Abstract
Did you know that in addition to the Sun and planets, our solar system is filled with millions of asteroids, which are chunks of rock left over from the early days of its formation, or from collisions between larger objects like planets? Agencies like NASA track asteroids, not only because they might pose a threat to humanity by colliding with Earth, but because they can provide us with information about the history of our solar system, and even be useful for mining raw materials in space! In this science project, you will explore some of the vast amount of data NASA has accumulated about asteroids, and use data analysis to see what you can learn about our solar system.Summary
Ben Finio, PhD, Science Buddies
The data used in this science project is from the NASA/JPL Small-Body Database Search Engine.
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Objective
Use data analysis to investigate NASA asteroid data and explain findings using your background research on the solar system.
Introduction
You may have seen science-fiction movies like Armageddon and Deep Impact, where life on Earth is threatened by a catastrophic collision with a comet or asteroid. Believe it or not, our solar system is actually filled with millions of comets and asteroids, and keeping track of all of them—including where they are, where they're going (including if they could possibly hit Earth!), how big they are, and even what they're made out of—is quite a daunting task. Luckily, the National Aeronautics and Space Administration (NASA) and the Jet Propulsion Laboratory (JPL) have an enormous, publicly accessible computerized database of information about hundreds of thousands of asteroids and comets in our solar system. In this astronomy science project, you will examine some of the information available in this database, and see what insight it provides into the formation and distribution of objects within our solar system. On the way, you will learn how to manipulate massive amounts of data. Before you get started, here is some more background information.
Comets and asteroids are both rocky chunks of matter, smaller than planets, that are either leftover pieces from the early days of our solar system's formation, or were created when two larger objects (like planets, moons, or even bigger asteroids/comets) collided, causing smaller pieces to break off. In general, asteroids are rocky objects that formed in the "warmer" region of the solar system, closer to the Sun (mostly inside the orbit of Jupiter). Comets formed farther out in the colder regions of the solar system, causing them to be icy; in addition to rocks and dust, they are made of frozen water and even frozen gases like methane and carbon dioxide. When they get closer to the Sun, the icy materials vaporize and create a visible "tail" on the comet. There can be some confusion between the two because, for example, rocky asteroids can sometimes travel farther out in the solar system (even all the way out by the planet Neptune!), and icy comets will sometimes travel closer to the Sun. The term "minor planets" can also be used to refer to both asteroids and comets. This science project will just deal with asteroids, so you do not need to worry too much about this distinction.
Figure 1 shows images of several different asteroids, and a picture of a comet with a visible tail taken from Earth:
Figure 1.High-resolution images of different asteroids. The largest, 4 Vesta, is 525 km in diameter. The smallest, 25143 Itokawa (barely visible in the image), is only 640 m long. The Hale-Bopp comet, visible from Earth in 1997. Hale-Bopp is approximately 60 km wide (Philipp Salzgeber, 1997).
So, what data does NASA actually track about asteroids? The JPL database contains information about their orbits, or trajectories through space as they rotate around the Sun. Asteroids' orbits are elliptical, as shown in Figure 2. Several different parameters define an asteroid's elliptical orbit. The basic parameters you need to know for this science project are:
- Semi-major axis is a distance equal to one-half of the major axis of an ellipse. The major axis passes through the center of the ellipse and ends at its widest points (see Figure 2). Semi-major axis is measured in astronomical units (AU). One AU is equal to Earth's average distance from the Sun.
- Perihelion is the object's closest distance to the Sun (see Figure 2). Perihelion is also measured in AU.
- Aphelion is the object's farthest distance from the Sun (see Figure 2). Aphelion is also measured in AU.
- Eccentricity measures "how circular" an orbit is. A perfect circle has an eccentricity of 0. As an ellipse becomes longer and skinnier, its eccentricity approaches (but never reaches) 1. Eccentricity does not have units.
- Inclination measures the angle between the plane of an asteroid's orbit and the plane of Earth's orbit around the Sun. Inclination is measured in degrees (deg).
- Orbital period is how long it takes the asteroid to orbit the Sun once. Orbital period is measured in years (y).
