Sciency Words: Kirkwood Gaps

July 31, 2015

Sciency Words PHYS copy

Sciency Words is a special series here on Planet Pailly celebrating the rich and colorful world of science and science-related terminology. Today, we’re looking at the term:


Some of the asteroids in the asteroid belt have gone missing.

Jy13 Milk CartonThe asteroid belt begins at a distance of 2.1 AU (astronomical units) from the Sun and stretches all the way out to 3.5 AU from the Sun. But there are empty regions at approximately 2.5 AU, 2.8 AU, 3 AU, and 3.3 AU. These empty regions are called Kirkwood gaps.

Jy13 Kirkwood Gaps

Kirkwood gaps are named after Daniel Kirkwood, the astronomer who first discovered them and correctly deduced what caused them.

Any asteroid orbiting the Sun at a distance of 2.5 AU would happen to be in a 3:1 orbital resonance with the planet Jupiter. This means Jupiter would complete exactly one orbit for every three orbits the asteroid completed. The 2.8, 3, and 3.3 AU distances happen to correspond to other orbital resonances with Jupiter.

Asteroids in these resonant orbits would experience nagging, persistent gravitational tugs by the Solar System’s largest planet. This would slowly drag them away from their original circular paths around the Sun and throw them into new, highly eccentric orbits.

Many of the asteroids that cross Earth’s orbital path are probably former residents of the Kirkwood gaps. So the next time an asteroid comes along and wipes out the dinosaurs, don’t get mad at the asteroid. It might not be the asteroid’s fault.

Jy13 Jupiter the Bully

Get mad at Jupiter.

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Today’s post marks the end of asteroid belt month for the 2015 Mission to the Solar System… and the beginning of Jupiter month!

How Crowded Is the Asteroid Belt?

July 28, 2015

Exact figures for the number of asteroids in the asteroid belt vary widely, but astronomers generally agree there are millions of objects of at least one kilometer in diameter. That sounds like a lot, but it turns out the asteroid belt is a fairly empty and lonely place.

Jy12 Asteroid Echo

The average distance between main belt asteroids is just shy of 1,000,000 kilometers (600,000 miles). For comparison, the distance between the Earth and Moon is a little less than 400,000 kilometers (240,000 miles).

But for hotshot space pilots like myself, there are still some opportunities to zigzag between and around dangerously close asteroids, perhaps while being pursued by hordes of Imperial TIE Fighters.

Over a hundred asteroids are known to have at least one tiny moon. Binary and trinary asteroids have also been discovered. Those are pairs or trios, respectively, of asteroids of roughly equal mass that are locked into orbit around their common center of gravity.

And of course when asteroids collide, they often form debris fields. This debris can linger for weeks before recombining as a rubble pile asteroid. Maybe these chaotic post-collision debris fields are just the perfect place to try evading the forces of the Evil Empire.  Assuming you can find one.

Jy12 Millennium Falcon


10 Things You Need to Know About Asteroids from Time and Date.

Images of Asteroid Ida and Dactyl from NASA Near Earth Object Program.

Binary Asteroid from The Worlds of David Darling: Encyclopedia of Science.

Sciency Words: Rubble Pile

July 24, 2015

Sciency Words BIO copy

Sciency Words is a special series here on Planet Pailly celebrating the rich and colorful world of science and science-related terminology. Today, we’re looking at the term:


“Rubble pile” is not a formal part of any asteroid classification system, but it appears so often in scientific literature about asteroids that it deserves special attention. A rubble pile asteroid is really multiple asteroids tenuously held together by their own gravity.

Jy11 Hello Rubble Pile

Rubble piles probably form in the aftermath of asteroid collisions. When two asteroids smash into each other, gravity starts pulling the resulting debris back together again. A rubble pile can then form in as little as a few hours.

Since they’re not single, solid objects but conglomerations of multiple objects, rubble piles tend to have empty spaces inside them. This lowers the overall density of the asteroid, which helps astronomers distinguish rubble piles from regular “monolithic” asteroids.

Rubble piles have a minimum rotation period of approximately two hours. If they start spinning faster than that, they’re liable to fling themselves apart. However, astronomers have observed some rubble piles rotating faster than they should be able to, suggesting that additional forces besides gravity may help hold them together.

