Asteroids, the reality

 kw: book reviews, nonfiction, science, astronomy, asteroids

Chances are, the word "asteroids" conjures up an image a lot like this for most people. We hear about the millions of rocks of all sizes roaming the spaces between the planets, especially between Mars and Jupiter. We think of that space as crowded with space debris.

The reality is somewhat different. Before getting into that, however, I want to recommend a book about the asteroids, about how we came to know about them and what they are like. Asteroids, by astronomer Clifford J. Cunningham, doesn't pretend to be a comprehensive survey. Rather, the author has two aims: to survey the history of our knowledge, and theories, of asteroids and "small bodies" in general; and to show how they are classified.

The telescope was invented in the early 1600's, just over 400 years ago. Galileo made it famous by seeing craters on the moon and discovering satellites around Jupiter. Although several asteroids are bright enough to be seen using small telescopes, even binoculars, you need to know where to look. Two centuries were required to gather sufficient knowledge of the skies, until the first "new planet" was seen January 1, 1801.

It took a number of years for astronomers to determine that this new planet, Ceres, was 1/11 the diameter of the Moon, and even longer to discern its mass to be 1/800 that of the Moon. By then a number of small, "new planets" had been found. Over the decades, the number grew to hundreds, then thousands, and the current number of asteroids whose orbits have been worked out is more than a million.

Astronomers also discovered that these little bodies weren't evenly spread out in the "asteroid belt", the realm between the orbits of Mars and Jupiter where more than 90% of them are. There were some gaps, which are caused by gravitational "pumping" by Jupiter either adding or removing orbital energy so that those special orbits stay clear. There are also certain "families" of asteroids, most famously a small number of Trojan asteroids that are in the L4 and L5 orbital points ahead of and behind Jupiter about 60° in (and near) its orbit. More recently, a small number of asteroids have been found to precede or follow Earth, Mars, Saturn and Uranus, so the designation "Trojan asteroid" has been expanded to include them all.

Other orbital subtypes are focused on the ones that could threaten Earth. Four classes of Near-Earth Asteroid (NEA) are defined by orbital parameters. The ones of most concern are those that pass through Earth's orbit ("through" meaning anywhere within a few thousand km of the exact orbit). It's just a matter of timing before one of them winds up on a collision course. So far, though, none are known with certainty. But we only know about half of the NEAs that are there, which are big enough (more than 140m, or 460 ft), to devastate an area 100 km across or more.

Even though there are tens of thousands of NEA's, we are saved by the bigness of space. At present, I see a notice at least every month in online news about some asteroid "as big as the Empire State Building" or "school bus sized" that is going to pass "near" the Earth. It always turns out that the "near miss" will be a million miles or so. This is not to discount that some big, possibly devastating asteroids are out there, and we may not know about them yet. But the last asteroid hit to cause a "nuclear winter" happened 65 million years ago. Our portion of "asteroid space" has a low population. (At this point, I'll stray from what's in the book.)

What if Earth sat right between Mars and Jupiter? Then we'd have between 10x and 100x the chance of getting a significant collision in our lifetimes. But that chance is still low. We know that because many spacecraft have been sent to Jupiter and beyond, right through "the Belt", without mishap. Let's see why. This table lists the approximate (more approximate with smaller size) number of asteroids in the main belt, from 100m (0.1 km) and larger:

The 100m sized ones are big enough to cause plenty of trouble if they hit Earth. But what about a spacecraft, such as Voyager or New Horizons? Even a centimeter-sized pellet that hits a craft going 20 km/s can destroy it. Spacecraft can be shielded from smaller bits, so we need to know how many tiny bits of millimeter size there are. It isn't easy to extend this table to smaller sizes, because there are a few theories about the size distribution. Many publications posit a "scale free" distribution, which I think is extreme, but we'll use that for one sideboard of our estimates. The Theory of Breakage by Andrey Komolgorov predicts a lognormal distribution, which some think is too conservative, because the tail of small objects dies away so much faster. I happen to favor that hypothesis; I'll use it fo rthe other sideboard. Here is a table of the sideboards:

Diam.  Scale free N   Lognormal N
100m     25 million    25 million
 10m      4 billion     1 billion
  1m    300 billion    35 billion
100mm    22 trillion  1.1 trillion
10mm  1.7 quadrillion  33 trillion
 1mm  125 quadrillion  1 quadrillion

The volume of the main belt is about 4 billion billion cubic miles, or 10 billion billion cubic km. If the lognormal hypothesis is correct, there are a quadrillion (million billion) sand grain size bits in that volume, each has 10,000 cubic km to itself. That puts it about 25 km from its nearest neighbors, on average. On the other hand, the scale free hypothesis has 125 grains in that same 10,000 cubic km, and the average spacing is "only" 5 km. 

However, we want to sail through this mess, hoping to hit nothing. The appropriate analysis is to figure the collision cross-section, as though everything along the path were pasted to a surface the craft must pass through. This is like wrapping a big, big ribbon 40 million miles wide between Mars and Jupiter, and sticking all those sand grains to it, pulling or pushing them along radii from the Sun. This ribbon has a total area of about 80 quadrillion square miles, or 200 quadrillion square km.

This puts each sand grain "in possession" of either 200 km² or 1.6 km². Now the spacing, for the lognormal case, is 16 km, and for the scale free case it is 0.7 km.

Whichever way one analyzes the distribution, there is either a "pretty good" spacing between possible collisions, or a huge space. In any case, plenty of fragile spacecraft have passed through the main belt without incident. That crowded picture above is just not the way things are. From any particular asteroid, you can't see any others without a good telescope.

A word about "kinds" of asteroids. Most asteroids are dark colored, and some are extremely black. Some are comparatively bright, but even the metallic ones have a dusty surface, so the albedo (reflectivity) of a few asteroids may be 0.25 (25%), but most are in the 0.1 to 0.05 range, with some as dark as 0.02. That makes them hard to see, and it is harder yet to find out how big they are. Is a new body, just spotted, dark and large, or bright and smaller? Gathering observations over several days and then several weeks, we can figure out how far away they are. Size and albedo are harder.

One tool to help determine this is the reflection spectrum. The darkest asteroids are akin to the darkest meteorites (because the latter originate as the former), the carbonaceous chondrites. They not only reflect less light than other types, the distribution in the spectrum is different; they are called "red" (really a blackish brown). The brightest are metallic, with their own spectral distribution; and in between are the stony asteroids, with spectral features all their own. Although the book discusses these types and several subtypes, much is still being learned. Spacecraft that have visited asteroids, and the one or two that have brought back samples, are increasing our knowledge of them.

Finally, there is no "lost" or "exploded" planet that once resided in an orbit where the main belt is now. The pre-planetary bits didn't get organized into a planet, and Jupiter is probably mostly to blame. The empty gaps testify to Jupiter's power to eject objects from certain areas. Over time, it must have ejected a lot; the total mass of all the asteroids is thought to be less than 1/250th that of our Moon.

As we learn more about them, perhaps we'll learn enough to be able to detect and deflect any NEA that is found on a collision course with Earth. Perhaps.

I greatly enjoy books like Asteroids. I didn't know what to expect, and I learned a few things about the different kinds and different "places" of asteroids.