Figure 2. Some of the parameters defining an elliptical orbit.
The database also includes physical information about each asteroid. Some of the physical information includes:
- Diameter of the asteroid is approximate (note that many asteroids are not perfectly spherical, so this is an average diameter). Diameter is measured in kilometers (km).
- Extent measures the dimensions (length, width, and height) of a "box" that would fit around the asteroid. For a perfectly spherical object, all three numbers would be the same. Extent is measured in kilometers (km).
- Albedo is a measurement of how much light the surface of the asteroid reflects. Albedo does not have units. A value of 0 means that no light is reflected, and a value of 1 is the amount of light that would be reflected off of a perfectly flat, white, diffusing surface (meaning light is scattered in all directions). Most asteroids have an albedo ranging from 0.01 (very dark) to 0.7 (very bright).
- Rotation period is how long it takes the asteroid to spin about its own axis. Be careful not to get this mixed up with orbital period. Rotation period is measured in hours (h).
- GM is an expression of the asteroid's mass multiplied by the universal gravitational constant G (see the Technical Note about converting this number to mass in kilograms). GM is measured in kilometers cubed per seconds squared (km³/s²).
- Spectral type is an asteroid's classification based on information about how it reflects electromagnetic radiation (like radio waves, visible light, microwaves, and X rays), which gives scientists an idea of what materials the asteroid is made of. There are two primary classification systems, the Tholen system and the SMASS system. While there are some differences between the two systems, in general, asteroids can be grouped into three main categories (there are additional smaller groups and sub-groups not listed here):
- C-type or "carbonaceous" asteroids, which contain carbon compounds. This is the most common type of asteroid, making up about 75% of known asteroids. They consist primarily of clay and rocks, but can also contain large amounts of water.
- S-type ("silacaceous" or "stony") asteroids. These are the second most common type, about 17% of known asteroids. They consist mostly of stony materials (silicates), and nickel-iron, but can also contain valuable metals like platinum.
- M-type (also called X-type depending on the classification system) or "metallic" asteroids. These asteroids consist mostly of nickel-iron and other metals.
GM is an expression of an asteroid's mass in units of the mass, M, times the gravitational constant, G, which is defined as:
or (converting the distance units to kilometers instead of meters)
So, in order to calculate the mass, M, of an asteroid in kilograms, you have to divide the product, GM, by the gravitational constant, G:
Be sure to keep track of units; the physical parameter table that you will use in the Procedure gives GM in km3/s2, so you should use the second value for G when calculating mass.
Throughout the course of this science project, you may need to do additional research about asteroids to help guide your data analysis, or learn more about some of the terms discussed. The Bibliography section provides some resources about asteroids to help get you started.
To do the Procedure section of this science project, you will need to know how to make a histogram (including how to adjust the bin size) and a scatter plot. If you have not made these types of graphs before, consult the references provided in the Bibliography. Advanced students may wish to apply further statistical analysis; for example, curve-fitting and determining R-squared values (for scatter plots), and determining whether histograms follow a normal distribution.
Terms and Concepts
- Asteroid
- Comet
- Orbit
- Parameters describing an asteroid orbit:
- Semi-major axis
- Astronomical unit (AU)
- Perihelion
- Aphelion
- Eccentricity
- Inclination
- Orbital period
- Physical information about each asteroid:
- Diameter
- Extent
- Albedo
- Rotation period
- GM
- Spectral type
- C-type
- S-type
- M-type or X-type
- Data-analysis and Statistics terms:
- Histogram
- Scatter plot
Questions
- Asteroids that share similar orbital parameters are sometimes categorized into "families," "groups," and "orbit classes." Can you do background research about these different categories?
- What are the differences between different spectral types of asteroids? Do different types tend to occupy different locations in the solar system?
- When did humans start discovering asteroids? Are certain types of asteroids easier to find than others? How many asteroids have been discovered?
- Are there estimates for how many undiscovered asteroids remain in the solar system?
Bibliography
These resources will help you get started learning more about asteroids and the solar system:
- Freudenrich, C. (n.d.). How Asteroids Work. Retrieved July 13, 2013.
