We asked Kristin Wirth, Rosetta Science Operations Manager, based at ESAC - ESA’s European Space Astronomy Centre, Spain, to give some tips to amateur astronomers interested in tracking Rosetta from the ground. Kristin is leading an observation campaign from ESA’s Optical Ground Station on Tenerife, Spain, and she readily provided a tonne of information.

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Kristin told us that for an observer on Earth the movement of Rosetta relative to the stars is slow, except for a few hours around perigee passage - Earth closest approach (CA) on 13 November (7:45 UTC/ 8:45 CET). Rosetta will be approaching from the southern part of constellation Cetus and receding from Leo.

The path of Rosetta through the constellations around CA is as follows (times are in UTC, 13 November):

7:00 Sculptor
7:20 Piscis Austrinus
7:30 Capricornus
7:40 Sagittarius
7:50 Ophiuchus
8:00 Serpens Caput
8:10 Bootes
8:30 Coma Berenices

At closest approach the distance of Rosetta from the surface of the Earth is about 2500 km. Rosetta is flying fast: at 00:00 UTC on 13 November, the distance to Rosetta will be 260 000 km.

During approach, the Sun-Earth-Rosetta angle will be roughly 140º, so that observers on the ground will be looking at the illuminated solar arrays. We can expect a magnitude of 17 - 12 mag. This is about the same brightness of Pluto (which is about 1,150 times fainter than naked-eye visibility, according to Wikipedia).

But during recession, the Sun-Earth-Rosetta angle will be about 70º so that we will be looking at the dark side of the solar arrays and the magnitude will be much weaker.

Kristin’s observations at the ESA OGS on Tenerife will finish before Rosetta sets over Tenerife, at 04:20 UTC on 13 Nov.

This means that even before closest approach, Rosetta will only be a weak source of light and a ‘serious’ telescope will be necessary – but still within amateur range. Kristin recommends a 300 mm aperture.

She has also provided tips for observing from Central Europe and the Canary Islands (she will be at Teide Observatory, much to the ire of some of her colleagues :) ). Her tips will follow in a separate post.

--Amruta