<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Mars Reconnaissance Orbiter orbits Mars at the mean altitude of (250 + 316) / 2 = 283 kilometers in 6720 seconds. The orbital velocity is then 7346 000m * pi / 6720s = 3434 m/s.Let's assume for a moment, that the Mars Reconnaissance orbiter was the "mothership" of the manned mission to Mars circling on orbit like Apollo was circling the Moon.Now the total Delta-V requirement for the Mars lander would be 2 * 3434 m/s = 6868 m/s (braking and descent to the surface, ascent back to orbit), plus some extra propellant to do a bit of manouvering near the surface when looking for a decent landing site. Compare that to SpaceX Falcon-1, which provided Delta-V of 5200 m/s to some 350 kilograms of payload, and you will start to see the magnitude of the challenge of building a manned Mars lander.Even if that 350 kilograms would have been the equivalent of 921 kilograms on the surface of Mars. <br />Posted by aphh</DIV></p><p>It is not quite as bad as that. As much of the dV needed to land (~2.6 of 3.4 km/s) can be shed by atmospheric braking. So you only need 800m/s to land.</p><p>Unless you are planning to return directly to Earth your ascent vehicle can be made very small.</p><p>Jon</p><p><br /><br /> </p> <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em> Arthur Clarke</p> </div>