Tuesday 6 January 2015

Observing HEO objects

In wintertime at latitude 51 degrees North, satellites in Low Earth Orbit are mostly invisible except for twilight, as all their passes are completely within the Earth shadow.

This season is therefore the season that I focus on HEO and GEO objects. HEO stands for Highly Elliptical Orbit and is almost synonymous with the more informal name 'Molniya orbit', after a class of Russian communication satellites employed in such orbits.

Military SDS COMSAT USA 198 (SDS 3F5), imaged in Cassiopeia on 4 Jan 2014

Satellites in a Molniya orbit have an orbital period of about 2 revolutions per day, an orbital inclination near 63.4 degrees, perigee at a few hundred kilometers altitude over the southern hemisphere and apogee at altitudes near 36000 km over the Arctic. They spend most of their orbital time near their apogee.The 63.4 degree orbital inclination ensures that perigee keeps at a stable position over the southern hemisphere.

US military payloads and 'unknowns' in Molniya orbit

The advantage of a Molniya orbit is that it allows a good, long duration view of high northern latitudes, including the Arctic region, which are not well visible from a geostationary orbit. This is ideal for communications satellites serving these regions, for SIGINT satellites, and other applications (such as infrared ICBM early warning systems, e.g. SBIRS) that benefit from a long 'stare' and good view of high Northern latitudes.

The US military has several systems in a Molniya orbit (see image above): communication satellites (e.g. two components of the SDS system), several SIGINT satellites (TRUMPET and TRUMPET-FO), and components of the SBIRS system (piggybacked on three TRUMPET-FO SIGINT satellites). Identifiable payloads include:

- TRUMPET 1, 2 and 3 (SIGINT);
- TRUMPET-FO and SBIRS USA 184, 200 and 259 (SIGINT and SBIRS);
- SDS COM satellites USA 179 and 198

There are a couple more which we cannot (yet) tie to a specific launch and function (see note at end of post).

Near their apogee, satellites in Molniya orbit are located high in the sky for my location, and because of their high northern position, they are sun-illuminated and hence visible (typically at magnitudes near +9 to +12) even at midnight and in winter. They move very slowly when near apogee, creating tiny trails on the images.

On December 13, the NRO launched (as NROL-35) a new SIGINT and SBIRS platform into a Molniya orbit: USA 259 (see a previous post). It is currently still actively manoeuvering to attain its final orbit, which makes it an interesting object to track. The image below was taken in late twilight of Jan 4, when the satellite was past its apogee and on its way to perigee. It was 4 minutes early against orbital elements based on observations of only a few days old.

SIGINT/SBIRS satellite USA 259 (NROL-35) imaged in Andromeda in the evening of Jan 4

I image these objects with an old but good Zeiss Sonnar MC f2.8/180 mm telelens (made in the former DDR and sturdy -and heavy- as a tank). This lens has a 67 mm aperture at f 2.8, which means it shows faint objects. As these objects move very slowly, the relatively small FOV is no problem. My observational data from January 4th can be found here and here.

Note: the 'unknowns' in the orbital plot above are objects we track that are not in public orbital catalogues and which we cannot tie to a specific launch. Although some of them certainly are, not all of these need to be payloads: some might be spent rocket stages from launches into HEO.