|These thoughts may sound a little premature, after the first successful
testing of the advanced propulsion system with Xenon ions by Deep Space
1 and nearly 30 years after the last time, when humans left the influence
zone of Earth.
But for now and here I will left aside the technical problems of reachability of the more remote celestial bodies than the moon and only consider, how likely or even possible a landing of humans on other planets or moons of the planetary system is.
Also I ignore all bodies with less than 2,000 km, a somewhat arbitrary limit of course, but not unusual in astronomy. These little ones generally pose no big, but all the same problems: a lack of atmosphere and very low gravitation, therefore a handicap to move around with the legs on them - remember the pictures of the astronauts on our moon! This is valid especially for the numerous minor planets and comets.
The Sorting begins
First we can ignore the four Jovian planets Jupiter, Saturn, Uranus and Neptune. These lack rigid surfaces and offer no chance for our aim. What now remains, are the five smaller planets and the seven biggest natural satellites in the Solar System.
Mercury: no desirable aim
The innermost planet is troublesome in two aspects: the huge temperature differences, these range from -180 up to 400 degree C. And alongside the solar radiation produces an extremely high ultraviolet and particle flux, which is at this intensity very dangerous. In this harsh environment humans had to use heavy protection suits, which would immobile them largely. Mercury is therefore not a likely target for direct human exploration.
Venus: an offspring of hell...
Our inner neighbor among the planets has a very dense atmosphere, shielding much of the solar radiations. But this is already the good news. Otherwise it has several very nasty features: an atmospheric pressure at ground level of about the same level as in Earths oceans 900 m below the surface, a temperature even higher than the highest on Mercury (the infamous running greenhouse effect) and large amounts of sulfur acid. This will clearly forbid for a very long time direct human exploration - if not forever.
Earth and Moon are trivial, but Mars...
...is not only in terms of distance, but also by conditions the most suitable aim for us. A thin atmosphere enables even atmospheric support for wings of special designed aircrafts and temperature, pressure and gravitation are all of acceptable values. The low gravitation is a special bonus, because as next logical place for an human landing this makes it easier for the astronauts to readapt from zero gravity conditions during the voyage to Mars. Our first item on the list.
Jupiters four Galileian Moons
share generally a problem created by Jupiters large magnetic field: besides Callisto they are all in the zone of extremely strong radiation of Jupiters pendant to the terrestrial van-Allen belts. This radiation is such strong, that even special constructed electronic like the one of the Galileo probe can't bear it more than few hours. Humans would die nearly immediately in this environment. So at most Callisto could be an aim, but even there the radiation is nothing, which could be taken easy. The worst place anyway is Io, not due to its huge volcanic activity, but Io circles Jupiter in the most dense part of the radiation belt. We may put Callisto on an intermediate list...
Saturns big Moon Titan
features a dense atmosphere with nitrogen as main component. Despite the lack of oxygen and the extremely low temperatures require space suits, they must not be pressurized and we already know, that there are a solid surface and probably liquid parts consisting of hydrocarbons. Besides at all times synthetic illumination is needed, because the aerosol haze blocks the feeble sunlight (10 times more distant from Sun than Earth, which means only one percent of illumination!) nearly totally. This creates deep night all day. Safe for position, Titan is the second entry without doubts on our list.
Uranus offers nothing to discuss (limit above!), but Neptunes satellite
Triton and the comparable Pluto, regardless if classified as planet or not, are potential aims: despite being very cold places, there are no real obstacles - unless better propulsion systems than todays are lacking. The very thin atmospheres offer neither protection nor a hazard. These are number three and four on our list.
These are the bigger bodies found to be realistically explorable by humans:
Mars, Titan, Triton, Pluto and eventually Callisto.
Not a very long list... And the problems arise already with Titan (and also Callisto). It's not imaginable today, how we will reach them; at now there are not even final plans for a manned mars landing! But Jupiter and Saturn increase the problems of the voyage by a large factor compared with Mars.
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