On request of one of the editors I have written a long guest post for ESA's Rocket Science blog titled:
"Predicting GOCE re-entry: a citizen- scientist’s view"
The post details how I tried to forecast GOCE's re-entry time and position, using Alan Pickup's SatAna and SatEvo software. It provides some information about what factors are involved, and what problems you bump into. Basically, it is a consolidation and extension of posts that earlier appeared on this SatTrackcam blog.
Read the post on ESA's blog here.
Tuesday, 24 December 2013
Sunday, 22 December 2013
USA 186 (Keyhole KH-11/Advanced CRYSTAL) is moving orbit, as expected
In a series of previous posts culminating in the October 12 summarizing post here, I scetched a scenario of what I think will happen to the Keyhole/KH-11/Advanced CRYSTAL constellation of high-resolution Optical reconnaissance satellites following the launch of USA 245 on August 28.
The first part of that scenario now seems to be happening: USA 186 has moved orbit.
This happened slightly earlier than I anticipated, but it does seem to be the first change in a series of changes right along the lines I expected.
The KH-11's are currently almost inobservable from the northern hemisphere (and hence my location) due to the "winter blackout". In the southern hemisphere, where it is summer, South African observer Greg Roberts has however been tracking them.
On December 10, Greg failed to recover USA 186 (2005-042A) in its old orbit. Earlier I predicted that this would happen at some point, as the satellite would likely be moved several degrees in RAAN from the primary West plane to the secondary West plane, which are 10 degrees apart in RAAN. See my earlier post here for a discussion of primary and secondary orbital planes.
This made Greg next search for USA 186 in orbital planes more west of the original one. Indeed, on Dec 17 he recovered USA 186 in a more westward plane.
The new orbit as calculated by Ted Molczan from Greg's orbservations shows that the satellite lowered its orbital inclination by almost a degree, to 96.9 degrees. This manoeuvre probably happened on or near November 12th.
As a result of the inclination change the orbit is no longer sun-synchronous and hence its rate of precession changed. As a result its RAAN is currently shifting westwards relative to the other KH-11's. On December 17 the RAAN of USA 186 had already shifted westwards by 4 degrees. I suspect it will keep precessing until it reaches a value 10 degrees west of what it initially was (see my earlier predictions here, where I predicted this shift in RAAN), close to the aged West plane secondary satellite USA 129 (1996-072A). This shift will have been accomplished by early February at the current rate of precession (0.868 degrees/day or -0.12 degrees/day relative to the sun. Taking into account the RAAN precession of USA 245, they will have a separation of 10 degrees in RAAN by February 5).
I also suspect that next the satellite will reduce apogee altitude to attain a near-circular 390 x 400 km orbit, after which it will be sun-synchronous again. Indeed, the change in inclination to 96.9 degrees indicates as much as this inclination value fits a 390 x 400 km sun-synchronous orbit. As a result, USA 186 would start to move in an orbit very similar to USA 161 (2001-044A) in the secondary East plane in terms of apogee, perigee, inclination and eccentricity as well as in ground-track repeat patterns.
The initiation of these moves comes two months earlier than I expected, suggesting that USA 245 (2013-043A) which was launched into the primary West plane last August 28, needed less check-out time after launch than was the case with USA 224 (2011-002A).
As USA 186 is now moving to take the place of the aged USA 129 satellite, I expect the latter to be de-orbitted any moment.
Below diagram depicts the current constellation (December 17th), with USA 186 on the move westwards between the primary West plane (now occupied by USA 245) and secondary West plane (occupied by USA 129). See my earlier post here for a discussion of primary and secondary orbital planes.
It will be interesting to see whether the drift in RAAN of USA 186 relative to USA 245 indeed stops at a 10 degree difference (the former separation of the orbital planes of USA 186 and USA 129), or whether it perhaps continues up to 20 degrees (the separation of the orbital planes of USA 161 and USA 224 in the East plane).
The first part of that scenario now seems to be happening: USA 186 has moved orbit.
This happened slightly earlier than I anticipated, but it does seem to be the first change in a series of changes right along the lines I expected.
The KH-11's are currently almost inobservable from the northern hemisphere (and hence my location) due to the "winter blackout". In the southern hemisphere, where it is summer, South African observer Greg Roberts has however been tracking them.
On December 10, Greg failed to recover USA 186 (2005-042A) in its old orbit. Earlier I predicted that this would happen at some point, as the satellite would likely be moved several degrees in RAAN from the primary West plane to the secondary West plane, which are 10 degrees apart in RAAN. See my earlier post here for a discussion of primary and secondary orbital planes.
This made Greg next search for USA 186 in orbital planes more west of the original one. Indeed, on Dec 17 he recovered USA 186 in a more westward plane.
The new orbit as calculated by Ted Molczan from Greg's orbservations shows that the satellite lowered its orbital inclination by almost a degree, to 96.9 degrees. This manoeuvre probably happened on or near November 12th.
