Wednesday, 21 March 2018

Updated Tiangong-1 reentry predictions (updated March 21)

[post last updated March 21, 12:15 UT and 14:05 UT]
click diagram to enlarge

I am currently issuing a daily estimate of the reentry date for the Chinese Space Station Tiangong-1 on Twitter. This current blog post consolidates these estimates and is daily updated. My current and previous predictions:

Date issued         Date predicted 
March 21           31 March +- 3 days
March 20           31 March +- 3 days
March 19            3 April +- 4 days
March 18            1 April +- 4 days
March 17            1 April +- 4 days
March 16            4 April +- 4 days
March 15            7 April +- 5 days
March 14            6 April +- 5 days
March 13           13 April +- 6 days

Currently indicated is the first week of April or last week of March. A geomagnetic storm on March 19, temporarily increasing the drag, seems to have bumped the prediction to a slightly earlier date. But I suspect that over the coming week the estimated date will shift towards a later date, in the first days of April.

The predictions are made using Alan Pickup's SatAna and SatEvo software, with current and predicted Solar F10.7 cm flux. The error margins are a standard 25% of the number of days between the last elset used for the estimate, and the estimated moment of reentry. This might be a bit conservative, certainly well before the actual reentry.

The diagram above shows you how the prediction develops. Still some 2 weeks away, there is still quite an uncertainty and day-to-day shift in the estimated moment of reentry, due to variations in the atmosphere and the attitude of the space station.

There is an initially ~5.5 day periodic variability in the drag parameter B* (perhaps pointing to a slow rotation of Tiangong-1), as can be seen in the diagram below. Over the last few weeks this periodicity seems to slow down, to an about 6.4 day periodicity currently. I expect the reentry prediction to rock back-and-forth by a few days with a similar periodicity.

click diagram to enlarge

For the orbital data of the past two weeks I calculated area-to-mass ratio's. Using a  mass for Tiangong-1 of 8500 kg, I initially got the following variation in drag surface:

click diagram to enlarge
In an e-mail discussion with Jon Mikkel, he convinced me that the mass I used (8500 kg) might be too high as that value likely refers to a fully fueled Tiangong-1. If we assume ~1000 kg of fuel initially at launch but now spent, i.e. a mass of 7500 kg, the resulting drag surface is lower, varying between 16 m2 and 31 m2 for a 7500 kg mass. In the diagram below, values for both a 8500 kg mass (blue) and a 7500 kg mass (black) are shown:

click diagram to enlarge

The calculation was done using the MSISE90 model atmosphere as incorporated in GMAT. For each elset, one full revolution was modelled in GMAT, and atmospheric model densities sampled over that revolution. These values were then averaged to get an average atmospheric density. This density was used in this area-to-mass equation:

A/m = 5.0237*10-9 * ndot/2 / ( Cd * rho * n(4/3)

(where n is the Mean Motion taken from the orbital elements; rho is the atmospheric density as modelled in GMAT; Cd a drag coefficient (2.2); and NDOT/2 is taken from the orbital elements)

The drag surface thus modelled from the data between March 4 and March 20 varies between 19 m2 and 35 m2 (for a mass of 8500 kg) or 16 m2 and 31 m2 (for a mass of 7500 kg). These seem reasonable values: the body of Tiangong-1 measures 10.4 x 3.35 meter (this is excluding the solar panels however), which gives an approximate maximum cross section of 35 m2. This would suggest that over the two week analytical timespan, the drag surface varied between ~90% and ~50% of the maximum surface, i.e. the attitude of Tiangong-1 indeed appears to be slowly varying.

A ~6.4-day periodicity is indicated as a red dashed sinusoid in the diagram above. It is superimposed on a much slower trend with a period of several weeks. This suggests a very slow tumble or rotation over multiple axis.

As can be seen in the diagram, near the end of the analytical period (March 18-19) the drag surface was near maximum values. It is therefore possible that the drag surface will decrease again over the next two weeks. If that is true, then the date of reeentry will shift towards a later date again the coming two weeks. The first week of April still seems to hold the best cards.

