Background

Near-Earth Objects (NEOs) are asteroids and comets with perihelion distances less than 1.3 au. The vast majority of NEOs are asteroids. NEOs are divided into four groups (Atira, Aten, Apollo, and Amor) according to their perihelion distance (q), aphelion distance (Q), and their semi-major axis (a). Each of these measurements are illustrated on the figure below:

Figure 1. Measurements along the major axis of an asteroid orbit. Credit: Rubin Observatory/NSF/AURA

More background about the four classes may be found here.

Potentially Hazardous Asteroids (PHAs)are a subclass of near-Earth objects (NEOs). A PHA must have an Earth Minimum Orbit Intersection Distance (MOID) of 0.05 au or less, and a minimum diameter of 140 meters. Objects of this size or larger are capable of causing serious damage if they strike Earth.

This is not to say that an Earth impact from an object less than 140 meters in diameter is inconsequential. The graphic below addresses the relative amount of damage caused as a function of asteroid size. Even an asteroid 25 meters in diameter is capable of devastating a city.

Figure 2: NEO size and hazard. NEO estimates as of 2022, pre-LSST start. Credit: Johns Hopkins University / Applied Physics Lab.

In 1908, an object approximately 50 meters in size exploded over Tunguska, Russia with the equivalent of 5-10 megatons of TNT (hundreds of times greater than the first atomic bombs), leveling over 2,000 square kilometers of forest. This is also the approximate diameter of the impactor that made Meteor Crater in Arizona. If a similar event occurred over a major metropolitan area, it could cause millions of casualties.

Figure 3: Equivalent area of destruction for a Tunguska-sized asteroid over New York City. Background map imagery from Mapbox; Damage pattern from Boyarkina, A. P., D. V. Demin, I. T. Zotkin, and W. G. Fast. 1964. “Estimation of the blast wave of the Tunguska meteorite from the forest destruction.” Meteoritika 24:112-128.

Objects smaller than 25 meters entering the atmosphere usually fragment into small pieces that drift slowly down to Earth’s surface without making any large craters. On a daily basis, about one hundred tons of interplanetary material accumulates on Earth’s surface.

The U.S. Congress Public Law No: 109-155 of 2005 directed that NASA should find, track, and characterize at least 90% of the predicted number of NEOs that are 140 meters and larger in size by 2020. By 2020, less than half of the estimated 25,000 NEOs that are 140 meters or larger in size were detected.

Rubin Observatory’s efforts over the first ten years of the Legacy Survey of Space and Time (LSST) should detect around 70% of the predicted number of PHAs, which is a significant contribution to the 90% threshold. Combined with the efforts of other large upcoming missions (such as NASA's NEO Surveyor, scheduled for launch in 2027), the 90% milestone should be achieved by the late 2030s.

Figure 4: Discoveries of Near-Earth Objects through May 2026.

So should you stay awake at night, wondering if you are about to be struck by a killer space rock? No. Potentially Hazardous Asteroids are a very small fraction of the number of asteroids. Statistically, even within the PHA group, impacts are very rare.

More information may be found on these pages:

The Center for Near Earth Objects Studies - Find news and data about close approaches of asteroids to Earth.

Planetary Defense at NASA

OpenStax Astronomy textbook:

• Potentially Hazardous Asteroids
• Asteroids and Planetary Defense

More interactive resources are located in the Videos and Auxiliary Content page.

Teacher Notes

1. Our investigations are designed so that students cannot proceed to the next page without answering each question. If you would like to quickly preview the entire investigation, you can create a free educator account and use the “logged in" mode on the Start page.
2. Each investigation includes some questions that invite students to share their world views and life experiences to make connections between science and the real-world. In this investigation, the question is on page 29, question 59. This may be an opportunity for a small group or class discussion, or if in an asynchronous setting, students can contribute to a discussion forum.
3. Absolute magnitude (H) used in this investigation has a different definition than the absolute magnitude (M) of stars. Asteroids generate no light of their own, instead they reflect sunlight. The brightness of the reflected sunlight from an asteroid varies as a function of the Earth-asteroid distance. An asteroid’s absolute magnitude has been arbitrarily defined as the visual magnitude an observer would record if the asteroid were placed 1 Astronomical Unit (au) away from the observer and 1 au from the Sun, and viewed at an angle that would place the object in opposition and on the ecliptic. (Note that this geometry is physically impossible.) The scale of H values trends the same as all other magnitude scales, with larger positive numbers indicating a dimmer object and smaller positive numbers indicating a brighter object.
4. The size of an asteroid is calculated from its albedo and its absolute magnitude. An object’s albedo is a measure of its surface reflectivity. A certain value for albedo might be due to a small object that is highly reflective or a large object that is much less reflective. Albedo also varies with the amount of surface roughness. An ice-covered surface may be smoother and reflect light more efficiently. The albedos for most asteroids have not been measured, so typically a model range for albedos (.03-.23) is used to determine the possible minimum and maximum size for an asteroid.
5. Questions 15, 18, 23, and 26 in this investigation deal with calculating probabilities. We intentionally do not specify in what format the answer should be recorded. For example, in Q. 26, “2 out of 500” orbits may be expressed as a fraction (2/500) or as a percentage (0.4%). Many students do not understand how to compare fractions or how to correctly express percentages from calculations. You may want to decide how you wish these answers to be specified and show an example of how to calculate and correctly express percentages.
6. Not all large asteroids that cross the orbit of a planet are potentially hazardous. Some asteroids share the orbit of a planet but are locked in stable orbital resonances so that they will never come close to the planet. The Trojan asteroids of Jupiter or asteroid 2016 HO3 (a quasi-stable Earth satellite) are examples. You can view the locations of Trojan asteroids in Orbitviewer by turning off all but the Planets and Trojans on the Filters menu.