Surveying the Solar System Teacher Guide

Introduction

Studying the small bodies of the Solar System provides a unique opportunity to investigate the dynamic nature of our Solar System, which is not revealed by studying the major planets. The sensitivity of the Rubin Observatory LSST Camera combined with the high frequency of repeated observations will result in the detection of millions of new Solar System objects, from near-Earth asteroids to dwarf planets.

In this investigation, students use mathematical and spatial visualization tools to characterize four main groups of small Solar System objects based on their orbital properties. They interpret how their observational evidence can be used to support the solar nebula theory. They then examine and classify some newly discovered objects from Rubin Observatory.

Learning Outcomes

• Students analyze and interpret Solar System objects' orbital properties to identify the group of objects to which they belong.
• Students construct explanations regarding the role of gravity in orbital motions and interactions between Solar System objects.
• Students provide evidence that supports the solar nebula theory of the Solar System’s formation.

Prerequisite Concepts

• Students should have been previously introduced to the concepts of Newton’s laws, including the law of gravity.
• Students should have been previously introduced to the concepts of Kepler’s Laws of Planetary Motion.
• Students should be acquainted with the idea that the Solar System formed from the collapse of a cloud of gas and dust.

Where This Fits In Your Teaching

• Solar System (objects and organization)
• Small Solar System objects (asteroids, TNOs, NEOs, Comets)
• Nebular theory of Solar System formation
• Kepler's laws
• Orbits
• Newton's laws of motion
• Gravity
• Comparative planetology
• Planetary geology

Suggested investigation which could come after this one:
Hazardous Asteroids helps students develop the practices and reasoning to predict whether a newly-discovered near-Earth asteroid is a potential threat to Earth. Students learn how repeated observations reduce uncertainty, and which factors can be used to predict the potential damage of an impact.

Investigation Timing

Online component: 45-80 minutes

Standards

Three-dimensional lesson summary

Students use models and graphical data to identify similarities and differences between groups of small Solar System objects and their orbital properties, then use patterns in their data to classify newly-discovered objects.

Building towards:

HS-ESS1-4 Use mathematical or computational representations to predict the motion of orbiting objects in the solar system.

MS-ESS1-2 Develop and use a model to describe the role of gravity in the motions within galaxies and the solar system.

MS-ESS1-3 Analyze and interpret data to determine scale properties of objects in the solar system.

Science Literacy and Critical Thinking Skills

• Analyzing and interpreting data
• Developing and using models

Background

Students may not think of the Solar System as dynamic, but gravitational interactions involving small Solar System objects have caused changes to their orbital motions, and these changes are still taking place. Asteroid orbits are changed by close encounters with more massive objects (especially the Sun and Jupiter). New comets making their first journey sunward often experience orbital and physical changes.

Studying the orbital properties and compositions of small Solar System objects provides clues to the formation and evolution of the Solar System. Near-Earth objects (NEOs) are detected and monitored for the purpose of protecting our planet. A long-term goal is to mine NEOs for valuable metals.

Openstax Astronomy textbook: Comets and Asteroids

Links to Videos

These resources are from a teacher workshop on this investigation.

Video: Meg Schwamb, "The Legacy Survey of Space and Time (LSST) and the Solar System"

Speaker slides

Teacher Notes

1. Solar System objects that appear to move faster across the background stars are closer to Earth and the Sun than objects that appear to move slowly. The observed motion is due to the object’s orbital speed and distance from the Sun, its distance from Earth, and Earth’s orbital speed around the Sun. This rule is generally true except for the brief period of time each year when the Earth passes a more distant object.

2. This investigation emphasizes three orbital elements: eccentricity, inclination, and semi-major axis. However, in order for an orbit to be accurately defined, six elements are necessary. More background about the orbital elements may be found here.

3. The semi-major axis of an orbit is used as a proxy for its distance from the Sun. This is a standard practice used for objects in the Solar System.

4. A variety of objects are included in the data for this investigation. They are officially designated by the International Astronomical Union as small Solar System bodies. The four main groups used here are near-Earth objects (NEOs), main belt asteroids (MBAs), comets, and trans-Neptunian objects (TNOs). In this investigation, all small Solar System bodies (with the exception of comets) at the orbit of Neptune and beyond are collectively referred to as TNOs. This includes objects in the Kuiper Belt and the inner Oort Cloud. Likewise, all asteroids between the orbits of Mars (1.5 au) and Jupiter (5.2 au) are considered to be part of the main asteroid belt, even though the majority of main belt asteroids are located between 2.1 au - 3.3 au.

5. In the current version of this investigation, the positions of comets in their orbits are not accurate. (The positions of other Solar System bodies are correct.) Comets and asteroids discovered in the last several years may not appear in the data.

Common Student Ideas

1. Students are taught that planet orbits are elliptical (not circular). Some students are surprised to see that the orbits of many small Solar System bodies and the major planets appear circular. In other words, their eccentricity is very low.

