Potentially hazardous object

A potentially hazardous object (PHO) is a near-Earth object – either an asteroid or a comet – with an orbit that can make close approaches to the Earth and large enough to cause significant regional damage in the event of impact.[1] They are defined as having a minimum orbital intersection distance with Earth of less than 0.05 astronomical units (19.5 lunar distances) and an absolute magnitude of 22 or brighter.[2] 98% of the known potentially hazardous objects are not an impact threat over the next 100 years.[3]

Most of these objects are potentially hazardous asteroids (PHAs), and a few are comets. As of October 2019 there are 2,018 known PHAs (about 10% of the total near-Earth population), of which 156 are estimated to be larger than one kilometer in diameter (see list of largest PHAs below).[4][5][a] Most of the discovered PHAs are Apollo asteroids (1,601) and fewer belong to the group of Aten asteroids (169).[6][7]

A potentially hazardous object can be known not to be a threat to Earth for the next 100 years or more, if its orbit is reasonably well determined. Potentially hazardous asteroids with some threat of impacting Earth in the next 100 years are listed on the Sentry Risk Table. As of October 2019, only about 42 potentially hazardous asteroids are listed on the Sentry Risk Table.[8] Most potentially hazardous asteroids are ruled out as hazardous to at least several hundreds of years when their competing best orbit models become sufficiently divergent, but recent discoveries whose orbital constraints are little-known have divergent or incomplete mechanical models until observation yields further data. After several astronomical surveys, the number of known PHAs has increased tenfold since the end of the 1990s (see bar charts below).[4] The Minor Planet Center's website List of the Potentially Hazardous Asteroids also publishes detailed information for these objects.[9]

The asteroid Toutatis is listed as a potentially hazardous near-Earth asteroid, yet poses no immediate threat to Earth. (Radar image taken by GDSCC in 1996.)


Potentially Hazardous Asteroids 2013
Plot of orbits of known potentially hazardous asteroids, with sizes over 140 metres (460 ft) and that pass within 7.6 million kilometres (4.7×106 mi) of Earth's orbit. Epoch as of early 2013.

An object is considered a PHO if its minimum orbit intersection distance (MOID) with respect to Earth is less than 0.05 AU (7,500,000 km; 4,600,000 mi) – approximately 19.5 lunar distances – and its absolute magnitude is brighter than 22, approximately corresponding to a diameter above 140 meters (460 ft).[1][2] This is big enough to cause regional devastation to human settlements unprecedented in human history in the case of a land impact, or a major tsunami in the case of an ocean impact. Such impact events occur on average around once per 10,000 years. NEOWISE data estimates that there are 4,700 ± 1,500 potentially hazardous asteroids with a diameter greater than 100 meters.[10]

Levels of hazard

The two main scales used to categorize the impact hazards of asteroids are the Palermo Technical Impact Hazard Scale and the Torino Scale.

Potentially hazardous comets

Short-period comets currently with an Earth-MOID less than 0.05 AU include: 109P/Swift-Tuttle, 55P/Tempel–Tuttle, 15P/Finlay, 289P/Blanpain, 255P/Levy, 206P/Barnard–Boattini, 21P/Giacobini–Zinner, and 73P/Schwassmann–Wachmann.


Detected NEAs by various projects. The broader class of NEAs includes all PHAs as a subset.[4]

In 2012 NASA estimated 20 to 30 percent of these objects have been found.[10] During an asteroid's close approaches to planets or moons other than the Earth, it will be subject to gravitational perturbation, modifying its orbit, and potentially changing a previously non-threatening asteroid into a PHA or vice versa. This is a reflection of the dynamic character of the Solar System.

Several astronomical survey projects such as Lincoln Near-Earth Asteroid Research, Catalina Sky Survey and Pan-STARRS continue to search for more PHOs. Each one found is studied by various means, including optical, radar, and infrared to determine its characteristics, such as size, composition, rotation state, and to more accurately determine its orbit. Both professional and amateur astronomers participate in such observation and tracking.


