Asteroid of the Half-Month: 253 Mathilde

253 Mathilde is seriously battered. Not as in breadcrumbs and oil vats, but that’s not completely off, either.

And finally- we have an up-close visit, by a recognizably-modern spacecraft. The Galileo mission had a digital camera, but of a late ’70s vintage. NEAR Shoemaker (Near-Earth Asteroid Rendezvous, later honoring scientist Eugene Shoemaker) launched in 1996, and after a fast development program. Its camera was 12-bit (not 8-bit) plus antiblooming features. This came in handy at Mathilde, known to be a dark object. NEAR Shoemaker also had an X-band transmitter; the Doppler shift of its signal told us of Mathilde’s gravitational pull, and therefore its mass. Galileo had such a radio too, but… the antenna stuck. Galileo only used a backup, S-band transmitter, not as useful. Still, Galileo got first-mover advantage: seeing a new type of body, up close, for the first time, upended many of our assumptions even via obsolete instruments. NEAR Shoemaker’s only first-mover benefit was the first C-type asteroid. (Galileo flew by two S-asteroids.) Even then, the real flood of data only came at NEAR Shoemaker’s primary target, 433 Eros. NEAR’s trajectory only aligned with Mathilde in the Main Belt, where the Sun appears farther and dimmer. With less light on its solar arrays, mission managers operated just the camera, not the other instruments.

As this was the wild west of small-body space flight, even just a camera reaped a bonanza. We have carbonaceous chondrite meteorites, thought to be Mathilde-like, and full of organic chemicals. NEAR Shoemaker passed within 1212 km (757 mi) of Mathilde on June 27, 1997, approaching from the night side and crossing into day. NEAR covered 60% of Mathilde’s surface, with 160 meter/pixel resolution at peak (the target was 500m/pixel). Only 60% was covered, because by chance Mathilde was a slow rotator. A day on Mathilde is about 417 Earth hours, and in the 25.2 minute encounter period the night side never came out of shadow.

Mathilde turned out to be slightly smaller and brighter, at least for a C-type. Its albedo (reflectivity) was found to be .047 instead of an assumed .035, and its diameter roughly 53 km instead of 60 km. This is darker than coal, darker than Mars’ moons Phobos and Deimos, and darker than carbonaceous chondrite meteorites (widely thought to come from C-type asteroids) but not much off other chondrite asteroids (C-type and most in the S-type).

The big surprise at Mathilde was big impacts. Not only is the top crater about equal to the radius of the asteroid, but… there are three more such impacts.

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Asteroid of the Half-Month: 15 Eunomia

We’ve seen 4 Vesta to be a differentiated body (core-mantle-crust). We’ve seen 16 Psyche to be a fragment, post-differentiation (likely metal core, with minimal rock layers left). In hand, we have HED meteorites as Vesta samples, and iron meteorites as Psyche-like samples (possibly actual samples of Psyche). But these aren’t the only differentiations seen in the small bodies. 15 Eunomia looks like a piece of differentiated body, going from deep mantle on one side, up to crust material on the other. If so, Eunomia may be our first “tree rings” asteroid.

In the course of astronomy, we refine our understanding of any one target. Solar System points of light are tracked (orbit determination) and measured in brightness (photometry), then sorted via their reflectance properties (colorimetry at first, then spectrometry). One pixel turns into a Solar System object; with enough orbit and color data, a target is sorted by our small-body types. Each type seems to be some variation of minerals. Some minerals, with prominent spectral features, can be found via good spectroscopes.

Sorting Eunomia, a rare thing happened. As nonround bodies spin, brightness changes- first broadside, then, end-on. Broadside reflects more. But if we break down overall light into its colors, each color may cycle differently. The sides must be different colors. As Eunomia spins, good enough spectroscopes see two minerals cycling. One side is metal-rich olivine, one, pyroxene. Meanwhile, Eunomia’s region has many asteroids in similar orbits- the Eunomia family, likely fragments of a parent body. Eunomia, at 200-300 km, is the largest fragment by far. Since olivine is mantle rock, and pyroxene is found in basalt (a lava, flowing onto surfaces), that says the parent body was even bigger, molten, and grew a core, crust, etc. Eunomia is then a “layer cake” of differentiation, and thus planet formation.