   Bridge to learning: Have students examine the histogram in the investigations that bins the number of asteroid orbits by their eccentricities. Note that although a few orbits approach an eccentricity of zero, they are not in truly circular orbits. Many asteroids and the planets have low eccentricities that make their orbits appear circular. (This tripped up many great minds in astronomy until Kepler came along.) It’s extremely difficult to achieve a circular orbit because of the many complex gravitational interactions in a multi-body system.

2. All Solar System objects orbit the Sun in the same direction.

   Bridge to learning: Some small Solar System objects orbit opposite the direction of most other bodies and planets (a retrograde orbit). Most often these are comets. Students may notice some of these retrograde orbits if they carefully observe the visualizations that are built into this investigation. If at first they don’t see them, advise them to try viewing only the comets. Objects with an inclination of more than 90० have retrograde orbits and are easily distinguished on the inclination histogram.

   Retrograde orbits are a result of close encounters of the object with another, more massive body. The orbits of TNOs are more easily altered because they move more slowly in their orbits due to their great distances from the Sun. Main belt asteroids with retrograde orbits may be comets that have been depleted of their volatile components, and had orbits altered by gravitational interactions with Jupiter.

   Retrograde orbits should not be confused with retrograde motion, which is when an object appears to move backwards with respect to the background stars, due to the greater orbital speed of Earth relative to the object. Retrograde motion is an apparent (not real) phenomenon, similar to the way a car appears to move backwards, relative to your car, when you pass it.

3. Asteroids are found only between Mars and Jupiter.

   Bridge to learning: Students will quickly discover through this investigation that asteroids are scattered throughout the planetary orbits. The main belt asteroids are the ones typically described in textbooks, occupying the space from roughly 2 to 3.5 au, placing them between Mars and Jupiter.

4. Students sometimes wonder how spacecraft are able to navigate through the main asteroid belt without crashing into an asteroid. Visualizations often contribute to this misconception; when viewed in a zoomed-out mode, the asteroids in the main belt seem densely packed.

   Bridge to learning: Asteroids are not that close together and most asteroids are very small, less than 1 km across. If you could make the densest part of the main asteroid belt (from 2.1-3.3 au) into a flat plane, it would have an area of about 6x10¹⁷ km² (and in reality, asteroids deviate far above and below this plane). In this scenario, each asteroid would have more than one million km² of real estate to itself, leaving lots of room for spacecraft to navigate through.

5. Asteroids in the main asteroid belt are the remnants of a planet that exploded, or never formed.

   How the asteroid belt formed is still an unresolved question. One idea suggests that when Jupiter and Saturn migrated outward to their current positions, they flung most of the asteroids farther out in the Solar System, and the current belt is what is left over. Another hypothesis is that most of the asteroid belt formed after Jupiter and Saturn reached their current positions, and has been filled by both material being flung outwards away from the Sun, and inwards towards the Sun. Jupiter’s gravity has both held the belt in place and prevented it from forming larger objects. (In the same way, Neptune’s gravitational influence stabilizes the orbits of objects in the Kuiper Belt.)
   
   

Common Student Questions

1. What’s the difference between an NEO (near-Earth object) and a near-Earth Asteroid (NEA)?

   NEOs are defined as small Solar System bodies whose orbit brings them into proximity with Earth’s orbit. Although the majority of these objects are asteroids (NEAs), some are comets.

2. When determining an orbit, why is the semi-major axis used instead of an average distance from the Sun?

   Since planetary orbits are elliptical, the semi-major axis is used (instead of the radius for a circular orbit). The semi-major axis also more accurately describes the changes in velocity and the orbital position of an object.
   

   Consider, for example, the case of a comet whose orbit extends from beyond Pluto to inside the orbit of Mercury. The time it spends beyond Pluto is much longer than the time it spends near Mercury, due to the the slower velocity at aphelion (its farthest point from the Sun) vs. perihelion (its closest point to the Sun).

3. Why are there so many more main belt asteroids, compared to the other groups?
   

   It is likely just an observational bias. There are probably lots more TNOs and comets, but it is harder to detect them at such far distances. Another factor is the influence of Jupiter’s gravitational field, which creates a stable reservoir for asteroids.

4. Why do many comets have eccentric and very inclined orbits?
   

   There are two main groups of comets: long period comets and short period comets (sometimes referred to as Jupiter family comets). The short period comets are as a group less eccentric and inclined than long period comets because they originated from the Kuiper Belt. The long period comets have orbits that are more eccentric because they originated from far distances in the Oort Cloud. They also have a higher range of inclinations, since they originated from a cloud (spherical) distribution instead of from a disk (Kuiper Belt).

5. Why didn't the objects in the main asteroid belt form a planet?

   The objects in the main asteroid belt can’t form a planet because there isn’t a high enough density for objects to collide and stick together. Even when collisions between asteroids do occur, if their mutual approach speed is too slow, they won’t stick together, and if it’s too fast, they may explode into more fragments, since many asteroids are just “rubble piles” of loosely consolidated materials.

6. Why aren't there a lot more near Earth objects (NEOs), compared to main belt asteroids (MBAs)?

   Students may reason that since the Sun's gravity causes an increase in the density of objects nearer the Sun, there should be a lot more asteroids closer to the Sun (like NEOs) instead of in the main reservoir of asteroids between Mars and Jupiter.

