21
votes
Accepted
Why is the sunny side of JWST's sunshield purple and covered with silicon?
From this presentation James Webb Space Telescope (JWST) Technology Discussion For APPEL Forum, dated August 7, 2008, I see this:
Low $\alpha_S$/$\epsilon_H$ to minimize absorbed solar heat load
...
- 8,188
18
votes
Accepted
Will it be possible to see BFR approaching the moon from earth, with naked eye?
tl;dr: at an apparent magnitude of about +18.5 you need a several meter telescope and a dark sky. Hubble can do it too. So by reflected sunlight, definitely not by eye. The exhaust from a Methalox (...
- 151k
15
votes
Accepted
Who proposed detonating gunpowder on the moon to prove a rocket reached it?
Looks like it was indeed Goddard. Key search term was 'flash powder' not gunpowder.
The Smithsonian has a
box built and used by American rocket pioneer Robert H. Goddard in experiments in 1916 to ...
- 189k
15
votes
Accepted
Is it feasible to paint SpaceX Starlink satellites black so as not to frequently saturate the CCDs of the Large Synoptic Survey Telescope?
In general satellites are not "painted". They are covered in a variety of Multi-Layer Insulation (MLI) blankets with varying optical qualities. I have seen MLI in silver, black, and gold - sometimes ...
- 4,504
15
votes
Will it be possible to see BFR approaching the moon from earth, with naked eye?
If you want an easy way to think about it, imagine how bright it might be in low Earth orbit, 240 miles up (which is just a bit lower than the International Space Station). However bright that is, it ...
- 571
14
votes
Accepted
Is the Starlink array really visible from Earth?
Yes, the Starlink satellites were visible from earth with your naked eye and have been seen and recorded by several people.
See also https://vimeo.com/338361997 and read about it here.
Gizmodo: ...
- 12.6k
14
votes
Accepted
How did the kerosene-burning Black Arrow have transparent exhaust? (seems to just "hover" in photos)
Typically, kerosene-LOX engines run fuel-rich, with an oxidizer:fuel mass ratio of about 2.5:1 (as compared to 3.5:1 for complete combustion), leaving significant amounts of unburned carbon soot in ...
- 172k
12
votes
Two week mark; has Mayak (Маяк) been spotted yet? Reflector deployed? Astronomy "ruined"?
No, the reflector hasn't been deployed.
Project head, Alexander Shayenko reported about it (RUS) (ENG) today.
Early report information
On July 17th the team reported about possible success (RUS) (...
- 373
10
votes
Accepted
Units of "pixels" in a research paper on digital streak formation for optical payloads
$$L_{st} =\frac{f \ v_{t, a} \ \tau}{\cos(\theta_{obj}) \ d \ p}$$
where tau is the exposure time, $f$ is the focal length, $v_{t,a}$ is the averaged value of the object's tangential velocity in the ...
- 151k
10
votes
Accepted
Will the Europa Clipper spacecraft be visible in 2026 when it flies by Earth again?
Tentative yes (w/small telescope & electronic tracking)
The current solution in Horizons is dated Oct 31, 2024. It differs from some of the information in Wikipedia because now it's launch time ...
- 151k
9
votes
Accepted
Have astronauts seen Starlink trains?
Yes, it happened at least on 13th April 2020 at 21:25:02 UTC:
https://sattrackcam.blogspot.com/2020/04/starlink-train-photographed-from.html
The image above (image ISS062-E-148365, original at high ...
- 994
9
votes
How did the kerosene-burning Black Arrow have transparent exhaust? (seems to just "hover" in photos)
I think Wikipedia's Bristol Siddeley Gamma; Advantages of kerosene / peroxide engines can shed some light on the missing light.
The combustion formula of kerosene and hydrogen peroxide is
$$\mathrm{...
- 327
9
votes
How "specularly" and therefore spectacularly will JWST reflect sunlight? Will we be watching for JWST flares?
How "specularly" and therefore spectacularly will JWST reflect sunlight? Will we be watching for JWST flares?
tl;dr Yes, some of us will, and it's up to the reader to decide if transition ...
- 91
8
votes
Accepted
Besides the video game and Nine Inch Nails box set, what is HALO-IV? How does it relate to the testing of ICBM interceptors?
Well, since I used to work for 'em (L3), I'll omit the argument about whether is this on-topic:
HALO stands for High Altitude Observatory. It's an airplane (several airplanes)
The existing contract ...
- 189k
7
votes
Can satellites or the ISS be seen from Earth during a Total Solar Eclipse?
