Friday, March 13, 2015

Dawn's Arrival at Ceres; Sims and "Chaos"

Dawn became the first spacecraft to orbit two asteroids or planets other than Earth last week. More to the point, we're learning more about these survivors from the early Solar System.

Meanwhile, from the world of infotainment, "chaos" and the early Solar System.
  1. Arrival at Ceres
  2. Solar System Drama, Plus Some Facts

First Stop: Vesta



(From NASA,JPL-Caltech,ASU; via Wikimedia Commons, used w/o permission.)
("The most ancient and heavily cratered regions are brown; areas modified by the Veneneia and Rheasilvia impacts are purple (the Saturnalia Fossae Formation, in the north)[93] and light cyan (the Divalia Fossae Formation, equatorial),[92] respectively; the Rheasilvia impact basin interior (in the south) is dark blue, and neighboring areas of Rheasilvia ejecta (including an area within Veneneia) are light purple-blue;[94][95] areas modified by more recent impacts or mass wasting are yellow/orange or green, respectively."
(Wikipedia))

A week ago, Dawn became the first spacecraft to orbit two different places other than Earth in the Solar System. Dawn's first port of call was Vesta.

Vesta is the second-largest asteroid in the Solar System's asteroid belt: or the largest, if you call Ceres a dwarf planet.

Either way, Vesta and Ceres are — most likely — the last remaining large protoplanets: which makes them valuable samples from the early Solar System.

Apart from size and distance from our sun, they're very different: which also makes them intriguing places to study. Scientists have ideas about how Vesta and Ceres ended up where they are, but those hypotheses may change when we learn more from the Dawn mission.

There's quite a bit of data to work with already, from Dawn's stopover at Vesta.

Vesta's Rheasilvia crater covers most of the asteroid's southern hemisphere. That's not a typo. The crater is huge, the impact probably made the Divalia Fossa, one of the Solar System's biggest rift valleys, and I'm drifting off-topic.

There will not be a quiz on these names, thankfully.

Dawn's NSTAR Electrostatic Ion Thrusters


After entering orbit around Vesta, Dawn had enough fuel left to reach Ceres because it has a very efficient engine.

Dawn's three xenon ion thrusters use NSTAR (NASA Solar electric propulsion Technology Application Readiness) electrostatic ion thruster — tech tested on the Deep Space 1 mission.

It's the first time a NASA mission used only ion engines for propulsion after getting thrown into space by chemical rockets.

Magnetohydrodynamic rocket engines, like NSTAR, shoot a thin cloud of superheated matter in one direction — pushing the spacecraft the other way. Newton's third law of motion, and all that.

Strictly speaking, only rocket engines that produce thrust by accelerating ions are "ion thruster"

"Ion engine" is a lot easier to say than Magnetoplasmadynamic thruster or Variable Specific Impulse Magnetoplasma Rocket, though, so folks generally say "ion engine" when they mean any sort of Magnetohydrodynamic rocket engine.

Again, there won't be a test on these words. I'm a recovering English teacher: emphasis on "recovering."

Tsiolkovsky, McDonnell Douglas, and Looking Ahead


Dawn's journey started September 27, 2007, from Cape Canaveral Air Force Station, Florida.

Ion engines are wonderfully efficient, since their high exhaust velocity gives them really high specific impulse. But it takes chemical rocket engines, like the McDonnell Douglas Delta II launch system's, to get from Earth's surface to orbit.

Never mind the geek-speak.

The faster a rocket's exhaust is, the more efficient it is at turning propellant into change of velocity.

Chemical rockets aren't as efficient as ion engines; but they're very good at throwing massive amounts of propellant out, very fast. That lets chemical rockets produce enough thrust to throw stuff into space.

I'm pretty sure chemical rockets will be lifting cargo and passengers into orbit for quite a while.

Physicists have some intriguing ideas about beamed core antimatter rocket motors, or warping space-time: but those are mostly at the stage Tsiolkovsky's ideas were a century back. I'd be surprised if we start building interstellar spaceships before 2100, and that's another topic. (October 3, 2014; May 24, 2013; May 17, 2013)


1. Arrival at Ceres



(From NASA, JPL-CalTech/UCLA, MPS, DLR, IDA, via BBC News; used w/o permission.)
("The Dawn probe will spend just over a year mapping and sensing the dwarf planet"
(BBC News))
"Nasa's Dawn probe achieves orbit around Ceres"
Jonathan Amos, BBC News (March 6, 2015)

"The US space agency's Dawn probe has gone into orbit around Ceres, the largest object in the Solar System between Mars and Jupiter.