Asteroids larger than approximately 35 meters across can pose a threat to a town or city.[11] However the diameter of most small asteroids is not well determined, as it is usually only estimated based on their brightness and distance, rather than directly measured from e.g. radar observations. For this reason NASA and the Jet Propulsion Laboratory use the more practical measure of absolute magnitude (H). Any asteroid with an absolute magnitude of 22.0 or brighter is assumed to be of the required size.[2]

Only a coarse estimation of size can be found from the object's magnitude because an assumption must be made for its albedo which is also not usually known for certain. The NASA near-Earth object program uses an assumed albedo of 0.14 for this purpose. In May 2016, the asteroid size estimates arising from the Wide-field Infrared Survey Explorer and NEOWISE missions have been questioned.[12][13][14] Although the early original criticism had not undergone peer review,[15] a more recent peer-reviewed study was subsequently published.[16][17]

Largest PHAs

With a mean diameter of approximately 7 kilometers, Apollo asteroid (53319) 1999 JM8 is likely the largest known potentially hazardous object, despite its fainter absolute magnitude of 15.2, compared to other listed objects in the table below (note: calculated mean-diameters in table are inferred from the objects brightness and its (assumed) albedo. They are only an approximation.). The lowest numbered PHA is 1566 Icarus.[9]

Brightest Potentially Hazardous Asteroids [9]
Designation Discovery (H) (mag) D (km) Orbital description Remarks Refs
Year Place Discoverer Class a
e i
(4953) 1990 MU 1990 413 R. H. McNaught 14.1 3 km 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 km 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 km 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 km 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 km 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 km APO 1.084 0.477 37.0 0.567 1.601 0.04686 MPC · JPL · catalog
(89830) 2002 CE 2002 704 LINEAR 14.9 3.1 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km 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 km AMO 2.231 0.541 39.6 1.024 3.438 0.00956 MPC · JPL · catalog
12923 Zephyr 1999 699 LONEOS 15.8 2 km 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 km APO 2.380 0.573 5.9 1.017 3.743 0.01573 MPC · JPL · catalog


Below is a list of the largest PHAs (based on absolute magnitude H) discovered in a given year.[18] Historical data of the cumulative number of discovered PHA since 1999 are displayed in the bar charts—one for the total number and the other for objects larger than one kilometer.[4]

Brightest PHA discoveries of each calendar year since 1989 [18]
Number Name Year (H) Refs
4179 Toutatis 1989 15.3 MPC · JPL · catalog
4953 1990 MU 1990 14.1 MPC · JPL · catalog
7341 1991 VK 1991 16.7 MPC · JPL · catalog
5604 1992 FE 1992 16.4 MPC · JPL · catalog
39572 1993 DQ1 1993 16.4 MPC · JPL · catalog
136618 1994 CN2 1994 16.6 MPC · JPL · catalog
243566 1995 SA 1995 17.3 MPC · JPL · catalog
8566 1996 EN 1996 16.5 MPC · JPL · catalog
35396 1997 XF11 1997 16.9 MPC · JPL · catalog
16960 1998 QS52 1998 14.3 MPC · JPL · catalog
137427 1999 TF211 1999 15.0 MPC · JPL · catalog
23187 2000 PN9 2000 16.1 MPC · JPL · catalog
111253 2001 XU10 2001 14.9 MPC · JPL · catalog
89830 2002 CE 2002 14.7 MPC · JPL · catalog
242216 2003 RN10 2003 15.7 MPC · JPL · catalog
242450 2004 QY2 2004 14.7 MPC · JPL · catalog
308242 2005 GO21 2005 16.4 MPC · JPL · catalog
374851 2006 VV2 2006 16.8 MPC · JPL · catalog
214869 2007 PA8 2007 16.2 MPC · JPL · catalog
294739 2008 CM 2008 17.15 MPC · JPL · catalog
369264 2009 MS 2009 16.0 MPC · JPL · catalog
381906 2010 CL19 2010 17.55 MPC · JPL · catalog
415029 2011 UL21 2011 15.7 MPC · JPL · catalog
482467 2012 LK9 2012 17.8 MPC · JPL · catalog
507716 2013 UP8 2013 16.5 MPC · JPL · catalog
533671 2014 LJ21 2014 16.0 MPC · JPL · catalog
2015 HY116 2015 17.5 MPC · JPL ·
2016 CB194 2016 17.6 MPC · JPL ·
2017 CH1 2017 17.9 MPC · JPL ·
2018 XV5 2018 17.6 MPC · JPL ·
2019 CE4 2019 18.0 MPC · JPL ·
PHA-KM: potentially hazardous asteroids larger than 1 kilometer since 1999 – Cumulative number of discovered PHA by end of year (first of December). As of September 2019, there are 155 known PHAs larger than one kilometer.[4]
PHA: total number of potentially hazardous asteroids since 1999 – Cumulative number of all discovered PHA by end of year (first of December). As of September 2019, there are 2014 PHAs.[4]



