The last swingby will take place soon. This will also be the last chance for all amateur astronomers to catch a glimpse of the spacecraft. The viewing conditions are not very good for central Europe.
Closest approach will be on Friday, the 13th of November 2009 (Uh-oh ... but of course, we're not superstitious, are we?) at 7:45:40 UTC (8:45:40 CET), according to current planning. The perigee location will be above 109 degrees Eastern longitude and 8.2 degrees Southern latitude, just south of the Indonesian island of Java. Rosetta will zoom past at 2481 km altitude and a speed of 13.34 km/s.
The diagram above (Note: Clicking on any of the images opens an increased-size view) illustrates the swingby geometry with respect to the Earth. On the left you see the ground track of the spacecraft traced during November 13.
Rosetta will approach from a direction in the celestial vault that can be described by a right ascension of 31.93 degrees (2h 8') and a declination of -18.21 degrees, placing it into the constellation Cetus. After the swingby Rosetta will be receding into right ascension 169.96 degrees (11 h 20'), declination +24.3 degrees, i.e., constellation Leo.
In the following diagram you see the Sun-Earth-Spacecraft geometry. Imagine a line that extends from the spacecraft to the Sun, and another that extends from the Spacecraft to Earth. If the angle between both lines is small, this means that the spacecraft "sees" the Earth close to the direction towards the Sun.
Conversely, this obviously implies that the spacecraft appears well illuminated and over the night side when viewed from the Earth. This is precisely the situation prior to the swingby, as you can see from the red curve in the diagram. This "Sun-Rosetta-Earth angle" is below 40 degrees. With its 64-square-meter solar array, these illumination conditions should lead to a nicely visible magnitude (apparent brightness). My colleague Tim Flohrer, who knows a lot about these things because it's his job, computed a an apparent magnitude of around +17 for 22:00 UTC on Nov. 12; Rosetta will grow brighter to maybe as much as +15 shortly before setting. If this turns out wrong, complain to him, not to me. On the outbound arc, following closest approach, the angle is close to 90 degrees, meaning that the solar arrays will be viewed edge-on, rendering them largely invisible.
Of course, magnitude is not the only thing that matters. I have computed the viewing geometry for the viewing location Darmstadt/Germany, but everything said here should apply reasonably accurately to anywhere in central Europe. (If you need the exact ephemeris for any viewing location in the world, please use JPL-Horizons!) On the left you see the elevation over the horizon of the viewing direction to the Rosetta spacecraft. The spacecraft will always rise in the early evening, culminates shortly before midnight at over 20 degrees of elevation and sets in the early morning. After the swingby manoeuvre, it will rise shortly before midnight, culminate at 65 degrees of elevation and set in the early afternoon ... which is just as bad for observation as the expected low magnitude. Note: All times referred to here are given in UTC!
Now only the distance remains. Although the hyperbolic velocity, with 9.4 km/s, is lower than the perigee velocity of 13.34 km/s, but the fact remains that Rosetta is one fast spacecraft! Only on November 12 at 0 h UTC will the distance decrease below 1 million km. On November 3, when my analysis starts, the distance is still almost 9 million km.
Right, so now let's look at the evolution of azimuth and elevation for the viewing location Darmstadt (or anywhere else in Central Europe) in a polar diagram. What we see are ... two friendly smiles. The lower one is the evolution of the viewing direction from horizon to horizon during approach, the upper one the same during departure. The streak in between represents the largely theoretical (because we will not be able to see anything at that time) pass during the closest approach phase on November 13. Well, who knows, perhaps some radio astronomers will try to track Rosetta...
The Situation on November 13
As stated, there is no chance of visibility around perigee. The spacecraft will rise at 8 h UTC (9 h CET), too late for viewing. At that time, the expected distance from the regarded viewing location will already be 10,000 km and growing fast.
In the afternoon, the spacecraft will set and then re-appear after 23 h UTC (i.e., after midnight CET). At that time it will be beyond the Moon's orbit and, as stated, unfavourably illuminated and therefore unlikely to be visible.
A last glance at the azimuth-elevation polar plot. Forget the long streak across the graph: plain daylight. Forget the late pass starting at an azimuth of around 60 degrees: low apparent magnitude. All that remains is the last pass during final approach, at distances of less than 250,000 km. Rosetta will appear to pass relatively close to the Mag. +4 star upsilon Ceti, which might aid in finding it.
Here are right ascension and declination for this pass in the night of 12/13 November 2009. I will definitely try to see Rosetta then, weather permitting.
ESA Web article on the swingby on 13 November 2009
Rosetta Blog on the ESA web site
Rosetta web pages in the ESA science web site
ESA-Specials: Rosetta web site
And of course, the indispensable tool for all astroomers is the JPL Horizons web site, which gives you the precide ephemeris exactly for the location you specify. The ESA Flight Dynamics experts provided the Rosetta trajectory data.
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