Unit 9 – Solar System Debris

Unit 9 – Solar System Debris ______________ contains the vast majority of the mass of the solar system. ______________ containsmost of the rest. ______________ containmost of the rest after that. Dwarf planets, asteroids, and comets make up the remainder which is a few thousandths of a percent of the total mass. We will cover the types of small bodies first, then where they are located, and finally how they could potentially affect us.

Dwarf Planets To be considered a planet, an object must meet all of the following criteria. Dwarf planets fail on the __________ point. There are presently less than ______ confirmed dwarf planets, a few candidates, and possibly hundreds scattered around the orbit of Pluto.

Dwarf Planets These are some of the largest known dwarf planets with Earth to compare.

Pluto Just as Neptune was discovered because it appeared that _____________ was not where it should be in its orbit, Neptune was found to not be where it should be either after its orbit was mapped. The search began for a ninth planet and Pluto was discovered in ______. It was quickly determined that Pluto was ___________ to perturb Neptune’s orbit. Then ________________ flew by Uranus and Neptune and determined their correct masses and using these masses Uranus’ and Neptune’s expected orbits were recalculated to be exactly what they are observed to be. This means that Pluto was discovered __________ and that there is no massive ninth planet – only small dwarf planets.

Pluto Pluto’s diameter is only ___________ miles which is just over half as wide as our Moon. Its orbit is highly elliptical but it averages about ___________ AU from the Sun and is inclined 17o from the plane of Earth’s orbit which is known as the _____________. One Plutonian day is about ______ Earth days. One Plutonian year is _________ Earth years.

Pluto It is assumed that Pluto and Triton are similar in composition so Pluto is about 75% ________ and 25% ___________. The surface has ices of water, methane, and nitrogen and is ____oF. Pluto is so small and so far away that it is impossible to get high resolution images of it even with Hubble. This is the best that Hubble can do. It has light regions that are probably ____ and dark regions that are probably organic compounds darkened by ____________.

Charon Charon is Pluto’s relatively large moon. It is over half as big as Pluto, tidally locked to Pluto, and Pluto is tidally locked to Charon. This means that both Pluto and Charon ____________________________________. Recall that Earth’s rotation is gradually ___________ and Earth and the Moon will eventually be like Pluto and Charon. Charon is so big relative to Pluto that the center of mass about which they revolve is a point in space between them. In effect, they both revolve around each other.

Charon How did Charon and Pluto get together? Either they were formed together, Pluto captured Charon, or Charon got knocked off of Pluto. Which is it? formed together – capture – impact –

Nix and Hydra In 2005, astronomers using Hubble discovered two much smaller moons orbiting Pluto. They are named Nix and Hydra. Here is a Pluto system family portrait taken by Hubble a few years ago, but…

One more… In 2011, Hubble took this picture. There will probably be at least a couple more moons discovered once _______________ gets there in __________.

And yet another… This is the second time I have had to edit these notes in three years of teaching astronomy. Last year, astronomers discovered yet another small moon around Pluto. The arrival of New Horizons is now even more anticipated because of this.

Titius-Bode Law In the 1700s, two astronomers observed that the known planets at the time (Mercury through ______) seemed to occur at intervals that fit a mathematical equation. an = 0.4 + 0.3 x 2n-2 where a is the distance from the Sun and n is the planet (1 is Mercury, 2 is Venus, etc.). Math is the language of science (especially the physical sciences) so if behaviors or observations fit a nice neat mathematical equation, then it is very likely that the equation is valid. That’s how all of Newton’s laws were derived along with Kepler’s laws, Einstein’s Theory of Relativity, and all other accepted theories.

Titius-Bode Law

The Gap On New Years’ Day in _________, a new planet was announced with an orbit between _______ and _________. It was named _________ and remained a planet for a few years until more __________ were discovered. It is the smallest and closest dwarf planet to Earth. Hubble can get a decent image of it but it will be visited by a spacecraft in 2015.