"A signal from the satellite confirming its status was received by ground stations at 13:36 GMT.

"Ceres is the first of the dwarf planets to be visited by a spacecraft.

"Scientists hope to glean information from the object that can tell them about the Solar System's beginnings, four and a half billion years ago.

"Dawn has taken 7.5 years to reach its destination. Its arrival has seen it pass behind the dwarf to its 'dark side'.

"Over the next month, controllers will re-shape the orbit to get it ready to begin the prime science phase in late April.

"Over time, the intention is to progressively lower the orbit until the probe is just a few hundred km above the surface. By that stage, it will be returning very high resolution pictures...."
Astronomers had been looking for a 'missing' planet between the orbits of Mars and Jupiter since 1772, when Johann Elert Bode — wait a minute. I'm getting ahead of the story.

Johannes Kepler noticed pattern in planetary orbits: with a gap between Mars and Jupiter. That was in 1596.

In 1766, Johann Daniel Titius said there should be a planet 2.8 Astronomical Units — the distance between Earth and our sun — out from Sol. Johann Elert Bode said pretty much the same thing in 1772. these days we call their idea the Titius-Bod law.

It isn't so much a "law," as an interesting equation describing the relative size of orbits, from Mercury out to Uranus: not exactly, but intriguingly close to the real numbers.

We still don't know why the inner Solar system, Ceres, Jupiter, Saturn, and Uranus, follow Bode's a=4+n equation. It probably has something to do with orbital resonance and insufficient degrees of freedom — or something else.

Major moons of the Solar System's gas giants have orbits spaced at regular intervals, too: but not following Bode's equation.

We may find other planetary systems that follow the 'Titus-Bode law,' but — it gets complicated, and doesn't have much to do with Ceres. Probably.

Around the time that zeppelins were cutting-edge transportation technology and Konstantin Tsiolkovsky was showing how rockets could carry us to other planets, astronomers were using long-exposure photographic plates to find asteroids. (October 3, 2014)

That helped them find more asteroids. By now we know of more than 200 asteroids in the asteroid belt that are more than 100 kilometers in diameter, roughly a million more than a kilometer across, and a lot more smaller ones. (July 26, 2013)

Visiting Vesta, Ceres: It's a Big Deal


(From NASA, JPL, courtesy Marc Rayman, used w/o permission.)
(Dawn'​s approximate flight trajectory)

Ceres is the second, and last, stop for Dawn. The robot spacecraft began orbiting July 16, 2011: and became the first spacecraft to orbit two asteroids — or planets — on March 6, 2015, when it went into orbit around Ceres.

The Dawn mission has been telling us about these two big asteroids. That's a big deal, since both are probably protoplanets that didn't keep growing. Studying data from this mission should answer a few questions about how planets form — and probably raise a whole lot more.

Orbits and Bright Spots


(From NASA/JPH-CalTech, Sky and Telescope, Greg G. Dinderman; used w/o permission.)
("Science Orbits Dawn will conduct four different orbits around Ceres, spiraling down from the outermost to the innermost over a few months."
(Sky and Telescope))


(From NASA, JPL-CalTech, UCLA, MPS, DLR, IDA, via BBC News, used w/o permission.)
("The bright spots inside a 92km-wide crater have been the big surprise of the encounter so far"
(BBC News))

What looked like a very bright spot inside a crater of Ceres turns out to be two very bright spots, close together. It might be recent outflow from a 'water volcano,' but most scientists apparently think it's more likely a divot from a recent impact.

One of the big questions is why Vesta is denser than Ceres. Scientists get a fairly accurate idea of how massive Ceres is — 940,000,000,000,000,000,000 — kilograms by how much its gravity affects other, smaller, asteroids. That's a lot of zeroes, and about a third of the asteroid belt's mass.

Anyway, since they know Ceres mass, and its diameter, scientists know that Ceres' density is right around 2 grams per cubic centimeter: compared to about 3.4 grams per cubic centimeter for Vesta, and 5.5 for Earth.

