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Emily LakdawallaAugust 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:

The nucleus of Halley's comet

Russian Academy of Sciences / Ted Stryk

The nucleus of Halley's comet
The Vega 2 spacecraft returned many images of the nucleus of Halley's comet on March 9, 1986. The nucleus is an irregular object about 16 x 8 x 8 kilometers and very dark, with an albedo of about 0.03. The next time Halley will approach the inner solar system is in 2062.
After Halley, the next tiny object we visited came in 1991, when Galileo flew past Gaspra. The Galileo flybys of Gaspra, and later, in 1993, Ida, were wonderful, and not just because Gaspra was the first spacecraft encounter with an asteroid. They also allowed Galileo to take full advantage of its capabilities. Galileo's high-gain antenna never deployed. But for the Gaspra encounter they were able to return data at a high rate anyway, because after Gaspra Galileo returned to Earth for another gravity assist, in November of 1992. They waited until then to return all the data.
Asteroid 951 Gaspra

NASA / JPL / Ted Stryk

Asteroid 951 Gaspra
Galileo flew by Gaspra on October 29, 1991. Gaspra is an S-type asteroid about 19 x 12 x 11 kilometers in size. Gaspra is reminiscent of Mars' moon Phobos, having many craters and occasional grooves.
Next, Galileo flew by Ida. During this flyby, Galileo serendipitously discovered Ida to have a tiny little companion satellite, later named Dactyl.
Ida and Dactyl in enhanced color


Ida and Dactyl in enhanced color
Galileo captured this image of asteroid Ida and its moon Dactyl about 14 minutes before its closest approach on August 28, 1993. The range from the spacecraft was about 10,500 kilometers (6,500 miles). This was from the imaging sequence that produced the discovery of Dactyl. The color is based on violet and infrared filters; it represents actual variations on the surface of Ida, but has little relationship to the colors that human eyes would see. Ida is an S-type asteroid, about 58 by 23 kilometers in size.
Here's a zoom on little Dactyl.

NASA / JPL / Ted Stryk

Also known as (243) Ida I, Dactyl is a satellite of the asteroid Ida. Dactyl was discovered by the Galileo spacecraft in images taken as it sped past Ida in an encounter on August 28, 1993. It is about 1.6 by 1.2 kilometers in size, orbiting Ida at a distance of 90 kilometers.
Next was NEAR. On its way to its destination of 433 Eros, it flew past asteroid 253 Mathilde:
Asteroid Mathilde

NASA / JPL / Ted Stryk

Asteroid Mathilde
Asteroid 253 Mathilde was visited by the NEAR spacecraft on June 27, 1997, on its way to Eros. The encounter was the first fast flyby of an asteroid; for the images to be captured, the spacecraft's targeting had to be updated less than 12 hours before the encounter, based upon the orbital information provided by pre-encounter images. Mathilde is a C-type main-belt asteroid, 59 x 47 kilometers in size.
After Mathilde, NEAR became the first spacecraft to enter orbit around an asteroid on February 14, 2000; a year later, it became the first to land on an asteroid, and functioned there for two weeks before being shut down. It visited 433 Eros, which is not huge, but it's one of the biggest near-Earth asteroids.
Eros rotation animation


Eros rotation animation
One full rotation of the asteroid Eros as seen by the NEAR spacecraft on February 16, 2000, two days after its arrival. Eros is an S-type asteroid, about 33 by 13 kilometers in size.Source
Next up is Deep Space 1, which flew by asteroid 9969 Braille (named in a Planetary-Society-run contest) on July 29, 1999. For an encore, they picked up a comet, 19P/Borrelly, on September 22, 2001, performing the highest-quality imaging yet of a comet nucleus.
Asteroid Braille

NASA / JPL / Ted Stryk

Asteroid Braille
Deep Space 1 flew by asteroid 9969 Braille (formerly known as 1992 KD) on July 28, 1999, at an altitude of only 26 kilometers. However, the only images of Braille were captured from much farther away, about 14,000 kilometers. Braille proved to be an irregularly shaped body about 2.2 kilometers long by 0.6 kilometers across. It is probably a Q-type asteroid, dominated by olivine and pyroxene. It is also unusually bright for an asteroid, with an albedo of about 0.34.
Comet 19P/Borrelly, target of Deep Space 1

