UARS reentry update 23 Sept (3)

UARS reentry update, 23 Sept 13:30 UTC
click here to check for more recent updates

The nominal projected times for the UARS decay are moving forward again, to later times (which is less favourable for Europe). The latest three orbits at the moment of writing suggest times between 2 UT and 3:45 UT on the 24th.

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UARS reentry update Sep 23 (2)

UARS reentry update, 23 Sept 8:45 UTC
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New orbital update, time shifted forward by 2 hours again. Shows that the uncertainty is still many hours. Nominal time now 24 Sep 02:00 UTC, but moving somewhat towards the right direction (for me in the Netherlands) . Here's hoping it will eventually move to near 23 Sep 21 UT.....

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UARS reentry update 23 Sept (1)

UARS reentry update, 23 Sept 7:45 UTC
click here to check for more recent updates

The projected time of decay is still shifting considerabely with each orbit update: over the past 10 hours, it has hifted back and forth by several hours. The last few orbital updates tend to shift the projected reentry to a later time, early on the 24th, bringing a decay near the USA in view again

At the moment of writing, the nominal projected  reentry time using SatEvo and current solar flux data is 4:15 UTC on the 24th. This still has an uncertainty of a couple of hours, though.

Below diagram depicts how the projected renetry time has fluctuated with each new orbit released over the past two days:

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The following UARS related NOTAM was posted by the FAA:

!FDC 1/2095 FDC .. SPECIAL NOTICE .. EFFECTIVE IMMEDIATELY UNTIL 1109252359 UTC. AIRCRAFT ARE ADVISED THAT A POTENTIAL HAZARD MAY OCCUR DUE TO REENTRY OF SATELLITE UARS INTO THE EARTH'S ATMOSPHERE. FURTHER NOTAMS WILL BE ISSUED IF SPECIFIC INFORMATION BECOMES AVAILABLE. IN THE INTEREST OF FLIGHT SAFETY, IT IS CRITICAL THAT ALL PILOTS/FLIGHT CREW MEMBERS REPORT ANY OBSERVED FALLING SPACE DEBRIS TO THE APPROPRIATE ATC FACILITY AND INCLUDE POSITION, ALTITUDE, TIME, AND DIRECTION OF DEBRIS OBSERVED. THE DOMESTIC EVENTS NETWORK /DEN/ TELEPHONE 202-493-5107, IS THE FAA COORDINATION FACILITY.

UARS update, September 22 pm

UARS update 22 September 20:30 UTC

The projected decay time for UARS keeps shifting back and forth with each orbital update. Over the course of today, it has oscilated between 19:00 UT on the 23rd to 01:00 UT on the 24th.

The last few orbits tend to favour the later part of this time window, but it could as easily swing back to the earlier part of the window again over the course of tonight and tomorrow.

This just goes to show that the uncertainties are still large and still amount to many hours. Over the course of tomorrow, it will become gradually more clear. The current time window is however exciting, as we have passes visible from Europe including the Netherlands during that time span, notably around 21h UT.

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UARS update, Sept 22 am

UARS update 22 Sept 08:45 UTC

Re-entry of UARS is now projected on the 23rd. Latest orbit plus current solar flux and Alan Pickup's fine SatEvo software suggest a nominal time near 19:00 UTC on the 23rd, but at this time that still has an uncertainty of several hours (I suggest up to at least 6 hours).

Areas with the best chances to see the reentry are currently for a swat that includes South-America, Europe, Russia, central Asia, southeast Asia and Australia, which all have passes in the hours around the nominal predicted time. It is increasingly unlikely that the USA will get to see anything of it.

UARS update, September 21 pm

UARS update, 21 Sept, 20:00 UTC

Based on the latest orbital elements from Space-Track, the projected reentry time is slowly moving earlier. The nominal value is now late September 23 (nominal 20:30 UTC, but with still many, many hours uncertainty). But within uncertainties, early September 24 (in UTC terms) is certainly not yet out of the picture. A lot can still happen in the two days left, e.g. changes in solar activity.

Below is an updated diagram of the observed and predicted  orbital evolution in terms of apogee and perigee altitudes.

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UARS update 21 Sept am

Note: forecast and diagram now superceded by update here.

All eyes of the satellite world are now firmly on UARS. Unfortunately, I have no visible passes here these last few days of its existence.

