Asteroids you can step off of vs. those you can't

Wednesday, July 14, 2010

I noticed in a few of the articles on Rosetta's flyby of the 21 Lutetia asteroid that some were not sure whether this asteroid was the type where one misstep could send you careening into space, or whether it had sufficient gravity to hold people to the surface. The answer is of course that with a diameter of 100 km it has more than enough gravity to hold people to the surface, and indeed it would be impossible to escape without a velocity of some 300 kph. But while calculating the surface gravity and escape velocity of a body isn't very complicated it still isn't one that can be done in one's head in the same way area, circumference and all the rest can, so I thought it would be helpful to come up with a quick chart of the surface gravity, escape velocity and surface area of asteroids at different diameters.

The calculation assumes a sphere, and while smaller asteroids are almost never perfectly spherical it's the only way to make a rough calculation. It also assumes an average density of 4 grams per cm3, and so keep in mind that this can vary a bit as well. I have also added equivalent regions or countries to the surface area in order to give a good idea of how much there is to explore.

You can see that it doesn't take much to have an asteroid where the escape velocity is greater than any slight misstep, and a diameter of 5 km is sufficient here, while by the time you get to a diameter of 15 km those on the surface are very secure. The asteroid 243 Ida has an escape velocity that varies depending on how far away you are from the centre, but even at the furthest tip it's still 52 kph and this can reach as high as 100 kph closer to the centre, so that asteroid is safe for human explorers as well.

Back to the chart: it goes up to 300 km and then straight to 4 Vesta (the most massive asteroid but not the largest; 2 Pallas is slightly larger), as once you reach 200 - 300 km it's easier to just look directly at the information for each asteroid as there aren't many left. Ceres, of course, is larger still though it is no longer considered to be just an asteroid.


DiameterSurface gravity (% Earth)
Escape velocity
Surface area
Equivalent city/country

100 m

0.0006%
0.27 kph
31415 m2

500 m
0.0028%
1.3 kph
0.78 km2Slightly more than
Vatican City
1 km
0.0057%
2.7 kph
3.14 km2Slightly more
than Monaco
2 km
0.01%
5.4 kph
12.56 km2Tokelau
3 km
0.017%
8 kph
28 km2Macau
5 km
0.029%
13.5 kph
78 km2Guernsey
10 km
0.057%
27 kph
314 km2Malta
15 km
0.086%
40 kph
706 km2Singapore
20 km
0.11%
54 kph
1256 km2Hong Kong
25 km
0.14%
67 kph
1963 km2Mauritius
30 km
0.17%
80 kph
2827 km2Luxembourg
35 km
0.2%
94 kph
3848 km2Cape Verde
40 km
0.23%
108 kph
5026 km2Trinidad and Tobago
45 km
0.26%
121 kph
6361 km2Palestinian Territories
50 km
0.29%
134 kph
7854 km2
60 km
0.34%
161 kph
11309 km2Qatar
70 km
0.4%
188 kph
15393 km2East Timor
80 km
0.46%
215 kph
20106 km2Slovenia
90 km
0.51%
242 kph
25446 km2Macedonia
100 km
0.57%
269 kph
31415 km2Belgium
125 km
0.71%
336 kph
49087 km2Slovakia
150 km
0.86%
404 kph
70685 km2Ireland
175 km
1%
471 kph
96211 km2South Korea
200 km
1.14%
538 kph
125 663 km2North Korea
250 km
1.42%
672 kph
196 400km2Segenal
300 km
1.7%
807 kph
282 740 km2New Zealand
Vesta
(529 km)
2.24%
1260 kph
840 000 km2larger than Turkey

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