- NASA Solar System Exploration. (n.d.). Asteroids — Overview. Retrieved July 13, 2013.
- Wikipedia Contributors. (2013, July 15). Asteroid. Retrieved July 17, 2013.
- Clavi, W. and Harrington, J.D. (2013, July 25). NASA's WISE Finds Mysterious Centaurs May Be Comets. Retrieved August 9, 2013.
- Jet Propulsion Laboratory (n.d.). JPL Small-Body Database Search Engine. Retrieved July 17, 2013.
- Williams, A. (2012, Sept. 20). Sol Planetary System Data. Retrieved July 17, 2013.
These resources will help you learn more about statistics, data analysis, and plotting:
- MathsIsFun.com. (n.d.). Histograms. Retrieved July 18, 2013.
- MathsIsFun.com. (n.d.). Scatter (XY) Plots. Retrieved July 18, 2013.
- MathsIsFun.com. (n.d.). Normal Distribution. Retrieved July 18, 2013.
Materials and Equipment
- Computer with internet access
- Spreadsheet program such as Microsoft Excel or Open Office Calc
Experimental Procedure
Downloading the Data
Although the asteroid data is publicly available on the internet, you will need to create a copy on your computer so the spreadsheet or data-analysis program of your choice can access it.
- The asteroid data can be downloaded from the
JPL Small Body Database Search Engine, however, this database is very large; it contains over 620,000 asteroids! This can be a problem for older computers, which might freeze up when trying to load and plot that many data points, and for older spreadsheet programs (for example, older versions of Microsoft Excel only allow 65,536 rows of data). So, we have compiled several files that you can download, depending on the speed of your computer.
Note: The comma-separated variable (CSV) data, which can be opened by most spreadsheet programs, is saved in zipped (ZIP) format to save space and download times. After you have saved the zipped file, you'll need to unzip it to access the CSV file. In Windows (versions XP and later) you can do this by right-clicking the file and selecting "Extract All". In Mac OS X you can just double-click the zipped file. If you have an older operating system, you may need to use a third-party program to open zipped files.
- Asteroid_Data_6000.csv.zip This file should work well on most computers and spreadsheet programs.
- Asteroid_Data_60000.csv.zip This file will open in older spreadsheet programs (Excel 2003, Open Office Calc 3.2 and earlier), but plotting operations may run slowly on older computers.
- Asteroid_Data_All.csv.zip Only use this file if you are using a newer spreadsheet program (e.g. Excel 2007 or later, Open Office Calc 3.3 or later) on a new computer. If this file causes your computer to freeze, try using a smaller file.
These files were downloaded from the JPL database in July 2013. In the future, the database will grow as new asteroids are discovered and added to it. Increases in computing power may also make it more feasible to deal with the entire database, instead of a smaller subset of the data. If you would like to download the most recent data, you can follow JPL's tutorial on using the Small-Body Database Search Engine. The files here contain the "asteroid - basic" and "asteroid - physical" pre-defined output field sets.
- Now that you have downloaded the database, open it in your spreadsheet program (many spreadsheet programs can open CSV files directly, but you may need to look at the Help section or do an internet search for how to import a CSV file into your spreadsheet program, e.g. search for "How to import a CSV file in Excel 2007"). The data is organized into columns, with a header at the top of each column. The CSV file uses abbreviations for these headers; Table 1 shows the relationship between the output variables from the search engine, and the abbreviations used in the CSV file. Note: not every variable listed in Table 1 was explained in the Introduction, but you do not need to worry about each variable. However, if you are curious, you can do additional background research to learn about them.
