Surface gravity
From CreationWiki, the encyclopedia of creation science
The surface gravity of any celestial body, like a star, a planet, a dwarf planet, or a moon, is actually the acceleration due to gravity of a falling object at or near the surface.
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Terminology
Surface gravity is an acceleration, and is commonly measured in the units of acceleration, which are mass per square unit of time. The SI units of acceleration are m/s².
But surface gravity has another unit, g. The lowercase g is always italicized in the technical literature, mainly to distinguish it from g for gram, and also from g as a symbol for the quantity called surface gravity as opposed to its value. The quantity g is the standard surface gravity of the earth. By definition this is 9.80665 m/s².
In the discussion below, body refers to a celestial body and object refers to a much smaller object that might rest on a body.
Mass v. Weight
Mass and weight are often confused, especially in the US Customary system of units or any other system of ancient or medieval origin. In such systems, mass and weight are regarded as fundamentally the same. But weight can change with altitude or even with latitude.
Weight and mass are related in the following manner:
where g is the surface gravity at any given point on any given body and m is an object's mass.
The concept of weight still has a practical meaning in the planning of missions, crewed or non-crewed, to various celestial bodies. Weight is the force that a spacecraft's engines will need to counteract during liftoff and landing. Moreover the difference in the weight of a man on Earth and on a body to be explored will dictate how much equipment an exploratory crewman can carry on that body.
Calculation of surface gravity
Surface gravity derives directly from the Newtonian law of gravity:
where G is Cavendish's gravitational constant, M is the mass of the body, and R is the radius of the body at any given point on its surface.
Table
The following table lists several celestial bodies for which the surface gravity is known:
| Name | Mass | Mean radius | Equatorial radius | Surface gravity | Escape speed |
|---|---|---|---|---|---|
| Deimos | 1.4E+151.9052e-8 M☾1.4e+15 kg 2.34271e-10 M⊕ |
62006.2 km3.853 mi | 0.250.0255 g0.025 m/s² 0.82 ft/s² |
5.60.0056 km/s20.16 km/h 0.00348 mi/s 12.527 mph |
|
| Phobos | 1.073E+161.4602e-7 M☾1.073e+16 kg 1.79552e-9 M⊕ |
1110011.1 km6.897 mi | 0.5810.0592 g0.0581 m/s² 1.906 ft/s² |
11.40.0114 km/s41.04 km/h 0.00708 mi/s 25.501 mph |
|
| Mimas | 3.79E+195.15766e-4 M☾3.79e+19 kg 6.34205e-6 M⊕ |
198800198.8 km123.529 mi | 6.4020.653 g0.64 m/s² 21.004 ft/s² |
159.50.16 km/s574.2 km/h 0.0991 mi/s 356.791 mph |