   There are fewer NEOs compared to MBAs because the “lifetime” of a typical NEO is only a few million years. NEOs orbit in the inner Solar System, where the potential for gravitational interaction with one of the inner planets or the Sun causes their orbits to become unstable. They eventually will either impact a planet or the Sun, or be flung farther out into the Solar System. In fact, there would be few NEOs today if they were not replenished by new arrivals from the main asteroid belt.
   
   Also, NEOs are harder to find as many of them are very small. Another factor is that some NEOs spend most of their time closer to the Sun than Earth, so you would have to look into the Sun’s glare to find them.
   

7. What causes the distance gaps in the distribution of the main belt asteroids?

   These are due to orbital resonances with Jupiter. More information about this may be found here.

Differentiation

Each investigation includes some (DEI) questions that invite students to share their opinions and experiences to promote diversity, equity and inclusion in the classroom. In this investigation, the DEI questions are 49-51. This may be an opportunity for a small group or class discusssion, or if in an asynchronous setting, students can contribute to a discussion forum.

The Extension Activity may be used for further exploration or reinforcement.

Options for extended exploration:

1. Some objects within the asteroid belt have an extended appearance, like an asteroid with a “tail”. These are known as active asteroids. Encourage students to locate a list and information about these objects (such as this list on wikipedia) and attempt to develop a profile for factors they have in common (inclinations, eccentricities, composition, etc.), or how they differ from other short-period comets or main belt asteroids. More information about the properties of each individual asteroid may be found by searching its name at the Minor Planet Center.

2. Subcategories of Solar System objects may pique students’ interest as they view data on histograms and scatter plots. Any of these could be its own topic for further study, such as the Trojans in Jupiter’s orbit, long- vs. short-period comets, or the asteroids found between Jupiter and Neptune (the Centaurs). Here’s a link to subcategories. Students can compare and contrast characteristics between these groups, and investigate why these differences exist.

3. Even though the data sets and plots in the investigation may not be modified to do freeform exploration at this time, many of the data sets we use can be directly downloaded from the IAU Minor Planet Center. You can then use a spreadsheet and plotting program to interrogate your own data sets.

The Minor Planet Center has some pre-generated plots that are interesting for exploration purposes. For instance, a histogram of the numbers of main belt asteroids vs. semi-major axes reveals that they cluster into groups (due to orbital resonances with Jupiter). Students may be able to investigate whether there is a way to further categorize these groups based on their other orbital parameters.

4. Here is a plot of asteroid orbit size vs.inclination. What might explain the groupings?

Ideas for Further Study

• Extension Activity

  This activity uses an infographic published in a blog post on February 8, 2020, by Nicholas LePan, an author at Visual Capitalist. While it is titled, “A Map of Every Object in our Solar System,” if you read the fine print, you will find that the title is not really accurate. Eleanor Lutz created this graphic in June, 2019 by mapping every object greater than 10 km in diameter from five NASA data sets. While not comprehensive, it provides a view of the many small objects orbiting the Sun.

  Here is a high resolution version of the graphic. It may be printed up to a size of 32.6” by 32.6”.

  Say before students examine the infographic: “This graphic shows every known object in the Solar System larger than 10 km in diameter. The map shows the position of each object in its orbit in 1999. The key is at the bottom left corner.”

  Give students some time to generate questions, comments or observations about the graphic, then use the questions below for discussion.
  
  

  Leading Questions

  • What is the object that the majority of Solar System objects orbit? Why?
  • What are the most common objects in the Solar System?
  • Are small Solar System objects distributed uniformly in the Solar System? (If not, what patterns do you notice?)
  • Can the shape of the Solar System best be thought of as a sphere or a disk? Why do you think it has this shape?
  • Do all orbits have the same shape?
  • Do you think we have discovered most of the objects in the Solar System? If not, why not?
  • Can you think of a reason it might be important or practical to look for new Solar System objects?
  • What questions do you have about Solar System objects as a result of studying this graphic?

  
  
  

  Landing discussion

  If you are using the “Launch, Learn, Land” technique, here are some questions that may be used for a follow up discussion:

  You may wish to remind students that this infographic does not show all objects in the Solar System, rather it shows all known objects in the Solar System larger than 10 km in diameter, and their positions, as of 1999.

  • What are some of the strengths and weaknesses of the infographic as a model to teach about small Solar System objects?
  • Are the orbital properties (distance and shape) you discovered for the NEOs, MBAs, TNOs, and comets consistent with what is shown in this infographic?
  • Could you have discovered the range of inclinations for NEOs, TNOs, MBAs, and comets by studying this image? Why or why not?
  • What is the dominant force that affects the distribution of objects, and the shape of orbits for the Solar System?
  • What can cause the orbits of small objects to become inclined or eccentric?
  • What could explain why most objects appear to be in the same plane orbiting the Sun?
  • If this graphic was updated to show the locations of all currently-known objects in the Solar System larger than 10 km in diameter, what do you think the most noticeable change might be? Explain your answer.