Yesterday, Marco Langbroek brought word of the vital clue that the fireball moved at orbital velocity, enabling the meteor hypothesis to be discarded: http://satobs.org/seesat/Sep-2014/0097.html Based on that news and the information I will present in this message, I am now confident that the object that re-entered was uncatalogued debris of Cosmos 2495 (14025A / 39732). 1. Ballistic Properties In my last post, I reported that a hypothetical piece of debris jettisoned at the approximate time of the de-orbit burn (2014 Sep 02 17:40 UTC), made to decay near the time the fireball occurred, would have passed at about the correct time, and in reasonably good agreement with the available trajectory data. Below is the TLE that I created: 1 70001U 14245.73611113 .13400000 00000-0 78682-2 0 01 2 70001 81.3770 299.0950 0034344 130.9033 134.4989 16.17169265 09 Subsequent analysis enables me to report that the corresponding ballistic properties would have been similar to those of several other pieces of catalogued debris from previous launches of the same model of spacecraft. 1.1 Area to Mass Ratio The area to mass ratio (A/m) that yields reasonable agreement with the time and trajectory of the fireball observed over the U.S. on 2014 Sep 03 UTC near 04:33 UTC has been estimated using GMAT R2013a (General Mission Analysis Tool), "developed by a team of NASA, private industry, public, and private contributors". The analysis was performed using GMAT's Dormand-Prince 78 numerical integrator, with a four degree, four order gravity field, and the MSISE90 atmosphere model, with actual space weather data entered manually. The above TLE was converted for GMAT propagation using TLE Analyzer 2.12. Assuming co-efficient of drag (Cd) of 2.2, the analysis yielded A/m = 0.0773 m^2/kg. USSTRATCOM (U.S. Strategic Command) has catalogued 12 pieces of debris from five of the previous eight Kobal't-M missions. TLEs are available for eight of those objects. The TLEs of four objects were sufficient in number and time span to reasonably estimate A/m using GMAT. GMAT was used to propagate the orbit of one TLE to the epoch of a later TLE. Typical spans were several hours. The A/m was adjusted by trial and error to obtain the closest match between the propagated and actual perigee and apogee of the later TLE, near its epoch. Cd was assumed to be 2.2. The resulting A/m in m^2/kg are summarized below: Cosmos SSN COSPAR A/m 2410 28502 2004-038D 0.0655 2420 29258 2006-017D 0.0686 2445 33969 2008-058C 0.0790 2445 33970 2008-058D 0.0685 A larger sample could be wished for, but it seems reasonable to conclude that the hypothetical debris of Cosmos 2495 had similar ballistic properties to the debris shed prior to landing by earlier spacecraft in the series. These values are much greater than those of large spacecraft or rocket bodies, but typical of small, low mass debris. 2. Time of Debris Jettison A significant unresolved question was the time of jettison of the debris. Since most such debris from past missions was first detected on or about the day that the spacecraft landed in Russia, it was clear that jettison must have been prior to the de-orbit burn, but the proximity to the burn was unclear. To determine the approximate time that the debris was shed relative the de-orbit burn, I attempted to perform conjunction analyses between debris and parent spacecraft. In the case of the first mission, 04038A / 28396, no special analysis was required to see that both pieces of debris were shed nearly one day prior to landing. This seems to have been the exception. On later missions, separation occurred closer to the de-orbit burn, but the relative time could not be determined with precision. Due to the combination of low orbit and high A/m, it was impossible to obtain close, reasonable-looking conjunctions. In a few cases, SGP4 yielded a not too terrible-looking result, that turned out to be physically impossible because it would have occurred later than the time of the de-orbit burn. 06017D / 29258 yielded the best result. Conjunction to within 0.95 km was obtained, but ~42 min. after de-orbit; however, even at de-orbit, the along-track separation was less than 15 km (a couple of seconds of time at orbital velocity). At de-orbit there was several kilometres of altitude mismatch, but ~15 min., before de-orbit, the altitudes nearly matched, and the along-track difference was less than 17 km. GMAT was tried with a couple of the cases for which it had been used to determine A/m. The propagation was simply performed in reverse, and positions manually compared with those of the parent object. In the case of 06017D / 29258, the along-track difference reached a minimum of ~15 s near the time of the de-orbit burn. For 08058C / 33969, this difference was ~34 s, and was still trending to a minimum hours after the landing! Although neither approach could reveal the time of separation with complete confidence, allowing for the unavoidable along-track errors due to unknowable and/or un-modelled variations in rate of decay, there is no reason not to believe that separation occurs at or shortly before the de-orbit burn. However, in view of the uncertainty and the exceptional case of 