| Output Fields | CSV Table Header |
| object full name/designation | full_name |
| [ a ] semi-major axis (AU) | a |
| [ e ] eccentricity | e |
| [ i ] inclination (deg) | i |
| longitude of the ascending node (deg) | om |
| argument of the perihelion (deg) | w |
| [ q ] perihelion distance (AU) | q |
| [ Q ] aphelion distance (AU) | ad |
| orbital period (years) | per_y |
| number of days spanned by the data-arc (d) | data_arc |
| orbit condition code (MPC 'U' parameter) | condition_code |
| number of observations (all types) used in fit | n_obs_used |
| number of delay-radar observations used in fit | n_del_obs_used |
| number of Doppler-radar observations used in fit | n_dop_obs_used |
| [ H ] absolute magnitude parameter (mag) | H |
| object diameter (from equivalent sphere) (km) | diameter |
| object bi/tri-axial ellipsoid dimensions (km) | extent |
| geometric albedo | albedo |
| rotation period (h) | rot_per |
| [ GM ] mass expressed as product mass and grav. const. G (km³/s²) | GM |
| color index B-V (mag) | BV |
| color index U-B (mag) | UB |
| color index I-R (mag) | IR |
| spectral taxonomic type (SMASSII) | spec_B |
| spectral taxonomic type (Tholen) | spec_T |
Example Plots and Data Analysis
This section will walk you through several example plots and data analysis, which will prepare you to do your own plots and analysis in the next section.
- Relationship between distance from the Sun and orbit period (scatter plot).
- Background research: Look at the following data table, which shows the time (in Earth years) it takes some of the planets in our solar system to orbit the Sun once:
Table 2. Orbital periods for some of the planets in our solar system. Data from this page by Alexander J. Wilman, Jr. at Princeton University.Planet Orbit Period (y) Mercury 0.24 Venus 0.61 Earth 1.00 Mars 1.88 Jupiter 11.86 Saturn 29.44 Uranus 84.01 Neptune 164.79
- Make a prediction: According to Table 2, as planets get farther away from the Sun, they take longer to orbit it. Do you think this will be true for asteroids as well, or will they show a different relationship?
- Test your prediction: The semi-major axis is chosen here as a measure of distance from the Sun. Use your spreadsheet program to make a scatter plot of orbit period vs. semi-major axis. If you do not know how to make a scatter plot, look at the Help section for your spreadsheet program or do an internet search (for example, "How to make a scatter plot in Excel 2007"). Your scatter plot should look something like Figure 3 (the exact appearance will depend on which data file you used).
Example scatter plot measures an asteroids orbit period over the semi-axis major in astronomical units. The scatter plot shows a relatively linear graph where an increase in the semi-major axis leads to an increase in the orbit period.
Figure 3. A scatter plot of asteroid orbit period vs. semi-major axis.
- Draw conclusions: Do you see a correlation between orbit period and semi-major axis? Was your prediction from step b. correct? Do asteroids with longer orbits take longer to orbit the Sun?
- Advanced students: Fit your data to a curve (e.g. the "add trendline" function in Excel; look at your spreadsheet program's Help section or do an internet search if you do not know how to do this) and check the R-squared value. What fits the data better (meaning, what has the highest R-squared value): a linear trendline, a quadratic trendline, or something else? Can you explain this using your knowledge about asteroids? Do you need to do more research to explain the trend?
- Background research: Look at the following data table, which shows the time (in Earth years) it takes some of the planets in our solar system to orbit the Sun once:
- Histogram of semi-major axis.
- Background research: In your background research about asteroids, you might have encountered discussions about different groups or regions of asteroids. For example, many asteroids are clustered in the main asteroid belt, between the orbits of Mars and Jupiter, but there is also a large group of asteroids called the Jupiter Trojans that share Jupiter's orbit around the Sun.
- Make a prediction: How do you think these groups would affect the distribution of asteroids throughout the solar system, in terms of rough distance from the Sun (semi-major axis)?
- Test your prediction: Make a histogram of the semi-major axis data. If you do not know how to make a histogram, look in your spreadsheet program's Help section or do an internet search (e.g. "How to make a histogram in Excel 2007"). It should look something like Figure 4. Depending on which data file you used, you may need to adjust the bin size; again, look at the Help section or do an internet search if you do not know how to do this.
Example histogram measures the most common semi-major axis values for asteroids. Large spikes in the histogram show that the majority of asteroids have a semi-major axis value between 2.1-2.5 AU. Another spike occurs between 2.55-2.85 AU, and the smallest spike between 3-3.3 AU.
Figure 4. A histogram of semi-major axis data for the asteroids.
