Last Chance to See ... the Rosetta Comet Chaser

from Michael Khan, 31. October 2009, 04:00
Attention astronomers: On November 13, 2009, the ESA comet chaser probe Rosetta will visit us Earthlings one last time. The spacecraft needs to gain momentum at Mars (once) and the Earth (three times) to boost its orbital energy sufficiently to reach the target comet 67P/Churyumov-Gerasimenko. 

The Rosetta Spacecraft, Source: ESA(Lesen Sie diesen Artikel hier auf Deutsch)

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.

Top View of Rosetta Swaingby Geometry, Source: Michael Khan/ESAI am not going into details about how a planetary swingby works. Suffice it to say that it requires a close hyperbolic pass at a celestial body, in this case, our Earth.

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.

Rosetta Groundtrack on November 13, 2009, Source: Michael Khan/ESA

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.  Sun-Earth-Rosetta Geometry around Nov. 13, 2009, Source: Michael Khan/ESA

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. 

Tabelle der Position von Rosetta beim letzten Pass vor dem Swingby, 12-13.11.2009, Quelle: MIchael Khan/ESABewegung von Rosetta durchs Sternbild Steinbock, 12-13.11.2009, Quelle: Michael Khan/ESA

More Information

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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  1. Gerhard HOLTKAMP Asteroid on Collision Course
    01.11.2009 | 19:16

    A few days before Rosetta's second Earth flyby a small asteroid was discovered heading toward a collision with Earth. It took a few days for someone to realize that the orbit of this new asteroid was remarkably similar to the Rosetta orbit - that in fact the two were the same!

    Of course Rosetta did not collide with Earth but passed it at the correct distance. From what I know the position of Rosetta will now be checked if a suspected new asteroid is being discovered. So we should not get any false alerts over the next few days of an asteroid threatening Indonesia!

  2. Michael Khan That was a funny story ...
    01.11.2009 | 22:33

    Here is more on the excitement that Rosetta occasioned prior to its second swingby two years ago.

    Articles on Mistaken Identification of ESA Rosetta Spacecraft as Asteroid

    The mis-identification even led to the assignation of an asteroid number!

    Misidentification as an asteroid

    I still remember the dry comment in a communique that stated that the presumed Earth-crossing asteroid discovered shortly before "[...] is in fact the Rosetta spacecraft".

    Still, it shows that everyone was alert and that the Space surveillance was in fact working. I prefer them to be slightly over-zealous than to miss a real asteroid.

  3. Michael Khan Notes on the apparent magnitude
    02.11.2009 | 10:33

    Computing the apparent magnitude (i.e., the brightness with which an object appears to an observer in the night sky) is non-trivial to a high degree for a spacecraft.

    Let's look at previous Rosetta swingbys, for instance the first one in March 2005, one year after launch.

    The above article shows the swingby geometry. Here is the diagram again for your convenience:

    It shows that Rosetta approached almost exactly from the anti-sun-direction, meaning that the illumination conditions were optimal and the chances for specular reflection (i.e., sunlight bouncing off some reflective surface and hitting the eye of the observer, again increasing the apparent brightness) were maximized. In fact, this means that the aspect angle I mentioned in my blog post was near-zero.

    Regarding the distance, in 2005, Rosetta's approach speed was only about one third of what it will be at the upcoming encounter.

    Therefore, n days before the 2005 encounter, Rosetta could be expected to be around nine times brighter than n bays before the current encounter, PLUS the added effect due to the then optimal illumination conditions.

    Here now are some images and magnitude assessments from the 2005 swingby.

  4. Michael Khan And then of course, there was this ....
    02.11.2009 | 12:48

    Son of Rosetta

    During Rosetta's previous Earth swingby in 2007, excitement as provided not just by the temporary mis-identification of the spacecraft itself as a hostile asteroid, but also by the fact that it appeared to be "shadowed" by a real asteroid on its path from Mars to Earth.

    That did cause some concern at first - not so much because anyone thought that the asteroid harbored any hostile intentions, but rather because any object spotted on a co-orbiting trajectory with one of your spacecraft very often means bad news, i.e., something came loose and broke off.

    Luckily, this was not the case.

  5. Roberto Flores Hasta pronto Rosetta !!....
    04.11.2009 | 15:25

    Les deseamos un éxito completo, a Michael y todo el equipo que trabaja en el satélite.

    Observatorio Astronómico
    Neuquén (Argentina)

  6. Michael Khan Muchas gracias
    05.11.2009 | 17:36

    Muchas gracias, Roberto!

    Pero el momento mas dificil para esta mision no sera el encuentro con la tierra. Esperamos con nervosidad el encuentro con el asteroide Lutetia y naturalmenta la llegada al cometa ....

  7. Michael Khan Media Event at ESOC on 13. Nov. 2009
    06.11.2009 | 10:04

    There will be a press and media event at the European Space Operations Centre on the morning of November 13:

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