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Why do asteroids come in Pairs? A surprisingly large fraction of small bodies come in binaries and triplets.

In 1977, johns hopkins University astronomer David Dunham organized a group of amateurs and professionals to observe a star occulting the asteroid 6 Hebe. The asteroid's shadow passed through central Mexico, where three observers witnessed the star briefly flicker out. But experienced Texas amateur Paul Maley visually observed a 0.5-second occultation at the same time--some 500 miles north of the other observers. Was this evidence for an asteroid satellite? Unfortunately, researchers had no way to confirm Maley's sighting.

Over the next 15 years observers found hints of binary asteroids in other occultations, odd light curves, and radar echoes, but nothing could be confirmed. All of the speculation became moot on August 28, 1993. On that day, NASA's Galileo spacecraft flew by asteroid 243 Ida en route to Jupiter and made a profound discovery--a tiny moon later named Dactyl (last month's issue, page 28).

Up to then, astronomers had discovered more than 10,000 asteroids using ground-based telescopes. None were known to have companions and only a few were suspected to have one. Now, after two asteroid flybys (Galileo flew past 951 Gaspra in 1991), one was discovered to have a moon. Which of these statistics reflected the true nature of asteroids? Were binaries a few in 10,000, or one in two?

Opening the Floodgates

Within months of Dactyl's discovery, planetary astronomers began to report more evidence of asteroid satellites. Most were orbiting near-Earth asteroids (NEAs), tiny objects only a few kilometers wide that cross the orbits of the inner planets. The companions were suggested in high-quality light curves that could only be explained if there were two objects orbiting each other. At least one was spotted when one member eclipsed its partner, just like an eclipsing binary star.

By 2000 two new techniques for binary discoveries became mainstream and important: adaptive optics (AO) and radar. Large telescopes fitted with AO to beat atmospheric distortion could now resolve asteroid companions (the Hubble Space Telescope could also accomplish this feat). Using the 3.6-meter Canada-France-Hawaii Telescope, William Merline (Southwest Research Institute) and his colleagues discovered the first asteroid satellite with AO in 1999 when they found that 45 Eugenia was orbited by a small moon now known as Petit-Prince. Likewise, the newly upgraded Arecibo radio telescope in Puerto Rico could use radar echoes to produce high-resolution asteroid images. The first radar binary discovered was the near-Earth asteroid (185851) 2000 [DP.sub.107].

In 2005 planetary scientists received another pleasant surprise when Franck Marchis (University of California, Berkeley) and his colleagues found a triple asteroid, the large main-belt asteroid 87 Sylvia. The primary body had two small companions--later named Romulus and Remus for the mythical founders of Rome (Sylvia was their mother). And in 2008, astronomers using Arecibo discovered the first triple near-Earth asteroid, 2001 [SN.sub.263]. The discovery floodgates had opened.

This brief history lesson would be incomplete without mentioning the other main reservoir of binary "asteroids"--the Kuiper Belt. Technically, the first discovery in this region was that of Pluto's moon Charon in 1978. Because Pluto was considered a major planet until 2006, the distinction of the first official Kuiper Belt binary belongs to 1998 [WW.sub.31], announced in 2001, also found with AO. Since then, more than 70 Kuiper Belt binaries have been reported.

Classifying the Zoo Animals

After the discovery of 1 Ceres in 1801, it took nearly 200 years to find the first binary asteroid. In the past 19 years, scientists have discovered some 200 more. The main belt and Kuiper Belt account for nearly 80% of these, and the bulk of those remaining are found among the NEAs. We're now at the stage where we can classify binary systems.

Planetary scientists estimate that 15% of near-Earth asteroids are binaries or higher-order multiples. Most primaries are small, with diameters less than 10 km, and they rotate rapidly, usually once every two to four hours. The primaries are nearly spherical, sometimes with an equatorial ridge, and their secondaries are considerably smaller. Planetary scientists report the relative sizes of the two components as a ratio of diameters, [D.sub.s]/[D.sub.p], where "s" refers to the secondary and "p" refers to the primary. In the NEA population, the diameter ratio is usually less than 0.5, meaning the secondary is 50% the primary's diameter or smaller. The classic example of this group is 1999 [KW.sub.4], with [D.sub.s]/[D.sub.p] = 0.34.

