One of the readers sent us the following question:
In short — no, this is not true, however, depending on the relative position of the Earth and Mars, the amount of fuel required for the flight may differ significantly. Approximately once every one and a half years, the most optimal conditions for a flight to Mars arise, and these optimal conditions are so much better than all others that any other time for sending spacecraft to Mars is simply not considered.
There are several possible trajectories for a flight on the Earth-Mars route, and not all of these trajectories imply a start from Earth, when the Earth and Mars are relatively close to each other, for example, in some flight trajectories on the Earth-Mars-Earth route, gravity assist is used around Venus, therefore the position of Venus influences the dates of the starting window to the same extent.
Return missions to Mars, however, are a matter of the future. Now probes and rovers are sent to Mars in one direction, and for such a flight, the launch should really take place when Mars and Earth are quite close.
Mars and Earth reach their closest approach at times called oppositions or oppositions. However, spacecraft are not launched precisely at moments of confrontation.
They are trying to send the spacecraft along a trajectory that requires the least possible amount of fuel. Best suited for this is the so-called.
Thus, our goal is for Mars to be at the same point as our spacecraft. The period of the Homan orbit is 520 days, but since the spacecraft travels only half the orbit, this means that the journey to Mars will have to take 260 days. The orbital period of Mars is 687 days, i.e. in 260 days Mars will travel an angular distance of 136 degrees, while our spacecraft will travel 180 degrees. This means that in order for the spacecraft and Mars to meet, the launch must take place when Mars is 44 degrees ahead of Earth (180-136 = 44). In practice, this means that we have to launch a rocket with a spacecraft about 3 months before Mars and Earth are in opposition.
The above reasoning refers to ideal conditions that do not always arise. First, the spacecraft rarely moves along a pure Homan trajectory, since the orbital planes of the Earth and Mars are tilted at an angle of 1.85 degrees, which requires additional maneuvers. Therefore, in practice, the start time and flight duration may differ from Goman’s.
In addition, there is a three-pulse scheme for launching spacecraft to other planets of the solar system called MEGA (Moon and Earth Gravity Assist).
The essence of this scheme is that the spacecraft is launched into a geostationary orbit, where the engines make an impulse that increases the apogee of the spacecraft’s orbit and sends it to a point beyond the Moon. At the apogee, the impulse of the guiding apparatus is again made back to the Earth in such a way that on its way it made gravitational maneuvers near the Moon and the Earth and gained additional speed. Finally, at the perigee of the orbit, the engines make the third impulse of the directing apparatus to a transitional trajectory to Mars or another planet.
This approach was used during the launch of the Japanese probe Nozomi to Mars. Unfortunately, the gravity assist near Earth did not go as expected by the Japanese scientists, and as a result, the probe was almost lost.