<p>I found an openly available <a href="https://aerospace.org/reentries/cz-5b-rocket-body-id-48275" rel="noreferrer">CZ-5B Rocket Body (ID 48275) reentry prediction</a>:</p>
<p><a href="https://i.sstatic.net/oBv3T.jpg" rel="noreferrer"><img src="https://i.sstatic.net/oBv3T.jpg" alt="Reentry Prediction" /></a></p>
<p>Most intersting I found the following graph which shows <strong>how slowly error bars of the predicted reentry time decrease over time</strong> (as obtained from <a href="https://twitter.com/AerospaceCorp/status/1390416330269020160" rel="noreferrer">The Aerospace Corporation Tweet</a>):</p>
<p><a href="https://i.sstatic.net/jbFiV.png" rel="noreferrer"><img src="https://i.sstatic.net/jbFiV.png" alt="error bars of the reentry" /></a></p>
<p>Last but not least, I also learned from a <a href="https://www.businessinsider.com/us-tracking-uncontrolled-chinese-rocket-due-crash-may-8-2021-5?r=DE&amp;IR=T" rel="noreferrer">Business Insider story called &quot;The US is tracking an uncontrolled Chinese rocket traveling at 18,000 mph that is expected to crash down around May 8&quot;</a> more on why the rocket was called &quot;uncontrolled&quot;:</p>
<blockquote>
<p>McDowell told Space News that letting debris reenter the atmosphere uncontrolled was &quot;unacceptable.&quot;
&quot;Since 1990 nothing over 10 tons has been deliberately left in orbit to reenter uncontrolled,&quot; he said.</p>
</blockquote>

CC BY-SA 4.0
2021-05-07T09:59:07.510Z
Yes, the location in Europe is here crucial. If you live in South Europe, you might have a possibility of seeing it. But if you live on greater latitude than 50°, you might not have this possibility.
User123
CC BY-SA 4.0
2021-05-07T11:12:57.733Z
I prefer seeing it in transit rather than in my backyard :-)
B--rian
CC BY-SA 4.0
2021-05-07T11:15:07.500Z
But rocket in backyard would be more unforgettable.
User123
CC BY-SA 4.0
2021-05-09T01:06:30.530Z
the aerospace.org link now redirects to their twitter page :-/
Aaron F
CC BY-SA 4.0
2021-05-07T08:39:10.940Z
2021-05-07T08:39:10.940Z
B--rian
<p>The name of the module is Tianhe (天和), and the name of the rocket is Long March 5B, but it got a special designation of 2021-035B. It is not really official, but I think it is good enough to get some data. I found <a href="https://orbit.ing-now.com/satellite/48275/2021-035b/cz-5b/" rel="nofollow noreferrer">this website</a> for current position (<a href="https://orbit.ing-now.com/3dlaunch/payload/data/2021-035/" rel="nofollow noreferrer">live on globe</a>). Your userpage says you live in Europe, so the rocket will fly over Greece at 00:30 CEST (22:30 UTC) tomorrow (in 1.5 hour if you have time to be awake for that long). I also calculated the maximum angular speed (on May 6) of around 1.85 °/s (on May 7, it is 2.7 °/s).</p>
<p>There are already some images; I found only <a href="https://spaceweather.com/images2021/06may21/tianhe1_strip.jpg" rel="nofollow noreferrer">that one</a>. The booster's visual magnitude is around –1, so pretty bright to spot.</p>
<p>So, if you have time, try to look to the South and spot the booster at 22:30 UTC today. Maybe you are going to have luck tonight : )</p>
<hr />
<p>There are some equations I derived with basic trigonometry about the rocket's visibility:</p>
<p>Distance <span class="math-container">$D$</span> at which the rocket can be seen right on horizon:
<span class="math-container">$$D = R \ \arccos\left(\frac{R}{R+h}\right)$$</span>
Where the calculator calculates radians, <span class="math-container">$R$</span> is Earth's radius and <span class="math-container">$h$</span> is the altitude of the booster (you can get that on previously given website; at the moment it is 170 km).</p>
<p>The angular height of the booster which is <span class="math-container">$D$</span> kilometers away from us (on Earth) is given by:</p>
<p><span class="math-container">$$\alpha = \frac{\pi}{2}-\arcsin \, \left(\frac{(R+h)\sin{\frac{D}{R}}}{\sqrt{2R^2+2Rh+h^2-2R(R+h)\cos{\frac{D}{R}}}}\right)$$</span>
Again, the calculator must be set in radians.</p>
<hr />
<p>Update: The rocket fell on May 9 at 02:14 UTC into the Indian Ocean, so no one was hurted.</p>

