Re: What planet comes closest to Earth?

Venus approaches us as close as 26 million miles (41 million kilometres) 
every 19 months whereas Mars is closest at 35 million miles (56 million 
kilometres) every 15 years.
 Astronauts will however visit Mars long before they visit Venus because:

1/ Mars has a surface temperature ranging from -90 to +10 degrees 
Centigrade - ie it at worst is colder than Antarctica and at best is as 
warm as a spring day in northern latitudes on Earth. 

2/ Atmospheric pressures are similar to those above Earth at 100,000 feet
Thus, in life support terms it is no harder to support a person on Mars 
than it is to support a spacewalker from the Space shuttle or an Apollo 
astronaut, whereas on Venus the prblem is not that of conserving heat but 
of losing it into a dense (90 bars) atmosphere, equivalent to the 
pressure faced by a diver at 3,000 feet under the Ocean) which is also 450 
degrees Centigrade. Thus on Venus there is nowhere to dump the waste heat 
since the environemnt is far hotter than the inside of most ovens.
On top of this the atmosphere on Venus contains significant amounts of 
corrosive sulphuric acid!

  3/ As if all the above were not enough, lift-off from Venus for the 
return trip is as hard in energy terms as lift off from Earth- you and your 
vehicle would weigh 0.87 of your weight here on Earth, while on Mars you 
would weigh 0.38 of your Earth weight. Your outgoing spacecraft then 
would have to take a fully fuelled return vehicle as massive as your 
initial vehicle at Cape Kennedy! Clearly the whole lot would have to be 
assembled in Earth orbit- a massive undertaking indeed, while a Mars 
lander- return vehicle would be correspondingly smaller; it could also 
quite possibly use fuel extracted from the Martian soil or atmosphere.
 
4/To sum up -for human expeditions to Venus we are going to need to 
completely alter the climate and conditions there BEFORE we go, while for 
Mars with technology now within reach over the next few decades we can go 
there, set up a base, and begin living off the land under perspex or 
similar domes before we make any attempt to change local conditions to 
suit us - if indeed we decided to do so at all!

References:
 Travel to Distant Worlds- Karl Gilzin, Moscow Publishing 1956 
 The Case for Mars, by Robert Zubrin
 The New Solar System  4th Edition