AotHM: 433 Eros Part 1

We’re here: 433 Eros, the first small body mankind had orbited. Before I go into the NEAR Shoemaker mission, though, let’s remember Eros was one of the best studied small bodies well before NEAR, Galileo, or even all the probes of the “Halley Armada.”

Eros was known almost a century before NEAR even launched. Main Belt asteroids had been found from 1801 to 1898, establishing a curious zone between Mars and Jupiter. All asteroids were being given female names. In 1873, the first Mars-crossing asteroid (132 Aethra) was found, then later 323 Brucia and 391 Ingeborg (hence, their rising numbering). But all these were still predominantly orbiting outside of Mars, and could be thought of as straying inside Mars’ orbit temporarily. In other words, still mostly Belt bodies, just a little weird sometimes.

The 1898 discovery of 433 Eros reordered, and arguably ordered, the Solar System. Eros not only crosses Mars but approaches Earth, and closely- a “Near-Earth Object.” At closest, it comes within 0.1505 astronomical units (or AU, the distance from the Sun to the Earth). At the far point of its orbit (aphelion), Eros doesn’t enter what most would call the inner edge of the Belt, either. Previously, only comets were thought to “pass” Earth via such sweeping orbits. Eros was neither comet, nor quiet resident of the Belt. Yet another blow to the dogma of celestis (the heavens) and mundi (earth) as separate “spheres.” This was dramatic enough that the discovery received the first male name.

Close passes meant that it was easily studied, and useful beyond itself. Perturbations of Eros, timed well enough, gave the mass of the Earth-Moon system. In addition, the size of the Solar System could be taken. Kepler’s 3rd law lets us find the sizes of different orbits, but only as ratios. We know Earth’s year, of course, and we can time the laps of other planets and bodies. But until we numerically measure one orbit (such as Earth’s, let alone another planet’s), we can’t know any other orbit, and thus the Solar System in general. Eros did that where Venus did not. Said Harvard astronomer S. Bailey: “Indeed, Eros, at the most favorable times, is perhaps as good an object as can be desired”

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The Coming Flood

This had been a major week, in a major period, of a new era. Nov. 13 specifically, the Zwicky Transient Facility had first light. It’s just one more “new telescope” in a series that will rip the doors off small-body science. The others will, by various approaches, deliver asteroid-comet data with Moore’s-Law-like gains.

There had been sky surveys before, of course. There had even been a Mt. Palomar wide field, digital-based telescope before. The Zwicky Transient Facility is a major upgrade, to what was already a significant resource, and will expand the search for faint objects. The only issue is that ZTF sees the same skies (Southwestern US) as many other search programs. A given body here may be cross-checked two or three times, versus newer, more “fertile” ground.

Meanwhile, progress continues on the LSST and GMT. The Large Synoptic Survey Telescope will, upon completion in ~2020, make the ZTF look like a spotting ‘scope. One of the issues: all that data from the LSST will demand bleeding-edge computers just to process it. The LSST will operate in the Chilean Andes, like the GMT (Giant Magellan Telescope). This means the LSST and ZTF fields of view will have only partial overlap on the sky, and not be competitors. The Giant Magellan Telescope on the other hand will not be a sky search. The GMT, under construction for 2020s operations, will have ultra-deep capability and will follow up on select, promising targets. So, too, will the James Webb Space Telescope when it launches in 2019.

This current “growth spurt” in sky coverage continues the efforts of Pan-STARRS in Hawaii, which in turn built upon the mid- to late-’90s boom in wide field telescope searches. It’s a bit wry to look back at astronomy in ~1979, and how they marveled at their ‘golden age’ of asteroid knowledge…

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Asteroid of the Half-Month: 951 Gaspra

I can’t get too far without Gaspra, the first asteroid ever seen via probe. October 29, 1991, the Galileo spacecraft flew within 1,600 km, on its way to primary target Jupiter. Prior to this, we had meteorites, and astronomical observations of “dots” in the sky, but nothing really closing the gap. Mars’ moons Phobos and Deimos were studied up close, but extrapolating from small moons might still be off. Mars could be a shield to them, a bully, a parent, or combinations or options unmeasured at the time. Conversely, relatively nearby but free bodies like Gaspra (slightly smaller than Deimos), then Ida (slightly larger than Phobos) might rule out (or in!) processes and events of the Mars environ.

Again, I must say expectations before 1991 were open. Other than being an S-type body (rocky to metallic spectral signs in our telescopes), it was unknown what form true low-gravity objects, in vacuum, would take. Is this asteroid bare rock, since little should stick? Solid, or a rubble pile? Would debris form moons, or rings? How about binary objects, with no obvious parent/satellite relationship? Might there be no parent at all- a swarm, like a shotgun blast of rock? The rock/metal question was still open, because metals show bland, featureless spectra to even our best telescope mirrors. Scientists were divided on S-asteroids as analogues to stony meteorites, or stony-iron ones (pallasites and mesosiderites).

1,600 km is not far for a probe, but not that close either. Ground telescopes were not urgently refining Gaspra’s position, and flyby targeting was not the harried Galileo staff’s highest priority. They would not make Jupiter if the craft suffered a major fault, including damage or destruction from rings or a swarm. First science result: Gaspra was a single rock, and speculation about asteroid moons would continue being speculative.

