The solar system consists of the Sun, eight planets, moons, asteroids, and other celestial objects. Understanding their order, characteristics, and key space facts will help you on the ASVAB science test.
1. The Eight Planets (Ordered by Distance from the Sun) ☀️
Mnemonic to Remember the Order:
👉 “My Very Educated Mother Just Served Us Nachos.”
| Planet | Type | Key Features |
| ☿ Mercury | Terrestrial | Smallest planet, closest to the Sun, extreme temperatures. |
| ♀ Venus | Terrestrial | Hottest planet (thick CO₂ atmosphere), rotates backward (retrograde). |
| 🌍 Earth | Terrestrial | Only planet with liquid water and known life. |
| ♂ Mars | Terrestrial | “Red Planet,” has Olympus Mons (largest volcano in the solar system). |
| ♃ Jupiter | Gas Giant | Largest planet, has Great Red Spot (huge storm), 79+ moons. |
| ♄ Saturn | Gas Giant | Famous for large ring system, least dense planet (could float in water). |
| ♅ Uranus | Ice Giant | Rotates on its side, has faint rings. |
| ♆ Neptune | Ice Giant | Windiest planet, dark spot storms, farthest from the Sun. |
💡 Pluto? Pluto was reclassified as a “dwarf planet” in 2006 because it doesn’t clear its orbit.
2. Planetary Types & Characteristics 🌌
Planets are divided into two main types:
🔹 Terrestrial (Rocky) Planets 🪨
- Mercury, Venus, Earth, Mars
- Have solid surfaces and thin atmospheres
🔹 Gas & Ice Giants 💨
- Jupiter, Saturn (Gas Giants)
- Uranus, Neptune (Ice Giants)
- Made mostly of hydrogen, helium, and ices, no solid surface
3. Important Space Facts for the ASVAB 🚀
- The Sun ☀️
- The Sun is a medium-sized star made mostly of hydrogen and helium.
- It generates energy through nuclear fusion (hydrogen atoms combine to form helium).
- It controls gravity in the solar system, keeping planets in orbit.
- The Moon & Its Effects 🌕
- Earth’s Moon causes tides due to its gravitational pull.
- The Moon has no atmosphere, so temperatures vary drastically.
- Phases of the Moon: New 🌑 → Crescent 🌒 → Quarter 🌓 → Full 🌕
- Gravity & Orbits 🌍⬇️
- Gravity keeps planets in orbit around the Sun and moons around planets.
- Stronger Gravity = Larger mass (Jupiter has the strongest).
- Weaker Gravity = Lower mass (Moon’s gravity is 1/6th of Earth’s).
🌎 Example: An astronaut weighs less on the Moon than on Earth due to weaker gravity.
- Astronomical Measurements & Light Speed 🌠
- Astronomical Unit (AU) – Distance from Earth to the Sun (~93 million miles).
- Light-Year – Distance light travels in a year (~5.88 trillion miles).
- The Speed of Light – 186,000 miles per second (fastest thing in the universe).
- Comets: Cosmic Snowballs with Glowing Tails ☄️
Comets are icy bodies that orbit the Sun and develop a glowing tail when they get close. They are often called “dirty snowballs” because they contain ice, dust, and rock.
- How Comets Work 🪐
- Orbit: Comets follow elliptical (oval) orbits around the Sun.
- Heating Up: As a comet nears the Sun, solar radiation melts its ice, creating a bright coma (cloudy atmosphere).
- Tails Form: Solar wind pushes gas and dust away, forming a long tail that always points away from the Sun.
- Parts of a Comet
| Part | Description |
| Nucleus | Solid core of rock, ice, and dust. |
| Coma | Cloud of gas and dust surrounding the nucleus. |
| Dust Tail | Reflects sunlight and curves behind the comet. |
| Ion Tail | Made of charged gases, always pointing away from the Sun. |
- Famous Comets & Fun Facts
- Halley’s Comet – Visible from Earth every 76 years (next visit in 2061).
- Comet Shoemaker-Levy 9 – Collided with Jupiter in 1994.
- Comets may have brought water & organic molecules to early Earth.
