Comments (0)

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!

Comments (0)

By Juan Ivaldi, August 10, 2011 6:57 am

Imagine an impact between the Earth and a tiny rock the size of a grain of sand.  The outcome of such an impact is one of the most exciting sights in the night sky, a shooting star.  The accepted term for a shooting star is a meteor.  When Earth passes through a stream of such particles in space, it produces what is known as a meteor shower.  One of the best observed of such showers is the annual Perseid meteor shower which will peak on the 13^th of August in 2011.  Following up on my last post, this meteor shower is a great opportunity to share the sky with kids.

The Perseid meteor shower is an annual shower occurring in mid-August when the Earth plows through the debris stream left behind by comet Swift-Temple.  The reason for the name is that the meteors from this shower appear to come from the constellation Perseus.  According to NASA, the little bits of comet dust slam into Earth’s upper atmosphere at 140,000 miles per hour.  At this high velocity, the dust burns up as it enters the atmosphere and the heat released produces a beautiful glowing streak in the sky as the particle vaporizes.  During a meteor shower such as the Perseid shower, you can see many meteors in a short time span.

One of the great things about observing meteor showers is that you don’t need any equipment to view them.  Just go outside and look up.  You are likely to see dozens of meteors per hour near the peak time of the shower.  This year the full Moon will be in the sky at the time of the Perseid shower.  The Moon’s glare will make it harder to see the fainter meteors, but you are likely to still easily catch the brighter fireballs.  These fireballs are memorable when they happen and are caused by the larger pieces of comet debris entering our atmosphere.  Some of these are bright enough to cast a shadow.

Perseid meteors will start appearing as soon as Perseus rises above the eastern horizon around 10pm and continue until sunrise.  The peak meteor activity is expected on the night of Friday, the 12^th of August through early Saturday morning (13^th).  Because the debris stream is wide, meteors can be observed on the days around the peak as well.  In fact, the meteor shower has already started as of the time of this post.  So, take the opportunity to go outside, share the sky with your child, and enjoy the meteors!

In summary:

What dates to look:  Peak Perseid meteor activity is on the 12-13^th of August.  Meteors can be seen from 10 August to 15 August.

What time to look:  You can start looking after 10pm.  More meteors are visible in the hours just before dawn.

Where to look:  Anywhere in the sky is good.  Once Perseus is above the horizon, meteors can appear anywhere so just look up.

Observing tip:  If you can, find a place where you can stand in a moon shadow and still see a reasonable expanse of sky.  Perhaps you can use a row of trees, your house, or a building to shadow the Moon.  This trick will help you to see more meteors despite the Moon glare.

Most importantly:  Have fun!

Comments (0)

By Juan Ivaldi, July 18, 2011 8:12 pm

Children and adults alike repeatedly demonstrate their wonder about the Sun, the Moon, and the stars by asking me a wealth of interesting questions at the public astronomy programs where I volunteer my services as an amateur astronomer and sky guide.  It never ceases to amaze me when I see how naturally inquisitive children are and how they love to discover Nature and the Universe.  It makes me think that our fascination with the sky is built into our DNA and is probably as ancient as the human species itself.

During astronomy programs, I interact with many parents who see the discovery moments their children experience and they want to recreate similar moments at home.  Parents often ask me about activities they can do to continue to stimulate their child’s interest.  If you have similar questions, then here are five easy things you can do with the sky to generate family fun time outdoors and feed your child’s innate fascination with Nature.

1)      Make a human sundial.  You will need a sunny day, a piece of chalk if you have a sidewalk or a paved driveway, or a stick (or a rock) if you are in a field.  Have a child stand on the ground straight and tall and face in the direction of their shadow and stand still.  Mark the location they are standing by tracing their feet or place a stone at the center of where they are standing.  Then trace the shadow of their head and shoulders or place a stick to mark one edge of their shadow of their head.  If a sibling or playmate is available, have them do the tracing.  The child can now step away from their spot.  Wait 10-15 minutes and have the child return to their exact position and again face their shadow.  Now retrace their head and shoulders or place a second stick where the new location of their shadow is.  The shadow has moved!  You can repeat this as many times as you like.  This demonstrates the apparent motion of the Sun in the sky.  Of course it is not the Sun which is moving, but the Earth rotating on its axis which makes the shadow change.  This same rotation makes the Sun rise and set each day and is the phenomenon upon which actual sundials are based on.

