Context

Astronauts on spacewalks are likely to encounter fast-moving particles called meteoroids. A meteoroid is usually a fragment of an asteroid consisting of rock and/or metal. It can be very large with a mass of several hundred metric tons, or it can be very small- a micrometeoroid which is a particle smaller than a grain of sand. Micrometeoroids are usually fragments from comets. Every day Earth’s atmosphere is struck by millions of meteoroids and micrometeoroids. Most never reach the surface because they are vaporized by the intense heat generated by the friction of passing through the atmosphere. It is rare for a meteoroid to be large enough to survive the descent through the atmosphere and reach solid Earth. If it does, it is called a meteorite.

In space there is no blanket of atmosphere to protect spacecraft from the full force of meteoroids. It was once believed that meteoroids traveling at velocities up to 80 kilometers per second would prove a great hazard to spacecraft. However, scientific satellites with meteoroid detection devices proved that the hazard was minimal. It was learned that the majority of meteoroids are too small to penetrate the hull of spacecraft. Their impacts primarily cause pitting and "sandblasting" of the covering surface.

Recently spacecraft debris is of great concern to spacecraft engineers. Thousands of space launches have left many fragments of launch vehicles, paint chips, and other "space trash" in orbit. Most particles are small, but they travel at speeds of nearly 8,000 meters per second. These space-age particles have become a significant hazard to spacecraft and to astronauts on extravehicular activities.

Engineers have protected spacecraft from micrometeoroids and space trash in a number of ways, including thick-wall construction and multi-layer shields consisting of foil and hydrocarbon materials. A micrometeoroid striking multi-layer shields disintegrates into harmless gas that disperses on inner walls. Spacesuits provide impact protection through various fabric-layer combinations and strategically placed rigid materials.

Although effective for particles of small mass, these protective strategies do little if the particle is large. It is especially important for spacewalking astronauts to be careful when they repair satellites or do assembly jobs on the International Space Station. A lost bolt or nut could damage a future space mission through an accidental collision. (Note: Low orbit tends to be clearer of particles than higher orbits because low orbit particles tend to decay and burn up in the atmosphere.)

Investigation

Pea Shooter Meteoroids

The effects of high-speed micrometeoroid impacts can also be simulated with a "pea shooter." The shooter is actually a plastic milkshake straw. The projectile can be dried peas, popcorn, dried lentils, etc. The object of the activity is to penetrate tissue paper with the projectile. As with the Potato Astronaut activity, the velocity of the impactor determines the penetration.
 

Materials and Tools Checklist Plastic milkshake straw Dried peas, popcorn, etc. Tissue paper (for wrapping presents) Cardboard box Tape Eye Protection Objective • To compare the effect on tissue paper penetration between low and high speed projectiles. Procedure Step 1. Cover the opening of a box with tissue paper. Stretch the paper tight. Step 2. Drop a pea or other projectile from a distance of approximately 1 meter on to the tissue paper. Does the pea penetrate? Step 3. While wearing eye protection, stand back a few meters from the box and blow the pea through the pea shooter at the tissue paper. Does the pea penetrate? (With a little practice, the pea should penetrate the paper.) Step 4. Investigate what happens when more than one layer of tissue paper is used to cover the box opening.

Safety Precautions Students must wear eye protection. Caution students not to inhale through the straw. Connections: Mathematics Refer to the "Potato Astronaut- Part One" activity that follows.

Extensions • Tape two straws to end. Does that increase the velocity of the projectile? • Experiment with projectiles that have a greater mass than the pea. • Add a second layer of tissue paper to the box to see what effect the second layer has on penetration. • Is there any relationship between the ability to penetrate the tissue paper and the distance the shooter stands from the box?

Investigation

Potato Astronaut-Part One

The effects of high-speed micrometeoroid impacts are simulated with a potato and a straw. Students hold the potato in one hand and stab it with the other using a plastic milkshake straw. The penetration depth into the potato relates to the speed of the stabbing action. A straw slowly pushed into the potato collapses. The plastic isn't strong enough to support the force exerted at the opposite ends of the straw. However, when the straw is thrust rapidly into the potato, the straw easily penetrates and passes through. The straw enters the potato before it has a chance to collapse. As it enters, the surrounding potato helps support the straw by shoring up its sides.

Materials and Tools Checklist Potato Plastic (milkshake-size) straw Objective • To investigate the relationship between velocity and penetration depth when a potato is struck with a plastic straw. Procedure Step 1. Hold a raw potato in one hand. (See illustration.) While grasping the straw with the other hand, stab the potato with a slow motion. Observe how deeply the straw penetrates the potato. Step 2. Repeat the experiment but this time stab the potato with a fast motion. Observe how deeply the straw penetrates the potato. Compare your observations with the results of step 1. Safety Precautions Be careful to hold the potato as illustrated so that the straw does not hit your hand. Work gloves will provide additional protection.

Connections: Mathematics
The kinetic energy output of an impact, given in Joules, is calculated with the following equation:

 KE = 1/2mv²

m = mass of impacting object
v = velocity of impacting object

Note: The mass in this activity is actually the combined mass of the straw and the hand and forearm driving it.

Investigation

Potato Astronaut-Part Two

In part one of Potato Astronaut, students found that "high speed" impacts enabled the plastic straw to penetrate the potato without collapsing. Challenge the students to design a way to protect the potato from damage caused by impacts using just the materials they brought to the classroom. Their solutions to the challenge should be flexible and light in weight.
 

Materials and Tools Checklist Plastic (milkshake-size) straw Potato Tissue paper, notebook paper, hand chiefs, rubber bands napkins, aluminum foil, wax paper, plastic wrap, etc. Impact Resistance Test Stand (from Teacher Tech Brief) Procedure Step 1. Students design a method for protecting potato astronauts from damage caused by the plastic straw when the straw is quickly stabbed into the potato. Step 2. After students have tested a method for protecting a potato, conduct a discussion to evaluate technologies developed. Refine the constraints for a protection system (e.g. the materials used must together be no thicker than _ mm). Step 3. Have students redesign their system based on the refined constraints. Conduct additional impact tests with the straw. Step 4. Test protection systems by using the an Impact Resistance Test Stand as described in the Teacher Tech Brief found earlier in the guide. Evaluate the effectiveness of the protection systems developed..

Extensions
• Compare technologies for protecting astronauts from micrometeoroid and space debris impacts to other protective technologies such as bullet-proof vests, suits of armor, shields on power tools, and windshields on vehicles. How does the function determine the form? (e.g. Motorcycle helmet--provide protection during crash . . . be streamlined . . . comfortable to wear . . . protect face from bug and rock impacts, etc.)

• Experiment with different fabrics and fabric combinations for protective garments