- Space Exploration & Human Achievements 🚀
- First Human in Space – Yuri Gagarin (1961, Soviet Union).
- First Moon Landing – Apollo 11, 1969 (Neil Armstrong & Buzz Aldrin).
- International Space Station (ISS) – Largest habitable space station, used for research.
4. Key Takeaways for ASVAB Space Science
✅ 8 Planets (MVEMJSUN) – Terrestrial & Gas Giants
✅ The Sun = Powers the solar system via nuclear fusion
✅ Gravity = Controls planetary orbits & weight differences
✅ Light-Year = Distance light travels in a year
✅ First Moon Landing? Apollo 11 (1969)
1. Earth’s Structure 🌎
Earth has four main layers, each with distinct properties:
| Layer | Composition | Key Features |
| Crust | Solid rock (continental & oceanic) | Thin outer layer where we live. |
| Mantle | Hot, semi-solid rock | Thickest layer, where convection drives plate movement. |
| Outer Core | Liquid iron & nickel | Generates Earth’s magnetic field. |
| Inner Core | Solid iron & nickel | Hottest part of Earth (9,000°F). |
🔹 Fun Fact: The crust and upper mantle together form the lithosphere, which floats on the semi-fluid asthenosphere.
<p><a href=”https://commons.wikimedia.org/wiki/File:Earth_layers_NASA.png#/media/File:Earth_layers_NASA.png”><img src=”https://upload.wikimedia.org/wikipedia/commons/1/1b/Earth_layers_NASA.png” alt=”File:Earth layers NASA.png” height=”280″ width=”294″></a><br>By NASA – <a rel=”nofollow” class=”external free” href=”https://www.nasa.gov/worldbook/earth_worldbook.html”>http://www.nasa.gov/worldbook/earth_worldbook.html</a>, Public Domain, <a href=”https://commons.wikimedia.org/w/index.php?curid=38756075″>Link</a></p>
2. Plate Tectonics: How Earth Moves 🌍➡️⬅️
The Earth’s crust is broken into tectonic plates that move due to mantle convection (heat-driven circulation). Plate movements shape Earth’s surface, causing earthquakes, volcanoes, and mountain formation.
Types of Plate Boundaries & Examples:
| Boundary Type | Motion | Example |
| Divergent ↔️ | Plates move apart | Mid-Atlantic Ridge (new crust forms) |
| Convergent ➡️⬅️ | Plates collide | Himalayas (mountains form), Ring of Fire (volcanoes) |
| Transform ↔️ | Plates slide past each other | San Andreas Fault (earthquakes) |
🔹 Earthquake Tip: Seismic waves (P-waves & S-waves) help scientists study Earth’s interior.
3. Three Main Types of Rocks & Their Formation
| Rock Type | Formation Process | Examples |
| Igneous 🔥 | Cooled magma or lava | Granite, Basalt (volcanoes) |
| Sedimentary 🌊 | Compacted sediment over time | Sandstone, Limestone (fossils found here) |
| Metamorphic ♻️ | Heat & pressure change existing rock | Marble, Slate (formed deep underground) |
🔹 Rock Cycle: Rocks can transform between types due to heat, pressure, and erosion over millions of years.
4. Key Takeaways for Earth Science:
✅ Earth’s Layers = Crust, Mantle, Outer Core, Inner Core
✅ Plate Tectonics = Causes earthquakes, volcanoes, & mountain formation
✅ Rock Types = Igneous (volcanic), Sedimentary (layered), Metamorphic (changed by heat/pressure)
The Water Cycle & Earth’s Atmosphere: How They Work 🌎💧
Water is constantly moving through Earth’s atmosphere, land, and oceans in a process called the water cycle. The atmosphere plays a critical role in weather, climate, and protecting life on Earth.
1. The Water Cycle: Nature’s Recycling System 💦
The water cycle describes how water moves through Earth’s systems by changing states (solid, liquid, gas).