The predictions for the moment of re-entry keep fluctuating between late September 23 and early September 24. Using Alan Pickup's SatEvo software and a 10.7 cm solar flux value of 145 plus this morning's latest elset, the nominal prediction is for the early hours of September 24 but this time still has an uncertainty of up to half a day.

Below diagram shows the changes in apogee altitudes (blue) and perigee altitudes (red) of UARS over September, plus a forecast of the future evolution of them (grey) untill re-entry.

click diagram to enlarge

UARS re-entry Update (and Nanosail-D, and IGS 1B)

UARS update: It seems that the re-entry moment for UARS, the defunct NASA Upper Atmosphere Research Satellite, is moving forward in time, as Harro Zimmer pointed out on the Seesat-L list.

NASA and SSC Space-Track both settle for a decay on or near 24 September, and so do independant analysis by Harro Zimmer as well as my own assessment (using Alan Pickup's SatEvo software with current 10.7cm flux values).

As the orbital development near decay is highly influenced by solar activity, there currently is an uncertainty of possibly up to a few days in these predictions. Yet it seems save to say that UARS has not much more than a week left.

Nanosail-D update: Meanwhile, that other NASA object up there slowly coming down, NASA's experimental solar sail Nanosail-D, seems not quite willing to give up. It's current orbit and current values for 10.7cm solar flux suggest it will hang on at least another month, untill late October. Same caveats as for UARS apply.

IGS 1B update: this defunct Japanese spy satellite which is steadily coming down (see earlier post here and earlier posts linked their in) since it malfunctioned in 2007, likewise seems intend on spending some more time up there. The current orbit plus solar activity now give a nominal re-entry prognosis for early 2013 instead of 2012. This date is certainly still going to shift back or forth considerably however, depending on how solar activity develops.

Wednesday's fireball over the US southwest was not UARS

A brilliant fireball was seen and filmed over the US southwest (Arizona, Nevada and California) last night. Some media suggest it could be connected to UARS, the US satellite about to plunge down in the atmosphere later this month.

It was definitely not UARS though. Not only was UARS still being tracked and hence in orbit after the fireball appeared (approx. 19:45 MST on the 14th, or 2:45 UTC on the 15th): but it was simply not passing over that part of the USA at that time. It was hence not UARS, or a piece of UARS.

Watch UARS - it's dropping!

Several news outlets are abuzz (e.g. here and here) about UARS, the defunct Upper Atmosphere Research Satellite, which is in the last days of its life.

click image to enlarge

Launched on Space Shuttle STS-48 in 1991, it was shut down in 2005 and it's orbit lowered to speed up decay.

That moment of decay is now near, with the 6 tonnes satellite expected to re-enter into our atmosphere in the last days of September or first days of October. At this moment , it is not possible to predict the moment of decay more exactly than this, and hence it is impossible to say where (over which part of the world) the re-entry will take place. At the moment of writing, the satellite already has come down to a 244 x 275 km orbit. The nominal decay date is currently projected to be around September 28-29 but has an uncertainty of several days.

UARS is that big, that parts of it might actually survive re-entry and impact on land or sea. Modelling by NASA suggests up to 532 kg of material, broken up into tens of pieces, might survive re-entry, with the biggest piece being perhaps in the order of just over 150 kg. The odds of this debris hitting someone are small however.

UARS is a large satellite that can be quite bright and easily seen by the naked eye: in the past, I have seen it attain brightnesses up to  mag. +0.5, as bright as the brightest stars in the sky.

The image above shows UARS photographed by me on 16 June 2010 from Leiden, the Netherlands, when it showed a small flare.

An update on IGS-1B

Along the line of expectations, our tracking data show that the Japanese spy satellite IGS 1B (2003-009B) which malfunctioned in March 2007, keeps coming down (see earlier coverage here and here).

Early July 2011, the perigee had come down to 450 km and the Mean Motion (the number of orbital revolutions per day) is steadfastily increasing as the orbit becames more narrow:





The predicted decay date keeps shifting back and forth, being highly dependant on solar activity. Solar activity has been back to modest the past two months. As a result, the decay date forecast has shifted further away in time.

If solar activity does not increase, forecasted decay will be in late 2013 (SatEvo with current solar flux F10.7 cm = 85, elset 11184.15154535). If it does increase - which is likely, as we are on the approach to a solar maximum - it will be earlier, possibly as early as mid-2012.



