Observation arc

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In observational astronomy, the observation arc (or arc length) of a Solar System body is the time period between its earliest and latest observations, used for tracing the body's path. It is usually given in days or years. The term is mostly used in the discovery and tracking of asteroids and comets. Arc length has the greatest influence on the accuracy of an orbit. The number, spacing of intermediate observations, and timestamps have a lesser effect.

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Short arcs

A very short arc leaves a high uncertainty parameter. The object might be in one of many different orbits, at many distances from Earth. In some cases, the initial arc was too short to determine if the object was in orbit around the Earth, or orbiting out in the asteroid belt. With a 1-day observation arc, 2004 PR107 was thought to be a trans-Neptunian dwarf planet, but is now known to be a 1 km main-belt asteroid. With an observation arc of 3 days, 2004 BX159 was thought to be a Mars-crossing asteroid that could be a threat to Earth, but was later found to be another main-belt asteroid.

A relatively modest observation arc may allow finding an older "precovery" photo, providing a much longer arc and a more precise orbit.

An observation arc less than 30 days can make it difficult to recover an Inner Solar System object more than a year after the last observation, and may result in a lost minor planet. Due to their greater distance from the Sun and slow movement across the sky, trans-Neptunian objects with observation arcs less than several years often have poorly constrained orbits. [1]

As a general rule objects discovered when they are currently farther from the Sun will have greater uncertainties in their initial orbits if the observation arcs are short.

2018 AG37 which was discovered when 100+ AU from the Sun and has only been observed 9 times over 2 years [2] will require an observation arc of several years to refine the uncertainties in the orbital period and aphelion (farthest distance from the Sun).

1999 DP8 with only 4 observations over 1 day [3] has uncertainties so large that the error bars are not really meaningful and just show that the uncertainties are very large. On its discovery date 1999 DP8 is estimated to have been 52±1500 AU from Earth. [3]

Oort cloud comet C/2017 K2 was announced when it had a short 2.6 day observation arc, was estimated to be 20 AU (3.0 billion km) from the Sun, and was estimated to come to perihelion around 10 AU from the Sun in 2027. [4] But it is now known that C/2017 K2 was discovered when it was 16 AU from the Sun and will come to perihelion 1.8 AU from the Sun on 19 December 2022.

It took an observation arc of about 200 days to rule out a Mars impact by Oort cloud comet C/2013 A1 (Siding Spring). [5]

Interstellar objects

Interstellar objects generally require an observation arc of 2–3 weeks using hundreds of observations to confirm that an interloper has a hyperbolic excess velocity (interstellar speed) of more than a few km/s. Comet C/2008 J4 (McNaught) was only observed 22 times over an observation arc of 15 days, and due to an insufficient number of observations generates a low inbound interstellar speed of 3.9 km/s, but the uncertainties in the eccentricity easily produce a closed orbit with . [6] Comet C/1999 U2 (SOHO) with an almost meaningless observation arc of 1 day shows a very dubious interstellar speed of 17 km/s, but could easily have a closed orbit with an eccentricity as low as 0.7. [7]

Earth approaches

Uncertainty in Earth approach distance by comets currently further than Uranus
that have not been observed in the last ~20 years
CometObservation
arc		Number of
observations		 Uncertainty
parameter 		Earth
approach date		Uncertainty in
distance from Earth		Reference
Comet Swift–Tuttle 257 years65202126-Aug-05±10 thousand km data
C/2001 OG108 0.9 years88622147-Mar-23±2 million km data
C/1991 L3 (Levy)1.6 years12532094-Aug-01±15 million km data

With an observation arc of 257 years, the uncertainty in Comet Swift–Tuttle's closest approach to Earth on 5 August 2126 is about ±10 thousand km. [8] With an observation arc of ~1 year, the uncertainty in C/2001 OG108 's closest approach to Earth on 23 March 2147 is about ±2 million km. [9] Even though C/1991 L3 (Levy) has a longer observation arc than C/2001 OG108, it has significantly fewer observations which generates a greater uncertainty.

In contrast, comet C/2022 A1 (Sarneczky) was discovered on 2 January 2022 when it was 1.3 AU from the Sun, and announced on 7 January 2022 with only a 5-day observation arc. [10] It made its closest Earth approach the next day with a 3-sigma uncertainty region of ±1 million km. [11] The large uncertainty was a result of the short arc and discovery distance.

See also

Related Research Articles

1991 BA is a sub-kilometer asteroid, classified as near-Earth object of the Apollo group that was first observed by Spacewatch on 18 January 1991, and passed within 160,000 km (100,000 mi) of Earth. This is a little less than half the distance to the Moon. With a 5-hour observation arc the asteroid has a poorly constrained orbit and is considered lost. It could be a member of the Beta Taurids.

