4
$\begingroup$

When the entire Lunar Gateway space station is built it would be a bit smaller than the ISS. We amateur astronomers and observers love to spot the ISS and be able to follow the history of space exploration from our backyards.

I was wondering if this time we will be able to spot the new station. I know that we don't have the angular resolution to see more than a simple point of light but in terms of brightness, would it be visible? Could we track it as it goes across he terminator and is still lit by the Sun? I guess the brightness would be so low that there is no way to spot it with the unaided eye, but at least with an amateur telescope? What aperture would be needed?

$\endgroup$
1
  • $\begingroup$ A cynical answer would be "The idea is so dumb and pointless, it will surely never actually be built, so no it will never be possible to see it ..." :O $\endgroup$
    – Fattie
    Commented Jan 7, 2023 at 17:12

2 Answers 2

7
$\begingroup$

Will the Artemis Gateway Lunar Space Station be visible from Earth by amateur observers?

If they can see +15 magnitude stars routinely, they'll probably be able to see Gateway.

Answers to Good source for the relationship between absolute magnitude, diameter, and albedo? give a simple expression for absolute magnitude for solar system (reflective) bodies which is defined as the apparent magnitude of the object at 1 AU from the Sun and 1 AU from the viewer. From @MikeG's well-sourced answer:

$$ H = C - 5 \log_{10} D - 2.5 \log_{10} p_V$$ where $H$ is absolute magnitude, $p_V$ is albedo, and $C$ = 15.618

and D is the spherical diffuse object's diameter in kilometers.

I'm going to model the gateway as a 5 meter diameter spherical cow and ignore any potential flares from any solar panels, heat radiators, windshields, or other flat surfaces for now.

With an average albedo of 0.5, that gives us an absolute magnitude $H$ of +27.9. Now Gateway will presumably stay at 1 AU from the sun, but remain a heck of a lot closer than 1 AU from us. We can scale using inverse square and get the visual magnitude as follows:

$$V = H + 5 \log_{10} \left(\frac{r}{1 \ \text{AU}}\right)$$

With a lunar distance $r$ of 400,000 km or 0.00267 AU, that gives an apparent spherical cow magnitude of +15.

If we can comfortably see +5 magnitude with a 6 mm night-adapted pupil, then to go another 10 magnitudes requires about a factor of 100 in diameter, or a 60 cm (24 inch) aperture.

Maybe Gateway will be brighter, maybe you can see +6 or +6.5 stars, but it's going to be a 30 to 60 cm (12 to 24 inch) amateur telescope if you want to see it in the eyepiece.

An excellent follow-up question would be how far might Gateway move relative to the background stars in a 10 minute exposure!

If you are an astrophotographer then a small telescope is all you'll need if you have a good camera and good astrophotography technique.

Per sources in my answer to Is it possible to capture geostationary satellites with DSLR? a big communications satellite in GEO can brighten by 5 or 6 magnitudes during a flare.

Certainly Gateway (like any space station) will be quite power-hungry like a global communications satellite, so I think we can expect flares -- if they do happen say during a full Moon when anything near the Moon that's pointing at the Sun will also point roughly towards Earth -- will be fun to watch for.

Could we track it as it goes across the terminator and is still lit by the Sun?

Now that's an interesting question!

1 arcsecond is sort of a nominal best resolution from the surface of the Earth for a telescope without dynamic wavefront correction. The technique of lucky imaging might help here.

The disk of the Moon has an area of about $2.5\times 10^6$ arcseconds. Its maximum (full) and minimum (new) magnitudes are -12.9 and -2.5. To get the area brightness in funky sounding but traditional units of magnitude per square arcsecond, we add $2.5 \log_{10}(2.5\times 10^6) \approx +16$ giving +3.1 and +13.5 mag/arcsec.

This means that the light reflected from the tiny, deep sub-pixel spacecraft will barely brighten a pixel in an image of even the unlit area of the Moon.

(Unless of course there is a flare!)

Deep sub-pixel means it will not significantly darken a pixel in an image of the bright, lit side of the Moon.

Therefore you'll probably need to wait until Gateway is away from the disk of the Moon and against dark space to see it.

Images of a satellites in Earth orbit crossing the Sun look the way they do because they are resolved images

or much closer and brighter than Gateway


Further reading:

$\endgroup$
6
$\begingroup$

Gateway will almost certainly be observable by lucky and ambitious amateur astronomers.

The station's magnitude is more relevant than its size.

By comparison, JWST is physically smaller than Gateway but, as a magnitude 14 object, it has been captured by amateur astronomers https://petapixel.com/2022/01/03/photographer-captures-james-webb-space-telescope-flying-across-nebula.

Since Gateway is larger and 1/4 the distance, it should be observable as well.

Most of the observable light will likely come from specular reflection off the solar panels. The best observing will likely be around full moon when the panels are perpendicular to both the incoming solar rays and the sight lines to back yard astronomers.

The Near Rectilinear Halo Orbit, as planned, takes Gateway 3000km "above" the moon's North Pole and 70,000 km "below" its South Pole, so at its farthest, it will be 10 degrees (handsbredth at arm's length) "below" the moon.

The magnitude will be variable, like JWST's. Since it is an irregular structure observed by reflecting solar light, brightness can change rapidly. It is quite different from astronomic bodies whose magnitudes are consistent.

Glare from the moon will depend on observing conditions, your location and equipment. I live in a rural area with very dark winter skies (Class 2 on Bortle scale) and access to a 14" Celestron so I'll likely find the rascal.

You will be unlikely to "track it as it crosses the terminator" since you will be viewing the solar panels edge-on.

$\endgroup$
5
  • 1
    $\begingroup$ Nice answer. I will select this one if you include some numbers: perhaps the expected visual magnitude of the station? Also, I guess the full moon aproach is nice but the glare of the full moon might impede the observation. Will the station be as separated at to be easily observed without all the moonlight washing away the image? $\endgroup$
    – Swike
    Commented Jan 6, 2023 at 20:11
  • 2
    $\begingroup$ That video was not taken when JWST was at L2. It was taken 26 hours after JWST launch when it was only 240,000 km from Earth, a little over half the distance to the Moon. It was closer than Gateway. See Where is the Webb Telescope? at Dec 26th 15:00Z $\endgroup$
    – Schwern
    Commented Jan 6, 2023 at 21:21
  • $\begingroup$ @Schwern I get +15 $\endgroup$
    – uhoh
    Commented Jan 7, 2023 at 1:05
  • 1
    $\begingroup$ @Schwern ... you caught me failing to chase that down. But other amateur astronomers have sighted JWST at L2. The ephemeris is available from JPL. $\endgroup$
    – Woody
    Commented Jan 7, 2023 at 1:35
  • 1
    $\begingroup$ very impressive answer $\endgroup$
    – Fattie
    Commented Jan 7, 2023 at 17:16

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.