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Potentially hazardous object facts for kids

Kids Encyclopedia Facts
Toutatis
The asteroid Toutatis is listed as a potentially hazardous near-Earth asteroid, but it's not a threat to Earth right now. (Radar image taken by GDSCC in 1996.)

A potentially hazardous object (PHO) is a space rock, like an asteroid or a comet, that comes close to Earth. These objects are big enough to cause a lot of damage if they ever hit our planet. Scientists call them "potentially hazardous" if their path brings them very close to Earth. This means they pass within about 7.5 million kilometers (4.6 million miles) of Earth's orbit. They also need to be larger than about 140 meters (460 feet) across.

Good news! More than 99% of all known potentially hazardous objects are not expected to hit Earth in the next 100 years. As of July 2025, only 21 known PHOs are still being watched closely for possible future threats. Over many hundreds or thousands of years, the paths of some of these asteroids can change. This means they might become a real threat in the very distant future.

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 (0.6 miles) wide. Many discovered PHAs are Apollo asteroids. Fewer are Aten asteroids.

If we know an object's orbit well, we can be sure it won't hit Earth for at least 100 years. Potentially hazardous asteroids that might hit Earth in the next 100 years are listed on a special "Sentry Risk Table." As of September 2022, only 17 PHAs were on this list. Scientists keep observing new discoveries to learn more about their paths. Thanks to many astronomical surveys, the number of known PHAs has grown a lot since the late 1990s. NASA astronomers said in May 2021 that it could take 5 to 10 years to prepare if we ever need to stop a potential impact.

What are Potentially Hazardous Objects?

Potentially Hazardous Asteroids 2013
Plot of orbits of known potentially hazardous asteroids. These are over 140 meters (460 feet) in size. They pass within 7.5 million kilometers (4.7 million miles) of Earth's orbit. This map shows their paths as of early 2013.

An object is 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 Earth to the Moon. Also, the object must be brighter than a certain level, which usually means it's larger than 140 meters (460 feet) across.

An object this size could cause huge damage to cities or even a large tsunami if it hit the ocean. Such impacts happen on average about once every 10,000 years. Scientists estimate there are about 4,700 potentially hazardous asteroids larger than 100 meters (330 feet).

How We Measure Hazard

Scientists use two main scales to measure how dangerous an asteroid impact might be. These are called the Torino scale and the Palermo scale. They help us understand the risk.

Potentially Hazardous Comets

A potentially hazardous comet (PHC) is a comet that comes very close to Earth. This means its path brings it within about 7.5 million kilometers of Earth's orbit. Some known PHCs include 109P/Swift-Tuttle and 55P/Tempel–Tuttle. Halley's Comet used to fit this rule before the year 837. It passed very close to Earth then. Now, its closest approach is farther away.

How Many Are There?

NEA by survey
This chart shows NEAs found by different projects. PHAs are a smaller group within NEAs.
      LINEAR
      NEAT
      Spacewatch
      LONEOS
      CSS
      Pan-STARRS
      NEOWISE
      ATLAS
      Others

In 2012, NASA thought that only 20 to 30 percent of these objects had been found. When an asteroid passes close to a planet, the planet's gravity can change the asteroid's path. This means an asteroid that wasn't a threat before could become one, or vice versa. This shows how much the Solar System is always changing.

Many astronomical survey projects are still looking for more PHOs. These include Lincoln Near-Earth Asteroid Research, Catalina Sky Survey, and Pan-STARRS. When a new PHO is found, scientists study it. They use tools like telescopes, radar, and infrared sensors. This helps them figure out its size, what it's made of, how fast it spins, and its exact orbit. Both professional and amateur astronomers help with this important work.

How Big Are They?

Asteroids larger than about 35 meters (115 feet) can be a threat to a town or city. But it's hard to know the exact size of most small asteroids. Scientists usually guess their size based on how bright they appear and how far away they are. They don't often measure them directly with radar. Because of this, NASA and the Jet Propulsion Laboratory use a measure called absolute magnitude (H). Any asteroid with an absolute magnitude of 22.0 or brighter is thought to be large enough to be a PHO.