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Comet of the Quarter: C/1996 B2 Hyakutake

You figured this was an asteroid site? Comets aren’t pure ice (turns out they can’t be) and the line between asteroids and comets turns out to be arbitrary and temporary. Besides, we study both, for the same reason. They are the seeds of our existence, not the scraps.

Still, comet studies lag far behind the asteroids; most comet orbits are energetically costly. Flybys aside, missions to comets need extreme trajectories; even a flyby is generally fast, brief, with less data collected. This is all assuming a good comet target. The “fresh” ones (fewer passes by the Sun) reach Earth space from obscurity in the distant Solar System, on fast trajectories. Generally, too fast to orchestrate a probe mission. Comet studies had generally relied on ground telescopes, and more recently ones in Earth orbit, not probes. (The few outliers are less-fresh bodies, in closer, less dim orbits.)

Two ground telescopes belonged to Yuji Hyakutake. His giant, 150mm-aperture Fujinon binoculars are modern versions of battleship binoculars. These spot enemy ships, then the splashes of one’s own shells, for calling out gunnery corrections. They also excel at areal sky coverage, such as comet hunting. Yuji, seeing C/1965 S1 Ikeya-Seki as a teen, sought to earn a comet of his own, even moving from central Japan to Kagoshima for better skies. In fact, he had found C/1995 Y1 Hyakutake, when a re-scan barely two months later (Jan 30) found C/1996 B2 Hyakutake. Those binoculars were clearly faster than probe operators.

Even large telescope operators would need to be fast. Once C/1996 B2’s orbit was well-determined, it was clear it would pass very near Earth. The comet would come within 0.1 AU (9.5 million miles, or 15 million km), and in just two months. Large, sophisticated telescopes are (over!)booked months in advance, and this target appeared ripe. No comet had come closer since 1983, and none particularly active (emitting lots of gas and dust) since Comet West in 1976. (Halley is an older, less-active body, and in 1986 did not pass Earth closely.) Sky & Telescope would go on to call Hyakutake ‘the finest comet in 20 years’.

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Asteroid of the Half-Month: 16 Psyche

16 Psyche is so metal… and it’s still a hydrated asteroid, valuable on multiple levels.

A young planet differentiates when metals drop out of magma, becoming a core. Psyche, at ~186 km across (116 mi), looks like the core of a “planet” that no longer exists. The young Solar System had many planetesimals. Most were destroyed in impacts, hurled into the Sun, or hurled out altogether. A few winners, though, grew from multiple planetesimals to be today’s planets. The best Psyche hypothesis is that a planetesimal, big enough to get a metal core, was small enough to lose in a collision. Yet the small, strong core survived; its rock layers got lost to the ages. If so, it’s our best way to study planet cores- better even than Earth’s own.

We had some prior grasp of planetary metal (small metal asteroids aside). Iron meteors fall to Earth. They’re mostly iron, at least 5% nickel, then iron-soluble elements like cobalt, pulled in by iron. We also see the signs of the elements in the Sun; they leave a spectral imprint in sunlight. Iron meteorites have element abundances like solar abundances, after you shed gases and rocks, as planet formation and differentiation does. The iron had to be in a core, as crystals take time to grow from liquid. More cooling time, larger crystals. Iron alloy crystals are inches, which would take a million years. Cores stay hot for a million years, boulders don’t.

And yet, a possibility exists that Psyche was never a core, but formed other ways. Solar system simulations yield a Psyche in a small percentage of runs. Was our system just lucky, or did something else go on? Is Psyche a coincidental ball of other iron meteoroids? Not the parent body of such meteorites, but really their monster child? NASA wants to find out, like others studying star-system formation… and others studying water resources.