- Draw conclusions: There are several prominent "bumps" in the graph, is this what you predicted in step b.? Can you identify different groups of asteroids based on your background research? For example, where are the Jupiter Trojans?
- Advanced students: Does the histogram for semi-major axis follow a normal distribution? Why or why not? In addition to distinct groups like the Jupiter Trojans, there are also several prominent "gaps" in the main belt area. Can you find out why these gaps exist? Hint: They also have to do with effects of Jupiter's gravity.
Making Your Own Plots
Now that you have run through two examples, you are ready to try your own data analysis. We will provide several suggestions, but you are free to explore the data any way you want. For your science project, just make sure that you always have a reason for what you are doing; do not just plot any two random variables together! You should either make a plot, then try to explain its appearance using your background research on asteroids, and/or do background research and then make your own plot to confirm the information that you read.
Note that not all of the output fields are defined for every single asteroid in the database (for example, there are 620,291 asteroids in the database as of July 2013, but only 2,509 of them have diameter measurements). So, the following data files might also be useful for your analysis:
- Asteroid_Data_diameter_defined.csv.zip All asteroids with diameter defined (2,509 asteroids)
- Asteroid_Data_SMASS_defined.csv.zip All asteroids with SMASS spectral type defined (1,665 asteroids)
- Asteroid_Data_Tholen_defined.csv.zip All asteroids with Tholen spectral type defined (980 asteroids)
- Asteroid_Data_albedo_defined.csv.zip All asteroids with albedo defined (2,450 asteroids)
- Asteroid_Data_rotation_defined.csv.zip All asteroids with rotation period defined (5,496 asteroids)
- Make scatter plots for the following sets of variables. Do you see any distinct clumps or groups of asteroids in the resulting plots? Try doing background research to identify named groups or families of asteroids that appear in your plots.
- Eccentricity vs. semi-major axis
- Inclination vs. semi-major axis
- Inclination vs. eccentricity
- Advanced students: Can you make a three-dimensional scatter plot of all three variables? Note: This may not be possible in some spreadsheet programs.
- Make a histogram of asteroid diameter. How is asteroid size distributed (e.g. are there more small asteroids, more large asteroids, even numbers of all asteroid sizes)? Can you explain this based on what you know about the formation of the solar system?
- Advanced students: Is the histogram normally distributed?
- Make a scatter plot of asteroid diameter vs. semi-major axis. Does it appear that different sized asteroids are distributed differently throughout the solar system? Does the data appear to follow a trend, or is there no trend at all?
- Advanced students: Can you fit a trend line (or curve) to the data? If so, what is the R-squared value?
- Pick at least one combination of variables for a scatter plot, or a single variable for a histogram, that is not listed here or in the example plots. Make the plot, and then try to explain it based on your background research (you may need to do additional research).
- Repeat steps 1–4 of this section, and the two example plots, but this time break the asteroids up by either Tholen or SMASS spectral type.
- Are the plots different for different types of asteroids? For example, S-type asteroids are more common toward the inner part of the main belt, and C-type asteroids are more common toward the outer part of the main belt. Can you confirm this using histograms?
- Note: there are many sub-groups within the Tholen and SMASS classification systems, which will each have different letters in your data file. For example, F, B, and G-type asteroids all fall under the "C-group" in the Tholen classification system. There are also different variations of C-type like CB, CF, and CP. In order to make your plots easier to understand, it might help to condense all of the asteroids into three large groups (C, S, and M). This will require some effort on your part to re-organize the data file. We recommend using the files that only contain asteroids with Tholen (or SMASS) classifications to make this job easier.
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Global Connections
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Variations
- This science project focused on asteroid data. There are far fewer comets in the JPL database (3,218 as of July 2013), but try repeating the science project using comet data.
- This science project used data files that contained the default "asteroid — basic" and "asteroid — physical" output fields from the JPL search engine. There are other parameters in the search engine that are not included in these default output fields. There are also some parameters that are included, but were not discussed in this science project (for example, the absolute magnitude parameter H, and the location on Earth where the asteroids were discovered). Can you do research on these other parameters and include them in your science project?
Careers
If you like this project, you might enjoy exploring these related careers:
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