Key Stages of the Water Cycle:
| Process | What Happens? | Example |
| Evaporation ☀️ | The Sun heats water, turning it into vapor. | Water evaporates from oceans, lakes, rivers. |
| Condensation ☁️ | Water vapor cools and forms clouds. | Clouds form in the sky. |
| Precipitation 🌧️ | Water falls back to Earth as rain, snow, sleet, or hail. | Rainstorm, snowfall. |
| Runoff 🌊 | Water flows over land into bodies of water. | Rivers carry rainwater to oceans. |
| Infiltration 🌱 | Water soaks into the ground, refilling groundwater. | Rain soaks into soil and underground reservoirs. |
| Transpiration 🌿 | Plants release water vapor into the air. | Trees “sweat” moisture into the atmosphere. |
🔹 Fun Fact: The same water molecules have been cycling for billions of years—you could be drinking the same water dinosaurs did!
<a title=”Atmospheric Infrared Sounder, Public domain, via Wikimedia Commons” href=”https://commons.wikimedia.org/wiki/File:Water_Cycle_(8265084166).jpg”><img width=”512″ alt=”Credit: NASA” src=”https://upload.wikimedia.org/wikipedia/commons/thumb/3/32/Water_Cycle_%288265084166%29.jpg/512px-Water_Cycle_%288265084166%29.jpg?20230925005734″></a>
2. Earth’s Atmosphere: Layers & Functions 🌎☁️
The atmosphere is a layer of gases surrounding Earth that supports life and protects us from space hazards.
The Five Layers of the Atmosphere:
| Layer | Altitude | Key Features |
| Troposphere ☁️ | 0-12 km | Weather happens here, contains most oxygen. |
| Stratosphere 🛫 | 12-50 km | Contains the ozone layer, which absorbs UV radiation. |
| Mesosphere ☄️ | 50-85 km | Burns up meteors (shooting stars). |
| Thermosphere 🛰️ | 85-600 km | Auroras (Northern Lights), ISS orbits here. |
| Exosphere 🚀 | 600+ km | Merges into space, satellites orbit here. |
🔹 Fun Fact: The ozone layer in the stratosphere blocks harmful UV rays, preventing skin damage and cancer.
Weather Fronts & Types of Clouds ☁️🌦️
Weather fronts are boundaries between air masses with different temperatures, humidity, and pressure levels. These collisions create weather changes, including precipitation, storms, and temperature shifts.
1. Weather Fronts: Air Mass Collisions & Weather Changes 🌡️🌬️
| Front Type | Description | Weather Effects | Symbol |
| Cold Front ❄️ | Cold air pushes under warm air | Thunderstorms, heavy rain, cooler temps | Blue line with triangles 🔼 |
| Warm Front ☀️ | Warm air slides over cold air | Light rain, warm/humid air | Red line with semicircles 🔴 |
| Stationary Front ⏸️ | Air masses stall | Cloudy, steady rain for days | Alternating red/blue |
| Occluded Front ☁️ | Warm air trapped between cold air | Heavy rain, storms, cooler temps | Purple line with triangles & semicircles |
🔹 Example:
- A cold front can cause sudden thunderstorms, while a warm front brings gradual light rain.
2. Types of Clouds & Their Weather Meanings ☁️🌦️
Clouds form when water vapor condenses in the atmosphere. Their shape and altitude determine the type of weather they bring.
- High-Level Clouds (Above 20,000 ft) ⛅
| Cloud Type | Description | Weather Indicator |
| Cirrus 🌤️ | Thin, wispy, feather-like | Fair weather, but a change is coming |
| Cirrostratus ☁️ | Thin, hazy cloud layer covering the sky | Halo around the Sun/Moon (rain soon) |
| Cirrocumulus ⛅ | Small, rippled clouds | Fair but cold weather |
- Mid-Level Clouds (6,500 – 20,000 ft) 🌤️
| Cloud Type | Description | Weather Indicator |
| Altostratus ☁️ | Gray cloud layer covering the sky | Steady rain or snow is possible |
| Altocumulus 🌤️ | White/gray puffy clouds | Fair weather, but storms may develop |
- Low-Level Clouds (Below 6,500 ft) 🌫️
| Cloud Type | Description | Weather Indicator |
| Stratus 🌫️ | Low, uniform gray layer | Drizzle, foggy conditions |
| Stratocumulus ☁️ | Low, lumpy clouds | Cloudy but usually dry |
D. Storm & Vertical-Developing Clouds ⛈️
| Cloud Type | Description | Weather Indicator |
| Nimbostratus 🌧️ | Thick, dark cloud layer | Steady, widespread rain or snow |
| Cumulus ☁️ | Puffy, white cotton-ball clouds | Fair weather |
| Cumulonimbus ⛈️ | Towering storm clouds | Thunderstorms, tornadoes, hurricanes |
🔹 Example:
- Cumulonimbus clouds bring thunderstorms and tornadoes, while cirrus clouds indicate fair weather but a front is approaching.