A smaller asteroid might have low density because it's basically a gravel pile: rocks touching each other, but with a lot of gaps. That can't be the case for Ceres, since it's big enough to squeeze what's inside solid — or maybe liquid.

One of the easier ways to explain why Ceres is such a lightweight for its size is to assume that there's an immense ocean of water deep under its surface. This isn't idle speculation. Scientists noticed water vapor around Ceres recently. (January 24, 2014)

Water is a basic requirement for life as we know it, so it's (remotely) possible that we'll eventually find critters living in Ceres.

It's even more remotely possible that Ceres was colonized by folks whose biochemistry works with a water/hydrogen peroxide mix. They might think Ceres is 'just like home,' and dismiss the crackpot notion that any sort of life could exist on the hellishly hot third planet from this star.

No, I don't think so: but it might make a good story. (Apathetic Lemming of the North (March 5, 2009))

More than you need, or maybe want, to know about:

2. Solar System Drama, Plus Some Facts



(From BBC News, used w/o permission.)
"Lucky Earth survived cosmic pinball"
Toby Macdonald, Producer and director, BBC Horizon; BBC News (March 2, 2015)

"Rogue alien planets are forcing astronomers to rethink the birth of our Solar System. What's emerging is a tale of hellfire, chaos and planetary pinball - and it's a miracle our Earth survived.

"Hunting for alien planets is big business. Since the first exoplanet was discovered in 1995, astronomers around the world have been searching for those elusive Earth-like planets that could harbour life.

"The tally of planets found is staggering. So far, more than 1,800 confirmed planets have been discovered orbiting around other stars in our galaxy (the latest figure from Nasa is 1,816 planets around 1,117 stars).

"Among them, are a few rocky planets in the magical "Goldilocks" zone where water can remain in the liquid state, and life could evolve.

"It's no surprise to astronomers that these planets exist. But what has come as a shock is that many of these exoplanets seem to break all the rules...."
Toby Macdonald's article is a lively review of what we know — and don't know — about planetary systems. As usual, new data has answered some questions: and shown that we have a great deal more left to learn.

BBC News posted Mr. Macdonald's article — but he doesn't seem to be one of their regular science correspondents. He's identified as the producer and director of "BBC Horizon, a "long-running British documentary television series on BBC that covers science and philosophy." (Wikipedia)

That, I think, helps explain why Mr. Macdonald says the Solar System's early days were "...a tale of hellfire, chaos and planetary pinball...."

He's right, sort of. I also think he's helping "chaos" and "chaotic" become cliches. (January 3, 2014)

"Chaos" is "a condition or place of great disorder or confusion." (thefreedictionary.com)

What's seen as "chaotic" depends, I think, on assumptions about what "order" means.

Rectilinearity, Euclid, and the Titanic


My father repeated a story he'd heard from one of his fellow-librarians.

One of the college's exchange students was working at the library, helping move shelving around. The librarian showed him how to use seams in the linoleum to line the stacks up precisely. The exchange student smiled and said, "ah, the Western obsession with rectilinearity."

He had a point.

I grew up in a part of the world where most roads ran along the sides of mile-wide squares: an exercise in Euclidean geometry, at least on a local level.

Maybe that's why I sympathize, a bit, with H. P. Lovecraft's apparent unease about living in a non-Euclidian universe. Then the 'unsinkable' Titanic sunk, and that's yet another topic. (November 21, 2014; June 27, 2014)

Where was I? Euclidean geometry, H. P. Lovecraft, eldritch horrors. Right.

Non-Euclidean, Yes: "Chaotic," No


It's easy for a Westerner to see that a checkerboard is ordered, not chaotic. But straight lines and right angles aren't the only way to organize things.

Fractal patterns aren't, pop science writers notwithstanding, "chaotic."

They're not the sort of orderliness Euclid described: but they follow well-defined rules.

Now, back to 'chaos,' drama and science, BBC Horizon style.

Earth: "the Perfect Place for Life...." — or Maybe Not



learned that the early Solar System probably went through "a period of chaotic upheaval," before the planet fell into their present orbits.

As he says, "Earth - in the perfect place for life to evolve - was lucky to survive."

Again, he's right: probably. Maybe. Possibly.

Earth is certainly a very good place for critters made of water, protein, and nucleic acid.

