The Alinda Family of Asteroids

The asteroid 887 Alinda has long been known to follow an orbit that is nearly resonant with the orbital periods of both Jupiter and Earth: its orbital period of 3.915 years is close to the 1:4 Earth resonance and close to the 3:1 resonance with Jupiter.  In recent years the rate of discovery of previously unknown asteroids has been enormous, with thousands of new asteroid discoveries each year, so it is not surprising that a number of other Alindas have been found.  Membership in this family requires an orbital period very close to 4 Earth years, which in turn requires that the mean distance from the Sun (the orbital semi-major axis) must be close to 2.54 AU.  That places these bodies in the inner asteroid belt—except for the excursions brought about by the eccentricities of their orbits. 
Orbits close to a Jupiter resonance are not only subjected to the gravitational perturbations exerted by Jupiter on all asteroids, but experience repeated perturbations with the same approximate geometry.  This allows, like the resonant pumping of a child on a swing, a constant buildup of self-reinforcing disturbances, which cause a constant growth in the eccentricity of the asteroid’s orbit, making an ever more elongated ellipse. Eventually, this growth in eccentricity imperils the asteroid by extending its orbit inward to perihelion distances ever closer to the Sun, crossing the orbits of one or more of the terrestrial planets, while also stretching the orbit outward so that its aphelion distance can approach Jupiter.  Close encounters with any planet can seriously disturb an asteroid’s orbit; the closest encounters, resulting in collisions, are fatal to the asteroid and may be seriously disruptive to the target planet.
The 23 Alindas now known include eleven in low-eccentricity orbits (e ranging from about 0.30 to 0.34).  These bodies roam the reaches of the Solar System from about 1.7 to 3.4 AU from the Sun, spending most of their time in the asteroid belt and never approaching any planet closely. They are the "young" Alindas, recently nudged into resonant orbits.  In such orbits their resonant relationship to Jupiter causes their orbits over time to gradually become more eccentric.  They are not in immediate danger except for the small probability of colliding with other asteroids, but they are in for serious trouble in the long run.
Three of the known Alindas (6318 Cronkite, 8709 Kadlu, and 6322 1991 CQ) have orbital eccentricities between 0.465 and 0.475, sufficient to have them cross the orbit of Mars.  These three Alinda Mars-crossers do not cross the orbit of any other planet; Mars has a small mass and cross-section area, and cannot remove these bodies as rapidly as Jupiter can replenish them and move them on to even more eccentric orbits.
Then there is the namesake of the family, 887 Alinda itself, with an eccentricity of 0.564.  Its perihelion distance (q) of 1.084 AU qualifies it as a near-Earth asteroid (NEAs by definition have q < 1.300 AU).  It grazes but does not cross Earth’s orbit, making it an Amor asteroid as well as an Alinda family member.
Even more pumped-up Alinda clan members include eight (with eccentricities between 0.57 and 0.75) that cross Earth’s orbit: at perihelion they are closer to the Sun than Earth is at aphelion, 1.017 AU.  They are therefore Apollo-family NEAs as well as Alindas.  Since all Alindas are Earth-resonant, they may fly by Earth repeatedly at close range at 4-year intervals for decades at a time, affording radar observation and spacecraft launch opportunities—and collision opportunities—over that time period.  One such asteroid is 4179 Toutatis, which was the target of a close flyby by the Chinese Chang-e 2 spacecraft in 2013.  Two members of this group, 7092 Cadmus and 8201 1994 AH2, could be termed Venus-grazers, having perihelia inside 0.76 AU.  The most eccentric of the Alindas is 3360 Syrinx, a Venus-crosser with e = 0.743.  Its orbit makes six crossings of planetary orbits every four years (twice each for Mars, Earth, and Venus), a highly unstable situation that suggests a short life expectancy.  Interestingly, all three of these most-eccentric Alindas have aphelia close to 4.3 AU.  None of the Alindas approach Jupiter closely, a wise precaution.  A close encounter with Jupiter could swallow the asteroid whole, kick it out of the Solar System permanently, or wreak other orbital havoc.
The Alindas serve as a reminder of the role Jupiter plays in sending hazardous bodies toward us; a fringe benefit is the opportunity to have many repeated launch opportunities to a given asteroid.  The Alindas are loose cannons, subject to disturbance by Jupiter, Mars, Earth, and Venus.  These asteroids are both carrot and stick, guaranteeing that we will hear a lot more about them in the future--such as when Toutatis comes by again in 2016! 

Extraordinary Near-Earth Asteroids I: 2014 PP69

April 2015

 We presently know of about 13,000 Near-Earth Asteroids, including nearly 1000 that are larger than 1 kilometer in diameter.  Typical NEAs range from Earth’s general vicinity out to the heart of the Asteroid Belt on each orbit around the Sun.  Their orbits typically have inclinations (relative to the plane of the Solar System) of 10 to 30 degrees, eccentricities of 0.2 to 0.6, and orbital periods of about 2 to 4 years.  The mean distance of any NEA from the Sun is usually near 2 AU.  But the NEAs are a wildly diverse collection of bodies that originated at widely separated locations in the Solar System.  The outliers of this population include some truly remarkable nonconformists.  One such asteroid is 2014 PP69. 