As a result of the inclination change the orbit is no longer sun-synchronous and hence its rate of precession changed. As a result its RAAN is currently shifting westwards relative to the other KH-11's. On December 17 the RAAN of USA 186 had already shifted westwards by 4 degrees. I suspect it will keep precessing until it reaches a value 10 degrees west of what it initially was (see my earlier predictions here, where I predicted this shift in RAAN), close to the aged West plane secondary satellite USA 129 (1996-072A). This shift will have been accomplished by early February at the current rate of precession (0.868 degrees/day or -0.12 degrees/day relative to the sun. Taking into account the RAAN precession of USA 245, they will have a separation of 10 degrees in RAAN by February 5).
USA 186: old orbit (red) and new orbit (white, December 17 plane)
The new orbit is still precessing westward over time. I expect
this will stop once it reaches the RAAN of USA 129 (grey)
I also suspect that next the satellite will reduce apogee altitude to attain a near-circular 390 x 400 km orbit, after which it will be sun-synchronous again. Indeed, the change in inclination to 96.9 degrees indicates as much as this inclination value fits a 390 x 400 km sun-synchronous orbit. As a result, USA 186 would start to move in an orbit very similar to USA 161 (2001-044A) in the secondary East plane in terms of apogee, perigee, inclination and eccentricity as well as in ground-track repeat patterns.
The initiation of these moves comes two months earlier than I expected, suggesting that USA 245 (2013-043A) which was launched into the primary West plane last August 28, needed less check-out time after launch than was the case with USA 224 (2011-002A).
As USA 186 is now moving to take the place of the aged USA 129 satellite, I expect the latter to be de-orbitted any moment.
Below diagram depicts the current constellation (December 17th), with USA 186 on the move westwards between the primary West plane (now occupied by USA 245) and secondary West plane (occupied by USA 129). See my earlier post here for a discussion of primary and secondary orbital planes.
It will be interesting to see whether the drift in RAAN of USA 186 relative to USA 245 indeed stops at a 10 degree difference (the former separation of the orbital planes of USA 186 and USA 129), or whether it perhaps continues up to 20 degrees (the separation of the orbital planes of USA 161 and USA 224 in the East plane).
Gaia launch event, Noordwijk SpaceExpo
On 19 December at 9:12 UT, a Soyuz rocket with a Fregat upper stage carrying ESA's 2-tonne astrometric space telescope Gaia lifted off from Kourou in French Guiana. It's destination: the L2 Lagrange point of the Sun and Earth, some 1.5 million kilometer from the latter.
Gaia artist impression (ESA)
(wikicommons)
ESA, the Dutch Research School for Astronomy NOVA, the Netherlands Space Office NSO and TNO organised a launch event at Noordwijk SpaceExpo on the morning of the launch, and Marieke Baan of NOVA was so kind to invite me for this event. As part of the event we all watched the launch Live on a big screen, with live narration by Juan de Dalmau, and next awaited confirmation of the separation of Gaia from the Fregat upper stage and the crucial last bottleneck, the deployment of Gaia's folded sun shield.
At the launch the audience was 'as silent as a mouse' as we say in Dutch. Applause was there following successful separation from the Fregat stage, and again upon confirmation of the solar shield deployment. The short video below shows the first two of these three moments:
The audience largely consisted of people from the Space Industry and journalists, and apart from watching the launch live on the big screen, they were treated on small lectures by a few of the people involved in the project. Marieke Baan (NOVA) acted as a moderator of the talks.
After an introduction by Juan de Dalmau we first watched the launch broadcast. This was followed by a very fine lecture by ESA astronomer Rene Laureijs, who detailed what work Gaia will do and what techniques it will employ. Next, Leiden astronomer Simon Portegies Zwart told us what 'revolution' Gaia will bring to astronomy. This was followed by a technical talk about the development of the equipment by TNO's Wim Gielesen.
Renee Laureijs (ESA) lecturing
Over the next five years, this telescope will record positions, distance and proper motion characteristics of one billion stars, creating a detailed 3D map (or 4D, given that movement in time is involved...) of our galaxy. It will do so 50 to 100 times more accurate than previous efforts, and for about 10 000 times as much stars.
In the course of this work, the satellite is expected to also record positional data on some 300 000 asteroids in our solar system, detect the tell-tale signs of exoplanets with other stars, as well as record Quasars and transient phenomena such as supernovae in other galaxies, not to speak of providing more insights into stellar evolution. It is also expected to measure the bending of starlight by the sun's gravitational field and in this way test Einstein's General Theory of Relativity.
The only thing it does not seem to do is brew a decent cup of coffee...
The sensor of the satellite is equivalent to a 1000 megapixel CCD. For about 1 billion stars, Gaia will determine on average 70 positions per object (and in some selected cases more), measured over a 5 year operational period. It will measure their positions, do photometry and determine the object's radial speed. It does so by means of parallax measurements with an accuracy of 0.026 milli arcseconds (!). To give you an idea of this accuracy: it means the positions are pinpointed with no more leeway than the diameter of a Euro coin at the distance of the moon.
Starting in May 2014, the satellite will produce 40 Gb of data per day, for five years. In other words: an incredible amount of data.
A Dutch-Italian girl named Gaia was a special guest
The Netherlands plays an important role in this mission. Part of the initial data processing will be done here. Much of the spacecraft's frame and optical assembly were developed and built by TNO, while Dutchspace built a crucial Real-Time Simulator (RTS).
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