2)      What goes around the Earth in just over 27 days?  The Moon of course.  This is a great way to talk about the duration of a lunar cycle, the basic phases of the Moon, and why a month is approximately one lunar cycle.  It is no coincidence that the word “month” comes from the word “moon”.  While looking at the Moon with your child you can share the concept that the Moon is illuminated by sunlight just like the Earth is during the day.  As it makes the trip around the Earth, the Moon position and the illumination angle change together.  The Moon’s orbiting motion is why we see the phases change night after night.

3)      Ask your child this question: “What is the Moon made of?”  No, it is not made of cheese!  You’ll see amazement in your child’s face when you pick up some dirt and some rocks at your feet and place this material in your child’s hands.  They will immediately understand that the Earth and the Moon were formed from the same stuff.  The Earth and the Moon are members of the family of the Sun which means they were formed out of the same starting material roughly 4.6 billion years ago.

4)      Find the brightest star in the night sky.  This is a fun question to ask to stimulate scanning the night sky and hunting down a star.  It is also a perfect time to explain that the points of light in the night sky are brilliant fireballs just like our Sun only very much farther away.  Even the brightest stars in the night sky are extremely far away.  The closest star to our Sun is about 25 trillion miles away (a trillion is the same as a million millions).

5)      Locate one constellation and try to see its shape.  This is an excellent way to spend a little time and acquaint yourself and your child with one part of the sky.  Use a map of the night sky to teach the skill of reading a star chart and comparing that to what is actually up there.  Most kids can recognize the constellations of the Big Dipper (Ursa Major) and Orion.  The shapes are distinct and memorable.

To learn what constellations and stars are in the sky tonight, consult any of the online sky charts available for free on the web.  These provide star names, constellations and other information about the sky.  Here is a short list of links:

http://Skymaps.com My personal favorite

http://www.kidsastronomy.com/astroskymap/constellations.htm A simple skymap

http://www.stellarium.org/ Free planetarium software for your computer

Have fun and enjoy the sky!

Comments (0)

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.

Comments (0)

By Juan Ivaldi, December 3, 2009 11:01 pm

Quick answer: The planets all orbit around the Sun in the same direction because they retain the rotation of the original cloud of gas and dust from which they formed.

If you had a spaceship and were able to blast off from the North Pole of the Earth and rise a few hundred million miles above the plane of the solar system, you could look down on the planets.  You would be able to see that the planets are all orbiting in a counterclockwise direction around the Sun.  This is no coincidence and neither is the fact that the orbits of the planets lie in nearly the same plane.  Yet another clue is that the equator of our spinning Sun is well aligned with the plane of the orbiting planets.

These facts were known to scientists in the 1700’s.  In the later part of that century, Immanuel Kant and Pierre-Simon de Laplace began to conclude that the situation could not have arisen by chance.  They proposed that the planets evolved out of a primordial whirling disk.  This helps explain the observations.  Astronomers today agree that the basic idea of Kant and Laplace is correct although their ideas needed refinement.

The modern version of their idea is known as the nebular hypothesis.  It is generally accepted by astronomers as the preferred description for how the solar system formed.  According to the nebular hypothesis, the solar system began roughly 4.6 billion years ago as an immense rotating cloud of gas and dust called the solar nebula.    Our Sun was born from the gas in the core of this protostellar cloud.  As material was pulled inward by the gravity, the speed of rotation increased.  The increased rotation rate caused the cloud to begin to flatten out into a disk shape.

Every planet started out as a grain of dust within the disk.  Dust particles began to stick to one another by electrostatic attraction.  As the clumps grew bigger they started to have larger and larger gravitational fields.  Gravity attracted the larger chunks to one another causing collisions.  The collisions became more numerous.  The gentler collisions resulted in objects staying stuck together into bigger objects whereas the higher speed collisions caused destruction.  The bigger objects, called planetesimals tended to survive since these were harder to destroy depending on the intensity of the impacts.

