debris swarms seen by smei

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Advances in Space Research 49 (2012) 162–176

Debris swarms seen by SMEI

Donald R. Mizuno a, Stephan D. Price b,1, Kathleen E. Kraemer b,⇑,1, Thomas A. Kuchar a,Janet C. Johnston b,2

a Institute for Scientific Research, Boston College, Chestnut Hill, MA, USAb Air Force Research Laboratory, Space Vehicles Directorate, Hanscom AFB, MA, USA

Received 17 May 2011; received in revised form 9 September 2011; accepted 12 September 2011Available online 16 September 2011

Abstract

The large 3� � 60� fields-of-view of the Solar Mass Ejection Imager (SMEI) instruments are oriented on the stabilized Coriolis satelliteto image most of the sky each Sun-synchronous orbit. Besides observing coronal mass ejections, the SMEI mission objective, SMEI alsohas detected a plethora of Earth-orbiting satellites (resident space objects or RSOs) brighter than �8th magnitude at a rate of about 1 perminute. Occasionally, SMEI sees an RSO swarm: a sudden onset of a large number of RSOs, many more than the nominal rate, uptodozens detected in a 4-s frame. These swarms usually last for a few minutes. A sample of six such RSO ensembles is analyzed in this paperin which the distance and the direction of the velocity vector for individual objects are estimated. We present the observational evidenceindicating that the swarms must be near-field objects traveling in orbits near that of Coriolis, and that the relatively speeds between theobjects and Coriolis are low. Further, analyses indicate that the RSOs are quite close (<20 m) and are generally moving radially awayfrom the satellite. The predicted encounter geometries for Coriolis passing through or near a small debris cloud is, generally, quite incon-sistent with the observations. The most likely explanation consistent with the observations is that SMEI is seeing debris being ejectedfrom the Coriolis spacecraft itself. An analysis of distance and brightness for a subset of the RSOs indicates that the median diameterof the debris particles is �80 lm.� 2011 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Space debris; Solar Mass Ejection Imager

1. Introduction

The mission objective of the Solar Mass Ejection Imager(SMEI) was to detect and follow coronal mass ejections(CMEs), especially the geo-effective events, from within�25� of the Sun to well past the Earth (e.g. Howardet al., 2006). The three SMEI cameras are located at thebottom of the Coriolis satellite as shown in Fig. 1. Thesatellite was launched in February 2003 into a 98.8� incli-nation, 840 � 820 km altitude Sun-synchronous orbit.The instruments look �20� to 30� in elevation above the

0273-1177/$36.00 � 2011 COSPAR. Published by Elsevier Ltd. All rights rese

doi:10.1016/j.asr.2011.09.006

⇑ Corresponding author.E-mail address: [email protected] (K.E. Kraemer).

1 Current address: Institute for Scientific Research, Boston College,Chestnut Hill, MA, USA.

2 Current address: #3 44th Street, Newbury, MA 01951, USA.

horizontal opposite to the velocity vector of the orbit(ram) and are oriented approximately 55� apart in azimuthon the spacecraft. Coriolis is gravity gradient-stabilizedsuch that orbital motion sweeps a �150� arc across thesky from the slightly overlapping 3� � 60� fields-of-view(FOV) of the cameras, producing a map of almost theentire sky with each orbit. Eyles et al. (2003) describe theSMEI images and give details of the camera design whileJackson et al. (2005) discuss the mission operations.

In addition to CMEs, SMEI also observed many othernatural phenomena such as very high altitude aurorae(Mizuno et al., 2005), the zodiacal light (Buffington et al.,2009), variable stars (e.g. Spreckley, 2008), and comets(Kuchar et al., 2008). Earth-orbiting satellites or residentspace objects (RSOs) were the most common movingobjects observed, being detected at the rate of about oneper minute. Often, the RSOs are far enough away that they

rved.

http://dx.doi.org/10.1016/j.asr.2011.09.006

mailto:[email protected]

http://dx.doi.org/10.1016/j.asr.2011.09.006

Fig. 1. Coriolis satellite with SMEI cameras highlighted on the left. Coriolis is in a Sun-synchronous �840 km altitude, SMEI looks in the anti-ramdirection, outward from the Earth with a line-of-sight 20�–30� above the local horizon to avoid Earthshine and reflections from other equipment onCoriolis. The rotating Windsat instrument is located above SMEI on the satellite. The layout of the fast two-mirror SMEI optical system is shown on theright. SMEI surveys as much as 90% of the sky each orbit; adapted from Jackson et al. (2005).

D.R. Mizuno et al. / Advances in Space Research 49 (2012) 162–176 163

appear to “walk” through the FOV against the backdropof stars over four or more 4-s frames. A unique angularposition, rate, and time of observation may be extractedfrom each frame for such an RSO “track”. About twiceas many RSOs, though, are sufficiently close to formwell-defined streaks in a single frame. An example of sucha streak is shown in Fig. 2; the very fast optics of SMEI

Fig. 2. An isolated streak in Camera 1 detected on January 26th, 2006 at 7:22:5the streak through the FOV nor the direction is known, although it is likelymajority of RSOs show that motion due to the fact that SMEI generally obsertriangular stars are a result of the very fast optics.

produces the curved FOV and quasi-triangular shapedstars that are evident in the figure.

Occasionally, there are sudden large increases in thenumber of RSO observations, up to dozens in a frame(e.g. Fig. 3). A swarm usually has a mix of streaks thatcross the array in one or two frames as well as multi-obser-vation tracks which typically occur as the swarm detections

0 UT. Since the ends of the streak are beyond the FOV, neither the speed ofthat the motion is from top to bottom, the same as the stars, as the vastves objects in higher orbits than Coriolis’. The curved field and the quasi-

Fig. 3. (Top) Nearly empty background-subtracted field in SMEI camera 3. A residual from the star subtraction can be seen in the upper left. The specklesin this image are due to detectors that flip between a high and low state; the problem is due to the camera 3 CCD running warmer than expected. (Secondfrom top) Two frames later, a debris swarm (5) has entered the field, with too many objects to count. The next three images are taken every other framefrom the one before and show the rapid reduction in the length of the steaks. This swarm is seen only in Camera 3, and the RSO streaks are entering thefield of view from almost due sunward. The characteristics of the swarm indicate that the debris came from the Coriolis satellite.

