Update: January 17, 2007

THE ALIEN IS HERE!

By Pam Eastlick for THE DEEP on line

Welcome to The Deep science and technology column where we cover topics from the deep sea to deep space and beyond.

If the weather cooperates (NEVER a given here!) you just might have the chance to see the comet of the century this week.  Of course, this isn’t much of an accolade since we haven’t had any visible comets in this century but this particular comet just may outshine the bright comets of the last century as well.  We all remember Comet Hale-Bopp, which was so bright in Guam’s skies, and even Comet Kahoutek, which was a bust in the brightness department, but had the distinction of being so close to Earth as comets go, that you could actually see it moving against the background of fixed stars.

The new comet is called Comet McNaught (comets are usually named for the people who discover them) and it already has the distinction of being the brightest comet for the last 40 years.  It rounded the Sun over the weekend and although it’s no longer visible to viewers at the latitude of North America, we should see it just fine from Guam.  It may even become so bright that it’s visible in the daytime!

Comet McNaught over the skies of Germany.  Photo by Jens Hackman

Start looking tonight around 6:30 p.m.  Go out and watch the sunset and you’ll see a VERY bright star above the western horizon.  That’s not a star; it’s the planet Venus.  Now you’re going to need your precision celestial distance measuring instrument and the good news is that you’ve all got one.  Just extend your arm straight out and make a fist.  Your fist spans about 10 degrees of sky and it makes no difference if you’re five or fifty.  If you’re five, your fist is smaller, but your arm is shorter.

Tuck your thumb into your fist and turn your hand so the ‘thumb side’ is pointing up.  Then sight down your arm and put Venus right at the top of your fist.  Mark the bottom of your fist with your other hand.  You’ve just measured 10 degrees below Venus.  Now, turn your fist so the ‘thumb side’ points left and measure two fist-widths to the left.  You should be able to spot the comet in that area.

If it’s cloudy tonight, try later in the week but be aware that the comet will be moving and since it’s close to the Sun; it will be moving quite rapidly.  It will be climbing higher above the horizon every night and moving to the left (toward the south).  By this Friday, it will be at about the same altitude above the horizon as Venus and will be roughly a fist-width (10 degrees) father to the left.

Comet McNaught rounding the Sun.  This picture was taken yesterday morning at 9:54 a.m. Guam time the time is given in UTC) by the SOHO spacecraft.  The circle at the center of the image is blocking the Sun’s disc.  The bright spot near the comet’s head is the planet Mercury. So . . . just what is this thing you’re looking at, and why did I call an alien? Throughout all of human history until about a hundred years ago, the entertainment options after sunset were limited.  There was always conversation by lamp and candlelight and tool making and storytelling were also possibilities.  But a big part of the evening’s entertainment, particularly in nice weather was the sky.

Stories were told about the pictures formed by the random patterns of stars, and the changing moon and those star pictures were important seasonal markers.  Our ancestors knew when to plant, when to fish, when to harvest and when to celebrate by watching the sky.  Star patterns appeared at the same time every year, the moon changed phases endlessly and predictably and for almost everybody, the sky was an old familiar friend.

But occasionally, at totally unpredictable intervals, new and terrifying things appeared in the sky.  They moved slowly like pale flaming torches among the stars and they were almost universally regarded as portents of war, destruction, doom and death.  Today we call them comets.  Just what are these apparitions of doom?  Where do they come from, and what does their appearance really mean?
Scientific research over the years, including some up close and personal visits, has revealed that comets are in fact great big, dirty icebergs.  They're made mostly of frozen water and carbon dioxide along with other substances like methane, nitrogen and carbon.  There are also rocks, dirt and dust frozen into the comet’s body.

There are millions of these clumps of rock, ice and snow in a spherical cloud that completely surrounds our solar system.  It’s called the Oort cloud after Jan Oort the Dutch astronomer who first proposed it in 1950.  The inner edge of the Oort Cloud is far beyond Neptune and the outer edge is probably a quarter of the way to the closest star, a very long distance indeed.  The sun’s gravity at that distance is tenuous at best and almost anything can disturb these icy lumps.  They may collide with each other or fall prey to the faint gravity tug of another star.

Their collisions may send them out of the Oort Cloud altogether into the endless reaches of interstellar space but often they are nudged slightly closer to the sun.  When this happens, they begin to feel the pull of the sun’s gravity more on one side than the other.  This minor tug starts these deep space denizens on an astounding journey that will eventually transform them from icy, uninteresting rocks to the largest objects in the solar system.

That’s the typical story for a typical comet; but Comet McNaught is NOT typical.  An Oort Cloud comet will leisurely fall toward the Sun at an ever-accelerating pace until the Sun’s gravity sends the temporary visitor hurtling back out to the Oort Cloud again.  These comets have very long oval orbits that are called parabolas and they will come back.  In other words, all Oort Cloud comets are periodic and return to the inner solar system at predictable intervals.  The most famous comet is Comet Halley which has a very short period for a comet; only 76 years.  The periods of most comets are measured in thousands of years.

But Comet McNaught doesn’t have a closed parabolic orbit; its orbit is a hyperbole, which means that it’s open-ended.  Comet McNaught is NOT an Oort Cloud comet.  This particular piece of space debris has come to visit from deep interstellar space.  Comet McNaught has whipped around the Sun at about 100,000 mph and is headed for the stars again.  Its trajectory has been significantly changed by its encounter with our star and it’s off to explore the galaxy on a totally new heading.

Hyperbolic comets are so vanishingly rare that we don’t even have a good idea about how often they wander through.  Only one or two other comets have ever entered the solar system on a hyperbolic trajectory.

So go out this week and have a look at a solar system visitor you will never see again.  Take your children.  It’s literally the opportunity of a lifetime!