Emily Lakdawalla • Aug 20, 2008
Things that probably won't ever be called planets, but maybe they should
EDIT: I have now posted a single-image version of the scale asteroids and comets montage for your downloading pleasure. --ESL
The longer I listened to the "great planet debate" last week, the more strongly I felt that if it were up to me, I would define "planet" to mean "everything in the universe that's smaller than a star." The fact of the matter is, every time I speak about any object visited by a spacecraft, I make frequent slips of the tongue. I call Titan a planet. I call Enceladus a planet. I've even called Tempel 1 a planet. They're all wanderers, all places to visit. Subdivide it however you like -- it makes sense to speak of giant (or Jovian) planets, ice giant planets, major planets, terrestrial planets, minor planets, dwarf planets, binary planets, whatever. But just look around at the people who call themselves "planetary scientists" and see what they study. It's all of the above.
In that spirit, I went around the Internet to locate all of the really tiny Sun-orbiting worlds that have been visited by spacecraft, the ones too lumpy and common to fit into nearly anybody's definition of "planet." Yet, by virtue of having been visited by a spacecraft, they have been elevated to special status. Here's the gallery. Many thanks to Ted Stryk for rummaging through his archives and sending me his versions of many of these images.
The first of these minor planets to have been visited by spacecraft was, fittingly, the first comet that was recognized to be a periodic one: Halley's comet. A flotilla of spacecraft was launched to visit Halley when it last visited the inner solar system in 1986, including ICE (USA, NASA), Vega 1 and 2 (Soviet Academy of Sciences, USSR), Sakigake and Suisei (ISAS, Japan), and Giotto (ESA). Of these, Vega 1, Vega 2, and Giotto returned photos of Halley's nucleus, finding it to be good-sized for a comet at 16 by 8 kilometers across, but incredibly dark and also extremely low-density (if you're curious, the density was 0.1 grams per cubic centimeter). Here's a photo from Vega 2:
One thing I always wonder when I look at such photos is: how big are they? In particular, how do their sizes relate to each other? Here you go, all of the minor planets pictured above at the same resolution, 200 meters per pixel. Itokawa really is there, it's just a nearly invisible 2-by-1-pixel speck.
Asteroids above and left;
58 x 23 km
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