Exerting even a tiny force on a rubble pile could cause the thing to break apart. You can’t easily land on a rubble pile’s surface for mining purposes, and deflecting a rubble pile from a collision course with Earth would be tricky. As a result, rubble piles could pose many challenges to humanity in the future.


Potentially Dangerous Asteroid is Actually a Pile of Rubble from

Cohesive Forces Prevent Rotational Breakup of Rubble-Pile Asteroid (29075) 1950 DA from Nature.

Rubble-Pile Asteroid from The Worlds of David Darling.

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Today’s post is part of asteroid belt month for the 2015 Mission to the Solar System. Click here for more about this series.


Colonizing Ceres

July 22, 2015

Let’s talk about a certain dwarf planet. No, not Pluto. We’ll save that for Pluto month. Right now, we’re talking about Ceres.

Jy10 Jealous Ceres

Totally legit Hubble image of Ceres with Pluto visible in the background.

Someday, perhaps someday soon, humanity will start spreading across the Solar System. When we do, the dwarf planet Ceres could become an important part of our interplanetary infrastructure.

Ceres has been in the news a lot lately (almost as much as Pluto). During a recent visit by the Dawn spacecraft, we discovered some surprising features:

  • A single, lonely mountain in the midst of an otherwise flat landscape.
  • An unidentified white substance (water ice?) in the basins of several craters.
  • Plumes of water vapor escaping into space, hinting at possible subsurface oceans.

Even if Ceres doesn’t have vast oceans of liquid water beneath its surface, evidence suggests it at least has plenty of water ice. Water in any form is an incredibly valuable resource for space travelers, and not only for the obvious reasons.

Water can be used for:

  • Drinking (obviously).
  • Washing (obviously).
  • Oxygen: through electrolysis, water can be separated into oxygen and hydrogen, providing a spaceship’s crew with breathable air.
  • Rocket fuel: hydrogen and oxygen, cryogenically stored in liquid forms, make excellent rocket fuel.
  • Radiation shielding: water is surprisingly good at blocking solar and cosmic radiation. Well positioned water tanks on a spaceship’s exterior could do a lot to protect the crew.

A colony (or at least an outpost) on Ceres could serve as a convenient refueling depot for spacecraft heading out beyond the asteroid belt or for valiant explorers returning to the inner Solar System. A watering hole in space, if you will.

At the very least, it could make a fun setting for a Sci-Fi story.

Molecular Monday: Turning Water Into Rocket Fuel

July 20, 2015

Welcome to Molecular Mondays! Every other Monday, we examine the atoms and molecules that serve as the building blocks of our universe, both in reality and in science fiction.

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Humans need water. Our spaceships, however, may need it more than we do.

Human space exploration will never succeed if we have to carry everything we need from Earth. Instead, we have to learn to exploit the material resources space provides.

The Electrolysis of Water

Electrolysis is the process of using electricity to trigger a chemical reaction. Stick a pair of electrodes in water and turn on the power. This will break the chemical bonds holding water molecules (H2O) together.

Hydrogen will then accumulate around the negatively charged electrode. Oxygen will gather around the positive electrode.

Jy09 Hydrogen and Oxygen

Hydrogen, the Ultimate Rocket Fuel

Hydrogen makes the best rocket fuel (in more technical lingo, hydrogen has the highest specific impulse of any known substance). All you need is an oxidizer for the hydrogen to react with… and oh look, you’ve got plenty of oxygen! Just put the two back together in a reaction chamber, and you’re good to go.

Ideally, you’ll want to store your hydrogen and oxygen fuel in liquid form, which means you’ll need a lot of refrigeration equipment to keep them both below their boiling temperatures (roughly 20 Kelvin for hydrogen and 90 Kelvin for oxygen).

Asteroid Hopping

Of course, you’ll have some trouble finding water in space. If you’re planning an extended voyage through the Solar System, plot a course that will take you near some asteroids, specifically carbonaceous asteroids. They tend to contain relatively large amounts of water in the form of ice.

So long as you can find a few places to refuel your spacecraft, and so long as you bring the appropriate equipment along with you, you should be free to travel wherever you like.

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Today’s post is part of asteroid belt month for the 2015 Mission to the Solar System. Click here for more about this series.

