April 2001 Volume 6, Issue 2. NEWS - NASA

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A publication ofThe Orbital Debris Program OfficeNASA Johnson Space CenterHouston, Texas 77058Volume 6, Issue 2.April 2001NEWSPAM-D Debris Falls in Saudi ArabiaAfter nearly eight years in orbit, a PAM-D(Payload Assist Module - Delta) upper stagereentered on 12 January 2001 with one mainfragment being recovered in Saudi Arabia. Thestage (US Satellite Number 22659, InternationalDesignator 1993-032C) along with a GPSspacecraft (USA-91) was launched 13 May1993 and left in an orbit of 180 km by 20,340km with an inclination of 34.9 degrees. Theheart of the PAM-D is a STAR-48B solidrocket motor (SRM) manufactured by ThiokolCorporation. After burn-out the STAR-48B hasa mass of about 130 kg, a length of 2.0 m, and adiameter of 1.2 m.The stage had been undergoing rapidcatastrophic orbital decay since the first of theyear, dropping from an orbit of 145 km by 800km during the last week. The evening ( 1900local time) reentry over the sparsely populateddesert was observed, and one large fragmentwas found about 240 km from the capital ofRiyadh. The object, about 70 kg in mass, wasthe main titanium casing of the STAR-48B,although most of the phenolic nozzle hadbroken off or burned away. A Boeing partnumber was clearly visible on the casing,further substantiating the identification.This was the second known Delta stage topartially survive reentry in less than a year. On27 April 2000 a Delta 2second stage (US SatelliteNumber 23834, International Designator 1996019B), also from a GPSmission, reentered theatmosphere over theAtlantic Ocean.Threeobjects were recovered inSouth Africa after theevent: a stainless steelpropellant tank ( 260 kg),a titanium pressurantsphere (33 kg), and atapered cylinder (30 kg)which served as part of themain engine nozzleassembly. A propellanttank and pressurant spherewere found in very similar condition after thereentry of a Delta 2 second stage over Texas inJanuary 1997 (Orbital Debris Quarterly News,Apr-Jun 1997). single track elsets were created.The Molniya 3 series of spacecraft arecommunication payloads, the signal feature ofwhich is the orbit pioneered and employed bythese vehicles. This event was the second suchevent in as many years. The Molniya 3-36vehicle (1989-094A, 20338) fragmented undersimilar circumstances on 19 May 2000. Bothwere undergoing catastrophic decay at the time,i.e. perigee height was low enough thatsignificant aerodynamic forces were present,resulting in probable ablative heating andsubsequent breakup. Molniya BreakupThe year 2001’s second fragmentationevent was that of the Molniya 3-26 spacecraft,1985-091A, 16112. Launched from the PlesetskCosmodrome on 3 October 1985 at 0726 UTaboard an SL-6 (Molniya) rocket; thisfragmentation event occurred on 21 Februaryafter approximately 5620 days on-orbit. TwoInside.ISS Space Shuttles Examined for Debris Impacts . 3International Space Station Debris Avoidance Operations . 4Overview of the NASA/JSC Debris Assessment Software (DAS) Version 1.5 . 5International Space Missions and Orbital Box Score . 121

The Orbital Debris Quarterly NewsNEWSThe Environmental Implications of Small Satellites Deployed in LEOP. Anz-MeadorSmall satellites have historically madeimportant contributions to Earth and spacescience, biology, communications, andtechnology. However, innovative concepts arebeing proposed which would significantlyreduce the size of spacecraft while increasingthe coverage and, hence, the number of smallsatellites deployed on-orbit. This article willexamine current and projected small satelliteprojects in terms of their physical characteristicsand the overall environmental implications ofsmall satellite utilization.The Russian delegation to the 18th IADCmeeting (June 2000, Colorado Springs) presented a comprehensive overview of current andprojected small satellite traffic; these data, aswell as general nomenclature, are available online at Surrey Satellite Technology’s web site,and so will not be discussed at length here.While most are on the order of 10 kg or greaterin mass, several satellites possess masses on theorder of 0.2 kg and a linear dimension as smallas 2 cm. Further, the NASA New MilleniumProgram foresees so-called “femtosatellites”with masses on the order of grams and dimensions on the order of 2 cm and smaller.Constellations of such satellites, with numbersin the hundreds of operational satellites, areenvisaged for space science, communications,and remote sensing work. The Russian analysisof small satellite traffic indicates that thesevehicles would differ in