<span class="nowrap">2007 VK<sub>184</sub></span>

2007 VK184 is a sub-kilometer asteroid, classified as a near-Earth object of the Apollo group, and estimated to be approximately 130 meters (430 ft) in diameter. It was listed on the Sentry Risk Table with a Torino Scale rating of 1 for a potential impactor in June 2048. It was removed from the Sentry Risk Table on 28 March 2014.

<span class="mw-page-title-main">Lost minor planet</span> Asteroids whose orbits are not known accurately enough to find them again

A minor planet is "lost" when today's observers cannot find it, because its location is too uncertain to target observations. This happens if the orbital elements of a minor planet are not known accurately enough, typically because the observation arc for the object is too short, or too few observations were made before the object became unobservable.

1994 WR12 is an asteroid and near-Earth object approximately 130 meters (430 feet) in diameter. As a member of the Aten group almost all of its orbit is closer to the Sun than Earth is. On 24 November 1994 it passed about 374100 km from the Moon. First imaged at Kitami Observatory on 26 November 1994, it was discovered two nights later by American astronomer Carolyn S. Shoemaker at Palomar Observatory on 28 November 1994. The asteroid then went unobserved from 1994 until it was recovered by Mauna Kea in March 2016. It was removed from the Sentry Risk Table on 2 April 2016.

(5496) 1973 NA, is a very eccentric and heavily tilted asteroid, classified as near-Earth object of the Apollo group, approximately 2 kilometers in diameter. It was discovered on 4 July 1973, by American astronomer Eleanor Helin at the U.S. Palomar Observatory in California. At the time of its discovery, it was the most highly inclined minor planet known to exist. It may be the parent body of the Quadrantids.

<span class="nowrap">1999 XS<sub>35</sub></span>

1999 XS35 is a near-Earth object discovered in 1999 having a comet-like orbit. Its semi-major axis is 17.8 AU. Its orbital eccentricity is 0.94, which means that at the perihelion 1999 XS35 comes as close as 0.9 AU to the Sun, while at the aphelion it reaches beyond the orbit of Neptune. 1999 XS35 is a damocloid. 1999 XS35 is a small object with an absolute magnitude (H) of 17.2, which implies a size of about 1 km.

(467336) 2002 LT38, is a sub-kilometer asteroid and suspected tumbler, classified as a near-Earth object and potentially hazardous asteroid of the Aten group, approximately 240 meters (790 ft) in diameter. It was discovered on 12 June 2002, by astronomers of the Lincoln Near-Earth Asteroid Research at the Lincoln Laboratory's Experimental Test Site near Socorro, New Mexico, in the United States.

<span class="nowrap">(367789) 2011 AG<sub>5</sub></span> Near-Earth asteroid in 2040

(367789) 2011 AG5, provisional designation 2011 AG5, is a sub-kilometer asteroid, classified as near-Earth object and potentially hazardous asteroid of the Apollo group. It has a diameter of about 140 meters (460 ft). It was removed from the Sentry Risk Table on 21 December 2012 and as such it now has a rating of 0 on the Torino Scale. It was recovered in December 2022 extending the observation arc from 4.8 years to 14 years. As of 2023, the distance between the orbits of Earth and 2011 AG5 is 0.0004 AU (60,000 km; 0.16 LD)

2010 XC15 (also written 2010 XC15) is an Aten near-Earth asteroid and potentially hazardous object that spends most of its time inside of the orbit of Earth. It has an observation arc of 10 years and an Uncertainty Parameter of 1. 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.

(31345) 1998 PG is an eccentric, stony asteroid and binary system, classified as near-Earth object of the Amor group of asteroids, approximately 900 meters in diameter. It minor-planet moon, S/2001 (31345) 1, has an estimated diameter of 270 meters.

2011 XC2 (also written 2011 XC2) is a near-Earth asteroid roughly 60–140 meters (200–460 ft) in diameter that passed less than 1 lunar distance from Earth on 3 December 2011.

<span class="nowrap">(388188) 2006 DP<sub>14</sub></span>

(388188) 2006 DP14, provisional designation 2006 DP14, is a sub-kilometer sized, peanut-shaped asteroid on a highly eccentric orbit, classified as near-Earth object and potentially hazardous asteroid of the Apollo group. This contact binary was discovered on 23 February 2006, by astronomers of the LINEAR program at the Lincoln Laboratory's Experimental Test Site near Socorro, New Mexico, in the United States. On 10 February 2014, it passed 6.25 lunar distances from Earth. The asteroid is approximately 400 meters in diameter and has a rotation period of 5.77 hours.

2010 GZ60 was originally estimated by JPL to be a near-Earth asteroid approximately 2 kilometers (1.2 miles) in diameter. But is now known to be an asteroid from the inner region of the asteroid belt that does not get closer than 1.5 AU (220 million km) to Earth.

<span class="mw-page-title-main">2018 AH</span> Near-Earth asteroid Christmas 2021

2018 AH is a sub-kilometer asteroid, classified as near-Earth object of the Apollo group, approximately 100 m (300 ft) in diameter. It was first observed on 4 January 2018, by the Asteroid Terrestrial-impact Last Alert System (ATLAS) on Mauna Loa and quickly followed-up by many other surveys, with precovery observations found from Pan-STARRS and PTF from the day previous.

