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Transcript
In This Lesson: Solar System Debris (Lesson 2 of 2) Today is Monday, May 16th, 2016 Pre-Class: Where do we find asteroids in our solar system? http://www.esa.int Comet 67P/Churyumov-Gerasimenko seen from Rosetta Today’s Agenda • • • • • The Kuiper Belt and the Oort Cloud Comets Asteroids Meteors, Meteoroids, and Meteorites Russian Dash Cams and Secret Spaceships • Where is this in my book? – Chapter 10 (pages 299-321). By the end of this lesson… • You should be able to define the edges of our solar system in terms of the Kuiper Belt and Oort Cloud. • You should be able to distinguish between asteroids, meteoroids, meteorites, meteors, and comets. • You should be able to identify the components of a typical comet. Dwarf Planets • We ended last lesson by talking a lot about Pluto. – Lest we forget… • And one of the major arguments against Pluto’s planetary status is that there’s a lot of other stuff out there with it, making Pluto not so unique. – If everyone’s special, then no one is… Dwarf Planets • Among lots of smaller debris, which we’ll discuss in a few moments, Pluto is joined by a few other dwarf planets, a term I first used last lesson. – Dwarf planets are essentially very large asteroids – also a term we’ll get to – but don’t quite meet the requirements of being a planet. • Reminder: Planets need to be rounded by gravity, orbiting the Sun, and clear of any massive neighbors in their orbit paths. Dwarf Planets • There are five dwarf planets out there you should know, ranked from smallest to largest: – Ceres (discovered 1801) • The only one located within Neptune’s orbit (in the asteroid belt between Mars and Jupiter). – Makemake (discovered 2005) – Haumea (discovered 2005) – Pluto (discovered 1930) – Eris (discovered 2005) • The only one bigger than Pluto. Dwarf Planets http://i.space.com/images/i/000/023/868/i02/dwarfplanets-121120b-02.jpg?1353517196 Kuiper Belt • Surrounding our solar system like a…uh…belt…is the Kuiper Belt, named for one of its “proposers,” Gerard Kuiper. – Technically, he suggested it didn’t exist. • It should be noted that Kenneth Edgeworth independently proposed the same thing at the same time. Gerard Kuiper Fred Leonard 1905-1973 1896-1960 – Perhaps it should have been called the “Edgeworth Edge.” • Also notable is that Fred Leonard may have proposed the same thing about 10 years before these other two. – The “Leonard Line?” http://upload.wikimedia.org/wikipedia/commons/thumb/d/d3/FCL1947.jpg/220px-FCL1947.jpg http://mail.colonial.net/~hkaiter/aa_newest_images/edgeworth.jpg http://www.windows2universe.org/people/images/kuiper.gif Kenneth Edgeworth 1880-1972 Kuiper Belt • As you can see, the Kuiper Belt is a flat disc at the edge of the solar system, filled with debris: http://scienceblogs.com/startswithabang/files/2013/10/Kuiperbelt-1.jpg Kuiper Belt • Notice from the image that the Kuiper Belt is relatively flat, like the rest of the solar system. • Also notice that Pluto’s orbit takes it kinda over and under the Kuiper Belt, but certainly out far enough. • Despite being really friggin’ far away (“really friggin’” = 30-50 au), the Kuiper Belt has immediate effects on life on Earth. – Like, giant space rock to the face, immediate. Kuiper Belt • Gravitational attraction on the part of the outer planets (gas giants) can sometimes fling an asteroid or comet our way. – They may also be flung further out into space. • These objects are known as Kuiper Belt objects (KBOs) or trans-Neptunian objects (NBOs). Oort Cloud • Named for Jan Oort, the Oort Cloud is a shell of icy objects orbiting very far from the Sun. • Like the Kuiper Belt, gravitational interactions can occasionally sling an icy rock toward the inner solar system. – Usually these effects are driven by other stars or passing nebulae instead of by planets, though. Jan Oort 1900-1992 http://www.phys-astro.sonoma.edu/brucemedalists/oort/oort.jpg Oort Cloud • There are two main differences between the Oort Cloud and the Kuiper Belt: – The Oort Cloud is much farther away. • Kuiper Belt = 30-50 au. • Oort Cloud = 10,000-100,000 au. – The Kuiper Belt is flat; the Oort Cloud is like a spherical shell. • Much like the celestial sphere. • Those are good to know. Dodgin’ Space Rocks • Either region may send something our way, and there are three main classes of space objects that may be directed on a collision course toward us: – Asteroids – Meteoroids – Comets • They each have their own details, so let’s explore them. Asteroids • As we saw in a video a little while ago, astronomers in the 1800s starting discovering what they thought was a whole raft of planets in between Mars and Jupiter. • Today, we know them to be asteroids orbiting the Sun in the Asteroid Belt. – The root word “aster-” means “star,” since early astronomers couldn’t tell the difference (they’re small). • There are occasionally asteroids spotted elsewhere in the solar system but the belt is the most common place. – Seriously, there are around 1.1-1.9 million asteroids greater than 1 km in diameter, and millions more that are smaller than that. https://solarsystem.nasa.gov/planets/profile.cfm?Object=Asteroids&Display=OverviewLong Asteroids in the Solar System • Below is NASA’s diagram of NEAs (near-Earth asteroids) and PHAs (potentially hazardous asteroids). – Yikes. https://solarsystem.nasa.gov/multimedia/gallery/PHA.jpg Asteroid Size, Types, and Companions • Asteroids are leftovers from the formation of the solar system. • Ceres (dwarf planet) is the largest of the asteroids at 950 km in diameter (590 mi). • The smallest are under 1 km (0.6 mi). – Combined, all their mass is less than the Moon, and Ceres is 1/3 of it. • Speaking of moons, some have them, as seen at the right. • They’re very widely spaced out (13 million km average). https://solarsystem.nasa.gov/multimedia/gallery/Ida_Dactyl2.jpg https://solarsystem.nasa.gov/multimedia/gallery/685735main_pia15678-43_full.jpg NASA’s Dawn spacecraft’s view of Vesta, one of the largest asteroids. NASA’s Galileo spacecraft’s view of 243 Ida and its moon Dactyl. Asteroid Size, Types, and Companions • There are three main types of asteroids*: – C-type asteroids (chondrite) are made of carbon-based stuff (clay and silicates). • They’re dark colored. – S-type asteroids (stony) are made of silicates and nickel-iron. – M-type asteroids (metallic) are made of nickel and iron. • Each discovered asteroid is individually named and, among others, there are funny ones out there (like Mr. Spock) or tributes (like Annefrank). – http://www.minorplanetcenter.net/iau/lists/MPNames.html *Depends on whom you ask. This stuff is hard to categorize. Asteroid Types http://www.philipmetzger.com/blog/wp-content/uploads/2013/05/asteroid-and-meteorite-types.jpg Other Asteroid Locations • Like I said earlier, the majority of asteroids are in the asteroid belt. • There are, however, a few other regions of concentrated asteroids. – Trojans – Hildas – Apollo asteroids – Near-Earth asteroids • Let’s take a look… http://upload.wikimedia.org/wikipedia/commons/f/f3/InnerSolarSystem-en.png Trojan Asteroids • The Trojan asteroids orbit just ahead of, and just behind, the orbit of Jupiter. – The Sun and Jupiter’s pulls combine to keep them in place. • The locations of the Trojan asteroid swarms are known as the L4 (ahead) and L5 (behind) Lagrangian Points. – The points are 60° ahead/behind. • FYI, the Jupiter Trojans are bestknown, but Mars and Neptune have Trojans, too. http://ase.tufts.edu/cosmos/pictures/Chapter13/Fig13_1.jpg The Earth Trojan? • In 2011, NASA announced the discovery of a single Earth Trojan asteroid. – You read that right. • Earth’s Trojan has the incredibly sexy name of 2010 TK7 and is only about 1 km in diameter. http://news.nationalgeographic.com/news/2011/07/1107128-trojan-asteroid-earth-planet-orbit-nasa-space-science/ Hildas • The Hildas are one of many other asteroid families out there. • They exist in a 3:2 orbital resonance with Jupiter, which means they orbit three times for every two orbits of Jupiter and keeps them in their own gravitational lock. Apollo Asteroids • The Apollo asteroids are the scary ones. – They’re the ones that pass across Earth’s orbit, but there are only about 700 of them and are typically under 1 km in diameter. • Still, they hit the Earth once every 10,000 years on average. • They may be dead comets. http://astronomy.swin.edu.au/cosmos/a/apollo+asteroids Near-Earth Asteroids • Remember a few slides ago when I showed you NASA’s visualization of NEAs and PHAs? – ↖ The one up there in the corner? • The Apollo asteroids are just the largest of a few groups of NEAs and one of two groups of Earth-crossing asteroids. • There are also the Atens and the Amors, along with IEOs (Inner Earth Objects) that don’t cross Earth’s orbit. Near-Earth Asteroids http://neo.jpl.nasa.gov/neo/groups.html Aside: Apophis • Apophis (officially 99942 Apophis) is a near-Earth asteroid that crosses our orbit path. – It’s the Greek name for an Egyptian god of chaos and destruction. Uh-huh. • In 2004, rough calculations suggested Apophis had up to a 2.7% chance of striking Earth in 2036 after a near pass in 2029. – Uh…huh. • Further calculations have greatly reduced that chance, though its pass in 2029 may be so close that it passes under geostationary satellites. Kirkwood Gaps • As we discussed, the Trojans are held in orbit partially by Jupiter (but mostly by the Sun). • However, Jupiter’s gravity also creates a weird phenomenon in the main asteroid belt: gaps. – In other words, the asteroids in the asteroid belt are not evenly distributed. • The gaps in the belt are caused by gravitational perturbations on the part of Jupiter. – So the planet will either pull a bunch of asteroids toward itself at that region, or it’ll clear it out. Kirkwood Gaps • In this case, Jupiter’s gravity and orbital period creates gaps known as Kirkwood Gaps after Daniel Kirkwood, who first observed them in the 19th century. • In order to understand them, we need to understand orbital resonance first. – Orbital resonance is the relative speed of orbiting objects. – If an asteroid has 3:1 orbital resonance with Jupiter, it means that the asteroid orbits the Sun three times in the time it takes Jupiter to pass once. • Kirkwood Gaps occur at orbital resonances 3:1, 5:2, 7:3, and 2:1 for asteroids in the belt. – These concepts also explain the gaps in Saturn’s rings. Kirkwood Gaps http://www.colinpower.com/wp-content/uploads/2014/05/kirkwood-gaps.jpg Kirkwood Gaps http://lasp.colorado.edu/~bagenal/3750/ClassNotes/Class23/KirkwoodGaps.gif Other Space Debris • So if those are all asteroids, what are meteoroids? • Asteroids are generally larger than meteoroids. – A large meteoroid could be considered a small asteroid. – A large asteroid could be considered a small planet. • In contrast, meteors are asteroids or meteoroids that burn up in Earth’s atmosphere. – Known to many as “shooting stars,” though if they were actually star-sized they’d be “shooting to kill.” • Fun fact: if a meteor explodes in the atmosphere it’s called a bolide. And what about meteorites? • If one of those space rocks should make it through the atmosphere and land on Earth’s surface, however small, it’s known as a meteorite. – So, 65 million years ago, an asteroid (because it was so big) became a meteor before it became a meteorite and wiped out the last of the dinosaurs. • Want to collect your own meteorites? – Evidence suggests that microscopic meteorites can be “harvested” with a magnet in a rain gutter over several weeks. The Difference in One Animation Meteors and Meteoroids of Note • On February 15, 2013, a superbolide (big fiery meteor that blows up) entered the atmosphere over Russia and exploded about 18.4 miles up. – It became known as the Chelyabinsk meteor. • The light was brighter than the Sun and those that were close enough could feel heat from the blast. • 1500 people were injured from effects of the blast (mainly broken/flying glass). • Thankfully, no one was killed and the incident was caught on a large number of cameras. – Chelyabinsk Meteor – February 15 2013 video The Tunguska Event • The Chelyabinsk meteor was only around 20 m (100 ft) in diameter, but it weighed more than the Eiffel Tower. • Bigger still was an explosion in 1908 by something suspected to be either a comet or asteroid, known as the Tunguska event. – It landed in Siberia and cleared an entire forest with a blast estimated at 1000x the energy of the atomic bomb dropped on Hiroshima. Murchison Meteorite • Of all the countless meteorites to have landed on Earth, one stands out above all in the “origins of life” department: – The Murchison Meteorite, which landed in Murchison, Australia, in 1969. – Know what makes it special? • When it originally landed, scientists detected the