Meanwhile, it is interesting to see how the still active sister-ship IGS 1A (2003-009A), launched in the same 2003 launch, is faring. Above diagram shows the evolution of the orbital inclination. IGS 1B's orbital inclination is clearly drifting, consistent with loss of control. IGS 1A's orbital inclination initially was allowed to co-drift with IGS 1B, but then altered in a manoeuvre mid-2008 that brought the inclination up again, to match the other IGS-es in the constellation. As of 2010, it is kept more or less steady, librating around a value of 97.39 degrees, the sun-synchronous value for a 487 x 498 km orbit.

More on the IGS 1B fuel tank, and the (reduced) risk of it re-entering

At the end of the previous weekend, I posted an extensive post on the malfunctioned Japanese spy satellite IGS 1B (03-009B). It malfuntioned in 2007, has subsequently lost orbital altitude, and is now expected to re-enter early 2012.

The concerns was, that it might have a still partly filled fuel tank - potentially creating a risk at re-entry, a concern similar to that for the re-entry of USA 193 in 2008 (which, for that reason, was destroyed on-orbit by a SM-3 missile). This greatly worried me, the more as no news on this was appearing from either the Japanese, or US Space Command (who no doubt must have been aware that IGS 1B was coming down - an object like this will certainly be tracked).

My assessment of a potentially still half-full tank, was, as I indicated, at best an "educated guess". Noted amateur observer Ted Molczan from Toronto has now made an independant assessment of the situation, notably on the absolute amount of fuel left. Below I quote from his analysis, in which he writes (after first noting that he gets similar results to mine as to the probable time of decay, i.e early 2012):

"I agree that IGS 1B could decay from orbit in 2012, with perhaps half of its initial fuel mass; however, I believe that its initial fuel mass probably was far less than that of USA 193 - between approximately 28 kg and nearly 100 kg, compared with the 450 kg of USA 193. If half of IGS 1B's fuel has been expended, then between 14 kg and 50 kg may remain - at most 10 percent of USA 193's fuel load. Only the Government of Japan knows for certain the quantity of fuel that remains, but if my estimate is in the ballpark, then the risk of an uncontrolled decay from orbit would be much less than for USA 193."

[...]

"USA 193 carried about 450 kg of fuel, none of which had been expended by the time of its impending decay, due to its failure soon after it reached orbit. I believe that IGS 1B may have considerably less fuel for the following reasons:

1. IGS 1B was designed to operate at a considerably higher altitude than USA 193 (485 km vs. 360 km), which means that it was subject to far less atmospheric drag, which would have decreased the quantity of fuel required for orbit maintenance.

2. IGS 1B's total mass is reportedly about half that of USA 193 (1200 kg vs. 2300 kg). For a given velocity change, the fuel expenditure varies in direct proportion to total spacecraft mass.

3. IGS 1B died four years into what was reportedly a five year mission, so might already have expended most of its fuel."  

[note from Marco Langbroek: but its sister ship IGS 1A is still maintaining orbit 8 years later, as I indicated in my original post, suggesting that these satellites carry more fuel than for a minimum 5 year mission]

"With respect to points #1 and #2, assuming that IGS 1B's ballistic coefficient (mass divided by cross-sectional area) is similar to that of USA 193, and that its fuel supply was designed to enable operating up to twice the reported 5 year design life, i.e. 10 years, then the total velocity change required to maintain 485 km altitude would have been about 53 m/s (metres per second). Assuming IGS 1B uses the same fuel as USA 193, then for its mass of 1200 kg, the required initial fuel mass would have been just 28 kg - far less than that of USA 193.

Factoring in point #3: assuming provision of fuel for 10 years operation, then IGS 1B might have consumed 40 percent of its fuel by the time it died, four years after launch. Considering that its first couple of years of operation coincided with the tail end of the  previous solar maximum, its fuel use could have been somewhat greater; assuming for the sake of argument that half its fuel has been expended, then 14 kg would remain.

I based this rough estimate on data found in the respected textbook/reference Space Mission Analysis and Design III, specifically the annual velocity change required to maintain low Earth orbits against decay, depending on altitude, ballistic coefficient and solar activity. I assumed that fuel for attitude control was negligible, and that IGS 1B was not designed to be de-orbited at the end of its useful life (the latter would have increased the initial fuel mass to nearly 100 kg, with perhaps 50 kg remaining after four years of operation, still far less than USA 193 carried.)"