You will recall that the first five characters in an asteroid’s name tell us when it was discovered, in this case in 2014 in the second half of July.  This provisional name will be used until there is a long enough history of accurate tracking (usually at least one full synodic period, the time needed to “lap” Earth in its orbit around the Sun), to certify a precise, accurately predictable, orbit.  The synodic period is about 2 years for most NEAs.  At that time the asteroid will be given a catalog number such as 155629, at which point it will be referred to as 155629 2014 PP69.  Once an asteroid has been cataloged the discoverer may propose a name for it, such as Eros or Ceres; let’s call this one Egbert.  Then it will be called 155629 Egbert; just plain Egbert to its friends.  But the object of this post is just plain 2014 PP69: in the nine months since its discovery there has been no opportunity for it to pass by Earth again, and therefore no chance to assign it a very precise orbit and enter it into the catalog of numbered asteroids.  Once the refined orbit is determined, the discoverer of the asteroid gets to give it a name.

So here’s what’s unusual about 2014 PP69: its perihelion distance of 1.25 AU, which qualifies it as an Amor asteroid, contrasts sharply with its aphelion distance of 41.79 AU, well outside the orbits of Neptune and Pluto and well into the Kuiper Belt.  Its orbital period is an incredible 99.84 years, longer than that of Halley’s Comet.  But that’s not all: the inclination of its orbit is 93.63 degrees, meaning that it orbits almost at right angles to the plane of the Solar System—in fact, the orbit is slightly retrograde, moving around the Sun in a direction opposite to that followed by the planets.  The eccentricity of its orbit is 0.942, higher than that of the typical short-period comet.  At perihelion, closer to Mars’ orbit than to Earth’s, it is traveling at a whopping 40 kilometers per second.

What do we know about the asteroid itself?  Almost nothing.  The discovery images show that it has a visual (H) magnitude of 20.17, which, by the crude “rule of thumb” used for newly discovered NEAs (an assumed average albedo of 0.14; 14% reflectivity in visible light) corresponds to a diameter of about 330 meters.  However, the orbit is cometary, suggesting that a more realistic albedo would about 0.035.  If it’s that bright and that black, then its cross-section area must be four times as large, and its diameter twice as large, as this crude guess would suggest.  That implies eight times the volume and about eight times the mass, raising the question of its impact hazard.  The good news is that, despite its large size and kinetic energy, the point at which it crosses the plane of Earth’s orbit is far outside our neighborhood. 

The body is almost certainly of cometary composition, similar to the Centaurs and the Kuiper Belt bodies and to short-period comets.  A reasonable guess would be that it is about 60% by mass ices and about 40% rock, which in turn contains perhaps 5-10% of organic matter, mostly complex polymers.

Sending a spacecraft to visit 2014 PP69 would be extremely difficult because of its very high relative velocity.  And then there is the problem that the next optimal launch opportunity is a century off.

How soon will 2014 PP69 qualify for a catalog number?  On its next pass through the inner Solar System we will have an opportunity to track it again with such a long span of observations (a century!) that a very accurate orbit can be calculated.  That will be in the year 2114.  The bad news is that the discoverer will no longer be alive to exercise the option of naming his baby!

“The Size of Nine Ocean Liners”: Asteroid 1998 QE2

The sky is falling!  And it’s full of ocean liners!

On 30 May 2013 the press reverberated with the news that the Near-Earth Asteroid 1998 QE2, a monster “the size of nine ocean liners”, was going to sail by Earth (without docking).  This is a terrifying image: picture nine ocean liners falling from the sky!  What a splash that would make!  And some sources say it is long as nine ocean liners, and some say the width of nine ocean liners, and some say as big as nine ocean liners…well, whatever.

Of course the comparison of an asteroid with an ocean liner was motivated by the asteroid’s moniker (QE2), and should be understood as poetic license, or at least as unlicensed poeticism.  But is an asteroid of this size really a big threat?  True, it missed us by 5.8 million kilometers THIS time, but its orbit will bring it back across Earth’s orbit over and over again.  We can’t guarantee we will always be so lucky! 

The realists among us will appreciate that the news media routinely push the envelope of truth in order to generate scary headlines.  This is usually done by means of liberal use of adjectives (huge, gigantic, etc.; strangely I haven’t seen anyone refer to it as Titanic) rather than numerical facts.  So, assuming we know about the existence and usefulness of numbers, does QE2 represent a real threat?  Good question… and thanks for asking!

1998 QE2 is in fact about 1.7 miles in diameter, a pretty decent piece of rock, and has a 2000-foot satellite in orbit around it.  Now, let’s see: 1.7 miles is about 2700 meters in diameter, or a radius of 1350 meters and a volume of 10.3 billion cubic meters.  At an average meteorite or asteroid density of 3 tonnes per cubic meter, this is 31 billion tonnes.  Now let’s compare it to a big ocean liner—for example the Queen Elizabeth II:  displacement 44,000 tonnes.  So, by the difficult mathematical operation known as long division, we see that this asteroid (never mind its satellite) would deliver a mass equivalent to 700,000 ocean liners onto our unsuspecting heads.  WHAT?  The sensationalist media have underplayed the story by a factor of 80,000?

Why the disconnect?  Because of the vague use of the word “size”.  Some people use it to mean length, or area, or volume, or mass.  The length of an asteroid tells little about the size of a threat it presents—unless, of course, we know how to do arithmetic. 

But the real measure of its potential for wreaking havoc is its total kinetic energy.  Let’s take an impact speed of 16 kilometers per second as an example (many NEAs are moving a lot faster than that).  That puts it in the million-megaton  (1 teraton) league.  That would be comparable to 100 World War IIIs. 

Nine ocean liners?  Get real!