Table 1Debris swarms observed by SMEI.

Swarm Year Day Start UT Duration (min) Cameras #RSOs

1 2009 062 03:32:08 7 1, 2 502 2009 200 08:11:11 8 1, 2 (3 missing) 110a

3 2009 327 10:38:32 8 1, 2, 3 161a

4 2010 016 22:23:56 <1 1, 2, 3 95a

5 2006 341 23:50:48 7 3 33a

6 2010 149 No individual detections, only reflections from optics7 2010 277 06:35:43 2.5 1, 2 41

a These four events have numerous faint and/or overlapping RSO signatures that cannot be reliably measured.

3 C language implementation of SGP4/SDP4 algorithm available athttp://www.qsl.net/5b4az/norad.html.

164 D.R. Mizuno et al. / Advances in Space Research 49 (2012) 162–176

taper off. Most of the six such swarms analyzed in thispaper lasted several minutes, although one consisted ofonly a few densely packed frames. Two of the six swarmsappeared in all three cameras, while three swarms were seenin two cameras, and one was detected only in Camera 3,the camera closest to the Sun. Table 1 lists the observa-tional parameters for the swarms.

The first test for the nature of these swarm RSOs is todetermine if they can be associated with known RSOs.We use the unclassified two-line orbital element sets (TLEs)available from the Joint Space Operations Command, andthe SGP4/SDP4 orbit propagation model (Hoots and

Roehrich, 2006; Vallado et al., 2006),3 to predict the loca-tions of the catalogued RSOs in the SMEI fields of viewand compare with the observed positions. For the swarms,we find associations for about 5% of the objects (16 out of300+ objects), consistent with the one-per-minute nominalrate. In contrast, for ensembles of non-swarm RSOs, wetypically can identify 70–90%. Clearly, the swarms arenot just happenstance clusters of catalogued RSOs.

http://www.qsl.net/5b4az/norad.html


This paper presents an investigation the nature of theswarm RSOs inferred from their observed properties. InSection 2, we describe how orbits for individual isolated(i.e., non-swarm) streaks are estimated from the observa-tions, for reference in the swarm analysis. The procedureis applied to the swarms to estimate general orbital distri-butions of the streaks, which are compared to those pre-dicted if Coriolis were intercepting debris fields thatproduced the swarm observations. The motion relative toCoriolis and parallax distances of the slower movingobjects are described in Sections 3 and 4. The individualswarms are discussed in Section 5. An estimate of the sizeof the objects is presented in Section 6. The conclusion ofthe analysis, summarized in Section 7, is that most of theswarms are well explained as debris ejected from the space-craft itself.

2. Streak analysis

2.1. Streaks from non-swarm RSOs

Streaks are the signatures of RSOs whose path during a4-s frame either crosses the entire FOV of one of the cam-eras, is split over two frames, or is barely contained in a sin-gle frame, with partial paths seen in both adjacent framesas the SMEI integration is essentially continuous. For thelatter two cases the direction of the RSO can be ascertainedfrom the partial paths seen in successive frames. Fig. 2shows a bright streak that traverses an angular distancegreater than the FOV in-scan extent in a single frame.The streak length is taken as the angular distance betweenthe endpoints at the edges of the FOV if the RSO crossesthe FOV in one frame, the longer partial streak if it tra-verses two frames, or the exact length if it is barely con-tained. The nominal length is thus equal to or anunderestimate of the true streak length. RSOs visible infour or more frames provide at least three values of timeand position and are called “tracks” (see Section 3). RSOstreaks usually cross the FOV in the same direction asthe stars, i.e., relative prograde motion.4 Prograde motionfor such streaks can generally be successfully fit with circu-lar orbits using a nonlinear least-squares procedure to opti-mize the orbital elements. In contrast, retrograde streakswould arise from RSOs that overtake Coriolis in a higherorbit and so, strictly speaking, their orbits cannot be circu-lar. Since the direction of an RSO crossing the FOV in asingle frame is unknown, prograde motion is assumed ifthe identifiable streaks for that camera in a given swarmare prograde; for the observed swarms, the identifiablestreaks in a given camera were all in the same direction,i.e., in no case are the streaks crossing the FOV from both

4 The terms “prograde” and “retrograde” as used with respect to the RSOmotion are relative to the direction of stellar motion across the SMEIcamera FOVs, which is from top to bottom of the image with a horizontalcomponent that depends on the camera and the orientation of the object’sorbit with respect to the FOV.

directly above and below a camera. The streak orientationis recorded for retrograde and otherwise unidentifiablestreaks but no specific analysis is done for them.

If both or either end of a streak is outside the FOV, thequestion arises as to the accuracy of the derived circularorbital elements because at least one of the selected end-points is not an exact position-time data point. To examinethis, we derived circular orbits for a simulated streak thattakes 4 s to cross the FOV during the 4 s frame time (thenominal case) and also at angular speeds 2� to 5� fasterthan that. The orbital planes estimated from the differentcases were found to vary by only a few degrees from thenominal to the 5� case. Since the appearance of the RSOas a streak implies that the object is relatively nearby, theorbital plane is essentially defined by the orientation ofthe streak, and not its length. In contrast, the distance tothe RSO is not well characterized as it is a relatively sensi-tive function of the angular speed, for which only a mini-mum is usually known. The nominal cross-FOV casegenerally produces an upper limit of �500 km, a result thatshould also apply to RSO orbits with modest eccentricity,although the uncertainties will be somewhat higher as themaximum distance will be a bit larger.

Fig. 4 shows a diagram in spacecraft azimuth–elevation(az–el) space for all the streaks seen in Cameras 1 and 2from isolated RSOs (i.e., not members of a swarm) foran arbitrarily selected 3-h period (starting at 02:32 UT 29May 2010). The curved lines represent the apparent pathsof objects traveling in circular orbits 100 km above Coriolisincoming from the ram direction (i.e., the spacecraft +xaxis); different heights above Coriolis give similar pathsacross the SMEI FOVs. That the observed streaks gener-ally follow these streamlines, particularly for Camera 1,suggests that they arise from RSOs that are mostly in aplane close to Coriolis’ orbital plane, but going in theopposite direction. RSOs going in the same direction asCoriolis do not have sufficient apparent angular speed toproduce streaks that cross the FOV. From the results ofthe circular orbit fits to these streaks, there is a preponder-ance of orbits near Coriolis’ orbital plane, although a num-ber of RSOs are also in approximately transverse orbitsand seven of these are actually in slightly prograde orbitsrelative to Coriolis. The overall distribution can beexplained by the fact that objects in planes close to Corio-lis’ but going in the opposite direction at higher altitudewill likely be observed twice per orbit, while those inapproximately transverse orbits require fortuitous align-ment to be seen at all in SMEI.