CC BY-SA 4.0
2021-05-07T06:38:58.983Z
@B--rian Have you seen it yesterday?
User123
CC BY-SA 4.0
2021-05-07T07:40:27.287Z
Nope, was too cloudy.
B--rian
CC BY-SA 4.0
2021-05-07T07:43:55.473Z
I also included the equations for visibility, and if you live in UK or in Germany, the Greece is too far. Also note that the booster is currently falling.
User123
CC BY-SA 4.0
2021-05-08T01:07:13.027Z
`+1` yay for equations!
uhoh
CC BY-SA 4.0
2021-05-08T03:17:52.250Z
"It is not really official..." It really is official for those who track and catalog objects in Earth orbit (governments, militaries, etc.) Just like 2021-035A is the [COSPAR International Designator](https://en.wikipedia.org/wiki/International_Designator) for [Tianhe](https://en.wikipedia.org/wiki/Tianhe_(space_station_module)), 2021-035B is the international designator for the LM 5B core stage from that launch.
uhoh
CC BY-SA 4.0
2021-05-08T20:44:51.650Z
Perhaps is too late for such a question... Would it be visible only by reflection or can it set a plasma trail?
Alchimista
CC BY-SA 4.0
2021-05-08T20:59:48.803Z
It can be normally seen just by reflection. But when it enters the atmosphere, it of course leaves a trail behind it. But you could only see that if the rocket would fall less than 800 kilometers away from you, which is pretty unlikely. Also, you could know when to go out because of the inaccurate predictions about the hour of fall.
User123
CC BY-SA 4.0
2021-05-06T21:08:17.137Z
2021-06-16T14:06:02.673Z
2021-06-16T14:06:02.673Z
User123
<p>SpaceWeather.com's newsletter states</p>
<blockquote>
<p>Sky watchers are reporting two new objects in the night sky: China's new space station and an out-of-control booster rocket that helped launch it. The tumbling booster is flashing brighter than some of the brightest stars in the sky.</p>
</blockquote>
<p>My usual star-gazing app does not have the rocket booster in its database (yet), and I did not manage to spot it yet (from cloudy central Europe). Is there an official name for the object? Could somebody help me how to find it in the remaining 2 days while it is still in orbit around Earth? Or maybe somebody could even share a picture or two?</p>
<p>Bonus question: Maybe somebody could also enlighten me in how far any single-use booster-rocket could be <em>in-control</em> after it ejected its payload? Wasn't that the default for many years? Also: Now that we can observe the rocket we know the trajectory and could also forecast when it will crash and where, aren't we?</p>
<h3>References</h3>
<ul>
<li><a href="https://www.reuters.com/lifestyle/science/us-space-command-tracks-chinese-rocket-uncontrolled-re-entry-orbit-2021-05-06/" rel="noreferrer">U.S. Space Command tracks Chinese rocket for uncontrolled re-entry from orbit</a></li>
</ul>

CC BY-SA 4.0
2021-05-06T21:05:09.230Z
I was too fast, my bad: Tianhe-1 is indeed the space station, I should have double-checked. @User123 Thanks for pointing that out.
B--rian
CC BY-SA 4.0
2021-05-07T16:17:30.340Z
We would love this question on the Space Exploration Stack Exchange! I'm sure you could also get some pretty cool answers there!
James Ervin
CC BY-SA 4.0
2021-05-07T16:42:11.773Z
I will ask a different flavor of the same question over there tonight...
B--rian
CC BY-SA 4.0
2021-05-07T20:37:22.687Z
That's awesome to hear!
James Ervin
CC BY-SA 4.0
2021-05-08T03:03:30.023Z
I just tried to see if I could spot it from Spain. I think I did - some brief flashes, as I would expect from a tumbling object - but not 100% sure. I definitely saw Tianhe, five minutes later, at 0447h GMT+1, and confirmed with Stellarium (the satellites plugin has been updated with CZ-5B and Tianhe since you posted this question)
Aaron F
<amateur-observing><celestial-mechanics>
Spotting the booster rocket of the Chinese space station?
B--rian
# Spotting the booster rocket of the Chinese space station?

(Asked by: username_0 on 2021-05-06)

SpaceWeather.com's newsletter states

> Sky watchers are reporting two new objects in the night sky: China's new space station and an out-of-control booster rocket that helped launch it. The tumbling booster is flashing brighter than some of the brightest stars in the sky.

My usual star-gazing app does not have the rocket booster in its database (yet), and I did not manage to spot it yet (from cloudy central Europe). Is there an official name for the object? Could somebody help me how to find it in the remaining 2 days while it is still in orbit around Earth? Or maybe somebody could even share a picture or two?

Bonus question: Maybe somebody could also enlighten me in how far any single-use booster-rocket could be *in-control* after it ejected its payload? Wasn't that the default for many years? Also: Now that we can observe the rocket we know the trajectory and could also forecast when it will crash and where, aren't we?

