Asteroid of the half-month: 243 Ida

Galileo looks bad today. Its camera (SSI- Solid State Imager) was one of the first digital (CCD) probe imagers. At 800×800 pixels (0.64 megapixels), and 8 bits, the builtin camera in your laptop or in your pocket embarrasses it. The SSI optics were the Voyager spares, themselves based on Mariner 10’s glass. Galileo’s infrared instrument was IRAS-era. To make matters worse, the jammed radio dish and limited tape recorder (remember those?) meant even 0.64 megapixel images came in very slowly. Yet, Galileo made important discoveries at 951 Gaspra and 243 Ida- that’s first mover advantage for you.

Ida, a ‘fast rotator,’ did a full pirouette before Galileo; the entire surface, except for some deep shadows, was seen down to 25 meters resolution in places. The stuck antenna meant that ‘jailbars’ (occasional lines of the full frame) were sent down first, to prioritize later downloads. Those jailbars already revealed a larger Ida than ground telescopes indicated; a very jagged shape; and the first asteroid satellite- 243 Ida I Dactyl. If a moon was found without even really trying, then asteroid moons, one would figure, should be common.

The albedo was darker, the mass and density lower, but the classification stayed the same. Ground observers had called Ida an S-type asteroid; a link to the ordinary chondrite meteorites was then announced. This was no stretch- S is a broad asteroid complex, not a declaration of a single type. It’s the most visible single complex in the sky. Meanwhile the ordinary chondrites are around 80% of all meteorites. Connecting the S asteroids to OC meteorites is hardly bold. Yet, LL chondrites were soon hypothesized too. Twenty years later, P. Heck would postulate that a recent, nearby collision of a chondrite explains the prevalence of such meteorites, not (necessarily) the S asteroids. And we’ll get to the Qs later.

Ida is saturated with craters, indicating an old surface. Yet, one crater, Azzurra, appears younger and bluer, along with its surrounding ejecta. It had been assumed asteroid surfaces age with exposure; Ida was visible evidence of reddening (‘optical maturation,’ or ‘space weathering’). In hindsight, it appears iron-rich grains turn to nanophase iron when exposed; other minerals turn to micro-glass beads. The Hayabusa samples of Itokawa already show this, even on the dust grains that mission returned. This space weathering confuses us on the ground, especially since asteroids with varying compositions will weather differently. Freshening events, such as large impacts or shaking (resettling, tidal force, or major outgassing) can turn over the top surface.

Speaking of major events, there’s the Dactyl question. How does an asteroid (or anything, for that matter) get a moon?

-Capture of a small, passing body

-Large impact throws up material

-A large, passing body pulls off material, which accretes

Planetary scientists have pretty much abandoned the first- there’s no plausible means for an asteroid to capture something, without one or both being destroyed. The second doesn’t make a lot of sense, either: a small body has a steep gravity well.  Little material would fall in the middle, neither thrown clear of the Ida gravity sphere, nor falling back down to the surface. (Not all scientists agree, though- see Davis, Granahan, or Giblin.) This also makes #3 unlikely; with neither Mars nor Jupiter nearby, other asteroids likely don’t have the reach to pull off material, but not pull the body apart completely.

A good number of solar system scientists favor yet another possibility: Ida and Dactyl formed together. Ida is a member of an asteroid family, the Koronis family. Even though “The Asteroids” collectively, did not form from an exploded planet between Mars and Jupiter, there are clumps. These Main Belt clumps indicate many families of asteroids came from many parents shattering. Ida, in particular, is a member of the Koronis asteroid family. The Koronis family is relatively tight, increasing our confidence that it’s actually a family, and indicating that the parent flew apart not that long ago- maybe a billion years prior.

Ida’s lineage would tell us more about more asteroids. If Azzurra is young for Ida, and Ida (in the Koronis family) is young for the S asteroids, then we now have three data points on space weathering with increasing age. Further, the Koronis family contains the Karin cluster or subfamily- a smaller, even tighter asteroid group. Presumably this makes it even younger, at only a few million years- younger by a factor of 100 or 200. If all these links hold up, plus OC meteorites, that’s a timeline of ~4.5 billion years, to today, in four or five neatly spaced data points.

We are particularly interested in the Q asteroids. Q-type resembles S-type, and the Karin subfamily. Imagine S asteroids are related to Karins, via Azzurra Crater on Ida. Then we know the composition of the Q asteroids: they’re just S asteroids that have been ‘freshened’ from S-type spectra. Or they’re too young to have space weathered at all. Pretty impressive for 0.6 megapixels, eh?

Of course, this depends on asteroid family assignments, and then we want to firmly connect OC compositions to Ida (or is it LL meteorites?). No further missions are planned to Ida, Koronis, or Karin, etc. As families, they’re all drifting silently through the middle and outer Main Belt. Except for pure chance, few missions will target the less accessible middle/outer Belt specifically.

Who knows. The coming age of large telescopes (Giant Magellan Telescope, Thirty Meter Telescope, and ELT, plus JWST in space) may find some key evidence I can’t even imagine right now. Just look at what Mariner 10 lenses with a 0.64 megapixel sensor did.