Trans-Neptunian object

439858

439858 (1999 ON4)

Where is it now

Distance from Sun: 41.152 AU
True anomaly: 30.53°
Distance from Earth: 40.312 AU
As of: 18 October 2026

Two-body Kepler propagation — accurate to ~0.1% for major planets over decades, less so for highly-perturbed bodies. Use JPL Horizons for high precision.

Seen from above the ecliptic, north up, to scale. Grid: Earth's orbit in blue. 439858 is 40.312 AU from Earth today (5.6 light-hours).

In the sky

Right ascension: 23h 35m 52s
Declination: -04° 53′ 53″
Constellation: Aquarius
Distance from Earth: 40.32 AU
Elongation from the Sun: 146.32° — well clear of the Sun

Visible from both hemispheres.

From your location

See how high it is right now, whether it is up after civil twilight, and when it rises and sets where you are.

Paste a location

How do I find my coordinates?

Google Maps on a computer

Right-click the spot where you'll be observing.
The first line of the menu is the coordinates, e.g. 51.50722, -0.12758 — click it and they're copied.
Paste them above.

Google Maps on a phone

Press and hold the spot to drop a pin.
The coordinates appear in the search bar (or on the pin's card) — tap to copy, then paste above.

A full Google Maps link works too, but the short maps.app.goo.gl share links don't carry coordinates — copy the numbers instead. We round to about a kilometre; that's all the sky calculation needs.

Know your what3words address? Paste it, e.g. ///filled.count.soap. This is optional — the three words are sent to what3words to convert them, and we don't store the address or the result.

Your location stays in your browser and is sent only for this calculation. Using a what3words address is optional; if you do, the three words go to what3words to be converted and we don't record them. Set a location first, then you can save an email alert for this object. See our privacy policy.

Two-body approximation, good to about a degree for the planets — enough to find the constellation and know whether it is up.

Orbital elements

Semi-major axis: 42.608 AU
Eccentricity: 0.0392
Inclination: 3.19°
Orbital period: 278.1 years
Perihelion: 40.937 AU
Aphelion: 44.279 AU
Longitude of ascending node: 133.11°
Argument of periapsis: 189.69°
Epoch · J2000: 2461200.5
Orbit class · TNO: Trans-Neptunian object
Earth MOID · 15,537.0 LD: 39.9233 AU
Tisserand (Jupiter): 5.832

Orbit quality

Orbit reasonably well determined (uncertainty code 4).

Observations used: 46
Data arc: 22.06 years
First observed: 19 July 1999
Last observed: 8 August 2021
Residual RMS: 0.28 ″
Solution · Otto Matic: 25 August 2021

Visual & observation

Absolute magnitude (H): 7.46

Discovery & classification

Discovered by: Mauna Kea
Discovery date: 19 July 1999
Discovery site: Mauna Kea

Also known as 1999 ON4

TNO

Sources

visual properties: JPL SBDB (lookup)
orbital elements: JPL SBDB