specializedmodules on the basis of the programmable logic matri-ces for fast image processing.

DATA PROCESSING

The data stream (approximately 16 Mbaud) is repre-sented by the frames of the sky produced by four tele-scopes with a period of 1 s. Such a stream can fill thewhole memory of the DPSU in several minutes. There-fore, the data should be compressed, and it is appropri-ate to do this in such a manner that the finished astro-nomical data, i.e., the maps, catalogues, “light curves”,etc., should be accumulated at the output. We classifythe sources separated from the input data stream andsubjected to the further processing

by the degree oftheir extent

:

—the “pointlike” source is a star or a starlike object(for a resolution of 1 angular min);

—the “track” is a trace in the form of a segment ofa straight line or going through the whole frame, whichcould be produced by an object moving with respect tothe stars;

—the “extended” source has characteristic dimen-sions from 1 angular minute to several degrees;

—the “background” is a source with dimensionslarger than the field of view of the telescope;

and

by the degree of time averaging

:

1

Avanesov, G.A.

et al

., Television star sensor, Inventor’s Certifi-cate no. 1591622, 1990.

Fig. 1.

Star sensor of orientation

BOKZ

, a prototype of thetelescopes for the

NORT

experiment.

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EXPERIMENT NORT ONBOARD THE INTERNATIONAL SPACE STATION 523

—the “model” source is specified a priori, itsparameters are refined by stages in the process of carry-ing out the experiment, if necessary;

—the “instantaneous” source exists in an individualframe;

—the “current” source exists on a map obtained bythe summation of the frames during a single sky surveynear the source;

—the “seasonal” source exists on a map obtained bya multiple sky survey near the source over severalmonths of operation (between the deliveries of theaccumulated information to the Earth).

The model of the instrumentation is specified as a“dark frame,” the frame of the “plane field,” and theform of the light distribution in a spot created by a star(PSF, the point spread function, i.e., the scattering func-tion of a point source). It should be taken into accountthat the PSF function depends on the orbital declinationat which the sky survey is carried out. In addition, thebackground sky chart and the star catalogue are storedin the memory. Together, they represent the model ofthe sky for the spectral range where the telescopes of

NORT

operate. The deviation from this model takinginto account the PSF function is detected in the processof the survey. The information about the tracks and

(a) (b)

(c) (d)

Fig. 2.

The diagram of measurement of the distance and velocity of a “fast” FSDM by its detection by four telescopes of

NORT

.The distance is determined by parallax displacement of the tracks in frames (a) and (b). The velocity is determined by the anglebetween the track in frames (a) and (c) and in frames (b) and (d). When the FSDM flew through the field of view, telescopes (a) and(b) were in “exposure” mode, and telescopes (c) and (d) were in “reading” mode.

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point sources will be accumulated and stored in theform of catalogues, while the images of extendedsources and of the background will be stored in theform of maps with a resolution of

~2

angular min.From the entire experimental video information

stream the frames that have tracks, i.e., images of theflying FSDM (Fig. 2), are sampled. According to thedisplacements of these tracks in relation to star images,one can determine the distance to an object by themethod of triangulation. The angular velocity of anobject can be determined either by the track length dur-ing exposure or by the slope between the tracks pro-duced in different modes of operation of CCD matrices(“exposure” and “reading”). Namely, since during the“reading” process the accumulated charge movesquickly along the lines of the matrix, the resulting trackis the sum of the vectors of displacements of the chargeand of the object along the “read” matrix. At the sametime in the “exposure” mode the velocity of the dis-placement of the charge at the frame is zero and theimage presents the actual path of FSDM against thebackground of stars. This method allows one to esti-mate the velocities of very fast FSDMs.

EXPECTED RESULTS

The accuracy of measurement of the angular posi-tion of an object with respect to the stars by a singleframe approximately corresponds to the dimension of apixel of the image and is equal to 1 angular min.Immovable and slowly moving objects are detected bythe instruments when they move in the field of view ofthe telescope approximately 100 times, which allowsone to reach the mean root square error of measure-ments of about 6 angular seconds. The same accuracyis reached in the measurements of the location of atrack (track of a fast FSDM) in the transverse directionto it, because the estimate is made by

~500

points.The telescopes’ sensitivity based on parameters of

the CCD matrix and optics corresponds to the unit ratioof the signal to noise for a “point” object of the 10thmagnitude. The accuracy of photometric measurementsby a single frame of sufficiently bright starlike objectsis

~10%

. The brightness of an object present in allframes obtained for two minutes in the process of a sur-vey and also for a “track” can be estimated within anaccuracy of 0.01–0.02 magnitude.