### References

* [U.S. Space Command tracks Chinese rocket for uncontrolled re-entry from orbit](https://www.reuters.com/lifestyle/science/us-space-command-tracks-chinese-rocket-uncontrolled-re-entry-orbit-2021-05-06/)

**Tags:** amateur-observing, celestial-mechanics
> **Comments:**
> **username_0**: I was too fast, my bad: Tianhe-1 is indeed the space station, I should have double-checked. @User123 Thanks for pointing that out.  
> **username_1**: We would love this question on the Space Exploration Stack Exchange! I'm sure you could also get some pretty cool answers there!  
> **username_0**: I will ask a different flavor of the same question over there tonight...  
> **username_1**: That's awesome to hear!  
> **username_2**: I just tried to see if I could spot it from Spain. I think I did - some brief flashes, as I would expect from a tumbling object - but not 100% sure. I definitely saw Tianhe, five minutes later, at 0447h GMT+1, and confirmed with Stellarium (the satellites plugin has been updated with CZ-5B and Tianhe since you posted this question)  

---

## 2 answers

### Answer by username_3 on 2021-05-06. Score: 8 | Accepted answer

The name of the module is Tianhe (天和), and the name of the rocket is Long March 5B, but it got a special designation of 2021-035B. It is not really official, but I think it is good enough to get some data. I found [this website](https://orbit.ing-now.com/satellite/48275/2021-035b/cz-5b/) for current position ([live on globe](https://orbit.ing-now.com/3dlaunch/payload/data/2021-035/)). Your userpage says you live in Europe, so the rocket will fly over Greece at 00:30 CEST (22:30 UTC) tomorrow (in 1.5 hour if you have time to be awake for that long). I also calculated the maximum angular speed (on May 6) of around 1.85 °/s (on May 7, it is 2.7 °/s).

There are already some images; I found only [that one](https://spaceweather.com/images2021/06may21/tianhe1_strip.jpg). The booster's visual magnitude is around –1, so pretty bright to spot.

So, if you have time, try to look to the South and spot the booster at 22:30 UTC today. Maybe you are going to have luck tonight : )

---

There are some equations I derived with basic trigonometry about the rocket's visibility:

Distance $D$ at which the rocket can be seen right on horizon:
$$D = R \ \arccos\left(\frac{R}{R+h}\right)$$
Where the calculator calculates radians, $R$ is Earth's radius and $h$ is the altitude of the booster (you can get that on previously given website; at the moment it is 170 km).

The angular height of the booster which is $D$ kilometers away from us (on Earth) is given by:

$$\alpha = \frac{\pi}{2}-\arcsin \, \left(\frac{(R+h)\sin{\frac{D}{R}}}{\sqrt{2R^2+2Rh+h^2-2R(R+h)\cos{\frac{D}{R}}}}\right)$$
Again, the calculator must be set in radians.

---

Update: The rocket fell on May 9 at 02:14 UTC into the Indian Ocean, so no one was hurted.

> **Comments:**
> **username_3**: @B--rian Have you seen it yesterday?  
> **username_0**: Nope, was too cloudy.  
> **username_3**: I also included the equations for visibility, and if you live in UK or in Germany, the Greece is too far. Also note that the booster is currently falling.  
> **username_4**: `+1` yay for equations!  
> **username_4**: "It is not really official..." It really is official for those who track and catalog objects in Earth orbit (governments, militaries, etc.) Just like 2021-035A is the [COSPAR International Designator](https://en.wikipedia.org/wiki/International_Designator) for [Tianhe](https://en.wikipedia.org/wiki/Tianhe_(space_station_module)), 2021-035B is the international designator for the LM 5B core stage from that launch.  
> **username_5**: Perhaps is too late for such a question... Would it be visible only by reflection or can it set a plasma trail?  
> **username_3**: It can be normally seen just by reflection. But when it enters the atmosphere, it of course leaves a trail behind it. But you could only see that if the rocket would fall less than 800 kilometers away from you, which is pretty unlikely. Also, you could know when to go out because of the inaccurate predictions about the hour of fall.  

---

### Answer by username_0 on 2021-05-07. Score: 7

I found an openly available [CZ-5B Rocket Body (ID 48275) reentry prediction](https://aerospace.org/reentries/cz-5b-rocket-body-id-48275):

[![Reentry Prediction](https://i.sstatic.net/oBv3T.jpg)](https://i.sstatic.net/oBv3T.jpg)

Most intersting I found the following graph which shows **how slowly error bars of the predicted reentry time decrease over time** (as obtained from [The Aerospace Corporation Tweet](https://twitter.com/AerospaceCorp/status/1390416330269020160)):

[![error bars of the reentry](https://i.sstatic.net/jbFiV.png)](https://i.sstatic.net/jbFiV.png)

Last but not least, I also learned from a [Business Insider story called "The US is tracking an uncontrolled Chinese rocket traveling at 18,000 mph that is expected to crash down around May 8"](https://www.businessinsider.com/us-tracking-uncontrolled-chinese-rocket-due-crash-may-8-2021-5?r=DE&IR=T) more on why the rocket was called "uncontrolled":

> McDowell told Space News that letting debris reenter the atmosphere uncontrolled was "unacceptable."
> "Since 1990 nothing over 10 tons has been deliberately left in orbit to reenter uncontrolled," he said.

> **Comments:**
> **username_3**: Yes, the location in Europe is here crucial. If you live in South Europe, you might have a possibility of seeing it. But if you live on greater latitude than 50°, you might not have this possibility.  
> **username_0**: I prefer seeing it in transit rather than in my backyard :-)  
> **username_3**: But rocket in backyard would be more unforgettable.  
> **username_2**: the aerospace.org link now redirects to their twitter page :-/  

---