As a check on these deductions, and of the fitting of cir-cular orbits to the streaks, we attempted to identify theRSOs in this sample using the SGP4/SDP4 orbit propaga-tion model and the known RSO TLEs valid for the day ofthe observation. Of the 70 streak RSOs in this sample, 45were identified. For these, the true orbits were determinedfrom the TLEs and compared with the results of the circu-lar orbit fits. The median error in the orbit plane orienta-tion was 3�, the average error 6�, and the worst error 21�.

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Fig. 4. Az–el plot of the non-swarm RSO streaks during a 3-h period. The black curved lines represent the streamlines for objects coming from the ramdirection (i.e., satellites in a higher orbit but in the same plane). The arced boxes show the projected FOVs of the cameras (here and in Figs. 6 and 12–14).


The general finding of retrograde motion was confirmed(only three were prograde), and more than half of the iden-tified RSOs had orbital planes within 10� of Coriolis’ orbit.

2.2. Streaks from the swarms

A series of Camera 2 images from Swarm 1, spanningabout a minute, is shown Fig. 5. The number of countsin the bottom panel shows that a perceptible increase ofstreaks was evident for 3 min. The az–el distribution forthe 31 RSO streaks in this swarm is shown in Fig. 6, asare the ram-direction streamlines for 20 and 100 km heightabove Coriolis. Although some of the streaks are close tothe streamlines, there is no obvious overall preference forthat orientation and there is much scatter in the directions.Discounting a handful of streaks that are close to thestreamlines as due to unrelated and isolated RSOs (i.e.,the �1 per minute background rate over the 7 min timespanned by this event), the remaining streaks have a largescatter in position angles. Fig. 7 shows the circular-fitderived orbits properly oriented with respect to that ofCoriolis, where the viewpoint is looking down on the tan-gent plane. If the orbital planes are accurate to within afew degrees, then they are inconsistent with the interpreta-tion that the swarm is a group of related objects travelingin nearly identical orbits. They cannot be a coherent debriscloud since the orbital planes are all over the map. TheRSO streaks for five of the six swarm events in Table 1show a similar scatter in orbital planes. Assuming thatthe swarms are composed of associated objects, not justfortuitous convergences of unrelated RSOs, then the orbitsdetermined from the circular fits are not realistic, a conclu-sion that also applies for slightly eccentric orbits. Thus, thestreaks in this swarm cannot be due to a coherent cluster ofRSOs with near-circular orbits.

Furthermore, since the RSOs in a debris cloud would bein approximately the same orbit, their velocity vectorsshould be nearly parallel and SMEI would observe thestreak paths across the CCD to follow a regular patternof streamlines, such as illustrated in Fig. 6 but alignedalong some other convergent point. The position anglesfor the Swarm 1 streaks clearly do not uniformly changeacross the cameras’ FOVs. If the RSO swarm has a largephysical extent, then the streamlines can be at different

angles with respect to a given line of sight and may evencross each other, as demonstrated in Fig. 6 for the two dif-ferent heights. However, the crossings occur only at dis-tances large relative to the size of the swarm. Forexample, if Swarm 1 were �100 km in diameter, thestreamline crossings would occur at about 700 km andmuch closer to the horizon than the SMEI FOVs. If theobserved scatter in position angles was instead caused bysome velocity dispersion of the streaking RSOs, then themagnitude of the dispersion would have to be on the orderof the orbital speed to produce what is seen in the observa-tions, which is counter to the assumption that the swarm isa coherent entity.

If the swarms are groups of related objects traveling innearly the same orbit, then the only reasonable explanationconsistent with the observations is that they are all in anorbit very close to that of Coriolis. In such a case, eitherCoriolis is passing through a small cloud of nearby objects,or the objects are particulate material being ejected fromCoriolis itself. The cloud explanation is unlikely as theobserved scatter in position angles implies a velocity disper-sion that is roughly of the same magnitude as the speed ofthe cloud relative to Coriolis. This is inconsistent with themuch smaller dispersion than relative speed that is neces-sary for a cloud to maintain coherence. This suggests thatthe swarms are not due to recent collisions or break-ups ofsatellites in low Earth orbit as was originally thought.

In contrast, the scatter in the position angles can beexplained by particles that are ejected from different partsof the satellite more or less simultaneously. To better dis-tinguish between these two possibilities, we examine otherglobal characteristics of swarm RSOs and then considerthe swarms individually.

3. Track analysis

Track RSOs, i.e., those with four or more observationsin consecutive frames, are less numerous than streak RSOsin the swarms by at least a factor of three: 110 vs. 354streaks for the 6 swarms. This is somewhat higher thanthe ratio for non-swarm RSOs, which ranges from �1.5to �3. The swarm RSO tracks have a mean apparent angu-lar speed about twice that of non-swarm tracks, whichtogether with the larger fraction of streaks, suggest that

Fig. 5. Camera 2 images of Swarm 1 (top). This series of 14 consecutive images spans about a minute. Debris is evident as the bright streaks in the lowerleft frames. However, about half the remaining frames contain fainter moving sources. The bottom figure shows the count rate per minute that indicatesthat the swarm was observed over 3 min, from 3:32 to 3:35.

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Fig. 6. Az–el plot of the streak RSOs for Swarm 1. The ram-direction streamlines for both 20 km and 100 km circular orbits above Coriolis are shown.Note that the streamlines have converged by the time the objects enter the camera’s FOV.


Coriolis Camera 2 Camera 1

Fig. 7. Results of the circular orbit fits for the RSO streaks in Swarm 1.The field spans about 2000 km on each side.