Main-belt binaries fall into two camps. Those with primaries smaller than 10 km in diameter have properties similar to the NEA binaries and are found in similar abundances, about 15%. Those with primaries much larger than 10 km are less common--only a few percent--and most have [D.sub.s]/[D.sub.p] ratios of less than 0.1. Ida-Dactyl is a quintessential member of this group [(D.sub.s]/[D.sub.p] = 0.04). These systems have no particular pattern of shape or rotation rate.

Because Kuiper Belt objects are so far away, we can only see bodies 100 km or larger. Of those binaries discovered, the companions are almost always comparable in size to the primary. A good example is 1998 [WW.sub.31], with a 130-km primary and a 110-km secondary [(D.sub.s]/[D.sub.p] = 0.85). The asteroid 617 Patroclus/Menoetius [(D.sub.s]/[D.sub.p] = 0.92), one of only four known binaries in the Trojan population (orbiting the Sun 60[degrees] ahead of or behind Jupiter), is also this type. When there are exceptions in the outer solar system--such as Pluto's four smallest moons--they tend to be much smaller than their primary, similar to the main-belt binaries with large primaries.

About 10 to 15% of Kuiper Belt objects are binaries, but they are oddly distributed. The Kuiper Belt has three distinct populations: the "cold classical" belt, a group of objects that have nearly circular and low-inclination orbits (within a few degrees of the ecliptic plane); the "hot classical" belt, objects with near-circular orbits, but inclined more than 6[degrees] or so to the ecliptic plane (the boundary is debated); and the "scattered disk," a population with far more eccentric and inclined orbits. Oddly, nearly half the objects in the cold classical Kuiper Belt are binary, whereas the ratio in the other two groups is less than 10%.

What Does This All Mean?

1. Asteroids Are Sandbags, Not Solid Rocks.

One of the beauties of a binary is that it allows astronomers to use Newton's Law to measure a system's mass and, with a few assumptions, estimate its bulk density. Most asteroids measured have bulk densities lower than solid rock--some as low as 1 [gram/cm.sup.3], the same as water. This means that the majority of asteroids are highly porous. The current consensus is that most asteroids were completely shattered by large impacts and later reassembled into rubble piles. This finding has two consequences--one for the way in which small binary asteroids form, and another for mitigating the threat of an asteroid impact. Rubble piles, like sandbags, absorb energy differently than solid objects, so the Hollywood version of deflecting a killer asteroid by detonating a nuclear bomb on or near it might not work. We have to rethink deflection strategies (S&T: December 2010, page 22).

2. Several Different Mechanisms Make Binaries.

The earliest hypothesis for binary formation assumed most were created by impact--just like our Moon. We know impacts were common in the early solar system, and presumably they're still occurring in the main belt. When scientists attempt to model binary formation with impacts, they can readily create binaries such as the Ida-Dactyl pair, but they have difficulty reproducing the other common types, especially the NEA systems. In the rarified Kuiper Belt, the opportunities for collisions are far less frequent and impacts appear unlikely to produce the many large, equal-sized pairs. Pluto's five known moons, however, appear to be the result of an impact, perhaps a smaller-scale version of the one that formed our Moon.

Another hypothesis for binary formation invokes tidal disruption of an asteroid during a planetary close approach, somewhat analogous to Jupiter pulling apart Comet Shoemaker-Levy 9. But models suggest that this process could only form a tenth of the near-Earth binaries we see. It's also unlikely to be important in the main belt or Kuiper Belt, where encounters with large planets are rare or nonexistent.

In 2000 David Rubincam (NASA/Goddard Space Flight Center) suggested that sunlight could create small binaries. Asteroids absorb sunlight, heat up, and then radiate that heat into space as infrared light. Because asteroids are never perfect spheres, they always radiate slightly more in one direction than another, imparting a mild torque. Depending upon the initial spin direction, this torque will either slow the rotation and eventually reverse its direction, or it will speed it up so high that a rubble-pile asteroid will fission, forming a binary.