Titius-Bode Law This law fit pretty well the known planets at the time even though there was a gap between Mars and Jupiter. _________________ was discovered in 1781 which fit the law almost exactly and later __________ was discovered which filled in the gap between Mars and Jupiter. Because of this, the Titius-Bode Law was universally accepted until _________________ was discovered to be nowhere near where the law predicted it to be. The Titius-Bode Law is no longer accepted and is considered a coincidence.

Other Dwarf Planets For a while, Eris was considered the tenth planet when it was discovered in 2005 since _____________ ______. This heralded the beginning of the end for Pluto’s planetary status. Sedna is currently the farthest known object to be orbiting the Sun.

Asteroids Asteroids are relatively small chunks of rock and metal that orbit the Sun. Ceres is by far the largest asteroid at 580 miles across which is still way smaller than our Moon. There are three main types of asteroids: S-type – made mostly of C-type – made mostly of M-type – made mostly of We probably know about 99% of the asteroids larger than 60 miles across. The large ones we don’t know about probably orbit beyond Neptune.

Asteroids We have close-up images of very few asteroids since they are usually observed by craft on the way to planets. This is Eros and in 2001, NASA landed a craft on it. It is a Mars-crosser.

Asteroids Ida orbits between Mars and Jupiter. Ida was photographed by Galileo on its way to ___________. What else is in this picture?

Comets Comets are familiar celestial objects. They are essentially “dirty snowballs” of _________ and ___________ whose highly elliptical orbits bring them close to the Sun for a relatively short period of time.

Comets When comets are close to the Sun, the ice sublimes forming the characteristic comet. • ___________________ – main solid body; very small • _____ – spherical dust and gas cloud around nucleus • ___________________ – very large but invisible cloud of hydrogen gas • ________ – visible elongated curved tail made of dust • __________________ – dim usually bluish elongated tail of ions that is much longer than the other tails The tails always point ___________ the Sun due to the ______________.

Comets Label this diagram.

Comets Comets’ orbits are extremely elliptical so their eccentricities are close to __. Kepler’s laws state that they move slowly the farther they are away from the Sun. Thus, they spend the vast majority of their lives away from the Sun and thus without comas or tails. After about 500 orbits, all ice has sublimated so the comet is _____.

Halley’s Comet Halley’s comet is the most well-known comet. Its orbital period is about ________ years but the gravities of the large planets alter its orbit slightly. It was last seen in ______ and the next appearance will be in _______. An astronomer named _____________ realized that a 1682 comet sighting was the same comet as previous sightings. After it reappeared in 1758 according to his prediction, historians traced it back and realized that it had been observed and recorded during every orbit since _______.

Halley’s Comet During the 1986 passage, several craft were sent to rendezvous with Halley even though it was on the other side of the Sun. Several close up pictures of the nucleus were sent back.

Asteroid Belt The vast majority of the known asteroids orbit in the asteroid belt which is between _________ and ________. The asteroid belt formed either as a result of a planet that broke up or a planet that never formed. Which is more likely? Why?

Asteroid Belt Most of the asteroids in the asteroid belt are in stable orbits but a few have orbital resonances with Jupiter and these resonances gradually elongate the asteroids’ orbits. Eventually, these asteroids will cross the orbits of the inner planets. More on this in a little bit…

Lagrange Points Lagrange points are regions of stability in a two-body system. Objects can remain in Lagrange points although some of the points are more stable than others. This diagram is Earth and the Moon but it can be any two objects in which one revolves around the other.

Lagrange Points The Hill sphere is the region of gravitational dominance of one body over another. The L1 Lagrange point is at the border of the Hill spheres of two bodies while the L2 and L3 points exist through more complex interactions. Theoretically, an object can remain in L1, L2, or L3 but any slight perturbation will cause it to fall towards one of the objects. Think of the points as being mountain peaks – a ball can rest on top of a mountain but a good shove and it will roll off. The L1, L2, and L3 points are ______________. Think of L4 and L5 as valleys – a good shove there and a ball will move but it will end up where it started. The L4 and L5 points ____________.