Radar image of the 360-meter PHA (308635) 2005 YU55

See also


  1. ^ An object's calculated mean-diameter is only a rough estimate. It is inferred from the object's varying brightness—observed and measured at various times—and the assumed, yet often unknown reflectivity of its surface. NASA's Asteroid Size Estimator is a tool for a generic absolute magnitude-to-diameter conversion for an assumed geometric albedo.


  1. ^ a b Task Force on potentially hazardous Near Earth Objects (September 2000). "Report of the Task Force on potentially hazardous Near Earth Objects" (PDF). Retrieved 22 January 2018. Cite journal requires |journal= (help)
  2. ^ a b c "NEO Basics – Potentially Hazardous Asteroids (PHAs)". CNEOS NASA/JPL. Retrieved 22 January 2018.
  3. ^ (44 Sentry risk-listed PHAs / 2016 PHAs) = 2%
  4. ^ a b c d e f "Discovery Statistics – Cumulative Totals". CNEOS NASA/JPL. Retrieved 25 April 2018.
  5. ^ "Unusual Minor Planets – Overview". Minor Planet Center. Retrieved 16 January 2018.
  6. ^ "JPL Small-Body Database Search Engine: PHAs and orbital class (APO)". JPL Solar System Dynamics. Retrieved 22 January 2018.
  7. ^ "JPL Small-Body Database Search Engine: PHAs and orbital class (ATE)". JPL Solar System Dynamics. Retrieved 22 January 2018.
  8. ^ "Sentry Risk Table". Retrieved 2019-08-15. (Click "Use Unconstrained Settings" AND select "H<=22" for list of PHAs)
  9. ^ a b c "List of the Potentially Hazardous Asteroids (PHAs)". Minor Planet Center. Retrieved 16 January 2018.
  10. ^ a b "NASA news – NASA Survey Counts Potentially Hazardous Asteroids". NASA/JPL. 16 May 2012. Retrieved 22 January 2018.
  11. ^ Will Ferguson (22 January 2013). "Asteroid Hunter Gives an Update on the Threat of Near-Earth Objects". Scientific American. Retrieved 2013-01-23.
  12. ^ Chang, Kenneth (23 May 2016). "How Big Are Those Killer Asteroids? A Critic Says NASA Doesn't Know". New York Times. Retrieved 24 May 2016.
  13. ^ Myhrvold, Nathan (23 May 2016). "Asteroid thermal modeling in the presence of reflected sunlight with an application to WISE/NEOWISE observational data". arXiv:1605.06490v2 [astro-ph.EP].
  14. ^ Billings, Lee (27 May 2016). "For Asteroid-Hunting Astronomers, Nathan Myhrvold Says the Sky Is Falling". Scientific American. Retrieved 28 May 2016.
  15. ^ NASA Administrator (25 May 2016). "NASA Response to Recent Paper on NEOWISE Asteroid Size Results". NASA. Retrieved 29 May 2016.
  16. ^ Myhrvold, Nathan (2018). "An empirical examination of WISE/NEOWISE asteroid analysis and results". Icarus. 314: 64–97. Bibcode:2018Icar..314...64M. doi:10.1016/j.icarus.2018.05.004.
  17. ^ Chang, Kenneth (14 June 2018). "Asteroids and Adversaries: Challenging What NASA Knows About Space Rocks - Two years ago, NASA dismissed and mocked an amateur's criticisms of its asteroids database. Now Nathan Myhrvold is back, and his papers have passed peer review". The New York Times. Retrieved 14 June 2018.
  18. ^ a b "JPL Small-Body Database Search Engine: PHAs and H < 18 (mag)". JPL Solar System Dynamics. Retrieved 2012-06-13.