04038A / 28396, I analyzed an additional hypothetical Cosmos 2495 case. If the debris had separated one revolution prior to the de-orbit burn, how would this have affected the correlation of the propagated trajectory with the fireball in question? I created a new hypothetical debris TLE based on separation on 2014 Sep 02 at 15:18 UTC. With GMAT, I found that for Cd of 2.2, an A/m of 0.063 m^2/kg yielded the best match to the fireball trajectory, but just about 1 min. too early. This seems to suggest that the separation could not have been much earlier than the time of de-orbit, certainly not ~90 minutes earlier; however, a firm conclusion requires more complete trajectory data than currently available, which I believe will be forthcoming eventually. This would be a good point to discuss the observational data and trajectory analysis results in greater detail. 3. Estimated Re-entry Trajectory Compared With Observations The analysis in my previous post was based on data from a meteor camera located in Denver, operated by the private Cloudbait Observatory. Before proceeding with the present analysis, I e-mailed the observatory's owner, Mr. Chris L. Peterson, seeking his advice regarding the reliability of the data, specifically from the camera named DMNS, which I had been using. Chris responded promptly, and informed me that when he began to look into this case, he discovered that the internal calibration database for that camera had become corrupted some time ago. Also, the time is consistently fast by over a minute. He advised that the azimuth accuracy is close, but could be off a couple of degrees. The altitude data is unusable. It is unclear to me whether or how much of the data may be salvageable. Fortunately, a second camera (named Cloudbait), located at Guffey, CO., was working properly. That data has yet to be fully reduced, but Chris informed me that from Guffey, "the event was recorded from 043213 to 043241 [UTC], and those times are accurate to the second. The maximum altitude observed at Guffey was 57° (az 100°) at 043225." For the GMAT analysis discussed in this report, I attempted to match the maximum altitude data for Guffey when I propagated the hypothetical debris orbit to final decay. The trial and error process that yielded the aforementioned A/m of 0.0773 m^2/kg, crossed azimuth 100 deg at 04:32:23 UTC, at 57.8 deg elevation. The observer who filed AMS Fireball Report #1973p from Alamogordo, NM, was observing Comet Jacques in 15X50 binoculars when the fireball entered the field of view. The estimated re-entry trajectory passed within 2.6 deg of the comet at 04:31:00 UTC, probably just outside the FOV, assuming the comet was centred. The estimated altitude above the geoid was ~102 km - close to the common rule of thumb of 100 km for initial self-illumination (I normally assume 96 km). As shown on the following plot, the estimated re-entry trajectory is a good match to the locations of known sightings, which span about 1,600 km: http://satobs.org/seesat_ref/misc/2014-025C_preliminary_track.png The fireball began over New Mexico, at about 100 km altitude. By the time it reached the northernmost observers, in Wyoming, South Dakota and Montana, it had descended to about 70 km. In the unlikely event that any debris reached the ground, the estimated toe of the footprint would have been near 45.40 N, 103.38 W, at the northernmost end of the plotted ground track. The blue streak that parallels the ground track over northern Colorado and parts of Wyoming and Nebraska depicts the suspected Nexrad radar detection that first appeared about 45 min. after the re-entry. The following plot provides a closer view: http://satobs.org/seesat_ref/misc/2014-025C_preliminary_Nexrad.png I have almost zero knowledge of the subject, so am unable to comment on the relevance of the Nexrad data at this time. The following Excel spreadsheet contains the estimated re-entry trajectory, and can be used to generate an ephemeris for any location at 5 s intervals: http://satobs.org/seesat_ref/misc/14025C_decay_ephemeris_GMAT_v1.xlsm The first sheet contains the ephemeris generator. Simply enter the observer's coordinates in the input cells at upper right, and the ephemeris is automatically computed. The second sheet performs no calculations, but displays ephemerides for a selection of sites that reported observations, mainly ones in the AMS fireball reporting database. The main selection criterion was to provide a wide geographic sample. The final sheet summarizes all 38 of the AMS sightings available at this time, including site coordinates that could be used to compute an ephemeris. Full details are available at the AMS site: http://www.amsmeteors.org/fireball_event/2014/1973 I look forward to analyzing additional meteor camera and other data when it becomes available. I would also like to try to determine whether the object could have been a solar array and estimate its approximate mass. Ted Molczan ssl3molcz_at_rogers.com _______________________________________________ Seesat-l mailing list http://mailman.satobs.org/mailman/listinfo/seesat-lReceived on Fri Sep 12 2014 - 22:04:24 UTC
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