Tuesday 28 June 2011

OT: more imagery of the earthgrazing asteroid 2011 MD

Like yesterday, I used the 0.61-meter F/10 Cassegrain of Sierra Stars Observatory (G68) in California again to image the earthgrazing asteroid 2011 MD, this time a few hours after closest approach.

Below animated GIF was made from three 30 second exposures, spaced 30 seconds, starting at 09:35:01 UTC (28 June).

Monday 27 June 2011

OT: Close Encounters of a Rocky Kind (2011 MD)

Today, a small asteroid, 2011 MD, made a very close pass by the earth, coming to within a distance of 12,400 km at about 13:30 UTC (June 27). That is well within the distance of geostationary satellites, and even within the distance of GPS satellites in MEO!

This morning, some 5 hours before closest approach, I used the 0.61-meter F/10 Cassegrain of Sierra Stars Observatory (G68) in California, to capture this PHA earthgrazer.

click image to enlarge

This image is a 30 second CCD exposure taken between 08:32:00 and 08:32:30 UTC (June 27, 2011). In these 30 seconds, the fast moving object left a notable bright trail on the image. Star field is in Serpens, image center approximately RA 15h35m57s, dec. +19.441 degrees.

UPDATE: more of my 2011 MD imagery shot on the 28th here.

Monday 6 June 2011

NanoSail-D: a brief note on how I construct these brightness curves

Some people have asked me how I get the brightness curves from my images. So here is a brief explanation.

On each image, the satellite has made a trail. Start and end times of the exposure are well known, and the start and end of the trail corresponds to these times.

Next it is as simple as using software (e.g. MaximDL, or IRIS) that can read pixel values along a line: let the software read a line that corresponds to the satellite trail on the image! With MaximDL or IRIS, this is as simple as drawing a line over the screen with your mouse, exactly over the satellite trail. The software then reads the pixel values along this line. The resulting data can be exported as a data table.

These data are then read into a spreadsheet I created, that for each data point interpolates the corresponding time (remember that the time of the first and last datapoint in the dataset is known: start and end of the exposure).

Now, this assumes the movement of the satellite on the image is linear. Strictly speaking, the apparent speed of the satellite is not linear. However, with short exposures (10s) that will not be a really big source of error.

Sunday 5 June 2011

NanoSail-D: evolution of the flash pattern during a pass

Yesterday evening at 23:00 CEST I observed a twilight pass of the experimental NASA solar sail NanoSail-D again (see earlier and later observations here).

This pass allowed me to capture a series of brightness curves, which document the evolution of the flash pattern during a single pass, as the looking angle is changing (looking "edge on" aroudn culmination, and then more and more "on the tail"as it is descending) . The change in flash pattern is profound: this is clearly a very complex matter where the flash pattern highly depends on the relative position of the object to the observer.

click images to enlarge

It starts (image and diagram 1) with a lot of irregular flashes, spaced 0.3 - 0.9s apart (average 0.49s but with large standard deviation).

Next (image and diagram 2), a nice semi-regular sinusoid pattern develops, flashes spaced 1.24 - 1.45 s (average 1.33s)

Then (image and diagrams 3 and 4), the period increases, the pattern transforming to a slower sinusoid with peaks first 5.61s apart, then somewhat decreasing again to 4.11s apart (edit: or maybe not: the firts "peak"might be a sub-peak. The valleys seem at similar distance to the previous diagram). Superimposed on this, a shorter cycle of minor subvariation can be suspected, with various periods.

This slower variation in the last two diagrams is why Bram, me and some other observers got the impression, on this and some past passes, of the period almost "disappearing" when NanoSail-D was descending on altitudes of ~35 degrees.

Two of the trail images in a bit more detail: note the difference in flash pattern:

click images to enlarge

Saturday 4 June 2011

NanoSail-D Galore

Yesterday evening, I observed a twilight pass (sun at -7 degrees altitude and a still bright blue sky) of NanoSail-D, the NASA experimental solar sail (see earlier posts here). It passed at 44 degrees in the east, and after culmination became very bright again, flashing to mag. 0.

Because of the bright twilight sky I had to tone down the ISO to 400 and diaphragm to F4.0. The images show the flashing behaviour very neatly, and I obtained two spectacular sinusoid brigthness curves (the second one is from the image shown):

click images to enlarge

The flash period is definitely slightly variable, varying between 1.2 and 1.5 seconds with an average of 1.35 +/- 0.12 seconds.

With the current orbit, decay is projected for mid-August.

note added: click the 'Nanosail-D' label below to see later posts on NanoSail.

Friday 3 June 2011

Yet more bright NanoSail-D, and an unidentified object from the same launch

Yesterday evening (Wednesday evening) was very clear and saw another fine pass of NanoSail-D (10-062L), the experiental NASA solar sail. As on previous occasions, it became very bright after culmination, while descending to the southern horizon: reaching an easy naked eye magnitude of +0.5. It is still flashing, but trail saturation on the images meant I could not get a reliable brightness variation curve this time. Below are two images: one that shows it just north of the Coma Berenices star cluster, the other shows it passing south of Bootes into Virgo somewhat later (bright star in the top is Arcturus):

click images to enlarge

Tonight (Tuesday evening) I had another pass, a low west pass at 35 degrees altitude this time. And....it was invisible, to the naked eye at least.

On April 27th, Russell Eberst observed an unidentified object that moves in the same orbital plane as NanoSail-D and appears to be "something" from the same launch (see also here). It was subsequently observed by a number of other observers (and perhaps earlier, on March 3, by Greg Roberts), and yesterday I photographed it:

click image to enlarge

Another object observed this evening was Lacrosse 5 (05-016A).

Wednesday 1 June 2011

More flashing NanoSail-D, and flaring KH-12 USA 224

A week after my May 24 observations, I observed NanoSail-D (2010-062L) again, the experimental NASA solar sail.

NanoSail-D (image: NASA/MSFC)

As a week ago, it became very bright after culmination, while descending in the south, and was rapidly flashing again. It was easily seen by the naked eye, reaching mag. +1 or possibly +0.5.

In fact it is so bright, that the pixel brightness of the trail reached saturation on two of the three images. The first image (below) did yield brightness information: the resulting curve is shown beneath it. The flash period is irregular, but periods of 0.5s and 1.0s pop up frequently in the diagram (for actual determined flash times, see here. Astrometry on the satellite itself can be found here).

click images to enlarge

As can be seen on the images, the satellite was in a race with an untimely field of clouds (the orangish streaks in the images), staying just ahead of it. Visually, the brightness fluctuation was much more apparent than it is on these images (due to the saturation of the latter): it was very clearly flashing.

Nanosail-D was not the only object flashing. USA 224 (11-002A), the new KH-12 Keyhole launched on January 20 this year, flared too, while passing through the zenith, with flares at 23:48:27.3 and 23:48:31.8 UTC (May 31). The "saddle" and elevated brightness between the two flares is interesting (the trail is notably fainter before the first flash, and subsequent images show it is fainter again after the second flash):

click images to enlarge

This was the second time I imaged USA 224 (The first time was May 24). In addition to USA 224 and NanoSail, I also imaged another KH-12 Keyhole, USA 161 (01-044A), and a Lacrosse SAR, Lacrosse 3 (97-064A).