2015 RN35

2015 RN₃₅ is a small, unusually red asteroid estimated to be around 41 metres (135 ft) in size. Its orbit crosses Earth's, and is therefore classified as an Apollo asteroid and a near-Earth object (NEO). It was discovered on 9 September 2015 by the Pan-STARRS survey at the Haleakalā Observatory in Hawaii, United States, during one of its close encounters with Earth.

2015 RN₃₅

An animation of 2015 RN35's radar echo, observed by the Goldstone Observatory.
Discovery[1][2]
Discovered by	Pan-STARRS 1
Discovery site	Haleakalā, United States
Discovery date	9 September 2015
Designations
MPC designation	2015 RN35
Minor planet category	NEO Apollo
Orbital characteristics[3] (JPL)
Epoch 5 May 2025 (JD 2460800.5)
Uncertainty parameter 0
Observation arc	2719 days
Earliest precovery date	9 September 2015
Aphelion	1.987 AU
Perihelion	0.962 AU
Semi-major axis	1.4746 AU
Eccentricity	0.3475
Orbital period (sidereal)	1.791 yr (654.028 days)
Mean anomaly	131.416°
Mean motion	1.7906° per day
Inclination	0.2343°
Longitude of ascending node	148.005°
Time of perihelion	8 September 2024 (UTC)
Argument of perihelion	269.669°
Earth MOID	0.00351 AU (525,000 km; 1.37 LD)
Jupiter MOID	3.3734 AU
Physical characteristics
Mean diameter	41±8 m[4]: 11
Synodic rotation period	0.31936±0.00008 h or 0.24889±0.000003 h [4]: 4, 6
Spectral type	A-type or Z-type[4]: 9
Absolute magnitude (H)	23.24 (JPL)[3] 23.9±0.2 (Beniyama et al.)[4]: 11

Discovery and observations

2015 RN₃₅ was discovered on 9 September 2015 by the Pan-STARRS 1 telescope as the asteroid passed within 0.1907 astronomical units (AU) from Earth.^{[2][5]: 327} Further observations were conducted to establish and refine the asteroid's orbit. These observations extended to March 2016, and once again in March 2018^{[1][5]: 327, 337}

In December 2022, 2015 RN₃₅ passed particularly close to Earth, coming within 0.0046 AU (690,000 km) on 15 December. This encounter permitted the Goldstone Observatory, alongside the Australia-based Canberra Deep Space Communication Complex and Australia Telescope Compact Array, to conduct radar imaging of the asteroid.^[6]

Orbit

A diagram of 2015 RN35's orbit, with the orbits of the inner planets shown. Yellow lines are the axes of the ecliptic plane.

2015 RN₃₅ is classified as both an Apollo asteroid (as its orbit crosses the Earth's) and a near-Earth object (NEO). It has an orbital period of around 1.791 Earth years. Its orbit is moderately elliptical, with an orbital eccentricity of 0.3475. It has a semi-major axis of 1.4746 AU, with its distance from the Sun varying from 0.962 AU at perihelion to 1.987 AU at aphelion. Its orbital inclination with respect to the ecliptic plane (the orbital plane of the Earth around the Sun) is very small, at 0.2343°.^[3]

Physical characteristics

The size of 2015 RN₃₅ has not been directly measured, but it can be estimated from its brightness (absolute magnitude) and how reflective its surface is (geometric albedo). Using an absolute magnitude value of 23.9±0.2 and assuming 2015 RN₃₅'s geometric albedo is 0.28±0.10, 2015 RN₃₅'s estimated diameter is 41 ± 8 m (135 ± 26 ft).^[4]: 11

During the December 2022 close encounter, 2015 RN₃₅'s lightcurve data was taken. Using the observed data, several teams of astronomers calculated its rotation period from fluctuations in the asteroid's observed brightness.^[4]: 4 A team of astronomers led by Lorenzo Franco calculated 2015 RN₃₅'s rotation period to be 0.3193 ± 0.0001 hours (19.1580 ± 0.0060 min). They also noted discrepancies in its lightcurve, possibly due to tumbling rotation.^[7] A different team, led by P. Koleńczuk, found a rotation period of 0.319487 ± 0.000115 hours (19.1692 ± 0.0069 min). Meanwhile, a team led by Milagros Colazo measured a longer period of 0.478 ± 0.008 hours (28.68 ± 0.48 min); however, discrepancies between the phase curve measured by Colazo's team means that this period is not likely.^[8][4]: 4 Further analysis in a 2023 study led by Jin Beniyama revealed two periodic signatures. The first period was calculated to be 0.31936 ± 0.00008 hours (19.1616 ± 0.0048 min), whilst the shorter second period was calculated to be 0.24889 ± 0.000003 hours (14.93340 ± 0.00018 min). The two periods agree with 2015 RN₃₅'s observed lightcurve, indicating that they both characterize the asteroid's rotation. However, it remains ambiguous which of the two periods corresponds to 2015 RN₃₅'s rotation period and precession period.^{[8][4]: 4, 6}

Unusually, 2015 RN₃₅'s visible spectrum suggests that it is strongly red. It is nearly as red as the visible spectrum of 269 Justitia, a similarly extremely red object located in the main asteroid belt. 2015 RN₃₅'s spectrum also agrees well with that of rare A-type asteroids and Z-type asteroids, the latter classification having been recently proposed in a 2022 study led by Max Mahlke.^{[4]: 9–12 [9]}

Exploration

2015 RN₃₅ serves as a potential target for a future spacecraft mission due to its proximity and unusual spectral properties. For a launch window between 2030 and 2035, a mission would need a delta-v budget of 11.801 km/s.^{[4]: 11–12} 2015 RN₃₅ is on NASA's automated Near-Earth Object Human Space Flight Accessible Targets Study (NHATS) list of mission-accessible target asteroids.^[6]

See also

• 203 Pompeja – Another very red main belt asteroid