It's only a rough guess of size from an object's brightness. This is because we have to guess how reflective its surface is. NASA's near-Earth object program assumes a reflectivity of 0.14 for this.

Largest Potentially Hazardous Asteroids

The largest known potentially hazardous object is likely the Apollo asteroid (53319) 1999 JM8. It is about 7 kilometers (4.3 miles) wide. Its brightness is 15.2. Keep in mind that the sizes in the table below are estimates. They are based on how bright the object is and how reflective we think it is.

Brightest Potentially Hazardous Asteroids
Name Discovery (H)
(mag)
D
(km)
Orbital description Remarks References
Year Place Discoverer Class a
(AU)
e i
(°)
q
(AU)
Q
(AU)
MOID
(AU)
[[(4953) 1990 MU{{{2}}}|(4953) 1990 MU{{{2}}}]] 1990 413 R. H. McNaught 14.1 3 APO 1.621 0.658 24.4 0.555 2.687 0.02640 MPC · JPL · catalog
3122 Florence 1981 413 S. J. Bus 14.1 5 AMO 1.769 0.423 22.2 1.020 2.518 0.04430 MPC · JPL · catalog
(16960) 1998 QS52 1998 704 LINEAR 14.3 4 APO 2.203 0.858 17.5 0.313 4.093 0.01443 MPC · JPL · catalog
4183 Cuno 1959 074 C. Hoffmeister 14.4 4 APO 1.982 0.634 6.7 0.725 3.240 0.02825 MPC · JPL · catalog
3200 Phaethon 1983 500 IRAS 14.6 5.8 APO 1.271 0.890 22.3 0.140 2.402 0.01945 MPC · JPL · catalog
(242450) 2004 QY2 2004 E12 Siding Spring Survey 14.7 3 APO 1.084 0.477 37.0 0.567 1.601 0.04686 MPC · JPL · catalog
[[(89830) 2002 CE{{{2}}}|(89830) 2002 CE{{{2}}}]] 2002 704 LINEAR 14.9 3.1 AMO 2.077 0.507 43.7 1.023 3.131 0.02767 MPC · JPL · catalog
(137427) 1999 TF211 1999 704 LINEAR 15.1 2.9 APO 2.448 0.610 39.2 0.955 3.942 0.01787 MPC · JPL · catalog
(111253) 2001 XU10 2001 704 LINEAR 15.2 3 APO 1.754 0.439 42.0 0.983 2.524 0.02934 MPC · JPL · catalog
(53319) 1999 JM8 1999 704 LINEAR 15.2 7 APO 2.726 0.641 13.8 0.978 4.474 0.02346 Likely largest PHO MPC · JPL · catalog
1981 Midas 1973 675 C. T. Kowal 15.2 2 APO 1.776 0.650 39.8 0.621 2.931 0.00449 MPC · JPL · catalog
2201 Oljato 1947 690 H. L. Giclas 15.25 2.1 APO 2.175 0.713 2.5 0.624 3.726 0.00305 MPC · JPL · catalog
(90075) 2002 VU94 2002 644 NEAT 15.3 2.2 APO 2.134 0.576 8.9 0.904 3.363 0.03010 MPC · JPL · catalog
4179 Toutatis 1989 010 C. Pollas 15.30 2.5 APO 2.536 0.629 0.4 0.940 4.132 0.00615 MPC · JPL · catalog
(159857) 2004 LJ1 2004 704 LINEAR 15.4 3 APO 2.264 0.593 23.1 0.920 3.607 0.01682 MPC · JPL · catalog
(85713) 1998 SS49 1998 704 LINEAR 15.6 3.5 APO 1.924 0.639 10.8 0.694 3.154 0.00234 MPC · JPL · catalog
4486 Mithra 1987 071 E. W. Elst
V. G. Shkodrov
15.6 2 APO 2.200 0.663 3.0 0.742 3.658 0.04626 MPC · JPL · catalog
1620 Geographos 1951 675 A. G. Wilson
R. Minkowski
15.60 2.5 APO 1.245 0.335 13.3 0.828 1.663 0.03007 MPC · JPL · catalog
(415029) 2011 UL21 2011 703 CSS 15.7 2.5 APO 2.122 0.653 34.9 0.736 3.509 0.01925 MPC · JPL · catalog
(242216) 2003 RN10 2003 699 LONEOS 15.7 2.5 AMO 2.231 0.541 39.6 1.024 3.438 0.00956 MPC · JPL · catalog
12923 Zephyr 1999 699 LONEOS 15.8 2 APO 1.962 0.492 5.3 0.996 2.927 0.02115 MPC · JPL · catalog
(52768) 1998 OR2 1998 566 NEAT 15.8 2 APO 2.380 0.573 5.9 1.017 3.743 0.01573 MPC · JPL · catalog