External links

Minor Planet Center

(177049) 2003 EE16

(177049) 2003 EE16, provisionally known as 2003 EE16, is an Apollo near-Earth asteroid and potentially hazardous object. It was discovered on 8 March 2003 by LPL/Spacewatch II at an apparent magnitude of 20 using a 1.8-meter (71 in) reflecting telescope. It has an estimated diameter of 320 meters (1,050 ft). The asteroid was listed on Sentry Risk Table with a Torino Scale rating of 1 on 2 April 2003.

(308242) 2005 GO21

(308242) 2005 GO21 is a large Aten near-Earth asteroid and potentially hazardous object. It has a well determined orbit with an observation arc of 7 years and an uncertainty parameter of 0. It was discovered on 1 April 2005 by the Siding Spring Survey at an apparent magnitude of 18.1 using the 0.5-metre (20 in) Uppsala Southern Schmidt Telescope.Based on an absolute magnitude of 16.4, the asteroid has an estimated diameter of 1.6 km (within a factor of two). (308242) 2005 GO21 is the largest potentially hazardous asteroid (PHA) discovered in 2005. On 21 June 2012 it passed Earth at a distance of 0.043963 AU (6,576,800 km; 4,086,600 mi). The 2012 passage was studied with radar using Goldstone and Arecibo.

(7341) 1991 VK

(7341) 1991 VK is a near-Earth minor planet in the Apollo group. It was discovered by Eleanor F. Helin and Kenneth J. Lawrence at the Palomar Observatory in San Diego County, California, on November 1, 1991. It is listed as a potentially hazardous object. Every 5 years (from 1946 through 2091) the asteroid makes a close approach to the Earth. The most recent close approach to Earth was on Jan 25, 2017; the next close approach will be on Jan 25, 2022 at a distance of 0.064 AU (9,600,000 km; 5,900,000 mi).

(8014) 1990 MF

(8014) 1990 MF is a near-Earth minor planet in the Apollo group. It was discovered by Eleanor F. Helin at the Palomar Observatory in San Diego County, California, on June 26, 1990. This object is approximately 0.7 kilometres (0.43 mi) in diameter, and listed as a potentially hazardous object. On July 23, 2020, it will come within 0.055 au of the Earth—about 21.3 times the Moon's distance.

(85989) 1999 JD6

(85989) 1999 JD6 is an Aten asteroid, near-Earth object, and potentially hazardous object in the inner Solar System that makes frequent close approaches to Earth and Venus. On the Earth approach in 2015, it was observed by the Goldstone Solar System Radar and found to be a contact binary with the largest axis approximately 2 kilometers wide, and each lobe about 200–300 meters large. Although 1999 JD6 in its current orbit never passes closer than 0.047 AU to Earth, it is listed as a potentially hazardous object because it is large and might pose a threat in the future.

The asteroid is well-observed, having been observed over 2,000 times over a length of over 25 years, and was assigned a numeric designation in August 2004.

1862 Apollo

1862 Apollo is a stony asteroid, approximately 1.5 kilometers in diameter, classified as a near-Earth object (NEO). It was discovered by German astronomer Karl Reinmuth at Heidelberg Observatory on 24 April 1932, but lost and not recovered until 1973.

