Herschel! Herschel! Wherefore art thou Herschel?
Since launch Herschel has made various trajectory correction manoeuvres, the last of them on 10 June and, of course, now has an accurately measured orbit, rather than the predicted orbit that had to be used initially. So, where exactly is Herschel now and where will it be in the over the next few months?
Herschel is now effectively at the 2nd Lagrange Point of the Sun-Earth system and slowing rapidly as it reaches the apogee of its orbit. At 00UT on 12 June it will be receding at 168m/s. At 00UT on 22 June at 95m/s. And at 00UT on 2 July, at just 40m/s. Eventually, at 09:30UT approximately on 7 July, Herschel will reach apogee at 1.576 million kilometres from Earth and will then start to loop backwards until 5 September when it will be at perigee at a distance of 1.248 million kilometres.
As it comes in towards perigee, Herschel will drop below the ecliptic, crossing from Ophiuchus to Serpens, then back again briefly into Ophiuchus, before spending the second half of July and first half of August in Sagittarius. Finally, two and a half weeks before perigee, it will reach a minimum declination of -30 degrees in the southern constellation of Microscopium. By perigee, Herschel will have climbed through Piscis Australis to reach declination -21 degrees in Aquarius.
Herschel's magnitude will vary between V=19.5 at apogee and 19.0 at perigee. During the summer, as it passes through the dense star clouds close to the Galactic Centre, Herschel will be an extremely difficult object to observe from Earth.
Daily updates of Herschel's position are given in the Herschel Twitter Page (ESAHerschel)
A Herschel ephemeris can be generated for any observing site on Earth or in space using JPL's Horizons system and setting the target as "Herschel Space Observatory".
[M. Kidger from HSC ESAC, posted 11 June 2009]