Sunday, 25 March 2018

The atmospheric reentry of the Soyuz upper stage 2018-026B on March 25

click map to enlarge

On March 21 at 17:44 GMT,  a Soyuz rocket (Soyuz MS-08) was launched from Baikonur in Kazakhstan, bringing three new astronauts to the International Space Station.

The upper stage from this rocket (2018-026B) reentered the atmosphere last night, producing a nice spectacle in the sky. The reentry was seen from southern Europe, and filmed from Italy. The still below is from video footage that you can find here on the Italian Ondanews website.

click image to enlarge. Link to newsitem with video

The US Military tracking network JSpOC gives a final TIP bulletin placing reentry at 25 March 1:25 UT ± 1 minute near 41.9 N, 8.1 E, depicted as a star symbol in the map in top of this page. The ± 1 minute indicates that this time and position come from an Infrared observation by one of the US Early warning satellites and hence should be very accurate.

I had been issuing forecast on twitter prior to this reentry, based on modelling in SatAna/SateEvo and GMAT. In addition to the JSpOC TIP position and time, the map above also gives some of my own modelling results for this reentry. The open circles were my two last proper forecasts, made before the actual reentry happened. The red dots are two "post-casts", i.e. forecasts made after-the -fact using orbital elements that were not yet available when I made my last forecast the evening before. The nominal position of the SatAna/SatEvo post-cast is only 10 minutes from the JSpOC TIP.

Friday, 23 March 2018

The reentry of Humanity Star (updated)

(This post was updated April 4, 2018, with the results of lifetime-modelling. The update is at the end of the post)

The Humanity Star. Image: Rocket Labs

The Humanity Star reentered into the atmosphere yesterday, 22 March 2018, near 13:15 UT.

Humanity Star (2018-010F) was a surprise payload launched on 21 January 2018 as part of the first successful orbital flight of fledgeling New Zealand space company Rocket Lab's Electron rocket. In addition to three cubesats, the launch featured an unannounced surprise in that it brought a 3-feet, 10.4 kg geodesic sphere into a 530 x 295 km, 82.9 degree inclined Polar orbit.

The idea was that the reflective surfaces would produce a conspicuous flashing object that would attract people's attention so that they would look up at the sky and ponder their place in the Universe. As a non-functional "art-for-arts-sake" satellite, it scooped (and was perhaps inspired by) a similar but much better thought through project by Trevor Paglen that is to be launched in August 2018.

Rocket Lab claimed that the Humanity Star would be visible as a very bright object in the sky. In reality, very few people have seen it. It mostly stayed faint, producing occasional very brief bright flashes (I saw one of these myself, at magnitude -1). Moreover, during the first 1.5 months of being on orbit, it stayed in Earth shadow, only becoming visible in twilight in March, when it already was close to reentry. The visibility window hence was short. As a project to attract public attention to the night sky, it largely failed. And the fuzz made by some astronomers about Humanity Star being "sky vandalism", clearly was over the top (and was in fact somewhat ridiculous from the start. Some people appear to take issue with everything nowadays).

Rocket Lab claimed the object would stay on orbit and be visible for nine months. Apparently, they had not realized that the area-to-mass ratio of this object was much different from a usual payload (it was a carbon sphere very lightweight for its size) and apparently they did not seriously model the lifetime. Because in reality, it lasted not nine months but only 60 days, a mere two months, on orbit. The orbital decay was very fast:

Apogee and perigee of Humanity Star over time. Click diagram to enlarge
I have modelled the last few days of Humanity Star's existence, producing reentry estimates in the two days leading to the reentry. I used two methods: one was the combination of Alan Pickup's SatAna and SatEvo software; the other was a simulation in GMAT.

click map to enlarge

The reentry occured in the early afternoon (UT) of  March 22, somewhere along the white line in the map above, and most likely near the two locations marked halfway that line, i.e. over southwest Asia.

JSpOC issued a final TIP bulletin estimating reentry at 13:15 UT  ± 29 min, nominally near 14 N, 61.8 E. My final GMAT simulation gives a result very close to that time and location, at 13:12 UT ± 45 min, nominally near 10.8 N, 61.9 E.

The final SatAna/Satevo result appears to be a bit early (indicating that I have to adjust some settings), placing reentry near 12:07 UT ± 28 min, nominally near 72 N, 126.5 W. For the upcoming Tiangong-1 reentry (see my daily updated post with reentry estimates) I am going to work with revised SatAna/SatEvo settings from now on.

UPDATE added 4 April 2018

I wrote: "apparently they [Rocket Lab] did not seriously model the lifetime".

To emphasize this, I ran a GMAT model for Humanity Star today, to see what modelled orbital lifetime would result.

I used the MSISE90 model atmosphere, a low solar activity regime, and modelled for a mass of 8.16 kg and diameter of 0.91 meter. Starting point was a TLE from 4 days after the launch.

The resulting lifetime was 51 days. My model has it reenter on March 13.
The real lifetime was 60 days. The real reentry was on March 22.

So my modelling resulted in a lifetime that was 85% of the real lifetime, which is not bad for modelling over a 2-month period.

[later added section] 
There are also other values for Humanity Star floating around: a mass of 10.34 kg and diameter of about 1 meter.

Running the model with those figures ads 2 days to the orbital lifetime, i.e. brings it at 53 days, i.e. 89% of the real lifetime.
[end of added section]

It also shows that applying a model (like GMAT) would have yielded Rocket Lab a much more realistic orbital lifetime than the 9 months which they claimed.

click diagram to enlarge

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).