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30th of March 2020
ASKAP observations of planets
by Emil Lenc
The ASKAP Survey for Variables and Slow Transients (VAST) and the Rapid ASKAP Continuum Survey (RACS) are being used to search for radio sources that vary over time-scales of minutes, days, months or even years. These surveys take repeated short (12-15 minute) snapshots of the sky to see what changes. While searching for transients we ended up following in the paths of the ancients and re-discovered a couple of local favourites which are compiled in the image above. Planets, the name itself derived from an ancient Greek word meaning "wanderers", move across the sky and so can appear as transients. Mercury lives in the fast lane and would normally be blurred over the course of a 12 minute ASKAP observation but when tracked we can clearly see this tiny planet. Venus is a fast mover but also bright - in this instance we see the trail as it moves relative to the background sky in just 12 minutes. The moon, also a fast mover, is big, bright, and rather annoyingly masks out background sources. In contrast, the outer planets take a more leisurely approach to their wanderings and appear as transients over periods of months. When we look at Jupiter at radio wavelengths what we actually see are its radiation belts - these appear as source on either side of Jupiter. Saturn appears as a simple dot because unlike in optical radio doesn't see the ring. Neptune is very distant and quite weak and cannot be seen in a single 12-minute observation. However, VAST observed Neptune a total of 13 times, by accounting for movement of the planet and then stacking these together the final image is just sensitive enough to detect Neptune. Note that Mars and Uranus were only briefly observed by RACS/VAST and were too weak to be detected.

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