Astronomy Picture of the Day
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2003 February 16 
Posted on 02/15/2003 9:57:52 PM PST by petuniasevan
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2003 February 16
Explanation: The planet Mercury resembles a moon. Mercury's old surface is heavily cratered like many moons. Mercury is larger than most moons but smaller than Jupiter's moon Ganymede and Saturn's moon Titan. Mercury is much denser and more massive than any moon, though, because it is made mostly of iron. In fact, the Earth is the only planet more dense. A visitor to Mercury's surface would see some strange sights. Because Mercury rotates exactly three times every two orbits around the Sun, and because Mercury's orbit is so elliptical, a visitor to Mercury might see the Sun rise, stop in the sky, go back toward the rising horizon, stop again, and then set quickly over the other horizon. From Earth, Mercury's proximity to the Sun cause it to be visible only for a short time just after sunset or just before sunrise.
The blank areas on the composite photo below are missing data.
Fully thirty years later we plan to go back for another look.
Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER)
The ESA (Europe) plans its own mission around 2009.
BEPICOLOMBO European Space Agency (ESA) Cornerstone mission to Mercury
| Mercury Statistics | |
|---|---|
| Mass (kg) | 3.303e+23 |
| Mass (Earth = 1) | 5.5271e-02 |
| Equatorial radius (km) | 2,439.7 |
| Equatorial radius (Earth = 1) | 3.8252e-01 |
| Mean density (gm/cm^3) | 5.42 |
| Mean distance from the Sun (km) | 57,910,000 |
| Mean distance from the Sun (Earth = 1) | 0.3871 |
| Rotational period (days) | 58.6462 |
| Orbital period (days) | 87.969 |
| Mean orbital velocity (km/sec) | 47.88 |
| Orbital eccentricity | 0.2056 |
| Tilt of axis (degrees) | 0.00 |
| Orbital inclination (degrees) | 7.004 |
| Equatorial surface gravity (m/sec^2) | 2.78 |
| Equatorial escape velocity (km/sec) | 4.25 |
| Visual geometric albedo | 0.10 |
| Magnitude (Vo) | -1.9 |
| Mean surface temperature | 179°C |
| Maximum surface temperature | 427°C |
| Minimum surface temperature | -173°C |
Atmospheric composition
|
42% 42% 15% 1% |
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