Potentially hazardous object facts for kids
A potentially hazardous object (PHO) is a space object, like an asteroid or a comet, that travels close to Earth. These objects are large enough that if they were to hit our planet, they could cause a lot of damage to a big area. Scientists keep a close eye on them.
To be called a PHO, an object must meet two main conditions:
- Its path must come very close to Earth's path. This is measured by something called its minimum orbit intersection distance, which needs to be less than about 7.5 million kilometers (or 4.6 million miles).
- It must be bright enough to suggest it's larger than about 140 meters (about 460 feet) across. This brightness is measured using something called absolute magnitude.
It's important to know that more than 99% of the PHOs we know about are not expected to hit Earth in the next 100 years. Scientists are constantly tracking these objects. As of February 2025, only a small number of known PHOs are considered potential threats in the next century, and their orbits are carefully watched. Over many hundreds or thousands of years, the paths of some of these "potentially hazardous" asteroids can change, making them a closer concern.
Most of these objects are potentially hazardous asteroids (PHAs). A few are comets. As of November 2022, there are 2,304 known PHAs. This is about 8% of all near-Earth objects. About 153 of these are thought to be larger than one kilometer (about 0.6 miles) wide. Many discovered PHAs belong to a group called Apollo asteroids.
If a potentially hazardous object's path is well-known, scientists can confirm it won't be a threat to Earth for at least 100 years. Objects that might pose a threat in the next 100 years are listed on a special "Sentry Risk Table." As of September 2022, only 17 PHAs were on this list. When new objects are found, their paths aren't always clear at first. More observations help scientists understand their movements better. Thanks to many astronomical surveys, the number of known PHAs has grown a lot since the late 1990s.
In May 2021, astronomers at NASA shared that it might take 5 to 10 years to prepare if we ever needed to stop a potential impact. This was based on a practice exercise they did.
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What are Potentially Hazardous Objects?
An object is called a PHO if its closest approach to Earth's orbit is less than about 7.5 million kilometers (4.6 million miles). This distance is roughly 19.5 times the distance from the Earth to the Moon. Also, its absolute magnitude must be brighter than 22, which usually means it's larger than about 140 meters (460 feet) across.
If an object of this size were to hit land, it could cause a lot of damage to cities or towns. If it hit the ocean, it could create a very large wave, like a tsunami. Such impacts are very rare, happening on average about once every 10,000 years. Scientists estimate there are about 4,700 potentially hazardous asteroids larger than 100 meters (330 feet).
Measuring the Hazard
Scientists use special scales to measure how risky an asteroid or comet might be. The two main scales are the Torino scale and the Palermo scale. These help them categorize the potential danger of an object hitting Earth.
Potentially Hazardous Comets
A potentially hazardous comet (PHC) is a short-period comet whose path currently brings it within 7.5 million kilometers (4.6 million miles) of Earth. Some known PHCs include 109P/Swift-Tuttle and 55P/Tempel–Tuttle. Halley's Comet used to fit this description before the year 837 AD, when it passed very close to Earth. Now, its closest approach is farther away.
How Scientists Find and Track Them
| LINEAR NEAT Spacewatch LONEOS CSS |
Pan-STARRS NEOWISE ATLAS Others |
In 2012, NASA estimated that about 20% to 30% of these objects had been found. When an asteroid passes close to another planet, the planet's gravity can pull on it. This pull can change the asteroid's path, sometimes turning a harmless asteroid into a PHO, or vice versa. This shows that our Solar System is always changing!
Many astronomical survey projects, like Lincoln Near-Earth Asteroid Research, Catalina Sky Survey, and Pan-STARRS, are always looking for more PHOs. When a new one is found, scientists study it using different tools. They use optical telescopes, radar, and infrared light to learn about its size, what it's made of, how fast it spins, and to figure out its path more accurately. Both professional and amateur astronomers help with these observations and tracking efforts.
Estimating Size
Asteroids larger than about 35 meters (115 feet) across could pose a threat to a town or city. However, it's hard to know the exact size of most small asteroids. Their size is usually estimated based on how bright they appear and how far away they are. This is not as precise as directly measuring them with radar.
Because of this, NASA and the Jet Propulsion Laboratory use a more practical measurement called absolute magnitude (H). Any asteroid with an absolute magnitude of 22.0 or brighter is generally thought to be large enough to be a PHO.
It's tricky to get a perfect size estimate just from an object's brightness. This is because we have to guess how reflective its surface is (called its albedo). NASA's near-Earth object program usually assumes an albedo of 0.14 for this purpose. Scientists are always working to get more accurate measurements of asteroid sizes.
Important Discoveries and Numbers
The asteroid (53319) 1999 JM8 is likely the largest known potentially hazardous object. It's about 7 kilometers (4.3 miles) wide.
Gallery
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Radar image of the 350-meter PHO (308635) 2005 YU55
See also
- Asteroid impact avoidance
- Earth-grazing fireball
- Global catastrophic risk
- List of asteroid close approaches to Earth
- Scout (NASA program); short term warnings
- Sentry (monitoring system)
- Spaceguard
