ingridscience

This activity is adapted from a NASA activity no longer available. Similiar activity also at https://www.teachengineering.org/view_activity.php?url=collection/cub_/…

Introduce gravity assist:
We have used rockets to send probes to many parts of the solar system, and beyond.
Show image of current position of probes: https://armchairastronautics.blogspot.com/p/solar-system-missions.html
For sending spacecraft these long distances, we need to keep the fuel usage to a minimum, as much of the weight of a rocket is fuel (90%). Once rockets have used much fuel to escape earth’s gravity, we can use the gravity of the sun or other planets to alter the path and speed of a spacecraft, with minimal further fuel usage. Using the gravity of other bodies to change the speed and direction of a spacecraft is called “gravity assist”, “gravitational slingshot”, or “swing-by”, and has been used to send probes to the outer reaches of, and beyond, the solar system. Gravity assists of close approach can last a few hours. The amount by which the spacecraft speeds up or slows down is determined by whether it is passing behind or in front of the planet as the planet follows its orbit. 

Students will model gravity assist by rolling a steel ball (the "spacecraft") down a ramp onto a plexi sheet, which has a magnet (the "gravity" of a planet) in its path. The spacecraft is deflected by the gravity of the planet as it passes by. The amount of deflection depends on the speed of the spacecraft, how close it passes by the planet and the gravitational strength of the planet.

Show students how to set up the gravity assist activity:
Draw a line down the centre of the plexi, along its length.
Tape the magnet onto the plexi, positioned just to the side of the central line and half way along it.
Turn the plexi over so the magnet is underneath, and place the plexi on the four supports inside the tray.
Tape the ramp at one end of the central line, so that a ball rolled from the top of the ramp rolls along the line towards the magnet. When the ball is released from the top of the ramp, it should reproducibly deflect a small amount around the magnet. If it does not, adjust the height of the ramp or the position of the magnet.

Structured format to activity:
Students are asked to change one variable at a time: the speed of the spacecraft can be changed by releasing the ball from the top of the ramp (faster) or lower down (slower). The gravity of the planet can be increased (i.e. a larger planet) by adding magnets underneath the taped magnet.
Show students how to measure the angle of deflection for the different conditions they try: make a mark where the ball rolls off the plexi, and repeat until the marks are consistently on top of each other. Then draw a line from this mark back to the centre line next to the magnet. Then use a protractor to measure the angle between the centre line and the angled deflection path.
See attachment for worksheet.

Discuss class data:
Although there will be much variability in the angles measured (depends on slope of desk, position of magnet etc) , decreasing the speed of the spacecraft by releasing at a lower position on the ramp, should generally produce greater angles of deflection. Increasing the gravity of the planet, by adding more magnets, should generally produce greater angles of deflection.
A graph can be made of the increase/decrease in angle when changing from low to high gravity and high to low speed.

Give students more magnets and allow them to freely experiment with planet and ramp placement. Show them images of real gravity assist trajectories (see below), that they might want to try and replicate.

Free experimentation format:
Discuss some of the variables that the students will be working with: the speed of the spacecraft (by varying where the ball is released from), the gravity of the planet (by varying the number of magnets), the position of planet(s).

Show students trajectories of real spacecraft that have used gravity assist (see below), and challenge students to replicate some of these, and design their own trajectories.

Discussion: Students will want to share how they made their trajectories, some of them employing methods not in the kit (e.g. tipping the plexi to combine gravity with magnetic force to direct their ball). Direct the conversation towards how even a small tweak in the speed of the spacecraft or the position of a planet makes the trajectory change dramatically. When space scientists plan the route of a spacecraft they use complex mathematical modelling to ensure that the spacecraft reaches the intended destination. In their calculations, they need to take in to account that the planets are moving, and they cannot vary the gravity of a planet as this model can, so the launch timing is critical for success of a mission. Also reinforce the years that it takes for a probe to reach its destination planet - the distances are huge.