<span class="mw-page-title-main">C/2018 F4 (PanSTARRS)</span>

C/2018 F4 (PanSTARRS) is a hyperbolic comet. It was discovered on 17 March 2018 when it was beyond the orbit of Jupiter, 6.4 AU (960 million km) from the Sun. It was quite far from the Sun and turned out to simply be an asteroidal object that was discovered before cometary activity was noticeable. As perihelion is inside the orbit of Jupiter, this object should become more active. In April 2018 it was determined to be a hyperbolic comet. Given that the incoming velocity was similar to that of an Oort cloud object, we can very confidently say that it is not of interstellar origin. C/2018 F4 fragmented around August 2020.

<span class="nowrap">2018 LF<sub>16</sub></span>

2018 LF16 is a small Mars crossing asteroid roughly 213 m (699 ft) in diameter. It was first observed by astronomers with the Pan-STARRS survey at Haleakala Observatory on 14 June 2018. It was removed from the Sentry Risk Table on 29 July 2021. With an observation arc of 15 years the orbit is very well known and it does not make any notable approaches to Earth.

2020 XR is an Apollo near-Earth object and potentially hazardous asteroid roughly 390 meters in diameter. With a 5-day observation arc it was briefly listed as having a 1 in 11,000 chance of impacting Earth on 1 December 2028 placing it at the top of the Sentry Risk Table with a Palermo scale rating of -0.70.

2020 AP1 is an Apollo near-Earth object roughly 5 meters (20 feet) in diameter. On 2 January 2020 it passed 0.00218 AU (326 thousand km; 0.85 LD) from Earth. With a short 1-day observation arc it was roughly expected to pass about 0.01 AU (1.5 million km; 3.9 LD) from Earth on 7 January 2022, but with an uncertainty of ±8 days for the close approach date it could have passed significantly closer or further.

2022 AE1 is a Tunguska event-sized asteroid, classified as a near-Earth object of the Apollo group, approximately 70 meters (230 feet) in diameter. It was discovered by the Mount Lemmon Survey on 6 January 2022, when it was 0.09 AU (13 million km) from Earth. On 9 January 2022 with an observation arc of 3 days, it was rated with a Torino scale of 1 for a virtual impactor on 4 July 2023 16:28 UTC. Nominal approach is expected to occur 1 July 2023 01:13 ± 1 day. With a Palermo scale rating of as high as –0.66 at the European Space Agency on 11 January 2022, the odds of impact peaked at about 4.6 times less than the background hazard level. NEODyS was the first risk-page to drop to Torino scale 0 on 12 January 2022 followed by ESA on 13 January 2022, but by January 14 both returned to Torino scale 1. On 14 January 2022 the waxing gibbous moon was as little as 3 degrees from the asteroid delaying observations of the asteroid from January 12–19. On 20 January 2022 with a 16-day observation arc, using JPL #11 the Sentry Risk Table dropped the asteroid to Torino scale 0 and then later that day JPL #12 resulted in it being removed from the risk table.

References

  1. ↑ TNOs really do require patience; 2-3 years is only just enough to say anything about the orbit parameters – Astronomer Michele Bannister (4 April 2018)
  2. ↑ JPL Small-Body Database Browser for 2018 AG37
  3. 1 2 JPL Small-Body Database Browser for 1999 DP8. Discovery date Ephemeris table setting: #39. Range & range-rate = 6.8E+11 / AU / 3-sigma = 1500 AU
  4. ↑ "MPEC 2017-K35 : COMET C/2017 K2 (PANSTARRS)". IAU Minor Planet Center. 2017-05-24. Retrieved 2017-10-21. (CK17K020) T 2027 Jan. 5
  5. ↑ "How to determine the orbit of a comet?". esa. 2014-03-07. Retrieved 2022-01-08. It took 44 days of observation to achieve even a semblance of an orbit determination – one that was still all over the place
  6. ↑ JPL Small-Body Database Browser for C/2008 J4 (McNaught)
    e = 0.9977 to 1.017
    semi-major axis = −58
    v=42.1219 √1/50000 − 0.5/−58
  7. ↑ JPL Small-Body Database Browser for C/1999 U2 (SOHO)
  8. ↑ JPL Small-Body Database Browser for Comet Swift–Tuttle
  9. ↑ JPL Small-Body Database Browser for C/2001 OG108
    (Close approach uncertainty: (MaxDist of 0.434) – (MinDist of 0.408) * 149597870.7 = 3.9 million km)
  10. ↑ "MPEC 2022-A59 : COMET C/2022 A1 (Sarneczky)". IAU Minor Planet Center. 2022-01-07. Retrieved 2022-01-08. (CK22A010)
  11. ↑ C/2022 A1 (Sarneczky) Close approach table at JPL SBDB and Uncertainty region archive
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