Asteroid of the Half-Month: 243 Ida I Dactyl

You didn’t think the first asteroid moon we’ve found would get off that easily, did you?

Asteroid satellites had long been suspected before Ida/Dactyl. Scientists had rejected “the exploded planet” origin for all asteroids, but later found “many, exploded parents” for many of the asteroids. Meteorites are pretty much assumed to be asteroid fragments; small asteroids appear in many ways to be fragments of bigger asteroids. Comets were seen to break apart sometimes. Double craters and multi-crater chains have been found on the Earth, Moon, etc. with the same age indicators, suggesting paired impactors, or swarms. A reasonable person would expect some fragments would stay around some parent bodies.

The Galileo mission then spotted Dactyl (formally, 243 Ida I Dactyl), around the second asteroid ever visited by a mission. (All this, despite not being originally designed to observe asteroids.) A moon would tell us Ida’s mass, by showing the pull of gravity between the two. More generally, craters and fragments should be an indication of the impact history and overall dynamics of that region of the solar system. But, like Lutetia, this body offers some answers and begs more questions. Some of this was just blind, dumb luck, not (just) the technology and skill of the Galileo spacecraft and operations team.

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Moonies II: That Edge of The Well

I described how costly it is to tangle with deep gravity wells. If it’s good to stay out of them, what constitutes ‘out’ and ‘in’? A surprising path forward, it so happens… or inward.

Picture a contour map of gravity. Like hilly terrain, one can climb ‘up’ the map contours with effort, or fall ‘downhill.’ Such a map would show Earth as a deep gravity well, and the Moon as a lesser mass, with lesser well depth. Hence the metaphor of falling in. However, note two points. Between bodies is a saddle area with a gravitational quirk- “Lagrange point 1,” or L1. An object there would orbit Earth at the same speed as the Moon. To us it would appear to hang there, stuck to the Moon. Less obvious is Lagrange point 2, behind the Moon. An object at L2 would seem to us to hide there as the Moon orbits. We can repeat this map with the Sun-Earth system, or any other two massive bodies.

Going to the Moon’s L1 point is easier than the Moon itself- after all, L1 is closer to us. But in 1972, physicist R. Farquhar1 showed that going to L2 is easier than the Moon. Basically, aiming slightly past L1 causes one to fall towards the Moon, overshoot, and reach L2. The aiming and extra fuel (measured in acceleration, or “delta V”) is less than that needed to slow down and land on the surface- a surprisingly high need. This creates a surprising but natural effect: L2 is the gateway to the greater Solar System. Not only should we skip the Moon in metaphorical terms. We should literally skip off the Lunar gravity well, and go past, because it’s cheaper. And cheaper than that? Bodies coming to us, through the gateway.

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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, and 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.

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Asteroid of the Half-Month: 21 Lutetia

Hooo boy… Lutetia’s a weird one. The body, like Uranus, is “tilted.” Either pole alternately faces the Sun for half its year. This isn’t too weird for a lander or settlement; if you’re near Lutetia’s equator, you would have solar power for the whole year by dialing solar panels into ‘summer’ and ‘winter’ angles. In spring and autumn, panels would be in between, and seeing day/night cycling every eight hours. If you’re on a peak or ridge, night shadowing would be even less.

It gets weirder- far weirder. When the asteroid classifications were first formalized in the early ’80s, Lutetia was put in the M-class based on its spectrum (color signature) and albedo (reflectivity or ‘shininess’). This is an oddball category, with only two other members at the time. It was suspected M asteroids were metallic, though an optical telescope alone won’t conclusively establish that. Nowadays ‘M’ is not officially short for metallic, and even at the time Lutetia was suspected of being borderline M- or C-class. The two classes have similar spectra, but M is brighter, C is dark.

Planetary radar was also getting big at the time. Some targets returned echoes that strongly indicate metal, but Lutetia gave no such ping. Passing asteroids did show a significant disturbance, indicating that Lutetia was a quite heavy object. Again, suspected but not conclusive. The radar return did indicate a surface layer of regolith (rock particles or ‘soil’), not bare slab. And there we sat for over twenty years, until the Rosetta comet mission passed Lutetia on the way to its primary target, Comet 67P/Churyumov-Gerasimenko.

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Well That’s The End of That (Moonies I)

It’s July. Time to sit in the shade, and be economically productive more than calorie destructive. It’s also the month where North Americans contemplate “a great nation,” whichever of those you sit in. Let’s contemplate the Augustine Report, or formally, Seeking a Human Spaceflight Program Worthy of a Great Nation. One of the Augustine Report’s takeaway points? Stay out of gravity wells!

Take the example of early lunar probes. The US first sent the Ranger program, to gather mapping imagery, then the Surveyor program, to perform geologic studies. Ranger probes took television stills on the way to their (fatal) impact, while the Surveyors made soft landings, survived, and returned data.

The last, successful Ranger probes weighed 804-809 pounds (~366 kg) at Earth departure. Some of that (several kg) was trim propellant, for a midcourse correction. With no trim burn on the outbound leg, the odds of hitting the Moon- based on launch vehicle accuracy alone- were low. Surveyor 1 and 2, designed just a year later, were 596 lb (271 kg) in landed configuration, but 1970-1980 lb (~895 kg) in flight- a difference of 1374 lb (~620 kg), almost all propellants. That’s right, Surveyor wasn’t just more propellant than probe- it was over twice more!

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Search Status July 2017

Hope you had a happy Asteroid Day. The tally so far (Jun 29):

-733268 total asteroids, 3986 comets
-16370 of those Near-Earth Objects (NEOs), including 106 comets
-1816 of those Potentially Hazardous Asteroids (PHAs)

…and counting. The discovery rate is a few bodies per day, and increasing. This means the totals will climb much higher before eventually leveling off.

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Asteroid of the Half-Month: 4 Vesta

We’ll start with an easy one: Vesta, the second biggest asteroid in the Main Belt (after 1 Ceres).

Vesta is big enough that it produces other asteroids; impactors do not destroy it, but in some cases spall off debris. These are the Vestoid family asteroids, and the HED meteorites (for Howardite-Eucrite-Diogenite). Between HED samples recovered on Earth, and the Dawn spacecraft that orbited Vesta and Ceres, astronomer A. Rivkin states that the body is as well-characterized as our Moon and Mars. And what do we now know?

Vesta is so big it was once molten rock- not too surprising. The magma separated, like Earth did, into core, mantle, and crust- “differentiation.”
Vesta took two massive impacts, spraying forth the Vestoids. The Main Belt was a violent place, and that wasn’t 4.5 billion years ago, but “only” one billion or so.
Yes, the HEDs are chips off the old space spud.
Vesta was damp; one intriguing observation is that not only is there residual moisture today, but possibly “mudslides.”
All in all, the body is a “planetesimal”- the embryos, very common in the early Solar System, that came at each other until only planets stood; everything smaller got railroaded. Vesta’s composition is surprisingly like Earth, Mars, the Moon, etc. which is part of the attraction.

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