I have high trust in Ted's assessment: and the result is somewhat of a reassurrance: 14 to 50 kg of fuel is an order of a magnitude less than the 450 kg of fuel of USA 193. While no uncontrolled re-entry is without danger, these figures from Ted's assessment lead me to think that IGS 1B is clearly less of a threath than USA 193 was.

Ted's assessement is exactly the kind of thing I called for in my earlier post, when I wrote:

Instead of watching this one quietly go down, I would prefer to see a good risk assessment done [...] a clear argument presented as to why it would not be a danger in this case, given all the fuzz created around falling fuel tanks with USA 193.

Ideally, this should of course have come from the Japanese themselves (which refused to say anything pertinent to one of the reporters that contected me over this, besides the simple statement that there was "no risk"). In absence of that, Ted's assessment is a good thing to have.

KH-12 USA 129 flaring: and IGS 1B again

A period of sunny weather commenced the past week, albeit with a trend towards increasing hazyness. I observed on the 19th, 20th and 21st of April, targets being the evening KH-12's USA 129 (96-072A) and USA 186 (05-042A), as well as IGS 1B (03-009B). An attempt to locate the geostationary satellite Mentor 2 (98-029A) on the 20th failed, probably because of the poor observing conditions.

Yesterday evening (21st April) the sky was quite hazy with, during twilight, abundant whisps of thin clouds. USA 129 (96-072A) slowly flared to mag. 0 at about 19:56:15 UTC, and the resulting picture of the flare amidst thin clouds in a still bluish twilight sky, looks very eerie:

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Some what later, I made this shot of IGS 1B (03-009B) gracefully sailing through Bootes:

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My story about the re-entry of the above pictured malfunctioned IGS 1B satellite in about a year from now, has been picked up, notably after Jim Oberg reposted it on the NASA Spaceflight Forum here. Two journalists contacted me with questions, and my post itself attracted some US Government attention (when the Executive Office of the President visits your weblog, you know it is being taken serious). So I guess some people have woken up now, and hopefully we will soon see a serious risk assessment and more pertinent information by the Japanese as to the tank content of the satellite.

[UPDATED] Another Malfunctioned Spy Satellite is Coming Down - with Hydrazine onboard

UPDATE (24 Apr 2011): in a separate post, I discuss a new analysis by Ted Molczan, who has done an independent assessment on the absolute amount of fuel left in the tank of IGS 1B. This assessment, in which I have a large degree of trust, suggests that the absolute amount of fuel carried by IGS 1B is substantially lower than was the case with USA 193. This is somewhat of a reassurance. Read more about it in the separate post here.

Summary - this long post discusses the imminent uncontrolled re-entry of the malfunctioned Japanese spy satellite IGS 1B (2003-009B) in the first half of next year (2012): and points out that there might be a potentially hazardous half-full tank of fuel still in the defunct satellite, mimicking the situation with USA 193 in 2008.

Prologue - Three years ago: USA 193 and 'Operation Burnt Frost'

Three years ago, a malfunctioned US Spy Satellite called USA 193 (2006-057A) made headlines, when it was destroyed by a modified SM-3 missile fired from the USS Lake Erie near Hawaii. This was done in order to avoid a potentially dangerous uncontrolled re-entry early 2008 (see my coverage of the story here). According to US Government officials, the tank with toxic Hydrazine fuel onboard the satellite was the main reason for this unusual and spectacular pre-emptive destruction code-named "Operation Burnt Frost", although a few independant analysts (e.g. Yousaf Butt) maintain that the real motives were instead to prevent cutting edge technology from falling in the wrong hands, and perhaps also to provide a symbol warning to the Chinese. The Chinese had conducted a succesfull anti-satellite test (ASAT) a year earlier which greatly worried the USA. The suggestion that it was not the potential hydrazine hazard but another motive that prompted the decision to destroy USA 193, was hotly debated, notably by noted Space journalist Jim Oberg who strongly defended the official position (for more examples of the heated discussion, see here).


2012: IGS 1B, Another Spy Satellite Coming Down

Now, three years later, another malfunctioned spy satellite is coming down. And like USA 193, it likely too has a significant reserve of fuel left in it's onboard tank.