Real gravity assist trajectories (as of Spring 2019):
1. New Horizons flew by Jupiter (and sent back detailed images) to direct it to Pluto. New Horizons is currently in the Kuiper belt.
2. Juno flew by Earth for its gravity assist out to Jupiter. On its gravity assist to orbit Jupiter, it was a slow-down. [Ingrid check]
3. Voyager 1 used gravitational assist from Jupiter and Saturn and in August 2014 entered interstellar space. Voyager 2 swung by Jupiter and Saturn and then also Uranus and Neptune and just entered interstellar space. Animation of the paths of Voyager 1 and 2, showing the trajectory changes from gravity assist: https://www.theplanetstoday.com/voyager_flight_path.html
News article when Voyager 2 entered interstellar space: https://www.cbc.ca/news/technology/voyager2-interstellar-space-1.5274614
4. Cassini (see attached file) was a 20 year mission, to orbit Saturn before descending beneath the rings and into Saturn's atmosphere. It arrived at Saturn with gravity assist from Venus, Earth and Jupiter: https://science.nasa.gov/resource/cassini-trajectory/ The rocket that launched Cassini in 1997 was the most powerful available to NASA, but it still wasn't powerful enough to send the nearly 6,000-kilogram (13,200-pound) spacecraft on a direct course to Saturn. Instead, mission designers planned multiple flybys of Venus, Earth and Jupiter, using each planet's gravity to boost Cassini's sun-relative speed and send the spacecraft out to Saturn.
5. Juice (‘Jupiter Icy Moons Explorer’) was launched in 2023. It will use gravity assist of the Moon, then Earth, Venus, Earth and Earth again, then 223 Rosa (an asteroid) to get to Jupiter’s moon Ganymede in 2031. For animations of trajectory around the Sun, Jupiter and Ganymede (Jupiter Moon): https://en.wikipedia.org/wiki/Jupiter_Icy_Moons_Explorer (scroll down)

Show a dramatic video of a real rocket taking off e.g. a Delta IV Heavy rocket launching the Orion spacecraft (to be used in deep space exploration) Dec 2015: http://www.universetoday.com/117197/bringing-you-there-intense-sound-of…

Discuss how real rockets work:
The "rocket fuel" is mixed with an "oxidizer" in the combustion chamber. They chemically react and make new molecules, including a gas. The huge amount of gas produced (called "exhaust") can only escape out of a nozzle built into the back of the rocket. The action of the exhaust shooting out exerts an equal and opposite force on the rocket, which propels the rocket upwards. (Newton's Third Law of Motion: Action and Reaction).
(If the baking soda/vinegar rocket activity or Alka seltzer activity has been done, remind students that it is the same mechanism.)

Model the chemical reaction in real rockets:
Give the students their molecule pieces.
Write the oxidizer and fuel molecules on the board, ask students to build them:
O2 (liquid oxygen or “LOX”) + 2 H2 (liquid hydrogen or ”LH2”).
These are usually gas molecules (as some students may know), but in a rocket engine, they are stored at very low temperatures, so are liquid.
Tell them that in the fuel and oxidant are injected into the hot combustion chamber, where the oxygen and hydrogen chemically react to make a new molecule. Ask them to take apart their oxygen and hydrogens and rebuild two identical molecules.
They should make two water molecules, 2 H2O. They might recognize this molecule when it is called by it's chemical formula.
In the rocket combustion chamber the temperatures high enough that this water is a gas, which escapes through the nozzle and gives the rocket thrust.

Other facts about rocket fuels:

The fuel is a significant portion of the rocket mass, so as the fuel is burned up, the rocket gets significantly lighter and accelerates upwards.
The boosters (on the side) run out of fuel first, then the main engine.

The propellants of a rocket are often a "fuel" and a source of oxygen "an oxidizer".
The most efficient fuel and oxidizer combination is liquid oxygen (the oxidizer, also called LOX) and liquid hydrogen (the fuel, also called LH2), as we modelled here.
Liquid oxygen ("LOX") is the oxidizer commonly used in rockets.
Other fuels used besides liquid hydrogen are kerosene or methane.
Chemical reaction for kerosene: 2 C12H26(l) + 37 O2(g) → 24 CO2(g) + 26 H2O(g)
Chemical reaction for methane: CH4 + 2O2 → CO2 + 2H2O

A rocket where the fuel or oxidizer (or both) are gases liquefied and stored at very low temperatures (below −150 °C) is called a cryogenic rocket engine. If the propellants had been stored as pressurized gases, the size and mass of fuel tanks themselves would severely decrease rocket efficiency.

Rockets have to carry their own oxygen into space, where there is no air.
This is in contrast to aeroplanes, which use atmospheric oxygen to oxidize their fuel.