5 A seventh swarm has just recently been detected but is not included inthis analysis.

6 For interpretation of color in Figs. 1–3, 5, 9–11, the reader is referredto the web version of this article.


the swarms are relatively close to Coriolis, in agreementwith the conclusions using a different train of logic in theprevious section.

Can the angular motion of the RSOs indicate the originof the swarms? Unfortunately, the circular orbit-fittingprocedure described above for the streaks does not giveaccurate motions at the inferred small distances betweenthe SMEI cameras and the debris, because the relativemotion is dominated by small deviations in the orbitalparameters from those of Coriolis, which itself is notstrictly circular. Instead, the relative motion is assumedto be approximately linear for brief periods in a coordinatesystem that co-moves with Coriolis. We perform a linearleast-squares fit of position vs. time in az–el space on anRSO track for up to 40 s, i.e., 10 measurements. The fititeratively optimizes the position and velocity vectors atthe time of the first observation. Initially, the first and lastobservations are set at unit distance and the correspondinginitial velocity is estimated from the distance and timebetween the observations. The fit is in terms of unit dis-tance since a linear position vs. time solution to a set ofaz–el data can be uniformly scaled in radius to producepositions that also identically fit the data. The directionof the velocity vector is invariant with respect to the radialscaling, and so the direction an RSO travels relative toCoriolis may be determined, but the distance informationcannot. Solutions with az and el rms errors greater than0.25� (about three times the positional measurement error)are discarded, eliminating RSOs that have poor fits or poorquality data.

Fig. 8(left) shows the 48 of the 110 RSO tracks in the sixswarms that have valid linear fits. The fits are normalizedto the position on the unit sphere at the initial observation

and projected into the horizontal or tangent plane of theCoriolis orbit, i.e., looking from above; the Coriolis veloc-ity vector is down in the plot, and the Sun is to the right.The right panel shows the results from Camera 1 for aset of non-swarm RSO track observations, in which 83 of393 tracks had valid fits.

The velocity vectors are expected to be primarily radialfor ejecta but the exact track paths depend, in part, on thedistance to the RSOs — very close by objects can be trans-verse to the line of sight but objects observed farther awaymust have increasingly radial motion. If a derived velocityvector within 45� of the initial position vector is consideredradial, then about 59% of the RSOs in the left panel ofFig. 8 are moving radially. The radial fraction in a randomensemble of velocity vectors, such as for non-swarm RSOtracks, should be about 15%. Indeed, the non-swarm tracksin Fig. 8(right) have a radial fraction of 12%, consistentwith a random distribution of relative velocities.

If the swarms are RSO clouds, there should be no pre-ferred incoming direction if the clouds are moving at smallspeeds relative to Coriolis, and the relative velocity vectorsover all the swarms should be approximately random.Since only 6 swarms were observed,5 some of the velocityvectors may be aligned by happenstance to give an appar-ent radial motion. Regardless, the velocity fits for a givenswarm would be expected to produce approximately paral-lel tracks. A common direction for a number of the longtracks does seems to exist at a 45� angle to the upper leftin the left hand plot of Fig. 8. All of these tracks are fromSwarm 3, but this swarm also shows several tracks in atleast two other common directions. Swarms 1, 4, and 7have only one or two valid fits each, which is insufficientto form a conclusion. The velocity vectors for Swarms 2and 3 are distributed over a large range of angles. In onlyone, Swarm 5, are the velocity vectors generally consistentwith an ensemble of objects with a uniform motion; thesecomprise most of the Camera 3 tracks in Fig. 8.

4. Distance estimates from parallax

The edges of Cameras 1 and 2 FOVs overlap by about15 deg2. If an RSO appears within the correspondingframes for each camera in the overlap region, a parallaxcan be derived to calculate the distance, or an upper limitto the distance can be determined if the RSO appears inonly one of the cameras.

Fig. 9 shows an example of an RSO that is observed intwo cameras, incidentally about 7 min prior to the onset ofSwarm 2. The anti-sunward end of Camera 2 is on the leftand the sunward end of Camera 1 is on the right with theoverlapping FOVs outlined in green.6 The green squareshows the observed celestial coordinates of the RSO seen

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Fig. 8. Results of the linear motion fits for the RSO tracks from all six swarms (left) and a sampling of an equivalent number of non-swarm track RSOs(right). Forty-eight of the 110 tracks in the swarms produced valid fits. The position vectors are projected into the tangent plane, and are scaled to unitdistance at the initial position. The spacecraft velocity is “down” and the sun is approximately to the right. The numbers 1–3 indicate which cameraobserves each sector. The 83 non-swarm tracks with valid fits at the left are from the Camera 1 observations displayed in Fig. 4.

Fig. 9. Overlap region between Cameras 1 (right) and 2 (left) (green trapezoidal outlines). The bright RSO near the lower left edge of Camera 1 maps intothe green square in the Camera 2 array. The white line shows the direction of the expected parallax shift, if such existed (i.e., the object would appear upand to the right). In this example, there is no detectable parallax shift, which implies a minimum distance of �200 m.


in Camera 1 mapped into Camera 2. The approximatedirection of a parallax shift is given by the white line, whichis roughly at constant elevation as the two cameras are atabout the same satellite “z” coordinate. A measurable par-allax would move the observed RSO position up and to theright of the green square but there is no measurable shift inthis example. Cameras 1 and 2 are separated by about0.72 m nearly perpendicular to the line of sight of the over-lap region. Assuming the maximum parallactic shift of 0.2�(the size of a pixel) that could occur and not be detected,the minimum distance to this RSO is about 200 m.

Fig. 10 shows a Swarm 2 streak visible in Camera 1 butmissing in Camera 2. The green boxes map the endpoints ofthe streak into the overlap region. Conservatively, there isnothing visible in the Camera 2 image within about 2� ofthe mapped positions of the endpoints along the directionof the parallactic shift. This implies that this streak is cre-ated by an object within about 20 m of Coriolis.

