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


Jig said...

Great writeup. Do we know if the reentry was at or shortly after a perigee?

Hephaestus said...

Perhaps most launches should carry an inflatable version of the "Humanity Ball" to provide a spherical, low-mass, calibrated sample to use for monitoring short-term atmospheric variation. If these were dropped into low orbits on launch, they could decay over a couple of months and improve the models.

jkennaw said...

Can someone explain why SatFlare, Heavens Above and N2yo had the wrong info for this satellite?

On its last pass over my area on March 20 I set myself up ready to view according to arrival times given by the three tracking sites. I saw nothing. I thought perhaps I'd missed the satellite. I checked Humanity Star's site and found instead the pass didn't yet happen; there was a difference of about 42 minutes or so from the three tracking sites. It arrived right on schedule.

I don't understand why so many sites would have it wrong. This has been bugging me ever since. Does it have anything to do with the drag? Any explanation would be much appreciated.

Unknown said...

Can we be sure that Humanity Star really did burn up? It is very light for its drag, and it would have lost a lot of speed at very high altitude before it reached denser air. My rough calculations give it a 70kph terminal velocity at sea level, so it may have even survived impact, and may be sitting on the ground, intact, in Siberia or Kazakhstan.