It is the namesake and the first recognized member of the Apollo asteroids, a subgroup of NEOs which are Earth-crosser, that is they cross the orbit of Earth when view perpendicular to the ecliptic plane (crossing an orbit is a more general term that actually intersecting it). In addition, since Apollo's orbit is highly eccentric, it crosses the orbits of Venus and Mars and is therefore called a Venus-crosser and Mars-crosser as well.

Although Apollo was the first Apollo asteroid to be discovered, its official IAU-number (1862) is higher than that of some other Apollo asteroids such as 1566 Icarus, due to the fact that it was a lost asteroid for more than 40 years and other bodies were numbered in the meantime. The analysis of its rotation provided observational evidence of the YORP effect.It is named after the Greek god Apollo. He is the god of the Sun, child of Zeus and Leto, after which the minor planets 5731 Zeus and 68 Leto are named.

2002 JE9

2002 JE9 (also written 2002 JE9) is an Apollo near-Earth asteroid and potentially hazardous object. It has a well determined orbit with an observation arc of 10 years and an Uncertainty Parameter of 1. It was removed from the Sentry Risk Table on 10 May 2002. 2002 JE9 was discovered on 6 May 2002 by the Lincoln Near-Earth Asteroid Research (LINEAR) project using a 1.0-metre (39 in) Reflecting telescope; at the time of discovery, the asteroid possessed an apparent magnitude of 19.1.The asteroid has an estimated diameter of about 200 meters (660 ft) based on an absolute magnitude of 21.3. 2002 JE9 is considered significant due to having previously passed closer to the Earth; on 11 April 1971, it passed Earth at a distance of 0.0015 AU (220,000 km; 140,000 mi). 2002 JE9 is one of the largest objects known to have passed inside the orbit of the moon. During the close approach in 1971 the asteroid reached about apparent magnitude 10, about the same brightness as Saturn's moon Iapetus.The asteroid will pass 0.0049 AU (730,000 km; 460,000 mi) from Venus on 25 November 2021.

2004 TN1

2004 TN1 is an Apollo near-Earth asteroid and potentially hazardous object discovered on 5 October 2004 by NEAT at Mount Palomar. It has the fourth smallest geocentric Minimum Orbital Intersection Distance of any asteroid, after 2008 TC3 which exploded in Earth's atmosphere in 2008, 1994 GV, and 2014 AA which also impacted the Earth in 2014. The asteroid, however, will not make any significant close approaches to Earth in at least the next century.However, its orbit is poorly determined, with only 58 observations over 30 days between 5 October and 4 November 2004, yielding an orbital certainty of 6, with 0 being a well-determined orbit and 9 being an extremely poorly determined orbit. More observations would be needed to determine whether the asteroid may impact the earth in the next several hundred years.Absolute magnitude estimates guess the asteroid to be approximately 115–260 meters (380–850 feet) in diameter. A theoretical impact into porous rock at 45 degrees, assuming the asteroid to have a density of 2 g/cm3, would yield a crater between 1.7 and 3.2 kilometers wide, slightly larger than Meteor Crater in Arizona.

2008 AO112

2008 AO112 (also written 2008 AO112) is an Apollo near-Earth asteroid and potentially hazardous object. It was discovered on 12 January 2008 by the Mount Lemmon Survey at an apparent magnitude of 21 using a 1.5-meter (59 in) reflecting telescope. The asteroid was quickly lost and had an estimated diameter of 310 meters (1,020 ft). On 25 June 2009, with an observation arc of only 1 day in January 2008, the asteroid had a 1 in 4 million chance of impacting Earth on that very day. The virtual impactor had not been eliminated from the Sentry Risk Table by the day of the potential impact.

The asteroid was recovered on 5 March 2013 as 2013 EM20. Precovery images from 7 April 1997 at Kitt Peak National Observatory were located. It was removed from the Sentry Risk Table on 30 March 2013. It is now known that on 25 June 2009 the asteroid was 1.45AU from Earth.