3. Key Takeaways:
✅ Cold Fronts = Storms, Warm Fronts = Light Rain.
✅ High Clouds (Cirrus) = Fair weather, but change is coming.
✅ Cumulonimbus = Thunderstorms & Tornadoes.
✅ Nimbostratus = Continuous rain or snow.
4. How the Atmosphere & Water Cycle Work Together 🌍💨
- The Sun powers the water cycle by heating water for evaporation.
- The atmosphere holds moisture that forms clouds (condensation).
- Weather patterns move precipitation across the planet.
- Greenhouse gases trap heat, helping maintain Earth’s climate.
5. Key Takeaways:
✅ Water Cycle = Evaporation → Condensation → Precipitation → Runoff.
✅ The Troposphere contains all weather & breathable air.
✅ The Stratosphere holds the Ozone Layer, protecting us from UV rays.
✅ Meteors burn in the Mesosphere; Auroras & satellites exist in the Thermosphere/Exosphere.
Oceanography: Understanding Earth’s Oceans 🌊
Oceanography is the study of the ocean, including its currents, tides, and ecosystems. The ocean covers 71% of Earth’s surface and plays a major role in weather, climate, and marine life.
1. Ocean Currents: Earth’s Water Circulation 🌍💦
Ocean currents are large-scale movements of seawater, driven by wind, temperature, salinity, and Earth’s rotation.
Types of Ocean Currents:
| Type | Description | Example |
| Surface Currents | Driven by wind & Earth’s rotation | Gulf Stream (warms East Coast USA) |
| Deep Ocean Currents | Driven by temperature & salinity differences (thermohaline circulation) | Global Conveyor Belt (regulates climate) |
| Upwelling Currents | Cold, nutrient-rich water rises to the surface | Coastal upwelling (supports marine life) |
🔹 Fun Fact: The Gulf Stream keeps Europe warmer than other places at the same latitude.
2. Tides: The Ocean’s Rising & Falling Water 🌊🌕
Tides are the rise and fall of sea levels caused by the gravitational pull of the Moon & Sun.
Types of Tides:
| Type | Cause | Effect |
| High Tide | Water rises due to Moon’s pull | Coastlines flood |
| Low Tide | Water recedes from shore | Exposes marine life |
| Spring Tide | Sun & Moon align (strong pull) | Extreme high & low tides |
| Neap Tide | Sun & Moon at 90° angle | Weaker tides |
🔹 Fun Fact: Tides occur twice a day in most coastal areas.
3. Marine Ecosystems: Life in the Ocean 🌊🐠
Marine ecosystems support diverse life forms, from microscopic plankton to massive whales.
Major Ocean Ecosystems & Features:
| Ecosystem | Description | Examples |
| Coastal (Intertidal Zone) | Where land meets ocean, harsh conditions | Tide pools, rocky shores |
| Coral Reefs 🪸 | “Rainforests of the sea,” highly biodiverse | Great Barrier Reef |
| Open Ocean (Pelagic Zone) | Largest ecosystem, few nutrients | Tuna, sharks, whales |
| Deep Ocean (Abyssal Zone) | Cold, dark, high pressure | Anglerfish, deep-sea vents |
🔹 Fun Fact: Coral reefs cover less than 1% of the ocean but support 25% of marine life.
4. Key Takeaways:
✅ Ocean currents regulate climate (Gulf Stream, Global Conveyor Belt).
✅ Tides are caused by the Moon’s gravity (Spring & Neap tides).
✅ Marine ecosystems include coral reefs, open ocean, and deep sea.