Asteroid of the Half Month: 617 Patroclus

In the last post, I give how outer Main Belt objects look damp or frosty. This, despite being inside the “snow line”- the orbit radius where something is far enough from the Sun, and thus cool enough, to let water ice survive. 617 Patroclus, a Trojan, is now outside the snow line.

Jupiter, the biggest planet by far, orbits at a radius of 5.1 AU from the Sun (i. e., 5.1 times the radius of Earth’s orbit). This is past the snow line, and the two are connected. Water is one of the most common substances in the Solar System, especially if going by mass. Since water, past the snow line, would stick, instead of boil and blow off, the giant planets stuck more water together, and had higher gravity. This high gravity pulled in more water, dust, gas, etc., which in turn pulled even more matter in, and so forth. The giant planets got that way because, once the water held, the process went runaway. But as one got further out in the early Solar System, the cloud of starting material tapered away. Saturn, Uranus, etc. are much smaller. It appears Jupiter, just over the line, was at some sweet spot, not too close yet not too far.

The Trojan asteroids are in a different sweet spot. Bodies have Lagrange Points, as I’ve stated. Two of them are 60 degrees ahead, and behind, a planet in its orbit. At these points (Lagrange point 4 and 5, or L4 and L5), a quirk of geometry causes a slight dip in gravity. Objects which fall into L4 and L5 tend to stay there, loosely bound by the bigger object. Jupiter, the biggest body in the area, has strong L4 and L5 points, and trapped large numbers of objects there. These are the Jupiter Trojans; in total, their number is a significant fraction of the number in the Main Belt. Jupiter is so gravitationally dominant, other planets have negligible numbers of Trojan objects (or zero), and one just assumes “Jupiter Trojan” on hearing the T-word.

If the outer Belt retained water despite being measurably closer to the Sun, it is simply taken as fact that Trojans have ice. Still, it’s worth the investigation. The nature and history of Trojan chemistry turns out to be the history of the Solar System itself.

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Asteroid of the Half Month: 24 Themis (etc.)

It’s simple: asteroids are rocks, comets are ice, right? So comets shed a coma and tail when they approach the Sun, and warm up, right? No, it’s simplistic thinking: the cosmos does not divide neatly for the convenience of human perception and notions.

The Themis family of asteroids was one of the original asteroid families identified by Kiyotsugu Hirayama in 1918. “It has been identified as a family in all subsequent works” (Florczak et al. 1999) including analyses by multiple sort criteria. We widely assume that families form when a parent body is smashed by collision; its pieces then orbit in scattered but visibly related orbits. In this case, Themis-family objects orbit as a group in the outer Main Belt. A century after Hirayama, we now see that 24 Themis and its kin not only have identical or related spectral types. Many have a similar- icy or otherwise hydrated- composition, too.

The past twenty years have seen the emergence (to humans) of a new Solar System class: main belt comets. The Minor Planet Center gives them dual status: both asteroid and comet. This is apart from 2060 Chiron, orbiting between the gas giant planets, not in the Main Belt. Chiron grew a coma when closest to the Sun, indicating water loss. That wasn’t surprising; gas giants orbit past the “snow line” (inward of Jupiter) where the dim sunlight lets water ice persist. But the entirety of the Main Belt is in the snow line; asteroids were assumed- by definition- to be pretty dry, and incapable of any coma or tail. Let the assumptions end, via Themis “comets.”

Carbonaceous chondrite meteorites test out to 10% water or more, sometimes past 20%. The asteroids of the C-complex, including Themis-family objects, seem to be carbonaceous-like material. Florczak et al. go on: of 36 Themises checked, “Indications of aqueous alteration…is clearly present in the spectra of 15 asteroids… If a more relaxed criterion is adopted, 11 more objects could also present aqueous alteration in their surface.” Main Belt aside, we found comets to be not white like snow, but black, like C-asteroids (plus D-, P-types, etc.).

The past twenty years were no surprise; growing evidence indicated wet asteroids…

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Aggies and Buffaloes, not Ashy Boondoggles

A NASA/Department of Energy press conference will plug nuclear power for big expedition programs. If there’s anything we’ve learned in the past 15-20 years, it’s get rid of big, expensive programs, like nuclear deadweight.

What will probably go unspoken at the press conference is that space solar potentials are so boundless, we’ve left them as an excercise to our students. University teams across the United States were going wide and deep, as competition and fresh minds will do. It’s a matter of letting your best young talent stretch the possibilities. A few weeks ago, the field was whittled down to five. By comparison, the Department of Energy is a big, secure, bound organization, and will brook a new idea like George R. R. Martin can release new novels. We’re waiting.