Trojans There are hundreds of asteroids in Jupiter’s L4 and L5 Lagrange points and are called Trojans. They are ________ ahead and _______ behind Jupiter in its orbit. Neptune, Mars, Earth, and Venus also have asteroids in their Lagrange points although the asteroids of the inner planets are much smaller. There are hundreds of confirmed Jupiter Trojans and definitely thousands more with Jupiter and the other gas giants.

Centaurs The around a dozen known asteroids with orbits between Jupiter and Neptune are called Centaurs. The orbits cross those of __________ and _________ so the orbits are ______________. At some point in the future they will be perturbed by the gas giants and either ________________________, _______________________________________, or ______________________________________. A couple Centaurs look fuzzy indicating that they are actually _____________. They are much larger than any ones known so if they ever get close to the Sun they will be the brightest that we have ever seen.

Comet Origins There are two types of comets: short-period and long-period. Short-period comets (orbits less than 200 years) tend to have prograde orbits with low inclination to the ecliptic. Halley’s comet is an example. Long-period comets, however, can have both prograde and retrograde orbits coming from all directions. Hyakutake has an orbital period of about 72,000 years.

Kuiper Belt In the early 1900s, an astronomer named Kuiper theorized that there must be a flat disk of material beyond Neptune that is the source of the short-period comets. He called this disc the Kuiper Belt. Why must the Kuiper Belt be flat? Why must it be beyond Neptune rather than in interstellar space? Kuiper Belt objects (KBOs) are perturbed slightly by gravitational influences of the gas giants become comets if they get near the Sun.

Kuiper Belt The inner edge of the Kuiper Belt is about 30 AU from the Sun while the outer edge is about 55 AU from the Sun. Using our football field scale model of 1,000,000 miles = 1 in, the inner edge of the Kuiper Belt would be ______ yards from the Sun. The outer edge would be ______ yards from the Sun. The existence of the Kuiper Belt was confirmed in the early 1990s by the discovery of several KBOs. Pluto and Charon are KBOs but it is theorized that there are tens of thousands of KBOs greater than 50 miles in diameter. __________________ used to be a KBO before it was captured by Neptune during Neptune’s migration. More on this later in the year.

Oort Cloud Long-period comets come from all directions with prograde and retrograde orbits and have farthest points from the Sun (known as _______________) of around 50,000 AU. Jan Oort surmised that there must be a huge comet reservoir surrounding the solar system. It came to be known as the Oort Cloud. The Oort Cloud is thought to be around ________ AU across which is _________________. It extends a significant part of the way to the nearest star other than the Sun. On our football field scale model, the Oort Cloud would be _____________ away.

Oort Cloud The Oort Cloud may contain upwards of ________ ________________________ with a total mass around the sum of all the planets combined. Unfortunately, comets are _______________ and ________________ that it is impossible to observe the Oort Cloud directly so the blue haze on the previous picture isn’t all that accurate. Even so, we are fairly certain that it exists and the comets out there are leftovers from the formation of the solar system.

Oort Cloud Comets in the Oort Cloud are bound very weakly by the Sun’s gravity. They are perturbed causing them to come towards the Sun by Even then, they very rarely get any closer than Pluto but the ones that do make spectacular comets since it is likely that they are making their first pass into the inner solar system. The longest long-period comets can take over a million years to complete one orbit.

Scattered Disk Very recently, another group of small bodies has been proposed beyond the Kuiper Belt but much closer than the Oort Cloud. It is called the Scattered Disk. Objects in the Scattered Disk are more rocky than Oort Cloud comets but are near the ecliptic like KBOs. Sedna is in the Scattered Disk.