2009 RR

2009 RR micro-asteroid, classified as near-Earth object of the Apollo group. It was discovered on 11 September 2009 by the Catalina Sky Survey at an apparent magnitude of 19.5 using a 0.68-meter (27 in) Schmidt–Cassegrain telescope. 2009 RR was the only asteroid discovered before 2014 that was predicted to potentially pass inside the orbit of the Moon during 2014. The asteroid has an estimated diameter of 26 meters (85 ft) and is listed on the Sentry Risk Table. It is not large enough to qualify as a potentially hazardous object.

2010 XC15

2010 XC15 (also written 2010 XC15) is an Aten near-Earth asteroid and potentially hazardous object. It has an observation arc of 2 years and an Uncertainty Parameter of 2. It was discovered on 5 December 2010 by the Catalina Sky Survey at an apparent magnitude of 17.5 using a 0.68-metre (27 in) Schmidt.Based on an absolute magnitude of 21.4, the asteroid has an estimated diameter of about 200 metres (660 ft). 2010 XC15 is noted for a close approach to Earth on 27 December 1976 at a distance of about 0.0062 AU (930,000 km; 580,000 mi). As of November 2011 with an observation arc of 40 days, the JPL Small-Body Database showed that the uncertainty region of the asteroid during the 1976 close approach could result in a pass anywhere from 0.001 AU to 0.018 AU from Earth. During the 1976 close approach the asteroid reached about apparent magnitude 14.The asteroid will pass 0.0051 AU (760,000 km; 470,000 mi) from Earth on 27 December 2022, allowing a refinement to the known trajectory. The uncertainty region as of 2013 suggests that the asteroid may have passed inside the orbit of the Moon in 1907, but the nominal solution suggests the pass was about 0.007 AU (1,000,000 km; 650,000 mi).The asteroid 2002 JE9, with a much larger observation arc, is known to have passed 0.0015 AU (220,000 km; 140,000 mi) from Earth on 11 April 1971.

2013 FW13

2013 FW13 is an Apollo asteroid and a potentially hazardous object, that was discovered on March 23, 2013 by the Catalina Sky Survey. Further observation of its orbital calculation was made by amateur astronomer Mohammed Alsunni of Sudan.2013 FW13 is a potentially hazardous asteroid (PHA) since its minimum orbit intersection distance (MOID) is less than 0.05 AU and its diameter is estimated to be greater than ~150 meters. The Earth MOID is 0.013 AU (1,900,000 km; 1,200,000 mi). On 18 September 2024 it will safely pass about 0.02 AU (3,000,000 km; 1,900,000 mi) from Earth.The Absolute magnitude of the asteroid is 21.70 giving the object an approximate diameter of 120–270 meters.

3671 Dionysus

3671 Dionysus is a small binary Amor asteroid, orbiting between Earth and the asteroid belt. It was discovered by Carolyn and Gene Shoemaker at Palomar Observatory on 27 May 1984. It is named after Dionysus, the Greek god of wine. Its provisional designation was 1984 KD. It is an outer Earth grazer because its perihelion is just within Earth's orbit.


AG5 may refer to:

2011 AG5 (also written 2011 AG5), a near-Earth asteroid and potentially hazardous object

The code of Pandan Indah LRT station, a Malaysian at-grade rapid transit station

AG5, a 5-speed longitudinal transmission manufactured by Aisin Seiki

Minimum orbit intersection distance

Minimum orbit intersection distance (MOID) is a measure used in astronomy to assess potential close approaches and collision risks between astronomical objects. It is defined as the distance between the closest points of the osculating orbits of two bodies. Of greatest interest is the risk of a collision with Earth. Earth MOID is often listed on comet and asteroid databases such as the JPL Small-Body Database. MOID values are also defined with respect to other bodies as well: Jupiter MOID, Venus MOID and so on.