This spin-up process is called the YORP effect (for Ivan Yarkovsky, John O'Keefe, Vladimir Radzievsky, and Stephen Paddack). It works best on small asteroids; the spin-up rate is inversely proportional to the square of the asteroid size, so a 2-km-wide asteroid will spin up four times faster than a 4-km asteroid. It also takes awhile to act. Given an NEA's typical 10-million-year lifetime, it's only expected to work on asteroids smaller than 10 km. But everything we now know about the near-Earth and small main-belt binaries--their size and shape, rubble-pile nature, and rapid rotation--suggests that YORP is the main formation mechanism.

3. Some Binaries May Be Primordial.

The Kuiper Belt binaries are dominated by large, roughly equal-sized pairs. At 40 or more astronomical units from the Sun and with diameters of hundreds of kilometers, these objects are unaffected by YORP. And as noted earlier, impacts are extremely rare in this vast region of space, at least today.

So what's left? Recent solar system formation models, and the unusually high percentage of binaries found in the cold classical Kuiper Belt, suggest they are primordial. The early Kuiper Belt probably contained a few hundred times more mass than it does today and mechanisms such as gravitational capture, impact, or some hybrid process may have been far more common. The original Kuiper Belt may have assembled much closer to the Sun and been full of binaries. But the proposed early outward migration of Saturn, Uranus, and Neptune (S&T: September 2007, page 22) disrupted most of these pairings. Only the cold classical belt was left relatively unscathed and still preserves that early primordial binary proportion.

No matter how a binary forms, the story doesn't end there, particularly for those that may have been created by YORP. The angular momentum of a fissioning system undergoes wild changes and is initially unstable. Tidal stresses between a primary and its secondary become important. And it appears that another sunlight-related effect, called Binary YORP (BYORP), also comes into play. Here, the asymmetric torques affect the system as a whole, leading to three possible end states: a long-lived, stable binary system; an escape leading to two asteroids sharing the same heliocentric orbit, but not mutually orbiting each other; or a gentle collision resulting in a contact binary, probably resembling 25143 Itokawa (recently visited by Japan's Hayabusa spacecraft).

We're Just at the Beginning

The current picture is certain to be refined or even overturned as astronomers discover more binaries and planetary scientists piece it all together. But we're only 20 years into this new field, and this is one area where amateurs with modest telescopes, CCD cameras, and careful observing habits can play an enormously important role (S&T: October 2010, page 60).

Although the Kuiper Belt remains out of reach for anyone without access to meter-scale telescopes, the near-Earth, main-belt, and Jupiter Trojan populations are readily accessible to today's amateurs. A number of dedicated observers around the world collect asteroid light curves and search for new binaries (see for more information). A slew of professional papers on binary discoveries in the near-Earth and main-belt populations include amateur coauthors. Indeed, this work wouldn't be possible without them.


A binary is any separated pair of mutually orbiting objects. If two bodies are physically joined at a small point or neck, we call it a contact binary. If the center of mass of the two asteroids (the barycenter) is inside the larger asteroid, it's called the primary and the other can rightly be called a moon, moonlet, or, less romantically, a secondary. If the barycenter is outside both objects, it's more appropriate to stick with the generic terms "primary" for the larger of the pair and "secondary" for the smaller body.

RELATED ARTICLE: Three Methods of Binary Asteroid Formation

FORMATION MECHANISMS Asteroid satellites probably form by a variety of mechanisms, three of which are illustrated above. Left: A satellite can form when an impact smashes fragments off of a large asteroid. Some of these shards eventually coalesce into a small satellite. Center: A moon can form when a low-density asteroid (a rubble pile) ventures too close to a planet and is tidally shredded into many smaller fragments, which can later reassemble gravitationally into more than one body. Right: An asteroid moon can form when the YORP effect spins up a rubble pile past its breaking point, allowing a small piece to break off. Collisions and fissioning events are more common in the inner solar system. Comet Shoemaker-Levy 9 was tidally disrupted, but this process occurs infrequently. Most Kuiper Belt binaries are probably primordial.

Michael Shepard is a Professor of Geosciences at Bloomsburg University of Pennsylvania. He regularly uses the Arecibo radar system to study asteroids in the main-belt and near-Earth populations.
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Title Annotation:Mini Moons
Author:Shepard, Michael
Publication:Sky & Telescope
Geographic Code:1USA
Date:Dec 1, 2012
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