Near-Earth Asteroids There are millions of asteroids in our near vicinity but fortunately the vast majority are the size of grains of sand or pebbles. Near-Earth asteroids (NEAs) are defined as those with perihelions less than 1.3 AU. What does that mean? Remember that Earth averages _____ AU from the Sun so the _____ AU difference may not seem like a long way from us but Venus at its closest is about 0.28 AU from us.

Near-Earth Asteroids So why bother with the asteroids that never get closer to us than Venus? Potentially hazardous asteroids (PHAs) meet the following criteria: • orbits get within __________ AU of Earth • are larger than _________ ft For comparison, the Moon is 0.0026 AU from Earth so a PHA can be up to _____ times farther from us than the Moon. We currently know of over _______ PHAs.

Near-Earth Asteroids In 1968, an NEA named Icarus passed Earth at a distance of just under 4 million miles which is about 0.04 AU. In 1994, an NEA designated 1994 XM1 passed within 66,000 miles of Earth which is just one fourth of the way to the Moon. Again, most NEAs are very small so there are undoubtedly dozens that pass between Earth and the Moon every year that we don’t know about. They are extremely hard to detect because they are ____________ and they are _________________.

Near-Earth Asteroids In 2004, astronomers discovered the NEA Apophis and determined that it would make a close pass to Earth in 2029 and may pass through the keyhole. A gravitational keyhole is a small circular region in space near Earth and if an asteroid passes through it, then the asteroid will __________________________. The size of the keyhole depends on the size, speed, and direction of the asteroid. Apophis’ keyhole is about 2000 feet in diameter which is incredibly small considering how big space is. If Apophis passes through the keyhole, then it will impact Earth in _____ but very recent measurements of its orbit place the current odds of an impact to be _______________.

Torino Scale The Torino Scale is a way of quantifying the potential hazards of near-Earth objects (NEOs). It is based on the ___________________ as well as ____________ which is a function of __________ and ___________. Apophis was briefly a 4 on the Torino Scale after its discovery but it was downgraded to a 0 shortly thereafter. No other object has been any higher than 4 and none are presently 2 or 3 but there are a few that are currently 1. Virtually all objects are 1 when they are discovered but they are quickly downgraded to 0 after their orbits are studied more closely.

Torino Scale

Meteoroids, Meteors, Meteorites meteoroid – space rock that hits the atmosphere meteor – streak of light in the sky from meteoroid that burns up in the atmosphere also known as a ______________________ meteorite – meteoroid that hits the ground On any given clear dark night, you can see a few meteors in the sky. During meteor showers, you can see up to 100 per hour. The meteoroids that created these meteors range from the size of pebbles to small cars. Meteor showers usually occur when Earth passes through the orbit of a ______________ and debris hits the atmosphere.

Meteoroids, Meteors, Meteorites A common misconception is that meteoroid _____________ with the atmosphere causes meteors when in fact it is the Meteoroids enter the atmosphere at speeds between 10 and 20 miles per second and dramatically decelerate as they enter the atmosphere. All that kinetic energy is transformed into _______ energy which usually vaporizes the meteoroid forming a meteor.

Meteoroids, Meteors, Meteorites Meteorites are meteoroids that impact the ground usually forming craters. Bodies must be at least one ton for them to reach the ground. It does not take a large meteorite to make a large crater. The meteorite that created the mile-wide Barringer Crater in Arizona was only about ______ ft wide – less than 3 % of the crater width.

Meteoroids, Meteors, Meteorites Most impact craters on Earth have been erased by _________ but a few are visible from space. This crater is in Quebec and is about 44 miles wide which is much larger than the crater in Arizona.

Meteoroids, Meteors, Meteorites In 1908, a massive explosion was recorded near Tunguska in Siberia. It was heard as far away as Europe and produced a measurable increase in atmospheric dust all across the northern hemisphere. It destroyed trees over an area of 830 square miles. No crater was found so the asteroid must have exploded in the air. Had the meteoroid been a couple hours later, it would have ___________________.

Unit 9 – Solar System Debris