Fig. 11 shows the single case of an RSO in the swarmdata that is visible in both cameras with a measurable par-allax. This is a track RSO visible in both cameras for 10consecutive frames. The measured position in a single

selected Camera 1 frame of the track is shown as the greensquares in each panel, while the observed Camera 2 posi-tion is indicated with a white square. The angular separa-tion is about 1.6�, giving a distance of about 26 m. Theangular separation decreases from about 2.1� to 1.3� overthe entire track, suggesting that the RSO moved outwardfrom about 20 m to 30 m distance during the 10 frames.This translates into a speed of about a quarter meter persecond. It must be noted, though, that the parallax shiftis in a direction of �45� from that expected and that thebrightness in Camera 1 seems to be less than half that inCamera 2. Thus, it is possible that these tracks are due tothe fortuitous alignment of two separate objects, each ofwhich is closer than 20 m from Coriolis. However, the full10-frame tracks for this object in the overlap region havethe same direction and similar spacing in both cameras.

A total of 31 RSOs from the five swarms that were seenin both Cameras 1 and 2 were observed in the overlapregion. Of these, 26 were seen in only one camera, indicat-ing minimum parallactic shift of 2�; at least threeunmatched RSOs were seen in each of the five swarms.Four RSOs were seen in both cameras with exactly

Fig. 10. A Swarm 2 RSO that appears in the overlap region in Camera 1 but not in Camera 2. The green boxes show the endpoints of the RSO streakmapped into the Camera 2 array. The absence of the RSO within about 2� along the parallactic shift direction indicates the RSO is closer than about 20 m.

Fig. 11. An RSO track that is (possibly) seen in both cameras with some parallactic shift. The green box displays the Camera 1 position in both camerasand the white box highlights the RSO in Camera 2. The parallactic shift is about 1.6�, giving a distance of about 26 m. However, the shift is not quite in theexpected direction, and the RSO has a very different brightness in the two cameras. The two tracks on subsequent frames, though, do have similardirections and spacing.


matching positions. However, two of these occurred a cou-ple of minutes after the swarm apparently had ended andthus could reasonably be considered “ordinary” RSOs.The last event is the case described in the previous para-graph, which is either one nearby RSO or two even nearer.If the RSOs observed in the overlap region are a randomsampling representative of the swarms, then the largemajority of the swarm RSOs are within about 20 m ofCoriolis when they are seen.

In contrast, only seven RSOs were observed in the over-lap region during the 3 h non-swarm period used for theresults in Fig. 4, plus an additional 40 min of observationduring a period of elevated RSO activity prior to the onsetof Swarm 2, with no apparent relation to the actual swarm.All seven were seen in both cameras without a measurableparallax shift.

Of the 26 unmatched swarm RSOs, three from oneswarm had positional associations with the same distantcatalogued RSO (�25,000 km), which seems to be a coinci-dental alignment as SMEI is generally unable to detectsatellites at that distance. The remaining 23 had no associ-ations. Of the four with exact matches, two had catalog

associations, and of the seven non-swarm overlap RSOs,four had associations.

5. Analysis of individual swarms

5.1. Swarms 1, 2, 7

These three swarms are very similar in that they are seenonly in Cameras 1 and 2, the RSOs enter the FOVs fromabove at position angles that range from slightly anti-sun-ward to due sunward, and they have a mix of streaks andtracks lasting several minutes. Fig. 12 shows the paths ofthe RSOs for these swarms. A small fraction of the streaksand tracks that are close to the ram-direction streamlines,shown for reference in the top panel, are likely normalRSOs that just happen to occur at the same time as theswarm. The remaining RSOs exhibit a large scatter of posi-tion angles, particularly for Swarms 1 and 2. This scatter inangle suggests that these swarms are not clouds of smalldebris that Coriolis is passing through. In contrast, a fixedvelocity field projected into az–el space would produce asmoothly varying pattern of streak and track directions.

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Fig. 12. Az–el plots of all RSOs in Swarms 1, 2, and 7. The ram-direction streamlines are shown on the Swarm 1 plot. Swarm 2 occurred during a northpolar passage, a period of enhanced RSO detectability, so several of these RSOs may be “background” objects.


The RSOs in Swarm 7 have more consistent positionangles, if the few with deviant angles are disregarded aspart of the ordinary population of RSOs, and could con-ceivably be a passing cloud of objects. There are 10 trackRSOs in this swarm, so the cloud must be large enoughto include objects that are far enough away to producethe tracks, which are at least 4 times more distant thanthe cross-FOV streaks, if a fixed relative speed is assumed.For a uniform cloud density, the number of objectsobserved at a given distance should increase as R2, andthe total number out to a given radius should increase asR3. The number of streaks that cross the FOV shouldtherefore be not much more than a few percent of the total,with the remainder being partial streaks and tracks. On thecontrary, cross-FOV streaks comprise more than 50% ofthe RSOs in this swarm. If this is a cloud, the density mustfall off very steeply with distance from Coriolis.

To demonstrate that the varying streak and track posi-tion angles can arise from ejecta, we assume that the parti-cles are simultaneously ejected from one or more locationson the satellite. Over all the swarms, all of the RSOsobserved in Camera 1 and 2 enter the cameras from a rangeof directions from directly above the cameras to due

sunward. It is geometrically possible for the source of theseRSOs to be on the satellite structure since Cameras 1 and 2are mounted near the bottom of the satellite body and thesolar panel is due sunward. Camera 3 is mounted about ameter higher on the Coriolis body than the other cameras(see Fig. 1). The RSOs detected by Camera 3 come fromabove, below, and sunward directions towards which eitherthe solar panel or the Windsat structure are located. For anensemble of objects being ejected in random directionsfrom the satellite, the averaged spatial density should falloff as 1/R2, where R is the radial distance from Coriolis.The observed volume of space at a given distance increasesas R2, so the number of objects in each distance rangeshould be roughly equal. Indeed, the proportion of fullstreaks, partial streaks, and tracks over all the swarms isabout the same.

Note that the fields of view for Cameras 2 and 3 areentirely outside the shadow of the satellite body, and forCamera 1, the solar panel puts approximately half a meterto two meters in front of the camera into shadow, depend-ing on the line-of-sight direction, time of year, and positionin the orbit. (There are a few examples in which a trackRSO abruptly appears or disappears in the middle of the


Camera 1 FOV.) Thus, objects in the immediate vicinity ofCoriolis will be visible, save for a small region close toCamera 1.