The problem is that both SatAna/SatEvo and GMAT do not take into account this kind of slowly varying drag surface. GMAT uses a constant drag surface. SatAna and SatEvo use the NDOT/2 value, which reflects the drag surface, for the (relatively short) data arc under consideration, and these NDOT/2 values might not well represent the feature. This means that long-term predictions by these software packages are not very accurate. We can expect that in the last 1-2 days before the reentry, the predictions become more reliable.

click diagram to enlarge

Perigee of the Tiangong-1 orbit is currently below 215 km altitude and rapidly decreasing. The average orbital altitude is currently decreasing by 2-3 km/day (see diagram above), and that value will likely increase over the coming days.

click diagram to enlarge

This diagram shows the frequent orbital raising manoeuvres, ending late 2015, after which the station goes steadily down:

click diagram to enlarge
The rate of decay, clearly going up:
click diagram to enlarge

The map below shows the area where Tiangong-1 potentially can come down: included land areas at risk are southern Eurasia, Australia and New Zealand, Africa, South America, Meso-America and the United States. Northwest Europe including my country (the Netherlands) is not at risk.

In theory, the extreme margins of this zone (i.e. near 42.8 S and 42.8 N) have an elevated risk. In reality, it is notably the position of the perigee which matters, as reentries tend to happen just after perigee passage.

click map to enlarge

I'll update this post daily.

(note: this post has been updated, and parts added or rewritten, repeatedly)

Tuesday, 13 March 2018

One month left for Tiangong-1 [UPDATED]

Note: a daily updated post with reentry estimates for Tiangong-1 is here.

image (c) Alain Figer, used with permission

The beautiful image above (used with kind permission) was made by Alain Figer and shows the Chinese Space Station TIANGONG-1 over the French Alps on 27 November 2017.

Tiangong ("Heavenly Palace") 1 was launched on 29 Sept 2011. It was the first Chinese Space Station and was visited by Taikonauts twice, first by the crew of Shenzou 9 in June 2012 and then by the crew of Shenzou 10 in June 2013: six Taikonauts in total.

All eyes are currently on this Chinese Space Station, as it is about to re-enter. Since the station was shutdown in 2016, it has steadily come down, especially so the past year and months. Its orbital altitude has currently descended below 250 km (it currently is ~240 km, with apogee at 251 km and perigee at 229 km on 2018 March 13):

click diagram to enlarge

click diagram to enlarge

Using SatAna and SatEvo, and under the assumption that the re-entry will be completely uncontrolled, I currently estimate it to re-enter one month from now, somewhere between April 7 and April 21  April 1 and April 12.

EDIT:  daily updated re-entry predictions are in a dedicated post here

The station has an orbital inclination of 42.8 degrees, and hence can come down anywhere between 42.8 N and 42.8 S. The map below shows the area that is at risk:

click map to enlarge

Note that newspaper accounts (e.g. this one) that single out a particular area as being at particular risk, are nonsense: At this stage, a month before re-entry, it is impossible to pinpoint a region. That will only be possible during the hours just before actual re-entry (and even then...).

The station has a mass of about 8500 kg and measures 3.35 x 10.4 meter. It is hence a large and heavy object, which is why this re-entry is of concern. It is likely that parts will survive the re-entry and reach Earth surface intact.

Land masses inside the risk zone include southern Eurasia, Australia, Africa, South and Middle America and the United States. It is however most likely that the re-entry will be over an ocean.

As can be seen from the map above, my own country, the Netherlands, is well outside the risk zone.

I will follow the orbital evolution and re-entry predictions for Tiangong-1 on this blog as they evolve.

Tiangong-1 image on 18 July 2017 by Alexandre Amorim from Brazil
this is a stack of 4 separate images
(image (c) Alexandre Amorim, used with permission)

NOTE: new reentry estimates, updated daily, are consolidated in this new blog post.

Thursday, 1 March 2018

On PBS Newshour, about Open Source investigation of the North Korean missile program

In December of 2017, I was interviewed by Miles O'Brien for PBS Newshour, about Open Source investigation into the North Korean missile program.

The item aired on 28 February 2018. It is 9 minutes in duration and alternatingly features Jeffrey Lewis of the Middlebury Institute and me showing what we can learn from analysing DPRK propaganda photographs and video imagery.

(the video above starts at the start of the item).