8
$\begingroup$

Ground observers using naked eyes see several LEO objects per minute under good observing conditions, and many times more with binoculars. These are usually referred to as “satellites”. Many are, truly, operational satellites. But what proportion are non-functional satellites or debris?

According to United Nations Office for Outer Space Affairs (UNOOSA) registry, there have been 12,293 registered objects launched into space. As of April 2022 there were 8,261 satellites in Earth orbit, but only 4.852 (59%) are still active.

The exact numbers vary between databases, but a large minority of registered objects have become junk. (In addition, there are 27,000+ objects tracked by NASA, most >10cm diameter debris.)

So, when Little Johnny looks up and asks, “Daddy, what’s that?”, what’s the chance it is garbage?

Improve this question
$\endgroup$
1
  • 2
    $\begingroup$ I like your question! Since the lower something is the brighter it is, the more likely to be reentering and no-longer-needed it is, and the shorter-lived it is (on average - big solar-paneled space stations excepted), it's a bit of a fun challenge to get an average. Of course, when things have served us well and done their jobs faithfully, we have better words than garbage - we distinguish between cemeteries and landfills for example. $\endgroup$
    – uhoh
    Commented Jul 18, 2023 at 0:16

1 Answer 1

Reset to default
4
$\begingroup$

Very little

Almost everything in orbit that can be seen with the naked eye is between 100 and 400 miles from Earth and bigger than 20 feet1. A relatively large satellite in a relatively low LEO. These orbits decay within 1 to 20 years, so almost everything you can see was launched this century. Therefore, most of it will still be operational.

1Space.com's How to Spot Satellites

Improve this answer
$\endgroup$
5
  • 3
    $\begingroup$ Can you give references for "20 feet"? Starlink satellites are easily visible. According to casi.ca/resources/Documents/ASTRO/2020/… , most of the ground-observed light from Starlink satellites is reflected from the bus, which is only about a meter long. The solar array is about 10 meters long , but it points radially away from ground observers. It would be expected to produce specular reflections tangential to the Earth’s surface, away from ground observers. Solar arrays typically have a diffuse reflectivity of 2%. $\endgroup$
    – Woody
    Commented Jul 18, 2023 at 22:39
  • 2
    $\begingroup$ Near your lower limit of 100 miles from Earth - which if any objects are still actually useful? Is it possible to name a proof-of-principle example? $\endgroup$
    – uhoh
    Commented Jul 19, 2023 at 1:14
  • $\begingroup$ Of course we can't actually "see" anything in space other than the Moon, the Sun, a few nebula and galaxies, and the occasional comet tail. Everything else including stars are far too small to resolve with the naked eye. What we see instead is the light from these objects which is bright when viewed against a near black sky, and when viewing with somewhat dark adapted eyes. When viewing objects in orbit the main determiner is the amount of light reflected in our direction. Size is one factor in that but not the only one. For example "flares" can make a satellite pop in and out of view. $\endgroup$
    – Steve Pemberton
    Commented Aug 9, 2023 at 16:22
  • $\begingroup$ @StevePemberton by that definition we don’t “see” anything, everything we see is light from objects whether or not they are big or small. $\endgroup$
    – Dale M
    Commented Aug 9, 2023 at 21:53
  • $\begingroup$ @DaleM - I said in my comment, "What we see instead is the light from these objects". I don't anticipate that anyone will misinterpret this as meaning that we don't see light from other objects. $\endgroup$
    – Steve Pemberton
    Commented Aug 10, 2023 at 1:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.