|
| Miranda | 6.59E+198.96806e-4 M☾6.59e+19 kg 1.10275e-5 M⊕ |
234200234.2 km145.525 mi | 240000240 km149.129 mi | 8.020.818 g0.802 m/s² 26.312 ft/s² |
1940.194 km/s698.4 km/h 0.121 mi/s 433.966 mph |
| Enceladus | 1.08E+200.00147 M☾1.08e+20 kg 1.80723e-5 M⊕ |
252300252.3 km156.772 mi | 256000256 km159.071 mi | 11.331.155 g1.133 m/s² 37.172 ft/s² |
239.10.239 km/s860.76 km/h 0.149 mi/s 534.851 mph |
| Tethys | 6.18E+200.00841 M☾6.18e+20 kg 1.03414e-4 M⊕ |
536300536.3 km333.241 mi | 14.31.458 g1.43 m/s² 46.916 ft/s² |
3920.392 km/s1,411.2 km/h 0.244 mi/s 876.879 mph |
|
| Dione | 1.1E+210.015 M☾1.1e+21 kg 1.8407e-4 M⊕ |
561600561.6 km348.962 mi | 22.32.274 g2.23 m/s² 73.163 ft/s² |
5000.5 km/s1,800 km/h 0.311 mi/s 1,118.468 mph |
|
| Umbriel | 1.17E+210.0159 M☾1.17e+21 kg 1.95783e-4 M⊕ |
584700584.7 km363.316 mi | 22.872.332 g2.287 m/s² 75.033 ft/s² |
5170.517 km/s1,861.2 km/h 0.321 mi/s 1,156.496 mph |
|
| Iapetus | 1.81E+210.0246 M☾1.81e+21 kg 3.02879e-4 M⊕ |
718000718 km446.145 mi | 748000748 km464.786 mi | 23.42.386 g2.34 m/s² 76.772 ft/s² |
5790.579 km/s2,084.4 km/h 0.36 mi/s 1,295.186 mph |
| Rhea | 2.309E+210.0314 M☾2.309e+21 kg 3.86379e-4 M⊕ |
764500764.5 km475.038 mi | 26.342.686 g2.634 m/s² 86.417 ft/s² |
634.60.635 km/s2,284.56 km/h 0.394 mi/s 1,419.56 mph |
|
| Ariel | 1.35E+210.0184 M☾1.35e+21 kg 2.25904e-4 M⊕ |
578900578.9 km359.712 mi | 272.753 g2.7 m/s² 88.583 ft/s² |
5410.541 km/s1,947.6 km/h 0.336 mi/s 1,210.183 mph |
|
| Charon | 1.9E+210.0259 M☾1.9e+21 kg 3.17939e-4 M⊕ |
586000586 km364.124 mi | 27.82.835 g2.78 m/s² 91.207 ft/s² |
5800.58 km/s2,088 km/h 0.36 mi/s 1,297.423 mph |
|
| Ceres | 476200476.2 km295.897 mi | 487300487.3 km302.794 mi | 27.872.842 g2.787 m/s² 91.437 ft/s² |
515.20.515 km/s1,854.72 km/h 0.32 mi/s 1,152.47 mph |
|
| Oberon | 3.01E+210.041 M☾3.01e+21 kg 5.03682e-4 M⊕ |
761400761.4 km473.112 mi | 34.693.537 g3.469 m/s² 113.812 ft/s² |
726.80.727 km/s2,616.48 km/h 0.452 mi/s 1,625.805 mph |
|
| Titania | 3.52E+210.0479 M☾3.52e+21 kg 5.89024e-4 M⊕ |
788900788.9 km490.2 mi | 37.813.856 g3.781 m/s² 124.049 ft/s² |
7680.768 km/s2,764.8 km/h 0.477 mi/s 1,717.967 mph |
|
| Pluto | 11950001,195 km742.539 mi | 585.914 g5.8 m/s² 190.289 ft/s² |
12001.2 km/s4,320 km/h 0.746 mi/s 2,684.324 mph |
||
| Eris | 13000001,300 km807.783 mi | 65.546.683 g6.554 m/s² 215.026 ft/s² |
13051.305 km/s4,698 km/h 0.811 mi/s 2,919.202 mph |
||
| Nereid | 3.09E+194.20505e-4 M☾3.09e+19 kg 5.17069e-6 M⊕ |
170000170 km105.633 mi | 71.37.271 g7.13 m/s² 233.924 ft/s² |
1560.156 km/s561.6 km/h 0.0969 mi/s 348.962 mph |
|
| Triton | 2.14E+220.291 M☾2.14e+22 kg 0.00358 M⊕ |
13534001,353.4 km840.964 mi | 77.9557.949 g7.796 m/s² 255.758 ft/s² |
1452.61.453 km/s5,229.36 km/h 0.903 mi/s 3,249.374 mph |