5.2. Swarm 3

This swarm displays the most convincing evidence thatthe RSO source(s) must be on the satellite itself. The RSOsseen in Camera 3 are all streaks crossing the short axis ofthe FOV from beneath the camera, while for Cameras 1and 2, the RSOs all enter from due sunward or a somewhathigher angle. Fig. 13 displays an az–el plot for this swarmin which the streamlines are shown for objects radiatingfrom a point on the trailing edge of the solar panel about0.1 m below the z-coordinate of Camera 2. The higher-angle tracks in Camera 2 can be modeled by moving thesource about 0.3 m higher on the solar panel, whichchanges the Camera 3 streamlines only slightly. Note thatthe streamlines are straight-line representations of the par-ticle motion so the paths depicted by the lines are onlyapproximate, particularly for the slower objects.

The swarm lasts for about 8 min in Camera 1 and 4 minin Camera 3; information is lost for Camera 2 after 3 mindue to saturation by the Moon. The fastest objects withthe longest streaks in each camera appear first and thenare followed by slower objects. This is also the case forSwarms 2 and 5. This behavior can be explained by a debrisfield that expands outward at a constant speed in which thenearest objects, which also have the highest apparent angu-lar speed, cross into a camera’s FOV first, before thosewhich are further away. This scenario requires that allthe particles were launched in a very short time relativeto the total time the swarm was observed; otherwise longstreaks would be seen throughout the duration of theswarm.

5.3. Swarm 4

Fig. 14 shows the Swarm 4 streaks, all of which cross thecameras in a relatively short time, about 20 s, with a fewstragglers for another 20 s thereafter. The debris entersthe FOVs of the cameras from above (for streaks thatappear on two frames, the upper portion always appears

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Fig. 13. Az–el plot for Swarm 3. The streamlines for objects radiating away froare shown. A point about a third meter higher will produce the higher-angle

first). This is consistent with a cloud of RSOs speeding pastCoriolis from a direction somewhere above. The shortduration and the absence of slower RSOs imply that thecloud may be compact enough that all of it passes throughthe camera FOVs without any object being far enoughaway to produce a track. How Coriolis could preciselyintersect such a small cloud of objects is, however,problematic.

On the other hand, the nearly direct cross-FOV paths ofthe debris in all the cameras indicate that the source ofejecta is a localized region somewhere high on the satellitebody, likely the rotating mechanism for Windsat. Fig. 14also compares the observed paths with streamlines fromthe cloud model in the middle panel and from the ejectamodel at the bottom. The cloud comes in from a radiantat az = 290�, el = �72� while the ejecta are modeled tooriginate from the rotating Windsat structure, 3 m aboveCameras 1 and 2 at a radius of 0.7 m from the rotationaxis, and released at the tangent velocity such as in a lawnsprinkler. The differences are subtle, but the ejecta modelappears to predict the position angles of the streaks moreaccurately than the cloud model; in particular it reproducesthe trending along the extent of Cameras 1 and 3 simulta-neously while the cloud model can be adjusted to matchone or the other but not both.

5.4. Swarm 5

Fig. 3 shows every other frame of the first eight framesof this swarm, which is almost entirely confined to Camera3. The very long streaks that extend along the long axis ofthe FOV enter the camera almost directly from the sun-ward direction. Subsequent frames are populated byshorter elongated tracks that move along the long axis,with numerous slower and mostly fainter tracks on laterframes.

Superficially, this is what is expected from a cloud: theRSO paths are all initially parallel and a much larger pop-ulation of slower tracks from the presumably more distantRSOs is seen compared to the fewer bright, fast streaks.But this behavior is also expected from the ejecta model,in which the ejection point is somewhere to the sunwardside of the camera, close to the sunward end of the camera

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m a point on the trailing edge of the solar panel about even with Camera 2tracks in Camera 2 and leave the Camera 3 tracks only slightly changed.

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Fig. 14. Swarm 4. A single Camera 1 frame near the maximum number of streaks is shown at the top. Streamlines for a “cloud” model are shown in themiddle az–el plot, with the cloud coming from a direction of az = 290�, el = �72�. The bottom shows the results of the ejection model with the particlesemanating from a point high on the rotating Windsat structure in which it was assumed that particles were release with a small tangent velocity, lawn-sprinkler fashion, to improve the subjective fit. The particle speed is estimated to be about 8 m/frame. The SMEI fields-of-view are <1 m across at theejecta plane, which accounts for the fact that nearly all the RSOs are complete streaks.


baffle, with a large number of particles traveling along vec-tors parallel to the various lines of sight in Camera 3.

The convergence point deduced for particles in a debriscloud from the longest initial streaks is near or beyond thefar end of Camera 3. Since the other cameras see very few(if any) RSOs, the cloud would be just grazing the Camera3 side of Coriolis. Nearby objects in a spherical cloudwould produce the longest streaks, which would be obser-vable for roughly the first half of the time that the cloudpasses through the FOV, while objects of all angular speedsshould be seen throughout. Instead, the longest streaksoccur only on the first frame or two while the streaks arenoticeably shorter in the second frame than those in thefirst, after which there is a steady decrease in angular speedof objects entering the camera’s FOV as time advances.There are also a small number of bright stragglers withhigher-than-expected angular speeds that appear later.

For a debris cloud, all the objects should converge to asingle radiant on the sky, whereas ejecta would converge toroughly over whatever range of velocity directions the par-ticles initially have. Unfortunately the brightnesses of theobjects generally fade before the convergence points canbe ascertained. Although the convergence point of thecloud must be at az P �210� at the end of or interior toCamera 3, the linear fits to the tracks, shown as the Camera3 results in the left plot of Fig. 8, are at greater azimuthswith an average of about az = 235�, el = �29�, just abovethe middle of Camera 3. If this swarm were a cloud, the lin-ear fits should give the approximate convergence point, butmany of the initial streaks and fast tracks cross, and do notconverge on, that coordinate.

The observations may be better explained as ejecta inwhich the particles are radiating outward with a constantspeed over a large range of angles from a point near thetop edge of the solar panel about a meter forward of Camera3. The first debris to enter the FOV are those with velocityvectors that pass closest to the Camera 3 baffle, and wouldthus have the highest angular speed. Particles appearing laterare those with vectors closer to the Camera 3 axis and wouldthus have increasingly smaller angular speeds.

