| Minorplanet: | yes |
| 3401 Vanphilos | |
| Background: |
|
| Discovery Ref: |   |
| Discovered: | 1 August 1981 |
| Discovery Site: | Oak Ridge Obs. |
| Mpc Name: | (3401) Vanphilos |
| Alt Names: | 1981 PA1946 DA |
| Named After: | Vanessa Hall Philip Osborne |
| Mp Category: | Mars crosser  |
| Orbit Ref: |   |
| Epoch: | 4 September 2017 (JD 2458000.5) |
| Uncertainty: | 0 |
| Observation Arc: | 70.38 yr (25,708 days) |
| Perihelion: | 1.5147 AU |
| Semimajor: | 2.3671 AU |
| Eccentricity: | 0.3601 |
| Period: | 3.64 yr (1,330 days) |
| Mean Motion: | / day |
| Inclination: | 21.797° |
| Asc Node: | 322.38° |
| Arg Peri: | 108.64° |
| Mars Moid: | 0.1190 AU |
| Dimensions: | km km 10.30 km |
| Rotation: | h h h h |
| Albedo: | 0.20 |
| Spectral Type: | SMASS = S S  |
| Abs Magnitude: | 12.312.65 |
3401 Vanphilos, provisional designation, is a stony, eccentric asteroid and sizeable Mars-crosser, approximately 7 kilometers in diameter. It was discovered on 1 August 1981, by and at Harvard's Oak Ridge Observatory (Agassiz Station) in Massachusetts, United States.
Vanphilos orbits the Sun in the inner main-belt at a distance of 1.5–3.2 AU once every 3 years and 8 months (1,330 days). Its orbit has an eccentricity of 0.36 and an inclination of 22° with respect to the ecliptic. In 1946, it was first identified as at the Finnish Turku Observatory, extending the body's observation arc by 35 years prior to its official discovery at Harvard.
In the SMASS taxonomy, Vanphilos is characterized as a common S-type asteroid.
In February and March 2008, three rotational lightcurves of Vanphilos were obtained from photometric observations by astronomers Petr Pravec, James W. Brinsfield and Robert Stephens. Light-curve analysis gave a well defined rotation period of 4.225 and 4.226 hours, respectively, with a change in brightness between 0.50 and 0.54 magnitude .
In August 2014, astronomer Brian Warner derived a concurring period of 4.227 hours with an amplitude of 0.62 magnitude from his observations taken at the Palmer Divide Station in Colorado . Light-curve plots were published on-line by the Ondřejov Observatory and the Center for Solar System Studies.
According to the survey carried out by NASA's Wide-field Infrared Survey Explorer with its subsequent NEOWISE mission, Vanphilos measures 7.02 and 7.10 kilometers in diameter, and its surface has an albedo of 0.377 and 0.31, respectively. The Collaborative Asteroid Lightcurve Link assumes a standard albedo for stony asteroids of 0.20 and calculates a diameter of 10.30 kilometers, as the lower the body's albedo (reflectivity), the larger its diameter, at a constant absolute magnitude.
This minor planet was named for Vanessa Hall and Philip Osborne, by astronomer G. V. William to celebrate their marriage on 3 August 1991. The approved naming citation was published by the Minor Planet Center on 25 August 1991 .