presence of little bits of DNA and other organic molecules, but for a long time no one knew if those came from the soil or from space. – Well guess what was discovered in 2008? – Article… Meteor Showers • Throughout the year, regions of Earth are treated to a sometimes-astounding light show known as a meteor shower: http://i.space.com/images/i/000/024/852/original/geminid-shower-kennethbrandon.jpg?1357457156 The Geminid meteor shower Meteor Showers • Meteor showers are named for their apparent constellation of origin: – The Perseids (August) come from…? • Perseus, and that’s the biggest one. – The Leonids (November) come from…? • Leo. – The Geminids (December) come from…? • Sagittarius. • Just kidding. It’s Gemini. • But they don’t actually come from the constellation, so what causes them? Meteor Showers • The majority of meteor showers are triggered by comets. – As we’ll see in a few slides, comets shed a bunch of debris into which Earth may then pass. – Dates are predictable and they’re viewable with the naked eye, so have yourself a nighttime meteor picnic if you want: • http://stardate.org/nightsky/meteors • And this all provides a great segue into the next topic, comets. Practice • Asteroids Meteoroids Meteors Meteorites worksheet Comet • Whoops…wrong “Comet…” http://www.themeparkinsider.com/photos/images/CIMG0617.JPG Comet Orbits • Comets fit into two different classes: – Short-period comets take less than 200 years to orbit the Sun and come from the Kuiper Belt. • Because they come from the relatively flat Kuiper Belt, short-period comets orbit in a plane similar to that of the solar system. – Long-period comets take more than 200 years to orbit the Sun and come from the Oort Cloud. • Because they come from the spherical Oort Cloud, longperiod comets orbit on random planes. Kuiper Belt Comet Orbits Short Period http://www.astro.virginia.edu/class/oconnell/astr1210/im/halley-orb-2.gif https://www.awesomestories.com/images/user/bb9048c84ec9fef6bd1f57920ae6db66.jpg Oort Cloud Comet Orbits Long Period http://lcogt.net/files/styles/twocol/public/sroberts/orbits_Jon.jpg?itok=akVpsVom http://images.gizmag.com/inline/comet-will-give-mars-a-close-shave-in-2014-2.jpg Comet Structure • We’re all familiar with the typical comet structure, especially in clip-art form: • That’s…kinda accurate. http://www.clker.com/cliparts/4/5/d/5/12921785611657057370comet%203.jpg Comet Structure • First off, the “core” of the comet is known as its nucleus. – It’s made of ice, dust, and frozen gases and is usually around 10 km in diameter. – It’s also generally very dark, giving it the common name “dirty snowball,” but recent evidence suggests it’s more of an “icy dirtball.” • Immediately around the nucleus is the coma, which is a cloud of gas…but that’s about it. – Possibly up to 100,000 km in diameter, but very little actual mass. http://pages.uoregon.edu/jimbrau/BrauImNew/Chap14/7th/AT_7e_Figure_14_08a.jpg Comet Structure • Surrounding and much, much larger than the coma is the hydrogen envelope, which is invisible and slightly irregularly-shaped due to the solar wind. • And then, of course, there are the tails. – Yes, tails. More than one. http://pages.uoregon.edu/jimbrau/BrauImNew/Chap14/7th/AT_7e_Figure_14_08a.jpg Comet Tails • Comets generally have two tails: – The dust tail is usually curved and generally faces directly away from the Sun. • It comes from sunlight pushing dust particles off the comet in a form of sublimation. • Sublimation demo! – The ion tail is usually straight and faces directly away from the Sun. • It comes from solar wind radiation that ionizes (charges) gases in the coma, so it often gives off its own light. Comet Tails • In both cases, the tails become larger when the comet is closer to the Sun. – The tails may even grow to be close to an astronomical unit in length, which is, you know, kinda big. • The tails may be absent at perihelion. http://www.tivas.org.uk/solsys/images/comet_hale-bopp.jpg Comet Hale-Bopp in 1997. Comet Tails http://www.newtonsapple.org.uk/comet-bart-simpson/ Comet Tails http://www.newtonsapple.org.uk/comet-bart-simpson/ Comets http://lasp.colorado.edu/education/outerplanets/images_kbos/big/cometorbit.jpg Comets of Note • • • • • [comet etymology] Comet 67P/Churyumov-Gerasimenko Halley’s Comet