Image below: the doomed malfunctioned satellite IGS 1B, a bright naked-eye object, photographed by the author from Leiden (the Netherlands) on 21 April 2011

click image to enlarge

The satellite in question is a Japanese spy satellite, IGS 1B (2003-009B), believed to weigh 1.2 tons (about one-third of the weight of USA 193). It was launched on a H-2A rocket on 28 March 2003 together with a sister satellite, IGS 1A (2003-009A). IGS stands for Intelligence Gathering Satellite, an English translation of the Japanese designation joho shushu eisei.

Both satellites, placed in similar ~488 km, 97.3 degree inclined Polar orbits, were meant to work in tandem, the IGS-A object being an optical imaging reconnaissance satellite, the IGS-B object a Synthetic Aperture Radar (SAR) reconnaissance satellite with imaging resolutions believed to be in the order of 1 meter. Their mission -and that of subsequent similar IGS satellites launched- probably was and is primarily to keep an eye on North Korea's Ballistic Missile program, as well as providing an imaging aid in case of natural disasters occuring in Japan.

In order to carry out their mission, these satellites carefully maintain a very stable sun-synchronous orbit by means of frequent small manoeuvres. While some sources (including the CIA) list an intended life-span of 5 years, the optical satellite of the pair (IGS 1A) appears to be still actively maintaining its orbit as of April 2011, over 8 years after launch of the pair, indicating that these satellites probably have a significant amount of fuel onboard to enable these orbit maintenance manoeuvres.

Both objects in question are classified, meaning that neither the Japanese government nor the US Government make orbital elements available. Amateur trackers, including this author, have however kept track of both objects since their launch, determining and updating their orbits (periodically published here).


March 2007: Loss of power, and loss of altitude, by IGS 1B

In the spring of 2007, the Japanese government made public that the radar satellite of the pair, IGS 1B, experienced a serious malfunction involving loss of power on or near March 25, 2007.

Indeed, amateur tracking data show that since March 2007 the satellite has stopped the careful maintainance of its orbit and instead has started to lose altitude. In addition, amateur trackers (including this author) started to report an irregular brightness behaviour of the satellite, including some spectacular flares not seen prior to 2007 (e.g. reports here, here, here, here, here and here), indicating a loss of attitude control.

The following diagram, created by this author based on published orbital updates calculated by Mike McCants from amateur tracking data (including data by this author) shows how the Mean Motion of the satellite, initially constant near 15.26 revolutions/day (the sun-synchronous value for inclination 97.37 degrees), has gone up steadily since late March 2007 (this date, the date of the reported malfunction, indicated by a vertical dashed grey line), indicating a loss of altitude. For comparison, the values of IGS 1B's still operational optical sister satellite IGS 1A, are shown as well (note how they remain constant due to the constant orbital maintenance manoeuvres this satellite continues to make):

click diagram to enlarge

Indeed, the perigee and apogee altitudes of the satellite as derived from the published amateur orbits, show a clear and increasing drop in altitude from March 2007 onwards (unlike the constant values of its still operational sister craft IGS 1A, shown as a reference in the diagram as well). Since the 2007 malfunction, the orbital altitude has already decreased by over 30 km, and the decrease is continuing at an increasingly fast pace:

click diagram to enlarge

As the loss of altitude starts right at the moment of the reported malfunction (late March 2007), it appears to be a malfunction affecting control of the satellite itself, not just it's radar system. With this is meant that the loss of altitude and start of orbital decay does not appear to be due to a controlled shut-down sometime after the remote sensing equipment malfunctioned. Instead, it appears that the Japanese operators have indeed truely lost control over the satellite.


When will it re-enter?

At the current increasing rate of orbital decay, it is clear that the satellite is now entering its last year of existence. Using Alan Pickup's orbital evolution software SatEvo and the latest IGS 1B orbit updates by McCants, IGS 1B's re-entry into the atmosphere is predicted to occur in about a year from now, around March, April or May 2012.

These predictions will probably shift a bit back or forth in the future, as the orbital evolution depends on a.o. solar activity (which is not constant and not well-predictable). But it is clear that somewhere in the first half of 2012, IGS 1B will come down.


Issues connected to the uncontrolled re-entry of IGS 1B

Similar to what was the case with the now infamous USA 193 satellite, the situation is that we have a satellite in a Polar orbit and likely containing a still significant reserve of fuel about to come down in an uncontrolled fashion.

