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By Juan Ivaldi, March 11, 2012 9:19 pm

In case you are wondering, I am referring to the planets Venus and Jupiter on the 11^th through the 15^th of March, 2012.   They are absolutely spectacular in the western sky just after the Sun has set. To see them, go outside after sunset but before the sky is fully dark and look up in the general direction of where the sun has set (west).  If skies are clear, you simply cannot miss the two bright planets near each other in the sky.

Venus is the brighter of the two.  The pair are so bright that they are easily spotted during twilight so don’t wait until dark. After sunset, Venus and Jupiter appear as brilliant diamonds against a velvet blue twilight sky.  They are putting on a dramatic show night after night as their positions can be seen to change.  This is one of those perfect times to share the sky with kids.

Over the next three days, Jupiter will appear to descend closer to the horizon as Venus rises higher above the horizon.  So, the two planets will pass each other.  The closest they get is about 3 degrees on the 12^th of March 2012.  This is about the same separation in the sky as 6 full moons end to end.  That may sound like a lot but when you see it for yourself, you may be amazed how close Venus and Jupiter appear.  If weather permits, keep observing the pair over successive nights to see the apparent motion.

Despite the fact that the pair are getting cozy in the sky, Venus and Jupiter are in fact hundreds of millions of miles apart.  Jupiter is the fifth planet from the Sun and Venus is the second.  Earth is the third planet so Jupiter is on an outside track (relative to us) and Venus is on the inside track as the planets orbit the Sun.  Jupiter is much farther away from us, but its huge size makes up for that and it still seems relatively bright.

In scientific terms, the alignment we see is called a planetary conjunction.  This happens when two planets have similar right ascension.  Alignments like these have happened before and will continue to happen because all the planets tend to stay close to a line in the sky called the ecliptic.  This is the line traced out by the motion of the Sun.  Eventually, two (and sometimes more) planets will appear in the same part of the sky.  As such, these conjunctions have no real significance other than the fact that they are marvelous to see.  This will be one of the better planetary conjunctions for some time so go out and see the dazzling pair in west.  All you will need are your eyes and clear skies to the west!

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By Juan Ivaldi, January 3, 2010 5:11 pm

Quick answer: It is still out there and continues to orbit the Sun as it has for eons.

Pluto is a distant and frozen world orbiting the Sun once every 249 years at an average distance of 3.7 billion miles away. To the best of our knowledge, it continues to be a frigid ball of rock and ice orbiting the Sun in the same way it has done for eons.   Since Pluto’s discovery in 1930, our understanding of the solar system has progressed significantly and many more outer solar system bodies have been discovered. Despite this increase in knowledge, much remains to be learned about the icy world we call Pluto.

Pluto’s largest moon Charon was discovered in 1978. The precise sizes of Pluto and Charon only became known in the late 1980’s.  In 2005, two additional and much smaller moons of Pluto were imaged with the Hubble Space Telescope.  These newly discovered moons received the names Nix and Hydra (respectively in order of their distance away from Pluto).

Pluto shares characteristics of other solar system objects orbiting in a broad flat ring of rock-and-ice bodies just beyond the orbit of Neptune known as the Kuiper Belt.  Named after astronomer Gerard Kuiper, the Kuiper Belt is home to the left over icy debris from an early time in the formation of the solar system.  Although Kuiper proposed the existence of this belt in the 1950s, it wasn’t until the 1990s that Kuiper Belt objects (KBOs) were discovered.  At the time of this writing, there are 1100 known KBOs.  For the vast majority of these, only the positions and orbits are known.

Because of their great distance away from the Sun, KBOs are very cold and retain the simple molecules of the solar system in the form of ices on their outer layers. Depending on the amount and type of ice on the surfaces, the objects can appear more or less bright in telescopes.  Pluto and the KBOs are so distant that the images appear as fuzzy dots or blobs even with the most powerful of telescopes. The best photographs of Pluto taken by the Hubble Space Telescope now show that the surface has light and dark markings which change seasonally.  Scientists think this is caused by ice which sublimates into a gas, leaving one location only to refreeze elsewhere on the surface.

Some astronomers believe that Pluto is the king of the KBOs, that is, it is among the largest, brightest, and nearest of them. Based on its density, Pluto is estimated to be composed of 70% rock and 30% water ice.  For comparison, water is much less than 1% of the total composition of Earth.

Another trait that Pluto has in common with other KBOs is a highly tilted and oblong orbit compared to the major planets. Size is yet another differentiator.  Although the largest KBOs are considered to have a significant amount of rock in their composition, they are generally much smaller in size compared to the rocky inner planets of the solar system, Mercury, Venus, Earth, and Mars.  The smallest of these four is Mercury.  Pluto’s diameter is slightly less than half that of Mercury.

In the past decade, planet-like objects similar in size to Pluto have been discovered in the Kuiper Belt. The largest of these, later named Eris, was discovered by the team of Michael Brown, Chad Trujillo, and David Rabinowitz in 2005.  Eris, with its single moon Dysnomia, lies further away from the Sun on average compared to Pluto. Measurements of the diameter of Eris show that it is slightly smaller than Pluto but not by much.  A handful of other objects have been found in a similar size range but no KBOs larger than Pluto have been discovered so far.

After its discovery in 2005, the community of astronomers and space scientists had great difficulty with the classification of Eris.  Astronomers became caught in a name game that turned out to be a long, controversial, and mostly empty debate about the definition of the word “planet”.  Does Eris become a tenth planet with potentially more planets joining the ranks or is it in a different class?  If Eris is in a different class then does this have implications for the classification of Pluto?