But life optimized for methane oceans wouldn't just die here: the critters would promptly boil away in Earth's 'intolerable' heat.

A world where life uses methane like we use water would probably use lipids where we've got proteins. Lipids like proteolipids and lipoproteins can be very complex, and are a part of our biochemistry — so that's not such a wild bit of speculation.

Another plausible life chemistry uses proteins in ammonia, with nitrogen doing what oxygen does here. And that's yet again another topic. (March 7, 2014)

Early Solar System: Bumper Cars Played With Planets


As Mr. Macdonald said, the Solar System didn't always look the way it does now. Scientists figure that the giant plants' orbits kept shifting for 500,000,000 to 600,000,000 or so years after the planets formed. Eventually, Jupiter and Saturn fell into a 2:1 resonance — a fancy way of saying that Saturn orbited the Sun once for every two Jupiter orbits.

Then things got lively. Or deadly, depending on your viewpoint.

The Jupiter-Saturn resonance set up a gravitational tug on the outer planets. Neptune moved outwards, past Uranus and into the ancient Kuiper belt. Most small icy bodies in the outer Solar System got scattered inwards, planets kept getting pushed and pulled into different orbits before settling into their current positions.

Jupiter, by far the Solar System's largest object besides our star, was a major player in this process.

Some of small fry got thrown out of the Solar System, some fell into our sun, and some hit other planets: including Earth.

The Late Heavy Bombardment is still a hypothetical event — but there's a growing body of evidence showing that a remarkable number of asteroid-size objects hit Earth and our moon 4,100,000,000 to 3,800,000,000 years ago.

The Solar System's first half-billion years are complex, and working out what happened takes more math than I learned in high school.

But "chaotic" in the sense of utterly random? Not so much.

Simulations, "Chaos," and Getting a Grip


Math used in Mesopotamia, back when Nabonidus lost the Battle of Opis, probably let folks work out what happened in the early Solar System. They were smart, but we've learned a few things since then: including how to build computers.

In the last century, simulation tech has gone from mechanical horses to sims using mathematical models for testing traffic networks, training pilots and soldiers, and entertaining folks.

I've been running The Sims on my computer. They don't act exactly like humans, but the simulation is pretty close. Then there's the Turing test, and that's still another topic. Topics.

I don't know how long unpaid interns running a Solar System simulation with abaci and clay tablets would take to get to the Jupiter/Saturn 2:1 resonance — but I'm guessing that whoever started the project wouldn't live long enough to see results.


(From AstroMark, via Wikimedia Commons, used w/o permission.)
("Simulation showing outer planets and Kuiper belt: a) Before Jupiter/Saturn 2:1 resonance b) Scattering of Kuiper belt objects into the Solar System after the orbital shift of Neptune c) After ejection of Kuiper belt bodies by Jupiter[2]"
(Wikipedia))

The Solar System is still "chaotic" in the sense that we can't predict exactly what Earth's axial tilt will be 4,500,000,000 years from now. On the other hand, we've got a pretty good handle on what's happening in the immediate future: like the next few million years.

I'm not surprised that the universe — and Solar System — follow knowable, rational, predictable, physical laws: and in that sense is not "chaotic." (Catechism of the Catholic Church, 32, 35-36)

On the other hand, I don't expect us to know everything. At some point, we must acknowledge that we have limits: or re-learn why pretending otherwise is a really bad idea. (Catechism, 396)

That doesn't mean thinking is a sin, or that folks who are curious go to Hell: and that's — what else? — another topic. (November 21, 2014; August 1, 2014; July 15, 2014)

My take on life, the universe, and everything:

2 comments:

Brigid said...

Feels like there's something missing: "In 1766, Johann Daniel Titius said there should be a planet 2.8 Astronomical Units — the distance between Earth and our sun."

Also something missing: "turns out to be two very bright spots, close."

Extra word: "probably use lipids where we we've got proteins."

The Friendly Neighborhood Proofreader

Brian Gill said...

Oops! Right. Found, fixed, and thanks!

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From time to time, a service that I use will display links to - odd - services and retailers.

I block a few of the more obvious dubious advertisers.

For example: psychic anything, numerology, mediums, and related practices are on the no-no list for Catholics. It has to do with the Church's stand on divination. I try to block those ads.

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