I spy...a spacecraft

The Herschel satellite is not only being monitored by the mission controllers at Herschel Science Centre.  It is also being watched by optical astronomers, who have captured images of the satellite against the starry background.  There are two methods of doing this, one is to stare at the area of sky which the spacecraft will pass through and watch for moving objects - this is the same method used to find near-earth objects such as asteroids and comets.  The other method, which relies on more equipment, is to track where Herschel should be in the sky.  Since it is a moving target, travelling around one arcminute (1/60th of a degree) per hour relative to the backround stars, taking a long exposure on a camera causes the stars to move across the field of view.

Animation of Herschel moving
Image of Herschel against the background stars

Left: animation of Herschel moving against the background stars. Right: image of Herschel, with background stars moving. Image credit: Gustavo Muler, Observatorio Nazaret

Gustavo Muler, of Observatorio Nazaret (Lanzarote), has captured the satellite moving against the background stars.  The very faint speck moves past the arrow in the animation on the left, which may need to be clicked on to show it fully.  On the right, Gustavo has used the second method, and tracked Herschel while allowing the stars to move through the image.

Images of Herschel and Planck
Images of Herschel (left) and Planck (right) with the background stars moving. Image credit: Peter Birtwhistle, Great Shefford Observatory

In the UK, Peter Birtwhistle of Great Shefford Obervatory in Berkshire has captured both Herschel and its sister satellite Planck, as shown on the left.  Planck appears brighter despite its smaller size, which is most likely due to the angle of the solar panels causing more sunlight to be reflected back to Earth.

The satellites are both very faint, appearing at about magnitude 18-19 - that's nearly a million  times fainter than the faintest stars visible to the naked eye!  To observe them, relatively large telescopes are needed, and even with a 0.4m diameter telescope such as Peter's requires an exposure over 20 minutes long.  Herschel is currently towards the constellation of Perseus, as seen from Earth, and is slowly moving into Taurus.  It is moving at around 1 arcminute (1/60th of a degree) per hour.