In a little over two weeks, just after Midsummer, the launch window for PoGOLite will open. In my last post, I talked about the conclusion of PoGOLite tests at Linköping airport. Since then, PoGOLite has been moved up to the Esrange Space Centre thereby marking the start of the launch campaign. Esrange is located some 40 km east of Kiruna and provides unique opportunities to launch large helium-filled balloons into the stratosphere. We are hoping to make a circumpolar navigation of the North Pole, overflying Norway, Iceland, Greenland, Canada and Russia before landing back in Sweden some 20 days later. Such a long flight will allow repeated observations of our primary science targets, the Crab and Cygnus X-1 and plenty of time to study backgrounds – which is essential for a counting rate anisotropy measurement such as polarisation. Obtaining permission to fly over Russia is a complicated business and we are still on tenterhooks waiting for the green light. If this is not forthcoming, we’ll have to cut the balloon down over Western Canada and strike out into the wilderness to recover the payload and, in particular, the valuable cargo of hard disks.
There is a lot of activity at Esrange right now with several large balloons being prepared for launch. First up are three NASA-sponsored payloads. The first two, AESOP and LEE, have flown many times before in their quest to monitor the effect of solar modulation on low energy cosmic rays. The relatively high latitude of Esrange means that cosmic rays are not screened by the Earth’s magnetic field. LEE (Low Energy Electrons), has already been launched and landed in Western Canada after 5 days. AESOP (Anti-Electron Sub Orbital Payload) will be launched as soon as weather conditions allow. The final payload in the queue is called Highwind and uses a Fabry-Perot interferometer to probe the upper atmosphere. Interestingly, LEE and AESOP have connections to my ‘other’ experiment PAMELA. You may recall that the low energy part of the well-known PAMELA positron fraction did not agree with the majority of other published balloon-borne data since these measurements were taken during a different solar configuration (polarity and activity). Measurements from AESOP collected around the time PAMELA was launched agreed with the PAMELA observations, however.
PoGOLite rounds off the summer launch programme for large balloons, with the late date chosen to maximise the angle between the Sun and the Crab. So, how large is large? Well, the balloon which will lift PoGOLite to an altitude of 40 km has a volume of a little over 1 million cubic metres – about twice the volume of the Globen Arena. Such a large balloon is needed since PoGOLite is, in reality, not so light and weighs in at about 2 metric Tonnes.
An exciting few weeks lie ahead at Esrange. On-ground tests of the polarimeter continue, with a polarised radioactive source replacing photons from the Crab. The attitude control system which keeps the polarimeter aligned to targets of interest is also being put through its paces. As PoGOLite drifts Westward, Esrange will eventually drop below the horizon and the only way to contact the balloon is through Iridium satellite links. This requires that the payload can operate autonomously, rather like a satellite. Not always so easy to test realistically on the ground!
For more details about PoGOLite, you may wish to take a look at a paper which was presented a couple of weeks ago at the 20th ESA Symposium on European Rocket & Balloon Programmes (arXiv:1106.1322).