The planet Uranus was discovered in 1781 by Sir William Herschel, after whom the Herschel Space Observatory is named. It is the seventh planet from the Sun and orbits at a distance of nearly 3 billion km - around 20 times further than the Earth. At this huge distance the planet is extremely cold, below -200 Celsius, making it very bright in the sub-millimetre light measured by SPIRE.
Today, the Herschel Space Observatory is still making use of the Uranus, as the extensive study of our Sun’s seventh planet means that it is very well understood. Herschel’s SPIRE instrument regularly observes Uranus so that astronomers can calibrate other measurements against the well-known brightness of Uranus.
Professor Bruce Swinyard, from University College London, said “Uranus was one of the first objects we observed with SPIRE, being imaged shortly after the lid over the instruments was opened. One of the reasons Uranus is particularly useful is that its spectrum is very smooth and well understood at our wavelengths, making it an ideal standard to compare other measurements to.”
Uranus is far too small for the Herschel satellite to see as more than a very bright point of light. It is so bright, in fact, that it appears as a six-pointed star. This is because of the way the Herschel satellite is constructed, with three struts supporting the secondary mirror. The diffraction of light around these structs causes the spikes seen in the image.
In the background are dozens of faint fuzzy blobs, each one a distant galaxy. This is an excellent illustration of Herschel’s power, as the faint galaxies are typically ten thousand times fainter than the much closer planet Uranus. The jagged edges to the image show the edges of the stripes Herschel observes as it methodically scans across the sky.
French Herschel site
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