"Monopropellants" are just one molecule (e.g. hydrogen peroxide or hydrazine) that can split into gas molecules, with a catalyst to speed up the reaction. Hydrogen peroxide decomposes into oxygen and water gases. Hydrazine decomposes into nitrogen and hydrogen gases. The gases produced are directed through a nozzle to create thrust.

First liquid-fuelled rocket in was launched in 1926. It flew 12metres. First satellite, Sputnik, launched in 1957 by the Soviet Union. 1961 was the first person in space.

For a free play activity using found objects:
One student adds an item to the tub e.g. pine cone, leaf etc. and snaps on the lid.
Their partner tries to guess what is in the tub, by shaking it and hearing the sounds made.

For a structured activity set up by the teacher:
Before the lesson, tape different items into shoe boxes, and leave one shoe box empty. I used metal cups in one, a ball of cloth in another, and rice in another.
Add a metal ball to each box. Seal the boxes. Make sure all holes are blocked, especially for a box containing rice grains.
Ask students to tip the boxes, and deduce what is inside from the sounds they hear.

Discussion on how scientists use sound to learn about the sun:
Scientists listen to the sounds coming from the inside of the sun, to learn about its interior structure. (Called "helioseismology".)
The Sun's sound waves bounce from one side of the Sun to the other in about two hours, causing the Sun's surface to oscillate, or wiggle up and down. Because these sound waves travel underneath the Sun's surface, they are influenced by conditions inside the Sun. So scientists can listen to the sun to learn more about how the structure of the Sun's interior shapes its surface.
The Sun's sound waves are normally at frequencies too low for the human ear to hear. To be able to hear them, the scientists sped up the waves 42,000 times, and compressed 40 days of vibrations into a few seconds.
What you hear in this audio track are just a few dozen of the 10 million resonances echoing inside the Sun:
http://solar-center.stanford.edu/singing/SOUNDS/three_modes_l_0_1_2.mp3

Review states of matter that the students know: often just solid, liquid and gas.
Review the spacing of the atoms/molecules in each state of matter (close in solids, further apart and free to move in liquids, even further apart and even more energetic in gases) and that in each state the particles are progressively more energetic.
Tell students that when gas is energized even more, the atoms split apart and the electrons become free from the nuclei. This is a 4th state of matter, called plasma.

In the plasma ball, plasma is produced when electrons moving at high speed (from the central electrode to the glass of the ball) bump into gas atoms and break them apart.

In the sun, the extremely high temperatures and pressure split the atoms to make plasma.

For classes where this activity is not to take up too much time, the teacher can demonstrate by holding the balloon/foam ball and walking around the circle.

Blow up a balloon for each student, only half inflating it, so that it is as round as possible.
Give each student a balloon to represent earth.
Ask students to hold the balloon with the tied neck at the bottom, then to draw on the equator with a sharpie.
Then draw on a rough outline of North America, using a globe or map as a guide. Add a dot for Vancouver.
Keen students can add other places that they know/are from.
(See notes and second photo for using a foam ball on a skewer instead.)

Place the light fixture in the centre of the room, on a desk/chair so that the bulb is near head-height for standing students.
Ask the students to make a circle around the light fixture, large enough so they have good elbow room between them.
Darken the room.

Ask students to point the North Pole of their "earth" balloons towards a high feature in the room e.g. a corner. The designated feature of the room is the "Pole Star" or "North Star". With the North Pole towards the Pole Star, each earth should be tilted, ideally at an angle close to the real tilt of the earth (24 degrees from the vertical).

Ask students to position their earth in front of their bodies so that the "sun" is shining on the earth, while maintaining the tilt towards the Pole Star.
Ask students which is east and west on their earth. If they look at the continent of North America, it should help them.
Ask students to rotate the earth in the correct direction: the sun rises in the east, so the light of the bulb should hit the east coast of North America first. They will be rotating their earths counterclockwise.
For the real earth, each rotation takes one day. As the sun hits our continent it is day time and as we continue to rotate it becomes night time.

