Above all, the route has an impact on Solar Orbiter's cost. In theory, you can build a spacecraft with enough on-board oomph to reach the sun and orbit at such a close distance. The trouble is you need so much energy that the launcher would be huge and so would its cost. "This isn't a feasible option for us," says SÁnchez PÉrez.
What is feasible is using what the solar system already provides: free energy courtesy of the planets' gravitational pull, and using a process known as a gravity assist manoeuvre.
A spacecraft directed close to a planet or a moon can use that body's gravity to redirect and boost its flight.
The plan for Solar Orbiter involves two fly-by boosts from Earth and up to six from Venus, depending on when the spacecraft launches. "Every time we perform one of these gravity passes we get closer to the sun, which is our goal," said SÁnchez PÉrez.
Missions designed around gravitational boosts do have drawbacks compared with the more expensive options, such as carrying a lot of propellant or launching on a heavy booster rocket. The mission design is more complex and operations are less flexible. "In flight, we wouldn't want to change anything unless absolutely required," SÁnchez PÉrez says.
The biggest drawback to gravity-assist missions is that they generally take longer. Unlike the Soyuz quick profile flights, Solar Orbiter depends almost entirely on taking longer to get from here to there. One look at the meandering, sling-shotting voyage designed for Solar Orbiter shows it is akin to the interstellar version of a ballroom dance diagram (see diagram, left).
Because of this, Solar Orbiter won't reach its operational orbit until about three years after launch. Once that orbit is established, the spacecraft is designed to function for another three to four years.
Clever trajectory design will also play a crucial role in any human mission beyond Earth orbit. Earlier this year, space tourist and entrepreneur
The proposed trajectory for the Inspiration Mars mission – a fly-by that will give humans a close look at the Red Planet, but not a landing – is a "free return".
The spacecraft would first use solar gravity to fling it towards Mars, where it would come within 160 kilometres of the planet's surface. That fly-by doubles as a Martian gravity assist manoeuvre that will help propel the spacecraft back towards Earth.
Tito himself will undoubtedly have something to say about the spacecraft's trajectory design: before making billions in the financial sector, he worked at the
Tito is adamant that the Inspiration Mars mission will not only have a US crew but will also use the country's rockets, whether tried-and-true workhorses like the Atlas or Delta, or the Falcon Heavy launch system being developed by SpaceX of
One launch scenario for the Inspiration Mars mission would first launch a propellant tank into orbit, to dock with the crew vehicle to transfer fuel. The faster and more efficiently that docking happens, the sooner humans are on their way to Mars.
In the meantime, future orbital travellers – whether tourist or professional – can look forward to quicker trips, perhaps sparing their ground controllers that eternal refrain: are we nearly there yet? n
It's normally a long and cramped ride to the space station on Soyuz
The other space race
Rivalry during the 1960s between the US and Soviets also extended to docking in space
The US set the crewed record in 1966 when
The Soviets set the absolute record in 1968 when the
Kosmos 212 and 213 vehicles docked just
47 minutes after launch. Both Kosmos spacecraft were uncrewed.
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