Over time the planetesimals aggregated into protoplanets.  These were big enough to sweep out their orbits.  The protoplanets then underwent a period of massive and violent mergers.  Ultimately the greatest survivors of this process became the known planets of the solar system.  Getting to this point is believed to have taken many millions of years.  A large amount of leftover building materials was still flying around the solar system causing heavy bombardment of the planets.  Today we see the evidence of these bombardments as craters.  Some of that left over material is still around in the solar system.  Although impacts still occur today, the really big collision events are much more rare.

The nebular hypothesis explains why the planetary orbits are all going counterclockwise as viewed from the north and why the Sun has a counterclockwise rotation on its axis.  The original rotation direction of the solar nebula is still here today billions of years after the Sun began to shine and the planets emerged from their beginnings as tiny grains of dust in a whirling cloud.

Comments (0)

By Juan Ivaldi, November 29, 2009 7:24 pm

Quick answer: What we see as the lunar phase is the portion of the Moon which appears illuminated by sunlight.  The angle of illumination changes as the Moon orbits around the Earth, thereby changing the phase.

The Moon is about 240 thousand miles away from the Earth.  As the Moon orbits around the Earth, we see a complete cycle of phases taking place every 29½ days.  It is easy to understand why the word “month” and the word “moon” have a common root.  The lunar cycle has been recognized since the time of ancient civilizations and became an important element in the construction of calendars.

When we look at the Moon, we can observe that the portion which appears illuminated changes throughout the course of a month.  As the Moon orbits the Earth, the angle of the Sun’s illumination changes and more or less of the Moon appears to be lit up.  The progression has a familiar pattern which repeats in nearly the same way each cycle.

At the very beginning of each lunar cycle, the Moon is new which means it appears relatively near the Sun in the sky.  During the new phase the Moon is not visible since the part of the Moon facing the Earth is in shade.  About two weeks after the new phase, the Moon appears in full phase in the sky.  During the full phase the Moon appears located opposite the Sun in the sky. This is why a full moon can often be seen rising in the east just as the Sun sets in the west. A full Moon appears as a fully round disk since the portion facing us is completely illuminated.

The other two important phases are called “quarters” which fall exactly midway between the full and new phases.  During a first quarter moon the disk of the Moon appears half illuminated.  The word “quarter” reminds us that the Moon is one quarter of the way through its cycle from either the new or full phase.  Observers may sometimes get confused by the fact that during quarter phases the moon appears half lit.  To avoid confusion it helps to remember that the full moon phase is the halfway point in the lunar cycle.

By observing the Moon on different days we can see the combined effect of the lunar orbit and the changing illumination.  The orbital motion of the Moon causes changes in where we see it in the sky and at what times.  That same motion is what changes the illumination angle of the sunlight from our viewpoint.  So the lunar phase and position are intimately linked.

A common misconception is that the shadow of the Earth cast on the lunar surface causes the phases of the Moon. Of course this is not true at all.  On rare occasions the shadow of the Earth does fall on the Moon.   This is a special and beautiful occurrence known as a lunar eclipse. Lunar eclipses are a completely different phenomenon from the regular observation of the lunar phase.

Comments (0)

By Juan Ivaldi, November 15, 2009 1:38 am

Quick answer: The seasons occur because of the tilt of the Earth’s axis of rotation.

Each year the Earth travels through space orbiting around the Sun.  As it does so we observe the passing of the seasons.  Although the amount of variation in the seasons depends on where we live, the trends are the same.

During summer the length of the day exceeds 12 hours.  So in summer, nighttime is shorter than daytime.  During summer is when we experience the longest days and shortest nights compared to any other time of year.  The Sun is higher in the sky at midday in summer than in winter.  The combination of longer days and the high Sun explains the heat of summer.

By contrast, winter is characterized by shorter days and longer nights.  Nighttime in winter exceeds 12 hours.  With the Sun lower in the sky at midday, the sunlight striking the ground is less concentrated.  This results in the significantly cooler temperatures of winter.