The five finalists: University of Colorado-Boulder; Norwich University; Princeton University; Texas A&M University; and the University of Virginia. One of the judges, Lee Mason, is head of power and energy storage at NASA’s Space Technology Mission Directorate. “I am really impressed by the number of proposals and the diversity of ideas,” said Mason, “I can honestly say that the proposals introduced new and innovative ideas for solar array packaging that we haven’t thought about before.”

Let’s not forget that the 2006 New Horizons mission to Pluto succeeded despite the Department of Energy, not because of it. The DoE could only deliver roughly half the plutonium asked by New Horizons’ tight launch window, limiting power and constraining mission operations…

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AotHM: Eros Part 4

For as long as there has been an S-type, 433 Eros was called an S-asteroid. See: Chapman Morrison Zellner 1975, Zellner Gradie 1976, Bowell et al. 1978, Tholen 1984, and Bus Binzel 2002. A probe to Eros- the closest, best-studied, big S-asteroid- was sought, to get many answers. What are S-types like, and made of? (Implying their history, and the conditions of the early Solar System.) Do we have samples, via meteorites? Which?

(This would be a good time to review meteorites, per my last post. There are stony, iron, and stony-iron meteorites. Stonys are further divided into achondrites (fully molten, once) and chondrites (never molten). A few are primitive achondrites (barely molten).)

S-type asteroids dominate the inner Main Belt, and the planet-crossing zone (that is, near-Earth objects). We backtrack the trails of meteors; they, like Eros’ orbit, lead out to the Belt. One would expect to find S-pieces on Earth. Of the meteorites, stonys dominate irons and stony-irons. Stonys (technically, ‘ordinary chondrites’, OCs) are ~80% of all falls. Yet, the most common asteroids do not look like the most common falls. S-asteroids’ minerals look like stony-irons, only ~1% of all meteorites. This would imply stony-iron fragments are missing or shy, but there’s a secret pool of stonys, bombarding Earth somehow.

The Galileo mission to Jupiter seemed to find an answer. Passing 951 Gaspra (in 1991) and 243 Ida (1993) on its way, it observed younger areas to be a bit bluer and more ordinary-chondrite-like, than old areas. S-types get ‘space weathering,’ a thin coat hiding the bulk minerals (which are OCs). A flyby of course gives a less than thorough view, while chronic Space Shuttle delays made Galileo fly with late ’70s/early ’80s technology… and a busted antenna. The NEAR mission (Near-Earth Asteroid Rendezvous) would orbit an S-type in 1999, using many, modern instruments, and to be sure a rigid antenna. What did it find?

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Meteor-Write: Chondrule Or Not

As we get closer to small bodies (including the first landing), we get deeper into Solar System science. I’m using more and more terms I haven’t defined; let’s remedy that.

The basics include chondrules. Chondrules are literally the basic building blocks of small Solar System bodies, and by extension both planets and meteorites, formed from small bodies. A chondrule (Greek for granule) is a millimeter-scale object; once molten and adrift in space, it is now hard and inside a small body or meteorite. These consist of chondrules, embedded in a matrix. Meteorites which still look like clumps of chondrules are called ‘chondrites’. When we cut open chondrite meteorites, many chondrules are clearly visible.

At the formation of the Solar System, there was just gas and dust- the Solar Nebula. This matter began sticking together- gases condensed to droplets, volatiles into tiny crystal grains, and dust into dust clumps. At some point, heat melted many of these bits, which pulls them denser and rounder. The process of ‘accretion’ continued- gas and dust kept sticking to chondrules, which stuck to each other, which made them better targets for more gas and dust and chondrules. And so on, forming small asteroids and comets, up to planetesimals, then planets. Eventually, most stuff had been consumed; those planetesimals left (now cooled, harder, and further apart) were just as likely to destroy each other in a collision, rather than stick.

In planets and large planetesimals, collisions and radioactive material generate heat. Planets and some asteroids then remelted, losing chondrules. In small bodies, heat is lost more easily, via their closer, relatively larger surface. So most asteroids and comets kept chondrules, a tangible record of the early Solar System and its processes. We value such bodies as time capsules or ‘baby pictures’; we also have samples of gently- and partly-melted asteroids. They, too, look like nothing on Earth. One could say it starts and ends with chondrules.

Chondrules then define our meteorite classification system, and by extension asteroids…

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AotHM: Eros Part 3

An asteroid orbiter (not flybys like Galileo, or even NEAR at Mathilde) had been long overdue. The software fault added a year to the NEAR trajectory, but the 1998 Eros flyby worked, and set the stage for orbital operations in Feb 2000.

