Starship Asterisk*

Hi all,

A puzzle for those more experienced than me ...

I was out last night in my backyard in the North-West of Brisbane, shooting 30 second sub-frames of Pluto in the dense star fields of Sagittarius. Normally, I would discard subs like these two, due to poor tracking accuracy, but can anyone identify the streak moving from right to left (imaged by pure dumb luck)?:
Unlabelled details of the above frames include:
1) 30 second exposure.
2) Shot from NW Brisbane (UTC+10) towards: Az 268°, Alt 45°.
3) Frame dimensions ~49x32 arcmin, in celestial coords (nearly), with North up and East on the left.
4) Centre of frame: RA 18h38m30s, DE -20°12' (approx).
5) Brightest star in frame about magnitude 7.5.
6) Reduced original resolution about 6x for web upload.

Using heavens-above.com and Stellarium, I have thus far failed to identify this streak, which is moving more slowly than most of the satellites I have observed before (not many). Can anyone help, and perhaps give me an education in the process?

Thanks in advance,
Owen.

I'd say it's a geosynchronous satellite. 10 arcmin per minute is about right for a satellite that distance from the equator.

Thanks Chris, but I measure it to be almost 30 arcmin per minute. (The ~30 second gap between streaks was me being lazy and wasting sky time, by setting the camera to take intermediate dark frames and automatically subtracting them from the open frames. Silly me.)

(Having said that, it doesn't convince me that it is not geosynchronous, as I haven't worked out the maths for that yet.)

Cheers.

Nitpicker wrote:Thanks Chris, but I measure it to be almost 30 arcmin per minute. (The ~30 second gap between streaks was me being lazy and wasting sky time, by setting the camera to take intermediate dark frames and automatically subtracting them from the open frames. Silly me.)

(Having said that, it doesn't convince me that it is not geosynchronous, as I haven't worked out the maths for that yet.)

Cheers.

Right. I'm off at a ranch now with just my tablet, so no decent image measurement measurement tools. I was thinking these were 60 second exposures. At 30 seconds your estimate looks about right. It's still perfectly reasonable for a geosynchronous satellite.

The object appears to be moving predominantly eastwards, meaning that if it is geosynchronous (and I understand correctly) it must be close to its southern extreme on its daily analemma.

From my southern hemisphere location, geostationary satellites appear about 4.5 degrees North of the Celestial Equator. From this I deduce that a geosynchronous satellite observed at a southern extreme of 20 degrees South, would appear to traverse almost 100 degrees in Declination (4*[20+4.5]) every 24 hours, say 150 total degrees to scribe a typical analemma. This would make the object appear to move at an average speed of only about 6 arcmin per minute, when I have measured 30.

Unless I have made a bad assumption or some other gross error in my rough calculations, it seems like this object might not be geosynchronous. But the damn thing keeps me very interested!

Nitpicker wrote:The object appears to be moving predominantly eastwards, meaning that if it is geosynchronous (and I understand correctly) it must be close to its southern extreme on its daily analemma.

From my southern hemisphere location, geostationary satellites appear about 4.5 degrees North of the Celestial Equator. From this I deduce that a geosynchronous satellite observed at a southern extreme of 20 degrees South, would appear to traverse almost 100 degrees in Declination (4*[20+4.5]) every 24 hours, say 150 total degrees to scribe a typical analemma. This would make the object appear to move at an average speed of only about 6 arcmin per minute, when I have measured 30.

Unless I have made a bad assumption or some other gross error in my rough calculations, it seems like this object might not be geosynchronous. But the damn thing keeps me very interested! :)

There are also near geosynchronous satellites orbiting not too far off the equator that have low apparent speeds. Some sort of MEO is another possibility. In any case, just because the motion is slow doesn't argue against a satellite. Visually most satellites are in LEO and move fast, but images record a much wider range.

Oh, I'd be very surprised if it isn't a satellite. I'm hoping someone can help me identify which one.

To my inexperienced eye, this object seems too fast for geosynchronous orbit and too slow for LEO. That would narrow it down to a MEO (probably).

But which one?

Is there a list of classified satellites or is that not allowed? I figure astronomers must spot them from time to time.

geckzilla wrote:Is there a list of classified satellites or is that not allowed?

Maybe on wikileaks ... I've never checked, as I don't like being watched.

...

I'm a newbie at this astrophotography hobby, and in the few months since I started taking shots like this (moon size frame and 30s exposure), I must have taken well over a thousand, and this is the first time I've ever seen a streak (which didn't match the streaky stars on the poor ones). Over the last few weeks, I've taken hundreds just of this particular part of the celestial sphere (and about 20% are good ones too).

I know there are hundreds if not thousands of satellites, and lots of Near-Earth asteroids too, so I suppose I'm a little surprised that it hasn't happened more often. But I bet there are boffins out there who live to keep tabs on long lists of satellites and asteroids, and I'd be very interested to hear from them (and learn).

One thing I've overlooked ... My scope would have been moving westwards at about 15 arcmin per minute. Relative to a fixed Alt/Az, the speed of the object would be closer to 15 arcmin per minute. This increases the probability of it being a geosynchronous satellite. It also increases its northward component in relative terms, meaning that it is less likely that I have observed it near the southern extreme of its (supposed) analemma.

geckzilla wrote:Is there a list of classified satellites or is that not allowed? I figure astronomers must spot them from time to time.

Yes, the orbital elements for most classified satellites are available. Determining them is a popular amateur hobby. Of course, it's much more likely this is a satellite with elements publicly available. Either way, those and an orbit propagator should make it possible to identify this object. I'll take a look next week when I get back home.

Much appreciated.

Assuming a geosynchronous satellite, my rough (and possibly wrong) calculations would put it at a longitude of E114, oscillating over Western Australia, Indonesia and Hong Kong.

Not sure if that helps, but I hope it does.

According to the theory of geosynchronous orbits, I should be able to photograph the same portion of sky every sidereal day and re-observe the streak.

Not that I am surprised, but I attempted this last night and failed. Some theory!

I have confirmed that this streak did not match any of the 14,650 civilian satellites in the NORAD TLE database published at the time of the observation.

The only other information I have to go on is from Wikipedia: in the year 2001, almost 20,000 objects were catalogued from all sources. So, in 2013, I imagine there to be more than 10,000 satellites not published in the NORAD TLE database (which hardly narrows things down).

So, I'm giving up on identifying this streak, and making a mental note to never again get too excited about unintentionally photographing a satellite. Poop, that has spoilt my afternoon.

...

Instead, a new question ... does anyone know how to estimate the apparent magnitude of an object like this, when it appears in a photograph as a streak? Would anyone care to estimate the apparent magnitude of the streak in my images? (Actually that's two questions.)

Nitpicker wrote:Instead, a new question ... does anyone know how to estimate the apparent magnitude of an object like this, when it appears in a photograph as a streak?

It's not simple for a slowly moving object. Assuming the streak is unsaturated in all channels (which appears to be the case here) you need to either integrate the intensity over the entire path, or you need to determine the exposure time for any single pixel (or pixel grouping) and then scale the brightness against a known star.

Chris Peterson wrote: It's not simple for a slowly moving object. Assuming the streak is unsaturated in all channels (which appears to be the case here) you need to either integrate the intensity over the entire path, or you need to determine the exposure time for any single pixel (or pixel grouping) and then scale the brightness against a known star.

Thanks Chris. Speaking as a slowly moving object, I agree it is not simple for me.

Also, the colour CMOS sensor on the "pro-sumer" (horrible marketing term) grade DSLR I use, is perhaps not the best tool for this job.

I might first do some tests on known stars which appear round, before taking this any further.