In 2006, the International Astronomical Union (IAU), an international society of astronomers, created a new classification called “dwarf planet”.  According to the new convention, the word “planet” is reserved for the 8 largest Sun-orbiting bodies of the solar system.  Pluto and Eris were assigned into this new dwarf planet classification by virtue of a vote by society members at an IAU meeting. Pluto was thereby downgraded and Eris never made it to full planet status.

The controversial vote received intense media attention and caused confusion in the general public. For now, many researchers have adopted the IAU nomenclature. Not surprisingly, the debate still rages on among astronomers and non-astronomers alike.  Perhaps it illustrates that we still have a long way to go in our understanding of the solar system.  Regardless of the naming convention, most would agree that Pluto and its cousins in the Kuiper Belt are important members of our solar system and greater study is needed.

Although no spacecraft has ever visited Pluto, one is on the way right now.  The NASA New Horizons mission spacecraft has already passed the half-way point and is on schedule to arrive at Pluto in 2015.  The purpose of the mission is to make measurements of the chemical composition of the surface and the extremely thin nitrogen atmosphere of Pluto.  NASA scientists also plan to take high quality photographs of the surface of Pluto and Charon to better understand the nature of the ice and the surface topography.  They will also look for other moons beyond the three that are currently known to orbit the remote icy world.  After visiting the Pluto system, New Horizons will move on to other targets in the Kuiper Belt.

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By Juan Ivaldi, November 1, 2009 11:23 pm

Quick answer:  Yes, this has happened many times over the history of the Earth.  It will happen again but no one knows when.  Fortunately, such big events are extremely rare and unlikely to occur in our lifetimes.

Earth is about 4.6 billion years old.  Throughout this enormous stretch of time, Earth has been impacted by countless solar system objects.  Fortunately, in recent Earth history, major impact events occur many millions of years apart.  So the chance of any of us observing one of these big impacts is incredibly small.  However, much smaller less significant impacts happen more frequently.

There is plenty of evidence for past impacts, although weather and active geology on planet Earth erases most of the obvious impact remains.  This is the reason why impact craters are so rare on the surface of the Earth.  The Moon, by contrast, lacks geological activity and weather.  The long impact history is therefore recorded in the cratered lunar surface for all to see with the aid of a telescope or binoculars.

Among the most famous reminders that impacts can still occur on Earth, is Meteor Crater in Arizona.  This crater is believed to have been formed by the impact of an asteroid about 80 feet across.  The impact is estimated to have occurred around 20 to 50 thousand years ago.  Early human beings were walking the Earth at that time and it is quite possible that early humans witnessed the consequences of this impact event .

To understand where these impacting bodies come from, it is important to review the history of formation of the solar system.  Between 4.6 and 3.8 billion years ago, when the solar system was forming, impacts occurred at a very heavy rate.  This period of heavy bombardment happened as material in the solar nebula clumped into bigger pieces and ultimately impacted the young planets and their moons.  As the planets grew larger the strength of their gravity fields became larger and they swept up ever more of this solar system debris.  Over time, the orbital paths of the planets were cleared out and the rate of impacts in the inner solar system reduced significantly.

Today, there is plenty of left over material from that early time which did not get swept up but stayed in stable orbits around the Sun.  The most important of these debris bands are the asteroid belt, which lies between the orbits of Mars and Jupiter, and the Kuiper belt which lies beyond the orbit of Neptune.

The asteroid belt is composed mostly of rocky bodies also called minor planets.  Because of the greater distance away from the Sun, the Kuiper belt is the cold home of millions of dirty ice balls.  Occasionally one of these Kuiper belt objects gets flung toward the Sun to become a comet.  According to the International Astronomical Union, over 60 million minor planets and comets have been observed in the solar system.

Astronomers using powerful telescopes and cameras are keeping an eye on a subset of about 6000 objects which are on a path of close approach to planet Earth.  They are called Near Earth Objects (NEOs).  About 1000 of these are on a special list of potentially hazardous objects.  At present, one asteroid named Apophis is due to make a close approach on April 13, 2029.  Apophis is predicted to fly by the Earth at a safe distance of about one Earth diameter away.

In 1994 the world observed Comet Shoemaker-Levy 9 slam into Jupiter creating massive impact scars in the atmosphere of the great planet which lasted for months.  These scars were large enough to be visible in amateur telescopes.  The event caused great excitement in the scientific community since no human had previously predicted and subsequently witnessed the collision of two solar system objects.   There was a flood of media coverage for the event.  A wealth of scientific data was collected but the famous collision was a grim reminder to the human race that violent impacts can still happen in the solar system today.

Impacts of a less spectacular scale happen more regularly.  The Tunguska blast which happened over Siberia on June 30, 1908 is believed to have been caused by an asteroid or comet that entered Earth’s atmosphere.  The object exploded high above the ground and never had a chance to impact.  However, the explosion blew down trees for hundreds of miles around the point of entry and carried the sound of the blast even further away.

A more recent event occurred on October 7, 2008 when a boulder sized asteroid named 2008 TC3 was tracked and predicted to enter Earth’s atmosphere over Africa.  It followed the predicted path, entered the skies over Sudan, and disintegrated high in the atmosphere.  In a rare find, a university team recovered fragments of the broken up asteroid.  Scientists are now studying these fragments for clues about the formation of the solar system.  This is a unique and rare opportunity to directly examine pieces of the left over building blocks which formed our solar system 4.6 billion years ago.