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Asteroid of the Half-Month: 10 Hygiea

Every Mars mission is actually an asteroid mission- and I don’t mean Phobos and Deimos as quasi-asteroids (though we’ll get to that eventually).

Gravity’s a funny thing, in slapstick, the cosmos, and our spreadsheets. It permeates space, even around objects, and in places we can’t see. Reading gravity fields can be a sixth sense for those who choose to become literate in it. Miners read higher gravity as one sign of ore deposits; lower gravity is one sign of oil. As we kept learning about our Solar System, the more astute among us learned to use gravity to mine more knowledge; we’ll use that knowledge to literally mine the Solar System.

10 Hygiea was the tenth asteroid to have its orbit determined (back in 1849), hence 10 in its official designation. It’s the fourth largest one, however, since it’s dark, and in an outer range of the Asteroid Belt. Less light reached our telescopes, and six smaller asteroids got seen before Hygiea. But don’t think some sort of boondock dim bulb would be unknown, or unimportant. The more astute of us know how to study even a single pixel. Like Mars, we can use one pixel to read into that body, and other asteroids too. Far more than six other asteroids, and I don’t mean just the Hygiea family (numerous nearby asteroids, likely pieces from it). Being one point of light, in the outer Belt, actually helps in some ways.

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No, X Will Not Kill You: 3122 Florence

I’m getting really tired of these scarebait headlines:

Monster asteroid Florence to skim Earth

HUMANITY is set to experience an incredibly close encounter with a gigantic 4.8 kilometre-wide asteroid this Friday.
How many things are wrong with this lede alone? It’s not gigantic, it’s likely not even 4.8 km, and 18.3 lunar distances is not that close. Certainly not “incredibly close.” The only monster here may be set before a keyboard.

The “4.8 km” number was rounded up, of course. Estimates for the size of Florence are 2.7-3 miles, and the measurement error may make the asteroid turn out smaller as well as larger. But of course that’s not scary.

As of months ago, the miss distance was 18.3 lunar distances (LD). That’s 4,370,000 miles (6,990,000 km), or over eighteen times further than the Moon (hence those units). That’s far; much further than the Sun-Earth L1 and L2 points I described previously. It’s far enough that an object is outside Earth’s sphere of influence, or Hill sphere. Within the Hill sphere, Earth’s gravity beats the Sun’s gravity, and retains objects. So, what’s the Hill radius? Barely 5 LD. Gravity falls off with distance squared. At 18 LD, Earth’s tug on Florence would be an order of magnitude too low to hold it.

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Moonies IV: A JU Hope

Previously: Moonies I, II, and III

I’ll say it: Moon mining’s a joke. No one’s due diligence would back lunar material extraction, because what material?

The Moon doesn’t just lack metals compared to asteroids. Asteroids beat it in both metals and volatiles at the same time. By comparison, the Moon looks like reheated leftovers. The 2009 news of lunar ice (at its poles) doesn’t upset any carts because that water is mostly secondhand asteroid water. Asteroid water which is easier to extract and process too. And if, for certain reasons, you want depleted rocks, a suitable choice of depleted asteroids is also there for the taking. This is all independent of the transport issue, which I’ve described in the previous entries.

All planets are differentiated. When forming, they were molten, and things separated by weight. Dense materials sank to form a core, and light ones floated to form a crust; in between, a mantle. The core is basically iron, while crusts are, to the layman, sand. A better question is how much is plain sand (silica), versus exotic stuff like some of the world’s glamorous beaches. In the Moon’s case, all surviving origin scenarios had rock spraying off the Earth, and re-forming in orbit. The new body is big enough to retain the heat of its violent birth for a while, and differentiate again. That is, the lunar crust is double depleted of metals.

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