|
| Callisto | 1.076E+231.464 M☾1.076e+23 kg 0.018 M⊕ |
24103002,410.3 km1,497.691 mi | 123.57712.601 g12.358 m/s² 405.436 ft/s² |
24502.45 km/s8,820 km/h 1.522 mi/s 5,480.494 mph |
|
| Europa | 4.8E+220.653 M☾4.8e+22 kg 0.00803 M⊕ |
15608001,560.8 km969.836 mi | 132.3913.5 g13.239 m/s² 434.35 ft/s² |
20202.02 km/s7,272 km/h 1.255 mi/s 4,518.611 mph |
|
| Titan | 1.34553E+231.831 M☾1.34553e+23 kg 0.0225 M⊕ |
25755002,575.5 km1,600.342 mi | 135.3513.802 g13.535 m/s² 444.062 ft/s² |
2640.52.641 km/s9,505.8 km/h 1.641 mi/s 5,906.63 mph |
|
| Ganymede | 1.48186E+232.017 M☾1.48186e+23 kg 0.0248 M⊕ |
26312002,631.2 km1,634.952 mi | 142.8214.564 g14.282 m/s² 468.57 ft/s² |
2741.52.742 km/s9,869.4 km/h 1.703 mi/s 6,132.561 mph |
|
| Moon | 7.349E+221 M☾7.349e+22 kg 0.0123 M⊕ |
17371001,737.1 km1,079.384 mi | 17381001,738.1 km1,080.005 mi | 16216.519 g16.2 m/s² 531.496 ft/s² |
23802.38 km/s8,568 km/h 1.479 mi/s 5,323.908 mph |
| Io | 8.93194E+221.216 M☾8.93194e+22 kg 0.0149 M⊕ |
18216001,821.6 km1,131.89 mi | 179.61118.315 g17.961 m/s² 589.275 ft/s² |
2558.052.558 km/s9,208.98 km/h 1.589 mi/s 5,722.195 mph |
|
| Mercury (planet) | 24397002,439.7 km1,515.959 mi | 24397002,439.7 km1,515.959 mi | 37037.729 g37 m/s² 1,213.911 ft/s² |
43004.3 km/s15,480 km/h 2.672 mi/s 9,618.826 mph |
|
| Mars | 33895003,389.5 km2,106.138 mi | 33962003,396.2 km2,110.301 mi | 37137.831 g37.1 m/s² 1,217.192 ft/s² |
50305.03 km/s18,108 km/h 3.125 mi/s 11,251.79 mph |
|
| Uranus | 2536200025,362 km15,759.216 mi | 2555900025,559 km15,881.626 mi | 86988.613 g86.9 m/s² 2,851.05 ft/s² |
2130021.3 km/s76,680 km/h 13.235 mi/s 47,646.743 mph |
|
| Venus | 60518006,051.8 km3,760.414 mi | 60518006,051.8 km3,760.414 mi | 88790.449 g88.7 m/s² 2,910.105 ft/s² |
1036010.36 km/s37,296 km/h 6.437 mi/s 23,174.66 mph |
|
| Saturn | 5823200058,232 km36,183.687 mi | 6026800060,268 km37,448.799 mi | 89691.367 g89.6 m/s² 2,939.633 ft/s² |
3549035.49 km/s127,764 km/h 22.052 mi/s 79,388.869 mph |
|
| Earth | 63710006,371 km3,958.756 mi | 63781356,378.135 km3,963.189 mi | 978.032799.732 g97.803 m/s² 3,208.769 ft/s² |
1118611.186 km/s40,269.6 km/h 6.951 mi/s 25,022.369 mph |
|
| Neptune | 2462200024,622 km15,299.401 mi | 2476400024,764 km15,387.636 mi | 1100112.169 g110 m/s² 3,608.924 ft/s² |
2350023.5 km/s84,600 km/h 14.602 mi/s 52,568.003 mph |
|
| Jupiter | 7149200071,492 km44,423.069 mi | 2087212.815 g208.7 m/s² 6,847.113 ft/s² |
5954059.54 km/s214,344 km/h 36.996 mi/s 133,187.187 mph |
||
| Sun | 696000000696,000 km432,474.35 mi | 695500000695,500 km432,163.664 mi | 274002,794.022 g2,740 m/s² 89,895.013 ft/s² |
617700617.7 km/s2,223,720 km/h 383.821 mi/s 1,381,755.548 mph |
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