Size and basic dimensions
Apophis is a near-Earth asteroid roughly 340 meters (about 1,115 feet) in diameter, based on radar imaging and optical observations. Early size estimates ranged broadly, but consistent radar and infrared studies since the 2000s converge on this value within small uncertainty. Its approximate dimensions are 340 m × 340 m × 340 m for quick reference, though shape models show noticeable asymmetry.
Radar vs thermal measurements
Radar observations from facilities such as Arecibo and Goldstone provide high-resolution shape and size data by measuring reflected radio waves. Thermal infrared measurements, often from spacecraft or Earth-based observatories, infer size from emitted heat. Combining these methods reduces uncertainty and supports the current ~340 m diameter estimate.
Shape and structure
Apophis has a somewhat elongated, contact binary-like shape in early models, with two lobes possibly connected by a neck or ridge. Later high-resolution radar and light-curve analyses refine this, suggesting a roughly bilobate body with a diameter near 340 m and surface features that include boulders, troughs, and relatively smooth regions.
Rotation and observational history
Apophis rotates with a period of about 30 hours, which has been measured from ground-based optical observations. Its shape model incorporates this spin state, helping explain radar brightness patterns and surface property variations seen over multiple close approaches.
Mass, density, and compositionQ
- Estimated mass: approximately 2.7×10¹⁰ kg (for the ~340 m diameter model).
- Bulk density: roughly 2.6–3.4 g/cm³, consistent with a mix of rock and metal.
- Spectral type: classified as a Q-type or related basaltic or stony composition, indicating silicate minerals and possible metal inclusions.
Implications for hazard assessment
Density and composition affect how Apophis responds to deflection strategies and how energy is transferred during a close approach. A stony, moderately dense body responds predictably to kinetic impactor missions, which underpins its classification as a well-behaved target for planetary defense studies.
Orbit and future approaches
Apophis follows an elliptical orbit that brings it inside Earth’s orbit and out past Mars. The semi-major axis is about 0.92 AU, with an eccentricity near 0.19 and an inclination around 0.2 degrees. The 2029 Earth flyby will alter its orbit enough to make a 2036 encounter possible, subject to continued monitoring.
Key close approaches table
| Date | Approach type | Miss distance | Purpose/notes |
|---|---|---|---|
| 2029 Apr 13 | Close Earth flyby | ≈31,600 km | Well within satellite orbits; will shift orbit and enable 2036 approach |
| 2036 Apr 13 | Potential close approach | ≈120,000 km (current estimate) | Depends on 2029 trajectory change; monitored by radar and optical campaigns |
| Impact risk | Effectively ruled out for 2068+ | — | Prior impact probabilities were devalued after refined astrometry and radar imaging |
Monitoring and measurement methods
Apophis is tracked by ground-based optical surveys, radar when in range, and space missions if approved. Astrometry refines orbit and size; radar resolves shape; thermal observations constrain surface properties. Continued tracking lowers uncertainty in future encounter geometries and improves hazard assessments.
Summary takeaways
- Diameter: about 340 meters (1,115 feet).
- Shape: elongated, bilobate-like with surface detail resolved by radar.
- Density and composition: moderate density, stony/quasi Q-type, consistent with planetary defense test targets.
- Hazard outlook: 2029 flyby will shift its orbit; 2036 approach remains the primary focus after 2029, with impact risk effectively eliminated for the foreseeable future.