Normally, when a spy satellite in Low Earth Orbit is at the end of its life, the last reserve of fuel is used to make the satellite deliberately re-enter in a controlled fashion, over a carefully chosen spot: usually the Pacific Ocean, where the re-entry can do no harm. This was recently done with the US radar spy satellite Lacrosse 2 for example (see here).

With a satellite that is out of control, like the infamous USA 193 and now this Japanese IGS 1B, that is however not possible. The satellite can basically plunge down anywhere on earth, and when remnants survive this re-entry, they can become a danger if the re-entry happens to occur over an inhabited area.

The latter danger was the official rationale behind the decision to destroy USA 193 in 2008 by means of a missile fired from the USS Lake Erie, just before the satellite would have come down on its own. Especially the fact that, due to the early malfunction of this satellite, there still was a tank with a considerable reserve of toxic hydrazine fuel on board, was given as a reason for the "shoot-down" (actually more of a "shoot-to-pieces"): the operation was called "Operation Burnt Frost" because the stated objective was to destroy the hydrazine reserve which, after two years of inactivity of the satellite, was likely frozen.

With IGS 1B, we might be facing a similar hazard in 2012. The satellite is bound to have a fuel reserve left, and quite likely a considerable reserve at that. (note added 24/04/2011: see however the post here, featuring an independant re-assessment by Ted Molczan)

IGS 1B passing through Canis venatici and the tail stars of the Big Dipper on 9 April 2011

click image to enlarge

As mentioned earlier, some sources list an intended life-span of 5 years for IGS 1B (and IGS 1A). It malfunctioned after 4 years, so one can expect that as a minimum there is at least enough fuel for a year left in the spacecraft.

But there are reasons to believe that the reserve of fuel left could in fact be considerably more than that.

The reason to think so is that, as mentioned earlier in this post, eight years after launch the IGS 1B sister craft IGS 1A is still actively maintaining it's orbit (see diagrams above). Mid-2008, the spacecraft manoeuvred to re-allign it's inclination to the 97.37 degree inclination orbital plane of subsequent IGS satellites launched from 2006 onwards. This indicates that 5 years after launch, it was (and up to this day probably is) still fully operational, and being primed for continued tasks. A CIA summary suggests an operational replacement by another IGS satellite was not effected untill at least mid 2010, over 7 years after its launch. As mentioned, amateur tracking data show that IGS 1A is still actively maintaining it's orbit as of April 2011, 8 years after its launch.

The implication is, that these IGS spacecraft actually have enough fuel reserves onboard for over 8 years of operation. As IGS 1B malfunctioned after only 4 years in operation, the implication of that in turn is that half or more of the original fuel reserves could still be left in the spacecraft (one factor however not easily calculated in with this, is the amount of fuel spent in the initial manoeuvering to obtain the desired orbit directly after launch).

That, a tank potentially still half full, is a considerable amount of fuel. (note added 24/04/2011: see however the post here, featuring an independant re-assessment by Ted Molczan)


Should action be taken?

The potential hazard of the onboard reserve of hydrazine fuel upon impact on earth was given as the primary reason to mount "Operation Burnt Frost" with USA 193 in 2008. As we might now be facing a similar situation with IGS 1B, it will be interesting to see if a similar drastic measure is taken, either by the Japanese (who own the same SM-3 missile system used for 'Operation Burnt Frost') or it's ally the USA, given that the latter has previous experience with such a complicated exercise. And if not, then the question will be: why in the case of USA 193, but not in the case of IGS 1B?




As was the case with USA 193 in 2008, the doomed IGS 1B satellite is in a polar orbit. It has a 97.3 degree inclined orbit, meaning that it is a potential danger to every latitude between 82.7 degree North and 82.7 degree South. This range of latitudes covers every inhabited spot on Earth, including all of the USA, Canada, Europe, Australia, Africa, Asia, South America and Japan.

While the amount of fuel left in IGS 1B is probably not as large (in the sense of amount of gallons) as it was in USA 193, a considerable amount of it nevertheless is very likely there, in the shape of what could be (note: in a "worst case scenario") up to a half full (and frozen) tank that might survive re-entry. Here, I should however mention that of course my assessment on the tank content is at best an "educated guess", and I could of course be wrong (only the Japanese can answer that point). (note added 24/04/2011: see however the post here, featuring an independant re-assessment by Ted Molczan)

Instead of watching this one quietly go down, I would prefer to see a good risk assessment done and either mitigating action taken, or a clear argument presented as to why it would not be a danger in this case, given all the fuzz created around falling fuel tanks with USA 193.