6. Estimate of particle sizes

We need the distance to the particles to calculate theirsizes, and we have a specific distance only for the one caseof parallax analysis. We can, however, make reasonableinferences of the distances for three other situations, albeitwith the caution that these will result in only very approx-imate values for the sizes.

The angular size of the observed particles can be calcu-lated directly, with certain assumptions made about thealbedo and illuminated portion of the object. For this pur-pose, we use brightness measurements of Jupiter and a scal-ing argument to determine angular size: a 24 June 2010SMEI observation of Jupiter, which subtended an angulardiameter of 3800 being 4.86 AU from Coriolis and 5.0 AUfrom the Sun, yielded an instrumental flux of 9.7 adu-steradians, where an adu (analog-digital conversion unit)is a SMEI image brightness unit corrected for flatfieldingand optical inhomogeneities over the focal plane with anestimated error of 10%. (The calibration to science unitsis 1 adu � 6.1 erg s�1 cm�2 sr�1 in in-band radiance). We


assume that the particles are half-illuminated (a nearbysphere observed in the center of Camera 2 is approximatelyhalf-illuminated by the Sun), and have the same albedo asJupiter (0.52; the albedo of RSOs is often assumed to be0.5). The measured flux of a particle is multiplied by 2 (illu-mination correction), and divided by 25 (conversion toJupiter’s solar radiation field). The angular size relativeto Jupiter is then the square root of the ratio of the twofluxes.

We have one case of a directly measured distance, theRSO seen in the overlap region between Cameras 1 and 2described in Section 4. In the Camera 2 frame at whichthe parallax-determined distance was 24 m, the measuredflux is 0.025 adu-steradians, giving an angular size of0.3800. Applying the distance gives a particle diameter of60 lm. A similar analysis for the Camera 1 image of theRSO gives a diameter of 40 lm, as the observed brightnessis only about 40% that in Camera 2.

There are also sufficient observations of tracks andstreaks over all the swarms that we can make the reason-able assumption that the very brightest RSO observationis an object at close to the nearest plausible distance. Thebrightest object overall is actually a track RSO in Swarm1 and has a flux of 9.2 adu-steradians giving an angular sizeof 7.400 (the few next brightest objects are all streaks and sothe measured fluxes are lower limits). This object isobserved near the center of Camera 1. Since the solar panelshadows this line of sight out to a little under 1 m from thecamera at the time of observation, taking the lower boundof 1 m as the distance gives a diameter of 50 lm.

We can use the modeled geometry of the Swarm 4objects to estimate distances. In this case, we assume thatthe particles are being flung from the rotating mechanismof the Windsat instrument at the tangent velocity, and thatthey are thus traveling outward in a horizontal plane. Tak-ing the release point as 3 m above Cameras 1 and 2, and ata radius of 0.7 m, as in Section 5.3, the particle speed is2.2 m per second or 8.8 m during a 4-s frame. For a givenstreak, the intersection point of the line of sight and theejecta plane can be calculated from the line of sight eleva-tion, and thence the distance and the fraction of the total 4-s particle path observed over the streak. The total flux isestimated by scaling the measured flux by the inverse ofthe fraction (i.e., as if the particle had been observedentirely at the center of the streak for the 4-s frame dura-tion). With this analysis, 70 streaks recorded over the threecameras had resulting particle diameters in the range from50 lm to 140 lm, with a median of 80 lm and the peak at70 lm. Fig. 15 shows the distribution. (One very brightCamera 3 streak gave 370 lm and is not included in the fig-ure.) There were also numerous streaks too faint to mea-sure reliably so the lower limit is likely 30 lm or less.

It should be noted that in this analysis, the error in thediameter depends on the square root of the fractionalerrors in both the height of the ejecta plane and the tangentspeed, each of which, considering the geometry of theWindsat structure should not be in error by more than a

factor of �1.5. The error in the albedo is certainly not morethan a factor of 2. Thus, the overall error in the diameterscale of this distribution should not be in error by morethan a factor of 2.

Finally, we estimate the location of the particle sourceon the satellite and the time of ejection as inputs to the lin-ear fitting procedure presented in Section 3, to approxi-mately position track observations in space and therebydetermine distances to the observed points of the track.The linear fitting produces relative distances to the obser-vations but lacks an absolute distance scaling factor. Theapproach here is to assign a particle ejection time as a freeparameter, and back project the linear fit to the ejectiontime. Applying the scaling factor then locates the particleejection point on the satellite. With plausible constraintson the ejection point, the distance scaling and the ejectiontime can be determined.

Swarm 3 and Swarm 5 have ensembles of track observa-tions suitable for this analysis: in particular tracks withhigh initial angular speed, and which cover a large totalangle, to give the most reliable linear fits. In Swarm 5,numerous long streaks and tracks enter Camera 3 fromthe sunward direction, nearly parallel to the long axis ofthe camera. We selected eight tracks that start within afew frames of the swarm onset, to give a reasonable initialestimate of the ejection time. The only plausible source ofthe particles from the due sunward direction in Camera 3is the solar panel. The long axis direction intersects thesolar panel at the nearest �1 m from the camera and atthe farthest �2 m. The solar panel is �0.3 m in the sun-ward (-Y) direction from Camera 3 and is parallel to theX–Z plane. The constraints applied in this case are thatthe ejection point is Y = �0.3 m and is 1.5 m from the cam-era in the X–Z plane. The initial ejection time is selected as1 frame prior to the initial appearance of the swarm andadjusted by hand for each track, along with the scaling fac-tor, to match the two constraints. The derived distances arethen combined with the measured fluxes as before (usingthe first or second observation in the track depending onwhich is less cluttered) to determine the particle diameters.In this situation, however, the observations are closeenough to the Sun (35�–50�) that the illumination fractionsare calculated case by case (�0.1 to 0.2), again assumingspherical particles.

For the eight Swarm 5 tracks used in this analysis, wefind particle diameters ranging from 20 lm to 140 lm, gen-erally consistent with the previous results. The median dis-tance and speed are 6 m and 0.3 m s�1. The primarysources of error are the distance estimate (�p2 from theuncertainty in the source location), the flux measurementsas they are obtained from generally very cluttered frames(�30% error), and the uncertainty in the illumination frac-tion as it is highly dependent on specific particle shapewhen viewing close to the Sun. A net error of at least a fac-tor 2 is likely.

