Satellites in their last Orbits
Objects in an orbit around the Earth are bit by bit slowed down due to atmospheric drag. This is caused by air molecules that are still present in the space around the Earth, even when a satellite is actually orbiting way above the atmosphere. This drag causes that the satellites eventually to enter the atmosphere and burn up. Imaging satellites in their last orbits around the Earth is an interesting part of spacecraft astrophotography, especially because it provides an opportunity to observe satellites at shorter ranges then usually is the case thus makes it possible to see the objects in more detail then when the same objects are in their initial orbits at longer distances. Though, in practice I have learned that this is not always a logical reasoning, due to technical difficulties and limitations. We should realize that the lower orbits of satellites cause them to move at much higher angular (relative) velocities across the sky compared to higher orbits.
Imaging Technique at high Angular Speeds
The difficulty of high angular speeds in telescopic imaging can be solved by choosing shorter exposuretimes. In fact, exposuretimes shoud be as short as the brightness of an object allows. In practice this means that we must correct the gain adjustment of the camera with the result that images of fast moving objects in general will become a little more grainy then for example images of the ISS or other bright satellites in higher orbits.
Objects and Properties
Among the objects we can observe from time to time in their last orbits before reentry, there is a wide variety ranging from recent objects from failed missions to really old objects that have been slowed down and entered lower orbits gradually over the years. A common feature of objects shortly before decay is their tumbling behave although there is no guarantee that this instability is always present. On March 20, 2012, I photographed the old 1960's weather satellite Meteor 1-1 just a few days before its reentry. A full report on this is presented at the page Historic Spacecraft & Rockets.
Meteor 1-1's tumbling motion shorty before reentry, on March 20, 2012
IGS-1B captured in its final Orbits
IGS-1B is a Japanese Information Gathering Satellite that was launched on March 28, 2003 together with its companion IGS-1A on board a H2A rocket. Both satellites were delivered into a 486 x 491 km orbit with an inclination of 97 degrees. The satellites orbited within 37 minutes of each other. In 2007 it was announced by officials that IGS-1B was malfunctioning due to a loss of power. Since that time the satellite’s orbit was observed to degrade. The expected announcement of its upcoming uncontrolled atmospheric reentry finally happened on Thursday, July 26, 2012 at 9:52 GMT. Reentry was located over the Pacific Ocean, about 1,300 kilometers north-east of New Zealand. It is plausible that a considerable amount of fuel was still onboard during reentry of IGS-1B. It’s companion IGS-1A is still in orbit. In the last years, I regularly pointed my camera to IGS-1A and IGS-1B without any spectacular results; the images revealed not much detail. These satellites appeared to be reasonably difficult objects to capture in their original orbits.
That changed July 25, when I was able to capture IGS-1B just 13 hours before its reentry in an almost overhead pass in favorable atmospheric conditions. The altitude of the satellite at that time had already dropped below 200 km. The upper set of images show original unprocessed color frames from my video-camera attached to a 10 inch aperture reflecting telescope. It was one of the objects with the highest angular speed I have captured so far using my fully-manual tracking method.
Model picture of IGS-1B. Compare colors to telescopic images below
We see clearly the typical golden color of the foil wrapped around the satellite. The stronger processing used in the grayscale images on the bottom show interesting detail. Beside the solar panels – which from this angle are seen only slightly illuminated by sunlight – we see some nice detail on the satellite-body appearing as some knots and ridges that are confirmed by the other images of this frame-set. Searching the web for any pre-launch images of this satellite to compare with, I found out that there actually doesn’t exist published factory-images of IGS-1B or a comparable satellite of this type, only some sketches illustrating the approximate configuration of the satellites (see illustration on top). Ground-Based images of classified spacecraft are extremely rare as most of these vehicles are in classified orbits,” making the efforts put into obtaining telescopic photography of this type of satellites all the more worthwhile.
GOCE, Satellite with Aerodynamic Properties
Many models and simulations are made of the low orbiting Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) satellite, in space since March 2009. But apparently, there weren’t existing any real images showing its real situation in orbit, until now. The spacecraft was designed with aerodynamic properties because it had to fly through the thin upper layers of atmosphere still present at the altitude of 260 to 270 kilometers above the surface of the Earth. The design included an arrow shape and several fins to help keep the spacecraft stable during the flight. As the ion-engine is now quickly running out of fuel, the satellite is not far from an uncontrolled reentry this month. Currently, the orbit of GOCE is lowered to 221 x 233 kilometers.
On September 22, there was a last favorable pass of GOCE almost straight over my observing location in the south of the Netherlands. With good weather – which was very welcome after a bad luck period for my Cygnus spacecraft imaging attempts – I was fortunate to run an observing session with the telescope on that date. GOCE passed as expected with a high angular speed, but as already predicted by the different satellite programs, its brightness was not impressive. It was barely visible to the naked eye. Together with these fast angular speeds, such objects are among the hardest to photograph, as high speed requires fast shutter speeds – a fact every normal photographer knows. But I was lucky that the brightness of GOCE was just sufficient to capture it in several frames and even with some structure as well. We even can see some signs of the attached fins . Probably we see one of the longer fins along the side of the spacecraft from a relatively short angle or partially in shadow. So now at the end of the mission we have finally some real views of this satellite with some properties of aircraft and how it actually looks as it flies very low above the atmosphere. The row of images shows nicely that its flight was still stable on September 22, since in every image the attitude of the spacecraft looks equal.
A sign of the aerodynamic fins attached to GOCE are visible in this high saturated processing
ROSAT shortly before Reentry
I have been actively trying to get last telescopic views of the European (mainly German) ROSAT (ROntgenSATellite) in the last weeks before its reentry in 2011. When I photographed the satellite in June 2011, the altitude was still 330 kilometers, just a bit lower then the minimal altitude of the International Space Station. On October 6, 2011, I managed to observe ROSAT in a twilight pass, flying from West-Southwest into eastern direction, but it entered Earth’s shade when it was already at 68 degrees height in the Northwest. I could track it with binoculars only until 40 degrees, due to the clouds. On October 13, 2011, I saw ROSAT again during a new observing window, the final visible passes of the Röntgen telescope over my location in the Netherlands. It flew from West-Northwest to East with 47 degrees elevation. Sight was disturbed by clouds again, but I could see it with the naked eye between the clouds as a bright magnitude +1 object in twilight. Angular speed was high, as anticipated. Altitude was now reduced to 240 kilometers. As far as I could observe, it looked still fairly stable, but the clouds prevented me from determining this with certainty.
On October 14, 2011 – last visible pass of the final observing window – the sky was predicted to be cloudless. The problem this time was that ROSAT would pass in very early twilight. Since I perform manual tracking, I need a visible star to focus the camera and the question was: would I pick up the satellite in the light twilight sky? Then, while preparing the session a day earlier, I realized that it would pass at 51.4 degrees elevation through the North, as seen from my location. These coordinates are very close to the Northern pole star (Polaris). Now, with a chance to rely on a fixed position for the telescope, I knew that I could get the image, provided that everything else was fine. But of course it was not as easy as may seems; to track objects along the pole, the telescope mount has to be adjusted in a special position. But everything went fine: I picked up ROSAT in the tracking scope – with Polaris in the same field – and I captured several frames before the satellite disappeared behind a roof. The final image shows clearly the main body and the shape of the panels, with the shapes clearly defined, with increased resolution compared to the earlier images. The frames confirmed also the interpretation of the previous observations: so far, no visible sign of tumbling for this relatively small, yet massive satellite.
My best imaging result of ROSAT, taken on October 14, 2011. This photo was published in various media