Note added: according to the Japanese press, a second IGS radar satellite (IGS R2, 2007-005A) suffered a system failure in August 2010, 3.5 years after launch.

note: this post has been slightly edited in the afternoon of April 20, to better reflect the point that the "tank half full" assessment for IGS 1B is a "worst case scenario". Japan should give some openness in information to replace these "educated guesses" by more solid facts.

Mexican "impact" / fireball event is NOT due to Kosmos 2421 debris

News is doing the rounds of a spectacular fireball/sonic boom near Mexico city on 10 Feb, 18:30 local time (= 11 Feb, 00:30 UTC).

Initial reports talked about an actual impact with a 30 meter wide crater and damage to a bridge and road. That seems not to be the case.

Subsequent news releases suggested that it was a piece of Komsos 2421 debris impacting (06-026 HK, #33006).

For a summary, see Phil Plait's Bad Astronomy blog post here.

However, this event was certainly NOT due to the mentioned piece of space debris. The object in question was, contrary to apparent statements by a spokesman of the Mexican Space Agency (?), no way near passing over Mexico in a window of several hours around the reported time:

click image to enlarge

In addition, there are elements available with an epoch 0.75 days after the event, suggesting it indeed was still in orbit after that time. I used Alan Pickup's fine SatEvo software with the current F10.7 solar flux parameter (94) to predict a decay near 12 Feb 9h UTC, 1.25 days after the Mexican event.

More on the USA 193 shootdown

The online Bulletin of the Atomic Scientists has published an essay by Harvard astrophysicist Yousaf Butt with a very critical view of the official reasons given for the USA 193 shootdown.

Butt filled a request through the Freedom of Information Act and obtained the report featuring the re-entry model and analysis that was used. And found it to be flawed and on closer look not quite supportive of the alledged 'danger' of the re-entry of USA 193's hydrazine fuel tank.

The report is very cautious and it's authors already note that some of the model assumptions are not realistic. Importantly, it shows that even with these assumptions maintained, much of the tank's titanium outer layer will ablate according to the model (remember how Oberg denied this in his essay?!), leaving only a very thin outer shell 1/5th or less of the original thickness. This assumes uniform ablation (which is not realistic).

Butt argues that when more realistic assumptions are made, this suggests the tank would likely have been destroyed upon reentry.

You can read the essay here, and it includes a link to the report pdf.

The essay highlights:

• A NASA study on the survivability of USA-193's hydrazine fuel tank used an oversimplified model, leading to an overly optimistic assessment of the tank's survival.
• But even this study showed how the tank would have burned up when reentering the atmosphere.
• Therefore, Washington's contention that the tank would have hit the ground intact, posing a health hazard, seems questionable.

Another thing to note is that the tank was not completely filled with fuel, but 76% filled. This turns out to be of importance in assessing the fate of the tank.


(with thanks to John Locker for te 'heads up')

Oberg on the USA 193 shootdown

The renowned veteran space journalist and former mission control engineer James Oberg has published another article about the reasons for the USA 193 shootdown in february (see my detailed coverage of the USA 193 saga here).

Like in an earlier article, Oberg is strongly opposing suggestions that there is more to this all than the official reason given for the shootdown - the danger of the tank with Hydrazine reaching earth intact. He argues that that reason given was the true and sole reason.

As much as I respect Oberg, I am still not convinced (but then, I am merely only what Oberg calls an "amateur specialist". I observe satellites and determine their orbits. I do not launch them).

First, about disintegration of the satellite. Oberg makes an argument from a comparison with meteorite falls. That argument, at least in the way he presents it, is flawed.

Oberg argues - and he is correct in this!- that it is a widespread misunderstanding that meteorites arrive on earth surface 'red hot'. He points out that in fact they are cool when reaching earth surface, and then tries to argue that they do not heat up during their fall:

Though a thin outer layer is briefly exposed to very hot air, for most of the descent that air is thinner than the purest vacuum inside thermal-shielding thermos bottles.