several major respectsfrom the current population. For example, thevehicle mass density portrayed in Figure 1 is incertain cases a factor of 4 larger than that of acorresponding “classic” satellite. Here, individual data points are compared to the generalrelation, first derived by Kessler, for massdensity of intact satellites. This leads to anenhanced orbital lifetime since atmospheric dragis inversely proportional to mass density.Enhanced orbital lifetimes lead to a largerspatial density at a given altitude and a longerexposure time for other members of the on-orbitpopulation.Operational experience indicates thatcurrent small satellites may be difficult to trackor collect radar cross section (RCS) data on;future small satellites may therefore presentconsiderable difficulties for the deterministictracking task. Tracking may be enhanced usingactive beacons (operational vehicles only) orpassive dipoles or corner reflectors. However,this is unregulated and not implemented (withthe exception of a dipole in the tetherconnecting the Picosat 1 and 2 vehicles) at present. The small sizes of current vehicles, coupled with their geometries and powerrequirements, result in (a) a short operationallifetime and (b) minimal, if any, maneuvercapability. Both factors preclude either activecollision avoidance or self-disposal.Considering these characteristics in toto, smallsatellites are analogous in many aspects toorbital debris clouds.One means of assessing the environmentalimpact of a large constellation of small satellitesis to compare the spatial density of theconstellation to the expected spatial density of anew on-orbit breakup. Figure 2 depicts such abreakup debris cloud. This figure (courtesy ofP. Krisko), generated using the currentEVOLVE 4.0 breakup model, portrays the modeled spatial density of the LANDSAT 1 rocketbody debris cloud shortly after the event. Thespatial density of any constellation may therefore be ratioed to the size-dependent debrisspatial density for a readily understood figure ofmerit. A suggested means of expressing thisratio is via units of Kesslers, abbreviated [Ks].A unit ratio is 1 Ks, a constellation density for 2cm characteristic length picosats twice that ofthe modeled LANDSAT spatial density wouldbe 2 Ks, and so on. Of obvious import is thatrelatively few constellation vehicles cancombine to create the same effective spatialdensity as a larger number of objects in ellipticalorbits. Not perhaps apparent in this chart,however, is the long-term effect. Whereas thedebris cloud will disperse and eventually decay,the small satellite constellation may be bothstrictly maintained in orbital parameters andreplenished at regular intervals.Thus, amaintained constellation may be considered tobe equivalent to satellite breakups at regularintervals, the intervals being determined by thecompeting factors of natural decay (orbitaldebris) versus decay and replenishment (smallsatellites).In summary, small satellites may differconsiderably from their larger cousins not onlyin terms of dimensional and mass magnitude,but also their orbital lifetimes. Conversely, theoperational lifetime of these vehicles are, atpresent, extremely short. This leads, therefore,to scenarios involving large numbers of small,derelict satellites. These are in many waysanalogous to a young debris cloud, and largeconstellations composed of very small satellitesmay be characterized as such in estimatingenvironmental impact. It is not too early tobegin educating small satellite owner/operatorsas to the consequences of their activities, so asto both minimize environmental impact whileyet maximizing their effectiveness. 350number 2 cmnumber 3 cm300250n 5 cmn 10 cm20015010050040060080010001200altitude [km]Figure 1. Small satellite mass densities (represented by dots) compared Figure 2. The modeled LANDSAT 1 R/B debris cloud for variousto that of “normal” vehicles (represented by the line).debris sizes.2