Shoemaker-Levy 9 Hale-Bopp Aside: Comet Etymology • Historically, comets were associated with bad things: – Famine. – War. – Death of rulers. (you get the idea) • The “bad star” is so strongly associated with disaster that we actually get the word disaster from it. – “Dis-” = bad. – “-aster” = star. Comet 67P/Churyumov-Gerasimenko • In 2014, the European Space Agency (ESA) successfully landed a probe on the surface of a comet. – Like, whoa. • Let’s talk about the comet first, then a little about the probe. • The comet is called Churyumov-Gerasimenko (or, boringly, 67P), and while you may think of it as probably very small, it’s not. Scale of Churyumov-Gerasimenko http://d1jqu7g1y74ds1.cloudfront.net/wp-content/uploads/2014/08/67P-LA-anosmicovni-credit-BEST.jpg Churyumov-Gerasimenko • That said, comets generally have low escape velocities. – You could jump off comets. – That’s why tails “leak” material. • As a result, when the Rosetta orbiter deployed the Philae lander, the lander bounced several times before landing, failing to harpoon the comet to hold itself in place. – Philae is now on its side and out of power. – It should be able to recharge if sunlight hits it at the right angle. Churyumov-Gerasimeko as seen from the Rosetta satellite. https://farm8.staticflickr.com/7364/16475672246_3493285c83_ o.jpg Churyumov-Gerasimenko from Rosetta http://www.esa.int/var/esa/storage/images/esa_multimedia/images/2015/04/comet_activity_31_january_25_march_2015/15355062-1-engGB/Comet_activity_31_January_25_March_2015.jpg Halley’s Comet • Halley’s comet has a 75-76 year orbital period and last appeared in 1986 (good year). – It’s named for Edmond Halley, who concluded that a comet seen in 1531, 1607, and 1682 was actually the same one. – He predicted it would again appear in 1758 and was right, though he died before it returned. • Remember him? He also observed the movements of the star Arcturus and concluded that the stars move. Edmond Halley (1656-1742) https://cbs3weather.files.wordpress.com/2014/04/wx-blog3.jpg Halley’s Comet Photos • For its 1986 perihelion, space agencies got very excited. – It was the first time it had passed Earth in the age of rocketry. • Among a number of other probes (including the Russian Vega probes that went on to Venus), the European Space Agency sent Giotto to gather close up images. • Here’s what it returned… Halley’s Comet Photos https://solarsystem.nasa.gov/multimedia/gallery/halley_giotto_big.jpg Halley’s Comet Photos http://www.wolaver.org/space/cometHalley.jpg http://media-1.web.britannica.com/eb-media/00/4300-004-932A3155.jpg Shoemaker-Levy 9 • Shoemaker-Levy 9 was named for its discoverers. – Carolyn and Eugene Shoemaker along with David Levy – they discovered a number of comets as you can tell. • Evidence indicates that it got captured by Jupiter’s gravity in the 1970s and ended up in a highly eccentric orbit…till the 1990s. • In 1992 it got very close to cloud tops of Jupiter’s atmosphere and started to fragment. • It soon became clear that its pieces would collide with the planet in July 1994, and that’s just what happened. – Jupiter and Shoemaker-Levy Scars Up Close Shoemaker-Levy 9 http://www2.jpl.nasa.gov/sl9/gif/calar.gif Shoemaker-Levy 9 http://www2.jpl.nasa.gov/sl9/gif/hst21.gif Shoemaker-Levy 9 http://www2.jpl.nasa.gov/sl9/gif/mssso4.gif Shoemaker-Levy 9 http://upload.wikimedia.org/wikipedia/commons/5/5d/Jupiter_showing_SL9_impact_sites.jpg The Roche Limit • The Shoemaker-Levy 9 collision is a good example of the Roche Limit. – Proposed by M.E. Roche in 1849. • The Roche Limit states that a body of equal density to the host planet (like a moon) will break up due to tidal forces if it comes within 2.44 planetary radii of the planet. – Obviously, Shoemaker-Levy 9 did so on that 1992 pass. • Interestingly, planetary ring systems are often located right on the Roche limit. The Roche Limit http://pages.uoregon.edu/jimbrau/BrauImNew/Chap12/FG12_12.jpg Hale-Bopp • Hale-Bopp was discovered in 1995 by Alan Hale and Thomas Bopp (separately) when it was still outside Jupiter’s orbit. – It helped that it was unusually bright. • It became one of the most watched comets by the general public when it made its pass by Earth in April 1997. – As such, there are a lot of photos… – …and one mass