The NEAR Shoemaker probe came 3827 km (2392 mi) from 433 Eros on Dec 23, 1998. That’s more than the prior flybys, but the close orbits gave a flyby relative speed under 1 km/s- a brush, not a slash. With a working X-band radio, NEAR felt a Doppler shift of .006 Hz (1 part in 1,400,000) due to Eros’ pull. This set the body’s mass; volume is derived from images. NEAR began shooting Nov ’98, for navigation, and to take lightcurve data as Eros turned. A search for asteroid moons began; any would be a hazard to the mission. Images reached 363 m/pixel resolution at the flyby, and a density (mass divided by volume) of 2.5 gram/cubic centimeter was given, about that of Ida. (There was an uncertainty term in that; part of Eros was in shadow, or simply “in the back” and not shot well.) Both Ida and Eros appeared rocky, with some porosity.

The long Eros shape was obvious, confirming ground data; here it was described as a “kidney bean” or “banana.” Two large craters were seen. Though not as battered as Mathilde, craters 8.5 and 6.5 km were about as large as one could expect without reshaping or destroying Eros. Overall, it was more cratered than Gaspra, but smoother than Ida. The giant depression or crater (informally named Shoemaker) was also seen to be an albedo feature- a patch 30% brighter than the surface average.

Scientists wanted chemical data; the debate of what meteorites match what asteroids was far from settled. The NIS (Near-Infrared Spectrometer) instrument was aboard, taking data in the flyby. But the X-ray and gamma ray spectrometers would have to wait until orbit; this flyby was still too fast to take good readings.

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Comet of the Quarter: 2P/Encke

If there’s a title ‘Dr. Comet,’ it would go to the late, great Fred Whipple. He called 2P/Encke “our favorite source of cometary knowledge”, only part jokingly. With all due respect to ESA, he billed it “the “Rosetta Stone” of comets” too. This may include bodies that are Encke fragments (or of Encke’s parent), and we may have actual samples of Encke.

Pierre Méchain “discovered” it in 1786, but only saw it over 3 days- not enough for an orbit solution in those days. Caroline Herschel saw a comet in 1795, and in 1805 Jean Louis Pons too. He and Bessel calculated a parabolic (open) “orbit.” Finally, Johann F Encke gave open, then closed, returning orbits. This had him suspect one comet perpetrated all three apparitions, with a 3.3 year orbit. Encke persisted; he found it would be perturbed (tugged) by planets; such tugs refined the timing of its returns. Including tugs, Herr Johann predicted an 1822 return, which happened. He also applied this as a test of Newton’s then-new laws. Like Edmund Halley before him, J.F. Encke thus won naming rights and a place in astronomical history. By 1838 though, two and a half hours of orbit still could not be explained. Encke (the man) figured a ‘medium’ or ‘ether’ was slowing the comet via drag, hmmm…

Most comets grow a coma- gas and dust spreading radially outward, then into a tail. 2/P Encke doesn’t have a tail, but a fan or multitail, and faint ones at that. It also shows aphelion activity- emissions when farthest from the Sun, and thus coldest. (Granted, at only 3.3 years, the shortest comet period, Encke doesn’t get that far and cold.) It’s billed as a Type I comet, gas-rich but dust-poor (symbolized by χ). We now see this is partly selection bias, as its emissions differ before and after perihelion (its point closest to the Sun). Sarugaku et al. 2015 call it “essentially inactive but…” We now know Encke weakly sheds some gases, and very little dust (literal microscopic bits), but disproportionate amounts of rock. Not .x micron dust (which we would see visually) but infrared-detectable sand (x0 microns) to gravel (cm-sized). Some boulders (.1 m) are possible too. Radar echoes indicate dust is there on its surface, likely with frost; they’re just not shedding into space.

Encke largely set our current comet theory. The mid-20th Century debate was ‘Flying Sandbank’ or ‘Icy Conglomerate.’ Flying Sandbank meant comets were not rigid bodies, but particle swarms. Some particles were volatiles, or had volatile content, and shed a coma/tail if heated. Dust shedding was just loss of particles. An Icy Conglomerate was rigid; the swarm had a ‘hive’ (its nucleus). While Encke seemed to slow down a bit, some comets appeared to speed up. There was no ‘ether,’ emissions made jets that sped up or slowed down nuclei, by pointing fore or aft. But only rigid bodies could form jets, and take their thrust. Fred Whipple illustrated the Icy Conglomerate model, which got corrupted to ‘Dirty Snowballs.’ Unfortunately, Dirty Snowball’s a bad name. Nuclei don’t look snowy or icy, as Star Trek and Armageddon depicted. Star Trek, at least, visited comets in interstellar space, presumably fresher. Armageddon has no excuse, and is inexcusable. We already knew that nuclei are black- they’re wet(ter) asteroids or ‘frosty dirtballs’.

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