NASA / JPL / Ted Stryk

Comet 19P/Borrelly, target of Deep Space 1
Deep Space 1 flew by comet 19P/Borrelly on September 22, 2001. It flew within 2,171 km of the nucleus at 22:29:33 UT. The nucleus is about 4 by 8 kilometers in size and incredibly dark, with an albedo varying from 0.01 to 0.03.
The next thing to do was to try to bring something back from one of these objects. That's what Stardust set out to do, flying through the coma of comet Wild 2 to collect samples. The flyby happened on January 2, 2004. On the way, though, it picked up an opportunistic encounter with an asteroid named Annefrank.
Asteroid Annefrank

NASA / JPL / Ted Stryk

Asteroid Annefrank
The Stardust spacecraft flew within 3,300 kilometers of asteroid Annefrank on November 2, 2002. Prior to the encounter, little was known about the asteroid except how much light it reflected. It turned out that the brightness of Annefrank had been interpreted incorrectly, leading mission planners to expect a smaller and brighter asteroid. Its size is 6.6 by 5.0 by 3.4 kilometers.
Animation of Stardust's flyby of Wild 2

NASA / JPL / Emily Lakdawalla

Animation of Stardust's flyby of Wild 2
This animation is composed of 24 frames captured by Stardust as it flew by Wild 2 on January 2, 2004.
On July 4, 2005, Deep Impact took a more direct approach to studying the makeup of a comet, smashing an autonomous spacecraft into the nucleus of Tempel 1 and flying through the debris cloud. This image was taken on the way in.
Mosaic of Tempel 1


Mosaic of Tempel 1
This view of the nucleus of comet Tempel 1 is composed of many frames captured from different ranges by Deep Impact as it approached for its July 4, 2005 encounter. Images of the whole comet nucleus were taken from a greater distance and so are blurrier; highest resolution images were captured of the bottom half of the nucleus.
Finally, we have, I think, my favorite minor planet of the bunch. This is Itokawa, the second asteroid to be orbited by a spacecraft, and the second on which a spacecraft has landed, but the first from which we are attempting to return a sample, and by far the smallest body ever visited by a spacecraft. It's so very weird and different -- looking at pictures of Itokawa tells me that there are untold wonders remaining to be discovered among the very smallest denizens of our solar system. Itokawa was studied by Hayabusa in November 2005.
Itokawa rotates under Hyabusa

ISAS / JAXA / Øyvind Guldbrandsen

Itokawa rotates under Hyabusa
This animation consists of 57 separate images captured by the Hayabusa spacecraft as the tiny asteroid Itokawa (535 by 294 by 209 meters in size) rotated underneath it. The images are actually from three separate rotations; they were sorted and lined up into this animation by Øyvind Guldbrandsen of the Norwegian Astronautical Society. Click here for a full-resolution movie containing 169 frames (AVI format, 3.8 MB)
The next tiny place to look forward to is Steins, which will be encountered by Rosetta very soon. There's a page on this website on the entire history and near future of asteroid and comet exploration, if you want to read more about these missions.

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.

Mathilde at a scale of 200 meters per pixel

Eros at a scale of 200 meters per pixel
33 x 13 km

Annefrank at a scale of 200 meters per pixel

6.6 x 5.0 x 3.4 km

Gaspra at a scale of 200 meters per pixel

19 x 12 x 11 km

Dactyl at a scale of 200 meters per pixel

1.6 x 1.2 km

Itokawa at a scale of 200 meters per pixel

0.5 x 0.3 x 0.2 km

Braille at a scale of 200 meters per pixel

2.2 x 0.6 km

Asteroids above and left;
comet nuclei below

Halley at a scale of 200 meters per pixel
16 x 8 x 8 km

Tempel 1 at a scale of 200 meters per pixel

Tempel 1
7.6 x 4.9 km

Wild 2 at a scale of 200 meters per pixel

Wild 2
5.5 x 4.0 x 3.3 km

Borrelly at a scale of 200 meters per pixel

8 x 4 km

59 x 47 km

Ida at a scale of 200 meters per pixel
58 x 23 km

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
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