Perseid Meteor Shower 2012

.....The most reliable meteor shower of the year is the Perseid meteor shower, usually peaking on the night between August 11th and August 12th.  In younger years, this was the one time during the year that I could get the whole family as interested in astronomy as I was, a situation that I found was pretty common.  I have a lot of good memories of this shower, both growing up in the southeast, and times that I've visited my wife's family.  Now that we live farther away from our families, and will see them less often, it is good to have this as a link to the past.

..... Every dark, moonless night not dominated by city lights, we can expect to see a few shooting stars per hour, flashing randomly across the sky. These typically come from one of three sources: Leftover bits of flotsam and jetsam that have been floating around the solar system for the last five and a half billion years (cool), little bits that have been boiled off of comets as they passed around the Sun (also cool), or nuts/bolts/heat shields/tool boxes that have come off of space craft and are crashing back down to Earth (less cool).  I lived in central Florida for several years, and when I realized that probably the bulk of the meteors I was seeing from there were bits and pieces left over from launches, I admit that some of the romance died.

.....Each time a comet passes through the inner solar system, if it still has much of its original ice, that ice will boil off, taking some dust pebbles with it, and the ice will reflect sunlight, resulting in the bright coma and tail. What happens to this once the comet goes back to the outer reaches of the solar system? Nothing. That comet rubble stays in orbit, resulting in the comet's orbit eventually becoming a dusty tube of gunk around the Sun. If the Earth should pass through this gunk, then when the particles hit the Earth's atmosphere they will light up from the heat of friction generated from going from a temperature of less than three hundred degrees below zero (Fahrenheit) to thousands of degrees. Since all of these meteors are coming from the same general area in space, they will appear to come from the same general area of the sky, meaning that the meteors will all seem to radiate out from the same point. (Called, reasonably enough, the "radiant".)

.....Each August, the Earth passes through the remnant trail of the comet Swift-Tuttle, generating the Perseid meteor shower because the radiant of the meteors (the dotted circle in the image below) is in the constellation of Perseus.  Meteor showers do not require a telescope or binoculars; just go outside and look (in this case to the northeast, especially after midnight).

.....A lot of this is part of the standard run up to a meteor shower.  Happily, this year the Moon will have a very small effect on the meteors this year.  The Moon will not rise over the horizon until after 1 AM, so it will have a very small effect.  This is good.  Looking at different sources, the number of meteors per hour for the Perseid Meteor shower is usually given as a number between 60 and 120.  (Wow!)  Now let's look at that as the sky gets brighter due to the Moon.  Even if we take the most generous version, that considers that we can see all the way down to our eyes limit.  With the bright Moon, we can't.  With an interfering Moon, even if we could see down to fifth magnitude (as opposed to sixth magnitude, our limit), we would go from 120 down to about 46.  The full Moon is much more limiting than this, however.  Even if we assume a third magnitude limit, we're down to about seven.  Per hour.  With any bad or humid air, this could limit us to second magnitude (maybe three meteors an hour, if we're lucky), or first (maybe three meteors every four hours).  This year, again, we don't have the Moon to worry about.

.....Looking for Perseids can still be done in the nights leading up to Saturday night, but the peak will be pretty concentrated on Saturday night/Sunday morning.  Here is a map of the northeastern part of the sky on Saturday at about midnight.

.....Do you have any family memories of the Perseid meteor shower?

Landing on Mars

.....Late tonight, (at 10:31 PM PDT / 12:31 AM CDT / 1:31 AM EDT) the NASA Mars Science Laboratory will reach Mars, and in a period of seven minutes will attempt to go from a speed of 13,000 mph to zero.  As far as this goes, I cannot do a better job describing the challenges involved than this video.  On top of this, all of this has to be completely planned out; there will be no chance to manage any of the entry from Earth; the time that the signals take to go from the Earth to where Mars is now is fourteen minutes.  If a problem showed up in the signals coming from the spacecraft, and someone on Earth tried to respond, it would be almost half an hour until that order got to the probe.

.....Because of the time it takes a probe to go from Earth to Mars (for this trip, 253 days), we cannot track the motion of this probe by drawing a straight line from the Earth to Mars, and simply assuming the probe traveled on this path.  We must trace out a curved path from where the Earth is at launch to where Mars will be when the probe reaches Mars' orbit.  In fact, when the probe was launched, Mars was rising after midnight, just getting into position for a decent observing season.  As the Mars Science Laboratory moved through the gulf between the planets, the Earth actually passed Mars on the inside (at the beginning of March), and then on for five more months until now.  As seen in the Earth sky, Mars, Saturn, and the star Spica form a triangle low in the west just as it is getting dark.  The base of the triangle will be parallel to the horizon.  Mars will be the base "star" on the right, and Saturn will be on top.  Go out and take a look, and consider for a moment just what we as a species are attempting.

.....You may also wish to read this piece (warning! naughty words!); I will have something to say on this line tomorrow.