The magnitude of a track of FSDM with albedo

~0.1

having characteristic dimension

D

, flying at distance

r

with the tangent velocity component

v

t

, may be esti-mated by the formula [8]

where

β

is the angular dimension of the pixel of theimage along the track,

~1

angular min =

3

×

10

–4

rad. It

m* 31.1–= 2.5r2

D2------

v t

r-----1

β---

log ,+

follows from this formula that a FSDM of centimeterdimension flying at a distance of 20 km from the

ISS

with a velocity of 40 km/s will be detected by tele-scopes of the

NORT

as a track of the 10th magnitude,i.e., with unit ratio of the signal to noise. When thevelocity or the distance of the FSDM flight decreases,the ratio of the signal to noise will increase inverselyproportional to these parameters.

If the length of the base between the instruments ofthe

NORT

is 5–6 m, the parallax displacement of theimage of a FSDM removed to 20 km is

~1

angular min,and it can be measured with an accuracy of 10%.

The angular velocity of “slow” FSDM (from 0.001to 1 deg/s) can be measured by the displacement of theimage of an object with respect to the stars during thesurvey of this part of the sky by the telescopes. By theangle between the tracks detected by CCD matrices in“exposure” and “reading” modes one can determine theangular velocities of the “fast” FSDM in the range from0.1 to 8000 deg/s; i.e., it will be possible to estimate thevelocity of order of 40 km/s for a meteoroid flying at adistance of 300 km from the

ISS

.

Thus, the instrumentation described will allow oneto detect and measure the brightness, angular velocity,and distance. Consequently, one can estimate (at leastroughly) the dimension for all FSDMs (larger than 1 cm)flying in the field of view of the telescopes at distances1–20 km. If the flux of such particles brings the hazardof collision with the station with a probability of oneevent per 10 years, they will be detected by

NORT

instruments several times per day of observations.

Polarimetric measurements will be carried out bycomparing the brightness of objects obtained by fourtelescopes with polaroids having different orientationsof the projections of their principal axes onto the celes-tial sphere. For extended objects with dimensions

~1°

,due to the large number of pixels taking part in the mea-surements, it is possible to detect the polarized radia-tion constituting

~10

–4

of the background [9]. Pro-longed cartography of the sky for many years will allowone to reduce this threshold to one more order of mag-nitude and to study the details of the galactic back-ground and the variations of the zodiacal light.

REFERENCES

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Astrophys. J.

, 2002, vol. 387.

2. Paley, E.S., Low, F.J., and McGraw, J.T.,

et. al.

, An Infra-red Optical Investigation of 100 Micron Cirrus,

Astro-phys. J.

, 1991, vol. 376, p. 335.

3. Cawson, M.G.M., McGraw, J.T., and Keane, M.J., CCDTransit Instrument,

Preprint of the Steward Observatory,

1986, no. 691.

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4. Maslov, I.A., A Possibility of Detecting the Light Echoesof Galactic Supernovae by the Method of Wide-AnglePolarimetry,

Pis’ma Astron. Zh.

, 2000, vol. 26, no. 7,p.

499.5. van den Bergh, S.,

A Preliminary Search for Light Ech-oes from Supernovae

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Centimetre-Sized Orbital Debris Observed with IRAS,

Proc. of the First European Conference on SpaceDebris, Darmstadt, Germany, 5–7 April, 1993

, (ESASD-01), p. 105.

7. Ziman, Y., Defining the Spacecraft Attitude,

Space Bul-letin

, 1994, vol. 1, no. 4.

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Okolozemnaya astronomiya (kosmicheskii musor)

(Around-the-Earth Astronomy: Space Debris), Moscow:Kosmosinform, 1998, p. 193.

9. Sholomitskii, G.B., Maslov, I.A., and Vitrichenko, E.A.,Large-Scale Polarimetry of Large Optical Galaxies,

Pis’ma Astron. Zh.

, 1999, vol. 25, p. 803.