PHA Discoveries Over Time

Here is a list of the brightest PHAs found each year since 1989. The charts below show how many PHAs have been discovered over time. One chart shows the total number. The other shows only those larger than one kilometer. PHAs brighter than absolute magnitude 17.75 are likely larger than 1 kilometer.

Brightest PHA discoveries of each calendar year since 1989
Number Name Year (H) Refs
4179 Toutatis 1989 15.3 MPC · JPL · catalog
4953 49531990 MU 1990 14.9 MPC · JPL · catalog
7341 [[1991 VH{{{2}}}|1991 VH{{{2}}}]] 1991 17.0 MPC · JPL · catalog
10115 [[1992 SK{{{2}}}|1992 SK{{{2}}}]] 1992 17.2 MPC · JPL · catalog
39572 395721993 DQ 1993 16.6 MPC · JPL · catalog
7482 74821994 PC 1994 16.7 MPC · JPL · catalog
243566 2435661995 SA 1995 17.4 MPC · JPL · catalog
8566 85661996 EN 1996 16.3 MPC · JPL · catalog
35396 1997 XF11 1997 17.0 MPC · JPL · catalog
16960 169601998 QS 1998 14.4 MPC · JPL · catalog
137427 1374271999 TF 1999 15.3 MPC · JPL · catalog
138095 1380952000 DK 2000 16.0 MPC · JPL · catalog
111253 1112532001 XU 2001 15.3 MPC · JPL · catalog
89830 898302002 CE 2002 15.0 MPC · JPL · catalog
242216 2422162003 RN 2003 15.7 MPC · JPL · catalog
242450 2424502004 QY 2004 14.6 MPC · JPL · catalog
308242 3082422005 GO 2005 16.3 MPC · JPL · catalog
374851 3748512006 VV 2006 16.7 MPC · JPL · catalog
214869 2148692007 PA 2007 16.5 MPC · JPL · catalog
294739 2947392008 CM 2008 17.1 MPC · JPL · catalog
523630 5236302009 OG 2009 16.2 MPC · JPL · catalog
458122 3819062010 EW 2010 17.6 MPC · JPL · catalog
415029 4150292011 UL 2011 15.9 MPC · JPL · catalog
746849 2012 HJ1 2012 17.9 MPC · JPL · catalog
507716 5077162013 UP 2013 16.4 MPC · JPL · catalog
533671 2014 LJ21 2014 16.1 MPC · JPL · catalog
2015 HY116 2015 17.6 MPC · JPL ·
620095 2016 CB194 2016 17.6 MPC · JPL · catalog
2017 CH1 2017 17.9 MPC · JPL ·
756316 2018 XV5 2018 17.7 MPC · JPL · catalog
2019 RU3 2019 18.1 MPC · JPL ·
2020 SL1 2020 17.7 MPC · JPL ·
2021 HK12 2021 17.7 MPC · JPL ·
2022 AP7 2022 17.3 MPC · JPL ·
2023 QF5 2023 18.3 MPC · JPL ·
2024 YU4 2024 19.0 MPC · JPL ·


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1999
2001
2003
2005
2007
2009
2011
2013
2015
2017
2019
Cumulative number of discovered potentially hazardous asteroids larger than 1 kilometer by end of year (first of December). As of September 2020, there are 157 known PHAs larger than one kilometer.
500
1,000
1,500
2,000
2,500
3,000
prev.
1999
2001
2003
2005
2007
2009
2011
2013
2015
2017
2019
Cumulative number of all discovered PHA by end of year (first of December). As of September 2020, there are 2115 PHAs.

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See also

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