As Gaspra got close enough to be resolved (~250-300 pixels on target), the speculation could end. It was a tumbling stone, its blocky shape (like facets of a cut gemstone) indicating mechanical strength, not a loose pile. Yet, it clearly had regolith, an outer layer of rock particles, just as Earth’s Moon had.

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2017 U1: Starlike Stardust

It’s here: the first unambiguous interstellar interloper.

Last month, a “comet” was spotted by search telescopes. An orbit determination was reached, after the object had been tracked over a sufficient arc. The determination showed the “orbit” was no orbit at all: 2017 U1 was not bound to the Sun. It had merely passed through the inner Solar System by chance, and is now passing out again. In other words, the asteroid (there is no emission of gas and dust) is a chunk of someplace else in the galaxy.

If this seems fantastical to you, think harder. The detection of 2017 U1 came about because we are now looking with many, sizable telescopes. It should not be the last, nor was it the first. We had seen suspects before, and we have tiny samples in hand. Meteorites contain tiny grains of material unlike any from this star system. In between, the Stardust mission obtained interstellar grains for cross-comparison. While cruising to Comet 81P/Wild 2 to collect comet dust, the Stardust probe collected tiny grains from a suspected dust stream. This stream is not flowing out from the Sun, nor does it lead back to any planet, moon, etc. in this System. We can look even further- other star systems show disks and belts of material, analogous to the asteroid belt in our System.

In other words, we have a clear and very long chain of material examples: condensed matter in meteorites, within the Solar System, entering the System, and in other systems. We can now compare and draw conclusions about the chemistry and formation of star systems, in multiple areas of multiple, independent examples. The word “asteroid” means starlike. Originally, it meant that these small objects did not show disks, even in the largest telescopes. Now, it is literal. Asteroids (and the related comets) are chemical samples of the stars.

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Asteroid of the Half-Month: 22 Kalliope

If 21 Lutetia’s a weird one, 22 Kalliope is a sibling “stone” in the bunch.

Based on its light spectrum (i. e., color curve), Kalliope is firmly in the M-type asteroids. Its spectrum is mostly flat- just a touch reddened- like the C-types, yet with a high albedo (reflectivity). This compares well to metallic meteorites. By these reasons both Kalliope and 16 Psyche, plus one or two other bodies, got held up as the prototype M-asteroids (“M” implying metal, in some reports). Yet, multiple passes with planetary radar show that some M-asteroids have metallic signatures (radar-bright), but some don’t (appearing more stony than metallic).

We found ways to break the ambiguity… somewhat. Where 21 Lutetia got a 2010 spacecraft encounter (the Rosetta mission), Kalliope has an asteroid moon as a “probe.” In 2001, two independent teams, at different telescopes, reported a natural satellite. Temporarily dubbed S/2001 (22) I, it got named Linus (formally, 22 Kalliope I Linus) by co-discoverer J. L. Margot. This was the fourth asteroid satellite found- the Galileo craft saw Ida has Dactyl, as I described. 45 Eugenia has Petit-Prince, as found by ground telescopes, using adaptive optics to counter seeing (air twinkling). 2000 DP107 had one, found by planetary radar. With better instruments, asteroid moons have turned up everywhere, including a second, smaller moon of Eugenia.

Unfortunately, then fortunately, Linus raised more questions about Kalliope before it solved some. We can “weigh” space bodies by Kepler’s laws- things orbit faster around heavier parents. By clocking Linus’ laps around Kalliope, the mass of both is 6.3 x 1018 kg. Linus appears quite large for a moon, yet it’s still a tiny amount of the total mass. We then find density, dividing mass by volume. A volume estimate from the IRAS infrared satellite then puts density around 2.0, maybe 2.2 grams per cubic centimeter. Not only is this too low for metal (about 7 grams/cu. cm), it’s too low for stony asteroids (~3.5 g/cc) as measured from meteorites. Asteroids have spaces, just like beach sand isn’t perfectly packed. But no plausible void content puts metal at 2.2 g/cc. The answer was… better Linus laps.

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Moonies V: Why We Don’t Go Back

Not only is Earth’s Moon less economical (in terms of propellant) and economic (in terms of accessible resources) than near-Earth objects. It’s also less interesting; there is literally less science to be found on the Moon.

Quick recap of differentiation: a large-enough body holds heat. Heat melts rock; molten rock splits into heavy and light parts. The heavy parts (metals) form a core. The light rock forms a mantle, and a lighter crust. The lightest stuff (volatiles) boils off completely. Earth is large, and clearly differentiated.

Before the Apollo program, scientists had ranged widely on the origin of the Moon. After Apollo, there still isn’t a lone model standing. But all options had some impactor hit Earth, and spray out rock which re-formed into a Moon. It’s the various details still being pursued.

Earth’s mantle is distinguishable from its crust; the mantle is rich in olivine, while here at the surface olivine is unstable. Apollo managers, studying the Moon, wanted mantle rocks for comparison. Astronauts found none; later, the South Pole-Aitken Basin was identified as a huge impact site, 2500 km (1560 mi) across. It’s the largest on the entire Moon. Time and again, missions got proposed to return Aitken Basin samples; surely the enormous impact must have excavated through crust, into the lunar mantle. Again, every proposal was rejected. Yet, on Feb. 8, 1969, the Allende meteor had fallen on Mexico. Allende is full of olivines.

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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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