Now he is correct in this: small meteorites indeed arrive cold on earth surface, and of the object which does reach earth surface, only a thin outer layer has been heated.

But this is only part of the story, and as such the meteorite analogy is a very poor one.

There are two reasons why meteorites arrive cold on Earth. One is that from 25 km altitude, after being slowed down by the atmosphere to subsonic speeds, they stop ablating and enter a free fall that takes minutes to complete. During this phase they cool, much like the air the ventilator in your pc blows over your computer CPU cools your CPU.

A more important factor however is that heat generated during the incandescent phase of a meteorite fall, the result of atmospheric friction when the object still has cosmic speeds, is carried away immediately with the ablating material. It is for this reason that heat generated does not transfer much into the meteorite. This is basically what Oberg points out, but he neglects to tell something which is quite relevant:

that in this process of meteorite ablation, at least 70% (and usually more) of the meteorite ablates and hence vanishes. What reaches earth surface is at best 20-30% of the original mass.

The implications for the USA 193 tank, if we properly use the meteorite analogy, is therefore this. Either one of these two things will happen:

1) over 70% of the tank mass ablates and at best 20-30% and probably less of the original tank mass will reach earth surface;

Oberg however argues specifically against the notion of the tank being destroyed by ablation. The alternative option which remains then is:

2) the tank, due to it's special construction, does not ablate. In that case however, the heat dissipation mechanism Oberg brings up in his meteorite fall comparison will be absent too. In other words: the tank will heat up in its interior, unlike a meteorite.

In this case, Oberg's analogy is flawed.

Now, if I understand Oberg's article correctly, modelling (and who am I to question this) of the USA 193 tank entry would have nevertheless suggested the frozen hydrazine to remain intact.

In that case, you can actually question what the real danger is of a solid chunk of hydrazine ice contained in a metal casing reaching earth surface. It will only be dangerous when someone directly handles it (but even then).

Here, we should realize that tanks with -unfrozen!- hydrazine fly through our airspace daily. Most fighter jets contain a tank with hydrazine as an emergency fuel backup. The effects of this falling down on you will not much differ from those of the USA 193 tank falling down on you. Such crashes are not rare. For example, our relatively modest Dutch airforce lost 32 of its F16 fighters, which carry a hydrazine tank, through flight crashes. Some of these aircraft came down in populated areas (one actually hit a house).

All commercial aircraft carry tanks with fuel too - not hydrazine, but still not pleasant stuff. Chances that one of these tanks will descend on your head - and this happens from time to time- are much larger than that the tank of USA 193 would have. And we don't quite bother about that. So why bother about the USA 193 tank then?

USA 193 was not the first failed fuel-carrying satellite to fall back to earth in an uncontrolled way. Nor will it be the last. In fact, launch failures where final rocket stages fail to fire are common. It will be interesting to see whether future cases will get a similar treatment.

In my opinion, the USA 193 shootdown was done for multiple reasons, and the "danger" of the hydrazine tank is only one of these. It is a convenient one to defend the exercise to outsiders, but not the only reason.

I am quite convinced that other reasons were of equal or even paramount importance in making the decision:
- that USA 193 presented a very convenient target for a practical test of ASAT capabilities (thus also making the money spent on the satellite at least partly pay off);
- that it would prevent new experimental technology falling (literally) into wrong hands;
- and that it was a timely moment to remind China, the US Senate and Congress and the US public that the USA has ASAT capabilities too and that the technology in a wider sense (missile defense) was worth further funding. Note that in April 2008, barely two montsh after the USA 193 intercept, the US Congress re-examined the status of missile defense of which the used Aegis system is part.


Note: considering the USA 193 shootdown, John Locker's summary and the links he provide are worthwhile reading

Two satellite fuel tanks survive re-entry

Two recent finds of satellite fuel tanks - both of carbon-wrapped titanium fuel tanks from the Atlas Centaur upper stages of two US military satellite launches probably - are in the news at the same time. One came down a week ago in Brasil, the other was found in the autumn of 2007 in Australia.

The Brasilian tank (source):




The Australian tank (source):




The Brasilian fuel tank is very likely part of the upper Centaur stage of the rocket that launched a US military communications satellite (Global SATCOM, 2007-046B) in October 2007. The Australian fuel tank could be a part of the upper Centaur stage of the rocket that launched another military satellite, USA 138 (1998-016A) in 1998.