Sciency Words: M-Type Asteroids

July 17, 2015

Sciency Words MATH

Sciency Words is a special series here on Planet Pailly celebrating the rich and colorful world of science and science-related terminology. Today, we’re looking at the term:


Previously on Sciency Words, we examined carbonaceous (C-type) and siliceous (S-type) asteroids, the most common and second most common asteroid types, respectively. We now come to the third most common type. Unfortunately, this is where nomenclature gets complicated.

Astronomers currently use at least two different systems to identify and categorize asteroids: the Tholen classification system and the SMASS classification system. The two systems overlap in some ways and diverge in others. What Tholen calls an M-type asteroid is, in SMASS, mixed into a broader category called the X-group.

I’d guess that eventually one of these classification systems will “win.” Either that or a new system will replace them both, simplifying matters. In the meantime, an M-type asteroid by any other name would smell as sweet.

Characteristics of M-Type Asteroids

The M in M-type stands for metallic. It’s unlikely that an asteroid of pure (or nearly pure) metal could form by itself. Therefore, M-type asteroids are assumed to be the metal cores of larger asteroids or maybe even dwarf planets that, for one reason or another, broke apart.

Most M-types seem to be composites of iron and nickel with traces of other metals—including precious metals like rare earths and platinum group metals. You can expect to find these valuable metals in much higher quantities than you would on Earth.

Jy07 Wealthy Asteroid

A typical M-type asteroid could be worth billions upon billions of dollars.

If someone were to capture one of these asteroids and safely bring it back to Earth, that someone would become extremely rich.

Or maybe not…

The Problems with Asteroid Mining

Before you hop into your personal spacecraft and fly out to the asteroid belt hunting for M-types, a few notes of caution:

  • There’s no way to know the exact composition of an M-type asteroid based solely on observations from Earth. There’s no guarantee that you’ll find substantial amounts of valuable metals.
  • M-type asteroids can have wildly different orbits from each other, making some much harder to reach than others. Fuel costs will stack up rapidly, seriously cutting into your profits.
  • M-types are basically solid lumps of metal. You’ll need more than a pickaxe or jackhammer to crack them open. The difficulties associated with mining operations will also seriously cut into your profits.

And yet it’s hard to resist the lure of M-type asteroids. Businesses in existence today (not some far-off Sci-Fi tomorrow) are greedily eyeing M-types, trying to figure out how to go get one. That may soon become a driving force in human space exploration. Or the hunt for M-type asteroids could turn into a huge economic bust.

Just something science fiction writers might want to think about.

Book Recommendation

Asteroid Mining 101: Wealth for the New Space Economy by John S. Lewis. There are plenty of books and articles out there about asteroid mining, but this one takes a serious look at both the pros and cons of the idea. If this is a subject that interests you, Asteroid Mining 101 offers a well-balanced view of the future of the asteroid mining industry.

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Today’s post is part of asteroid belt month for the 2015 Mission to the Solar System. Click here for more about this series.

Kicked Out of the Planet Club: The Story of Ceres

July 15, 2015

In 2006, the I.A.U. officially demoted Pluto from planet to dwarf planet. At this point, pretty much everyone knows that, and most people seem to have strong opinions on the matter (especially right now, as New Horizons sends back the first ever detailed images of Pluto). However, Pluto was not the first planet to be demoted. That particular dishonor goes to Ceres.

Discovered on New Year’s Day, 1801, Ceres was initially identified as the long sought missing planet between the orbits of Jupiter and Mars, making it the second planet discovered in modern times (Uranus was discovered about twenty years prior).

A few months later, however, astronomers discovered another object (named Pallas) sharing Ceres’s orbit. Within a few years, they found another (Juno) and another (Vesta). This started getting awkward. How could we have so many planets in the same (or nearly the same) orbit?

Jy06 Demoting Ceres 1

And so Ceres went from being the fifth planet to the first known asteroid.

Jy06 Demoting Ceres 2

What happened to Pluto is almost the exact same story. When Pluto was discovered, we had no way to know that it was one of a great many trans-Neptunian objects. Once we realized our mistake, Pluto had to be reclassified.

On the upside, the creation of a new category of celestial object allowed us to promote Ceres from asteroid to dwarf planet. So Pluto’s loss was Ceres’s gain.

But what do you think? Do you agree with the I.A.U.’s decision, or do you think Pluto and Ceres should have retained their planethood?

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Today’s post is part of asteroid belt month for the 2015 Mission to the Solar System. Click here for more about this series.