suicide. Hale-Bopp http://www2.jpl.nasa.gov/comet/gif/pach15.jpg Hale-Bopp http://www2.jpl.nasa.gov/comet/gif/oet5.jpg http://www2.jpl.nasa.gov/comet/gif/dru64.jpg Hale-Bopp: Heaven’s Gate • Heaven’s Gate was a UFO religious Millenarian group (read: “cult”) that believed the Hale-Bopp comet was being closely followed by an alien spacecraft. • On March 26, 1997, San Diego police found 39 members all dead of suicide in a mansion. – They had poisoned and asphyxiated themselves over a period of a few days, and each had $5.75 in their pockets. Why? – They believed the only way to reach the comet was to kill themselves, and the $5.75 was the “interplanetary toll.” • I am not kidding. https://en.wikipedia.org/wiki/Heaven%27s_Gate_(religious_group)#/media/File:Heavensgatelogo.jpg A closing note on comets… • Comets, it turns out, carry one other special ability with them. – Besides amazing people and causing small groups to kill themselves. • Comets may have delivered water to planet Earth and other places. • This is very much an active scientific debate, though, and there’s a lot of evidence to say that most didn’t come from comets and instead came from asteroids. • Still other evidence suggests it was here during Earth’s formation. http://www.space.com/27256-earth-water-older-than-sun.html Satellites • We’ve been launching satellites for a long time now. – Technically, these are artificial satellites. – Natural satellites are known as moons. • The first satellite to orbit Earth was Sputnik, launched on October 4, 1957 by the USSR. – The American people collectively pooed their pants. – Remember that shape…(sphere with four radio antennae). You’ll see why. • The Soviets launched two satellites (Sputnik II) before the Americans launched Explorer I on January 31, 1958. http://airandspace.si.edu/webimages/highres/2006-25353h.jpg http://d1jqu7g1y74ds1.cloudfront.net/wp-content/uploads/2010/06/Sputnik-1.jpg Satellites • Today, we’ve got over 1000 functioning satellites. – That doesn’t count dead ones. – http://satellitedebris.net/Database/LaunchHistoryView. php • Just how many go by without us even noticing? – Stellarium satellite view – This doesn’t include all the geostationary satellites – ones that orbit the Earth at the same speed as the Earth rotates. – Geostationary satellites stay put over one spot on Earth all the time. Space Junk • But what about the dead ones? – You know, like in WALL-E? [WALL-E clip] – Recognize that one satellite there? • They’re still up there – most of them anyway – along with a whole bunch of other interesting stuff: – – – – UniverseToday – How Many Satellites are in Space? Lost in Space: 8 Weird Pieces of Space Junk article Tool Bag Lost During Space Walk Astronaut Loses Camera in Space Space Debris • However, we’ve got huge amounts of debris orbiting Earth as well. • These bits of debris can cause massive damage to spacecraft, so to avoid them, the ISS and other satellites actually have to be moved from time to time. – The Department of Defense and NASA each have offices/systems devoted to tracking all of them. – Debris in Motion video – There’s even a newsletter: Orbital Debris Quarterly News • Sound familiar? – Gravity – Space Debris Space Debris • If you’ve seen Gravity, you know that the debris in space comes from the intentional destruction of a satellite by Russia in a demonstration of military might. • Know where they got the idea? – China Destroys Satellite in Test – 60 Minutes – The Battle Above • There are also very real collisions that occur. – U.S. Satellite Destroyed in Space Collision article • So how to clean it up? • Here’s a thought: – The Mad Plan to Clean Up Space Junk with a Laser Cannon article Practice • The Solar System Review worksheet Closure • Let’s do a little WhipAround of the whole unit so far. • Find a free margin in your notebook. • Independently, write down two things you learned over the course of this lesson (solar system debris). – Don’t look at your neighbor’s paper. • When you’ve got the two written, stand up. Closure • I’ll Bag-o’-Evil someone to read one of their two facts. • If you’ve written the same thing, check yours off. • Once you’ve either read yours, or both of yours have been read by others, sit down. • We’ll then repeat for the first lesson (Planets).