For Swarm 3, the tracks enter Cameras 1 and 2 fromroughly the sunward direction (see Section 5.2); again,

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Fig. 15. Distribution of particle sizes derived from the streak and track observations for Swarm 4 using the methods described in the text. A single datumat 370 lm is omitted.


the only plausible source is the solar panel, and addition-ally the streak directions in Camera 3 suggest a sourcealong the trailing edge. For the constraints in this case,we specify the ejection X and Y coordinates as the trailingedge of the solar panel relative to each camera. The eighttracks analyzed from this swarm produced in particle sizesfrom 1 lm to 16 lm, with a median distance and speed of2 m and 0.15 m s�1. These much smaller sizes result fromgenerally lower measured fluxes than those for Swarm 5(about a factor of two), larger assumed illumination frac-tion (about a factor of three), and nearer distances fromthe fits (also about a factor of three). Note that the bright-est streaks and tracks in this swarm are about an order ofmagnitude fainter than the brightest seen in these camerasin Swarms 1, 2, and 7.

7 http://lasco-www.nrl.navy.mil/index.php?p=content/debris.

7. Conclusions

The SMEI cameras monitor almost the entire sky eachorbit and observe the background of RSOs at the rate ofroughly one per minute. However, occasionally (at an aver-age of �once per year), a burst of objects is seen with manystreaks and tracks detected in single frames. These bursts lastfrom less than a minute to several minutes. Severalapproaches were used to determine where these swarms werelocated: circular orbit fits to the streaks show that the objectsare not in a common orbit distinct from Coriolis; rectilinearfits to the tracks indicate the objects are generally movingaway from Coriolis; and parallax considerations suggestthe objects are nearby, within tens of meters. Together theseanalyses indicate that most if not all of the objects that con-stitute the swarms are nearby objects and arise from theCoriolis spacecraft itself. Indeed, the location on the space-craft from which the debris is produced could be estimatedin some cases. For example, Swarm 4 was most accuratelymodeled by ejecta from a rotating source on Coriolis,

namely the Windsat instrument, and Swarms 3 and 5 arelikely from the solar panels.

The sizes of the particles that constitute the swarms aredetermined from their brightnesses and distance estimatesto have diameters of between 20 and 100 lm, althoughone swarm possibly consists of particles less than 20 lmin diameter. The ejected particles therefore have little mass,which is consistent with the spacecraft attitude and healthstatus, that indicated that there were no anomalies at thetimes of the swarms. Thus, impacts on the spacecraft orthe vibrations from Windsat’s rotation or flaking of surfacematerial due to aging that causes the debris were not vio-lent enough to perturb normal operations. Swarms ofstreaks have also been observed by the Large Angle andSpectrometric Coronograph Experiment (LASCO, Brueck-ner et al., 1995) that have also been interpreted as debrisfrom an impact on their parent spacecraft.7 The observed�1 event per year over a large area of sky (>500 deg2)observed in the SMEI data is consistent with the numbersa more thorough examination of the LASCO observationsfound, a few to several per year, and provides a baseline forwhat is expected for optical sensors in Earth orbit.

References

Brueckner, G.E., Howard, R.A., Koomen, M.J. The Large Angle andSpectrometric Coronograph Experiment (LASCO). Solar Phys. 162,357–402, 1995.

Buffington, A., Bisi, M.M., Clover, J.M., Hick, P.P., Jackson, B.V.,Kuchar, T.A., Price, S.D. Measurements of the gegenschein brightnessfrom the Solar Mass Ejection Imager (SMEI). Icarus 203, 124–133,2009.

Eyles, C.J., Simnett, G.M., Cooke, M.P., Jackson, B.V., Buffington, A.,Hick, P.P., Waltham, N.R., King, J.M., Anderson, P.A., Holladay,P.E. The Solar Mass Ejection Imager (SMEI). Solar Phys. 217, 319–347, 2003.

http://lasco-www.nrl.navy.mil/index.php?p=content/debris


Hoots, F.R., Roehrich, R.L. Models for propagation of NORAD elementsets. Spacetrack Report No. 3, ADA093554, 2006.

Howard, T.A., Webb, D.F., Tappin, S.J., Mizuno, D.R., Johnston, J.C.Tracking halo coronal mass ejections from 0–1 AU and space weatherforecasting using the Solar Mass Ejection Imager (SMEI). J. Geophys.Res. 111, A04105, 2006.

Jackson, B.V., Buffington, A., Hick, P.P., Altrock, R.C., Figueroa, S.,Holladay, P.E., Johnston, J.C., Kahler, S.W., Mozer, J., Price, S.,Radick, R., Sagalyn, R., Sinclair, D., Simnett, G.M., Eyles, C.J.,Cooke, M.P., Tappin, S.J., Kuchar, T., Mizuno, D., Webb, D.F.,Anderson, P.A., Keil, S.L., Gold, R., Waltham, N.R. The Solar MassEjection Imager (SMEI) mission. Solar Phys. 225, 177–207, 2005.

Kuchar, T., Buffington, A., Arge, C.N., Hick, P.P., Howard, T.A.,Jackson, B.V., Johnston, J.C., Mizuno, D.R., Tappin, S.J., Webb,

D.F. Observations of a comet tail disruption induced by the passage ofa CME. J. Geophys. Res. 113, A04101, 2008.

Mizuno, D.R., Buffington, A., Cooke, M.P., Eyles, C.J., Hick, P.P.,Holladay, P.E., Jackson, B.V., Johnston, J.C., Kuchar, T.A., Mozer,J.B., Price, S.D., Radick, R.R., Simnett, G.M., Sinclair, D., Tappin,S.J., Webb, D.F. Very high altitude aurora observations with the solarmass ejection imagers. J. Geophys. Res. 110, A07230, 2005.

Spreckley, S.A. A study of variable stars and search for extrasolar planetswith the space based photometers SMEI and STEREO. PhD Thesis,Univ. Birmingham, UK, 2008.

Vallado, D.A., Crawford, P., Hujsak, R., Kelso, T.S. Revisiting Space-track Report No. 3, AIAA-2006-6753, 2006.

debris swarms seen by smei

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