An object is classified as a potentially hazardous object (PHO) – that is, posing a possible risk to Earth – if, among other conditions, its Earth MOID is less than 0.05 AU. For more massive bodies than Earth, there is a potentially notable close approach with a larger MOID; for instance, Jupiter MOIDs less than 1 AU are considered noteworthy since Jupiter is the most massive planet.A low MOID does not mean that a collision is inevitable as the planets frequently perturb the orbit of small bodies. It is also necessary that the two bodies reach that point in their orbits at the same time before the smaller body is perturbed into a different orbit with a different MOID value. Two Objects gravitationally locked in orbital resonance may never approach one another. Numerical integrations become increasingly divergent as trajectories are projected further forward in time, especially beyond times where the smaller body is repeatedly perturbed by other planets. MOID has the convenience that it is obtained directly from the orbital elements of the body and no numerical integration into the future is used.The only object that has ever been rated at 4 on the Torino Scale (since downgraded), the Aten asteroid (99942) Apophis, has an Earth MOID of 0.000316 AU. This is not the smallest Earth MOID in the catalogues; many bodies with a small Earth MOID are not classed as PHO's because the objects are less than roughly 140 meters in diameter (or absolute magnitude, H < 22). Earth MOID values are generally more practical for asteroids less than 140 meters in diameter as those asteroids are very dim and often have a short observation arc with a poorly determined orbit. The only objects that have been detected and had their Earth-MOID calculated before Earth impact were the small asteroids 2008 TC3 and 2014 AA. 2008 TC3 was listed with a MOID of 0.00001 AU in the Minor Planet Center database, and is the smallest MOID calculated for an Apollo asteroid. It is even smaller at the more precise JPL Small Body Database (0.0000078 AU).


PHO or pho may refer to:

Phở, a Vietnamese noodle soup

Primary health organisation, New Zealand

Potentially hazardous object, an asteroid or comet that could potentially collide with Earth

Planetary Defense Coordination Office

The Planetary Defense Coordination Office is a planetary defense organization within NASA’s Planetary Science Division. Its mission is to lead the coordination of interagency and intergovernmental efforts to plan responses to potential impact threats. Announced by NASA in January 2016, it is given the job of cataloging and tracking potentially hazardous near-Earth objects such as asteroids and comets, which are larger than 30 to 50 m in diameter (compare to the 20-m Chelyabinsk meteor) and coordinating an effective threat response and mitigation effort. The office will continue to use the polar orbiting infrared telescope NEOWISE to detect any potentially hazardous object.

R. Scott Dunbar

Roy Scott Dunbar is an American astronomer, planetologist and discoverer of comets and minor planets.Dunbar played an active role in the Palomar Planet-Crossing Asteroid Survey. The Minor Planet Center credits him with the (co-)discovery of 10 numbered minor planets during 1981–1987.His most notable discoveries include the potentially hazardous object and Aten asteroid 3362 Khufu, which he co-discovered with Maria A. Barucci, as well as the near-Earth object, Mars-crosser and Aten asteroid, 3551 Verenia. Together with Eleanor Helin he co-discovered the minor planets 3360 Syrinx, 6065 Chesneau, 6435 Daveross and 7163 Barenboim.

Dunbar and Helin also claimed the discovery of comet 1980 p, which turned out not to exist. It was a ghost image of Alpha Leonis.

The main-belt asteroid 3718 Dunbar, discovered by Eleanor Helin and Schelte Bus, is named after him. Naming citation was published on 2 April 1988 (M.P.C. 12976).

Yoshiaki Oshima

Yoshiaki Oshima (大島 良明, Ōshima Yoshiaki) (born 1952) is a Japanese astronomer at Gekko Observatory and prolific discoverer of 61 asteroids as credited by the Minor Planet Center, and include the binary asteroid 4383 Suruga, the potentially hazardous object (7753) 1988 XB and the Jupiter trojan (4715) 1989 TS1.

Minor planets
On Earth
On Jupiter
See also
≥20 km diameter
Main topics
Space probes
NEO tracking
Potential threats
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