Another fact is that summer in the northern hemisphere coincides with winter in the southern hemisphere.  Likewise, winter in the northern hemisphere coincides with summer in the southern hemisphere.  In between summer and winter we have the seasons of spring and autumn during which the length of day and night are roughly equal over most of planet Earth.

All these effects are linked to the same cause, the tilt of Earth’s rotation axis.  The rotation axis of the Earth is inclined by about 23½° away from the axis of Earth’s orbit around the Sun.  Like a spinning top, the Earth’s rotational axis points in the same general direction in space.  So as Earth orbits around the Sun, one hemisphere (northern or southern) may be tilted toward the direction of the Sun while the opposite hemisphere tilts away.

Summer in the northern hemisphere occurs when the north pole is the most tilted toward the Sun.  At this same time the southern hemisphere is in winter with the south pole tilted away from the Sun.  Six months, or half an Earth orbit later, the northern hemisphere points away from the Sun, and we observe winter in the northern hemisphere and summer in the southern hemisphere.

There are special dates on our calendars to precisely mark the key locations in the orbit of the Earth which coincide with the seasonal changes.  The summer solstice occurs around June 21 and marks the beginning of summer in the northern hemisphere.  It is also the longest day of the year.  At the summer solstice the north pole is tilted as much toward the Sun as it ever gets.  The winter solstice occurs around December 21 and is the beginning of winter in the northern hemisphere.  It is also the shortest day of the year.  The winter solstice is when the north pole is tilted as far away from the Sun as it ever gets.

It is easy to think of the equinoxes of spring and autumn as the midway points between the solstices.  The name equinox implies equal nighttime and daytime.  At the moment of equinox, the poles are pointing neither toward nor away from the Sun.  This is because the spin axis of the Earth is at right angles to the Sun’s illumination during an equinox.  The vernal equinox occurs around March 21 and marks the start of spring in the northern hemisphere.  The autumnal equinox occurs around September 21 and is the beginning of autumn.

Comments (0)

By Juan Ivaldi, November 5, 2009 7:38 pm

Quick answer: Stars shine because they are hot.

Stars shine because their surface temperatures are very hot.  The temperatures of stars are so high that they defy human comprehension.  The Sun has a surface temperature of about 5,700°C.  Many stars visible to the unaided eye in the night sky are known to have surface temperatures which range from around 3,000°C  to 40,000°C.   The lowest temperature stars appear red in color whereas and the highest temperature stars are bright blue.

The incredible temperatures of stars are much hotter than what is generated in household ovens, stove tops, or wood fires.   Even volcanic lava is not as hot as the surfaces of stars.  However, temperatures of 20,000 to 30,000°C  can occur naturally within lightening strikes.  Man can artificially generate star-like temperatures in specially engineered and relatively small environments.  Examples of those environments are halogen arc lamps, camera flash bulbs, arc-welders, or plasma science apparatus.  A tungsten filament used in household light bulbs can reach temperatures just over 3000°C.

The surface of stars visible to observers is called the photosphere.  The photosphere is a spherical shell where light can escape away from the star.  Just beneath the photosphere a star is opaque.   Astronomers can estimate a star’s temperature by measuring the amount of light at different colors in the spectrum of starlight.  The spectral signature obeys certain physical laws which relate the temperature of the photosphere.

The process of nuclear fusion is the energy source that keeps stars shining.  Deep within the core of a star, temperatures can reach millions of degrees.  The initial source of heat is the crushing weight of the star’s great quantity of hydrogen compressed by the force of gravity.  When core temperatures reach about 10 million degrees, hydrogen fusion occurs spontaneously.

The great energy released in the core is exchanged with the intermediate layers within the star.  Eventually the energy reaches the photosphere where it can be released as starlight.  Those photons which fly away from the star begin a journey traveling the vast distances of interstellar space.

Some of the photons released from distant stars reach our man-made telescopes on Earth today.  The journey of these photons is ended when they are captured by the retinas of human eyes or by the detectors in astronomical cameras.  Some of these photons are known to have traveled for billions of years through space.

Comments (0)

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.