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SYFY WIRE Bad Astronomy

Santa Clausteroid

By Phil Plait
WISE image of an asteroid

Itâs Christmas morning, so by now youâve probably heard someone in your family telling you that the yule log, Christmas tree, festive lights, and half a dozen other holiday traditions arenât actually Christian in origin but were absorbed by the religion from a bunch of different cultures.

Thatâs fine; most traditions we celebrate today have evolved one way or another over the centuries. But I learned a new one this week that really surprised me. Itâs about Santa, and itâs amazing: Santa is not a jolly old elf with a white beard and red suit. Heâs actually a 35-kilometer wide asteroid orbiting the Sun between Mars and Jupiter!

And I have proof:

Thatâs an image from NASAâs WISE spacecraft, which observes in the infrared. You can see stars and galaxies as blue (actually, light at about 3 microns, roughly five times the wavelength the human eye can see), and on the left a series of red dots. Thatâs Santa! Actually, 1288 Santa (1933 QM), the asteroidâs official name. The picture was posted yesterday by my friend Amy Mainzer, who is the principal investigator of NEOWISEâthatâs the name of the mission now that itâs been rededicated to hunt for near-Earth objects.

Santa is no threat to us, though; it orbits the Sun at a distance of about 435 million kilometers (270 million miles), far, far from Earth. The picture is actually a composite of several exposures, and Santa moves a bit between each as it orbits the Sun, which is why it looks like a series of dots. But why does it look red?

Actually, in visible light Santa is darker than a lump of coal! Many asteroids only reflect about 3-5 percent of the sunlight that hits them, about the same amount as coal. But that means it absorbs a lot of sunlight, warming itâif a temperature of about -90 C (-130 F), way colder than even the North Pole, can be considered âwarmââand at that temperature it glows in the infrared, at a wavelength of about 20 microns where WISE can see it. Traditionally, that color is designated red in WISE pictures, which is why Santa looks the way it does in the image.

Unless, of course, thatâs actually a red suit. Or, as Amy noted to me, itâs Rudolphâs nose!

Either way, and whatever your beliefs or lack thereof, I hope you have a good holiday. May you have clear skies and boundless wonder for the Universe.

Tip oâ the white puffball-tipped red tuque to Amy, who spent time on the day before Christmas chatting with me about this. Oh, and note: She found a total of 11 asteroids in the picture. One more and I couldâve titled this post âThe Twelve Asteroids of Christmasâ. But honestly, I hoped sheâd only find two more besides Santa. Then it wouldâve been âThree WISE Asteroidsâ.

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