As an answer to the question in your title, NASA photographer Joel Kowsky captured this stunning composite earlier today in Banner, Wyoming. The positioning required to perfectly frame Station during ...
- 1,585
7
votes
Accepted
Why does Unity look transparent?
Much of the Unity spacecraft appears (almost) transparent in this image; you can see the blue sky right through it.
This is absolute nonsense; you cannot see the sky through Unity's airframe. It is ...
- 172k
7
votes
Accepted
Where will the March 4, 2022 impactor hit the Moon? Will it be visible from Earth?
The object will hit the Moon on the far side, not visible from Earth (emphasis added):
There have been a few folks who, for one reason or another, have asked me not to post their data. This includes ...
- 10.1k
7
votes
Units of "pixels" in a research paper on digital streak formation for optical payloads
Per the first link uhoh provided in their edit, $p$ appears to be the size of one side of a pixel, with units of length/pixel. I draw this conclusion from:
Figure 6 in the paper, where p is labeled ...
- 15.4k
6
votes
Have any satellites had lights visible from Earth besides FITSat-1?
Humanity Star
According to Wikipedia:
Humanity Star was a passive satellite designed to produce flares
visible from Earth. Its shape was a geodesic sphere about 1 metre (3
ft) in diameter, ...
- 181
6
votes
How do rocket propellant combinations rank in terms of "brightness"?
The different brightness is caused by the fuel. The liquid fuel rockets burn hydrogen with oxygen, both are gases and the reaction product is hot water vapour, also a gas. But pure gas flames emit ...
- 49.5k
6
votes
Accepted
Why were early satellites produced with polished "mirror-like" surfaces?
Per the official NASA history of Vanguard it was for visibility and thermal control.
(Bolding mine)
Tousey had made some of the first calculations in the fall
of 1955, contributing ...
- 189k
6
votes
Are crewed spacecraft missions that appear stationary from the ground possible?
To be stationary above ground we need an orbit with a period of about 24 hours ( exactly 23 hours, 56 minutes and 4 seconds, one sideral day). Such an orbit is called GEO and is 35,786 km above ground)...
- 49.5k
6
votes
What would JWST's apparent magnitude be?
A very back-of-the-napkin calculation, appropriating the calculations from here:
Assuming the JWST is in fact a 17m sphere (actual 21 x 14), with perfect reflectivity (as pointed out in the comments ...
- 963
6
votes
Accepted
Has anyone in space beyond LEO seen or has anything photographed a non-pointer laser from Earth?
Check out Surveyor 7. "Laser beams from Earth are successfully detected by the craft's television camera in a special test of laser-pointing techniques."
See also:
https://nssdc.gsfc.nasa....
- 1,124
6
votes
Can we watch the object move towards the Moon and hit it this week at home? Will there be live telescope feeds?
Big thanks to @PM2Ring for pointing us to the HORIZONS data!
Looking at the Earth referenced state vectors, the data is (relatively) low precision. The position resolution is in "steps" of ...
- 10.1k
5
votes
Accepted
Two week mark; has Mayak (Маяк) been spotted yet? Reflector deployed? Astronomy "ruined"?
Some breaking news on this, according to this source (not the most reputable, to be sure) the reflectors have failed to deploy. They link to this source which appears to be from the creators of Mayak,...
- 550
5
votes
Accepted
Has the Humanity Star been spotted yet?
Yes, Humanity Star was spotted at "0106 on [January] the 27th New
Zealand time (1206 on the 26th UTC.)" The sighting was made from Pukehina, Bay of Plenty, New Zealand.
It was observed again on ...
- 23.8k
5
votes
Accepted
Will WFIRST and JWST be able to resolve each other?
tl:dr: ...neither telescope could begin to resolve the other when on opposite sides of the orbit, but only by a factor of ten. So the question really boils down to one of orbital mechanics, mission ...
- 151k
5
votes
Accepted
Efficient method to compute rise and set times of satellites (visibility problem)?
From what I can tell, you're right -- every article I've read on the topic cites Escobal, because they all feel they have to acknowledge the existence of an analytical solution, but then they all ...
- 8,649
5
votes
Accepted
Project Pluto asks: "Should high-flying space junk be better tracked? Perhaps by an 'official' agency of some sort?" What would it take to implement?
Honestly if there isn't much there, we really don't need to track it, unless it is some kind of spy satellite.
LEO is very important to track, as collisions are likely. GEO is also similarly important,...
- 122k
Only top scored, non community-wiki answers of a minimum length are eligible