The Orbital Debris Quarterly NewsNEWSISS Space Shuttles Examined for Debris ImpactsTwo Space Shuttles, Discovery (OV-103)and Endeavour (OV-105), have recently beenexamined for orbital debris and meteoroidimpacts following missions to the InternationalSpace Station (ISS) late last year.Bothexhibited numerous impacts on a variety ofinspected orbiter surfaces, covering more than200 m2.Discovery visited ISS last year on the STS92 mission for seven days of its 13-day flight inOctober 2000. A total of 38 impacts wereidentified on the orbiter window thermal panesfrom orbital debris (9), meteoroid (7), andunknown (22) particles. The largest impactfeature with a diameter of nearly 1 cm wasapparently caused by collision with a smallpaint particle. Three of Discovery’s thermalpanes were subsequently replaced.Six impacts ( 3 orbital debris, 1 meteoroid,and 2 unknown) were found on the radiatorswith three of these achieving penetration. Thelargest radiator impact site was approximatelythree-quarters of a millimeter in extent and wascaused by a meteoroid strike. Four otherimpacts were also discovered: three on theflexible reusable surface insulation (FRSI)covering the external payload bay doors and oneon the vertical stabilizer. Of these, two weremeteoroids, one was orbital debris, and one was evenly divided between orbital debris andof unknown material.meteoroids. However, a significant number ofIn December Endeavour conducted the 11- impactors cannot be identified by type,day STS-97 mission, which again included particularly for the smaller window strikes.seven days docked to ISS. Although the Complete inspection details are provided innumber of identified window impacts decreased “STS-92 Orbiter Meteoroid/Orbital Debristo 30, the number of impacts to the radiators and Impact Damage Analysis”, JSC-29318, Januarythe FRSI (12 and 6, respectively) actually 2001, and “STS-97 Orbiter Meteoroid/Orbitalincreased compared to the longer duration STS- Debris Impact Damage Analysis”, JSC-29373,92. A total of two windows were replaced. Of March 2001. the 12 radiator impacts,only one penetrated thethin aluminum sheet, buttwo struck the silverteflon-aluminum doublerinstalled recently toprotect the radiatorcoolant loops.Fouradditional impact siteswere found on theleading edges of theorbiter wings, two fromorbital debris particlesand two from unknownsources.Overall, the numberof identified impactors Figure 1. Impact damage from a stainless steel debris particle on theof all sizes is roughly STS-97 right cabin window.Figure 2. Close-up of hole in the STS-92 vertical stabilizer caused bycollision with a stainless steel debris particle.Figure 3. Overview of the STS-92 vertical stabilizer impact area.Supporting Role of the NASA Orbital Debris Program Office in theDeorbiting of the Mir Space StationAfter 15 years of historic operations, theRussian Mir space station was successfullydeorbited over the South Pacific on 23 March2001. NASA JSC played a supporting role byfacilitating the exchange of tracking data on the135 metric ton complex among the U.S., Russia,Germany, France, and the European SpaceAgency.On 19 January Yuri Koptev, the DirectorGeneral of Rosaviakosmos (Russian Aviationand Space Agency), sent a letter to the NASAAdministrator asking for his assistance inproviding space surveillance trackinginformation on Mir and solar activity data in the3final days before the reentry of Mir. A similaragreement was signed between Rosaviakosmosand the European Space Agency on 23 January.In response to this request, the U.S. governmentagreed to submit the requested data through theestablished communications lines between the(Continued on page 4)

The Orbital Debris Quarterly NewsNEWSSupporting Role of the NASA Orbital Debris Program Office in theDeorbiting of the Mir Space Station, Continued(Continued from page 3)International Space Station Mission ControlCenters in Houston and Moscow. In return,Russia pledged to provide the U.S. with Mirdeorbit plans, including the schedule andcharacteristics of the planned Mir maneuvers.In February, NASA reached an agreementwith the German space agency DLR and theFrench space agency CNES to exchangetracking data on Mir in a similar manner. TheOrbital Debris Program Office at JSC served asthe central node in this real-time exchangewhich began on 19 March and concluded withthe reentry of Mir four days later. Trackingdata in the form of two-line element sets (TLEs)from the U.S. Space Surveillance Network(SSN) were transmitted from JSC to DLR,CNES, and the European Space Agency. Thesame data were also posted on NASA GoddardSpace Flight Center’s Orbital InformationGroup (OIG) website for access by the public atlarge, including all the nations of the world. Inreturn, DLR and CNES forwarded to the OrbitalDebris Program Office tracking data on Miracquired by their respective national resources.More than 70 different TLEs were exchanged inthis manner during the final 96 hours of Mir’sexistence. Project ReviewsInternational Space Station Debris Avoidance OperationsL. Foster, J. Frisbee, M. Wortham, and L.HoworthA debris avoidance process based uponcollision probability has been developed for theInternational Space Station (ISS) usingcovariance information supplied by the UnitedStates Space Command (USSPACECOM).Given t