Now, we will show what happens during the year, as the Earth orbits the Sun once.
Ask students to walk in a circle around the "sun", always tilting their North Pole towards the Pole Star, while continually rotating their earth counterclockwise (as viewed from above).
Once they have walked completely around the sun and ended up back where they started, a year has passed.
Now walk through another year while asking students to keep an eye on how much sun Vancouver on their Earth receives through the year.
Ask where in the circle does Vancouver get the most time in the sun? (The northern hemisphere will be tilted towards the sun at this position in the circle.) This is the position of midsummer - the summer solstice. Mark this place in the circle with "June" (on the floor, or on the wall behind the circle).
Ask where in the circle does Vancouver get the least sun exposure? (The northern hemisphere will be tipped away from the sun at this position in the circle.)
This is midwinter, or the winter solstice. Mark this position with "December". It should be exactly across the circle from June.

(Optional:
Add that the vernal (spring) and autumnal equinoxes fall between, in March and September.
Students can locate where their birthday month falls in the circle.)

Now ask students to orbit the sun once more, with their tilted, rotating earths, and watch what happens to the north and south poles.
Each of them get sun all day for part of the year.
The sun never sets in Northern Canada in the summer as it is so close to the North Pole.

(Optional:
Discuss seasons in the northern and southern hemispheres, especially if a child in the class is from the southern hemisphere.
In the northern hemisphere, mid summer is in June, whereas in the southern hemisphere, midsummer is in December.)

To review:
In our summer, the northern hemisphere is tilted toward the sun, so has more hours of sunlight through the day (as shown by the above activity). More hours of sun exposure makes it warmer. See this image to review this point: http://www.srh.noaa.gov/srh/jetstream/global/images/seasons.jpg
In addition, the hemisphere that is tilted toward the sun also receives more direct rays of sunlight (or rays that are closer to perpendicular or a 90° angle), which means more intense radiation, so more heat. See this image to show clearly: http://astro.hopkinsschools.org/course_documents/earth_moon/earth/seaso… (backup image: http://web.gccaz.edu/~lnewman/gph111/topic_units/Earth_Sun/01_23.jpg)
The combination of more direct rays of sunlight and more hours of daylight causes the hemisphere that is tilted toward the sun to receive more solar radiation and to have warmer temperatures.

Please note:
A common misconception is that our seasons are caused by the distance from the sun. This is not the case. Our seasons are caused by the the hours of sunlight, and the angle of the sun's rays, as described above.

The surface of the moon is covered in moon regolith (moon dust), a few metre to tens of metres deep. image at https://en.wikipedia.org/wiki/Regolith#/media/File:Apollo_11_bootprint…
Earth regolith is formed by erosion - the wind and rain brush tiny particles from rock and erodes them.
Moon has no running water and no wind - how is the regolith formed? By the impacts of meteorites.

Simulate the formation of moon regolith by banging one rock (the surface of the moon) with another rock (a meteorite).
A fine dust is formed. Collect the dust in the tray with the white lining.

To look a property of the regolith, lay the plexi sheet over the tray of regolith, and rub a hand over it.
Students will find that the regolith jumps up to the plastic, and dances up and down, and they can spend a while experimenting. If they cannot get the dust to jump, try using a drier (non-sweaty) hand and rubbing faster (a piece of cloth may also help).
The regolith jumps because of electrostatic charges. Rubbing the plexi sheet gives it a charge, which attracts the tiny grains of regolith that are also charged.

Moon regolith is even finer (1/100 mm) than the dust made here, and also more charged (because it is bombarded by charged particles from space). When we land on the moon, the dust coats space suits, solar panels and camera lenses. (See image at http://www.space.com/18907-schmitt-covered-with-lunar-dirt.html
_GPN-2000-001124.jpg). Moon regolith is also sharp enough to wear away space suits and scratch visors. (Image of regolith magnified at https://media1.britannica.com/eb-media/30/76830-004-38A20966.jpg)
Technology has overcome some of these challenges, for example electric fields are wired on camera lenses and surfaces to attract the moon dust to the side of the lenses.

To do the activity with a camping stove:
Push the straightened paper clip into the cork, to make a flame test tool.
Dip the wire in water, then in salt, then put it in the flame. The flame should be yellow.
Burn everything off the wire.
Dip the wire in water, then in copper sulphate if available, then put in the flame. The flame should be blue/green.
Potassium chloride, in large enough amounts of powder in the flame, burns purple.

To do the activity with a campfire:
Throw copper sulphate into the fire, to make green-blue flames.
Boric acid crystals make green flames.

The colours appear when electrons in the metals, excited by the heat, fall back to lower energy states and release light.
Fire can be different colours depending on what is burning.