When sending spacecraft to Mars, the current, preferred method involves shooting spacecraft towards Mars at full-speed, then performing a braking maneuver once the ship is close enough to slow it down and bring it into orbit.
Above: Artist's depiction of the Mars Curiosity Rover starting its line of entry, descent, and landing on Mars. A new proposal for sending craft to Mars could save money and offer more flexible launch windows. Credit: NASA
Known as the "Hohmann Transfer" method, this type of maneuver is known to be effective. But it is also quite expensive and relies very heavily on timing. Hence why a new idea is being proposed which would involve sending the spacecraft out ahead of Mars' orbital path and then waiting for Mars to come on by and scoop it up.
This is what is known as "Ballistic Capture", a new technique proposed by Professor Francesco Topputo of the Polytechnic Institute of Milan and Edward Belbruno, a visiting associated researcher at Princeton University and former member of NASA's Jet Propulsion Laboratory.
In their research paper, which was published in arXiv Astrophysics in late October, they outlined the benefits of this method versus traditional ones. In addition to cutting fuel costs, ballistic capture would also provide some flexibility when it comes to launch windows.
Above: MAVEN was launched into a Hohmann Transfer Orbit with periapsis at Earth's orbit and apoapsis at the distance of the orbit of Mars. Credit: NASA
Currently, launches between Earth and Mars are limited to period where the rotation between the two planets is just right. Miss this window, and you have to wait another 26 months for a new one to come along.
At the same time, sending a rocket into space, through the vast gulf that separates Earth's and Mars' orbit, and then firing thrusters in the opposite direction to slow down, requires a great deal of fuel. This in turn means that the spacecraft responsible for transporting satellites, rovers, and (one day) astronauts need to be larger and more complicated, and hence more expensive.
As Belbruno told Universe Today via email: "This new class of transfers is very promising for giving a new approach to future Mars missions that should lower cost and risk. This new class of transfers should be applicable to all the planets. This should give all sorts of new possibilities for missions."
The idea was first proposed by Belbruno while he was working for JPL, where he was trying to come up with numerical models for low-energy trajectories. "I first came up with the idea of ballistic capture in early 1986 when working on a JPL study called LGAS (Lunar Get Away Special)," he said. "This study involved putting a tiny 100 kg solar electric spacecraft in orbit around the Moon that was first ejected from a Get Away Special Canister on the Space Shuttle."
Above: The Hiten spacecraft, built by the Institute of Space and Astronautical Science of Japan, was Japan's first lunar probe. Credit: JAXA
The test of the LGAS was not a resounding success, as it would be two years before it got to the Moon. But in 1990, when Japan was looking to rescue their failed lunar orbiter, Hiten, he submitted proposals for a ballistic capture attempt that were quickly incorporated into the mission.
"The time of flight for this one was 5 months," he said. "It was successfully used in 1991 to get Hiten to the Moon." And since that time, the LGAS design has been used for other lunar missions, including the ESA's SMART-1 mission in 2004 and NASA's GRAIL mission in 2011.
But it is in future missions, which involve much greater distances and expenditures of fuel, that Belbruno felt would most benefit from this method. Unfortunately, the idea met with some resistance, as no missions appeared well-suited to the technique.
"Ever since 1991 when Japan's Hiten used the new ballistic capture transfer to the Moon, it was felt that finding a useful one for Mars was not possible due to Mars much longer distance and its high orbital velocity about the Sun. However, I was able to find one in early 2014 with my colleague Francesco Topputo."
Above: India's Mars Orbiter Mission (MOM) was one of the most successful examples of the Hohmann Transfer method. Credit: ISRO
Granted, there are some drawbacks to the new method. For one, a spacecraft sent out ahead of Mars' orbital path would take longer to get into orbit than one that slows itself down to establish orbit.
In addition, the Hohmann Transfer method is a time-tested and reliable one. One of the most successful applications of this maneuver took place back in September, when the Mars Orbiter Mission (MOM) made its historic orbit around the Red Planet. This not only constituted the first time an Asian nation reached Mars, it was also the first time that any space agency had achieved a Mars orbit on the first try.
Nevertheless, the possibilities for improvements over the current method of sending craft to Mars has people at NASA excited. As James Green, director of NASA's Planetary Science Division, said in an interview with Scientific American: "It's an eye-opener. This [ballistic capture technique] could not only apply here to the robotic end of it but also the human exploration end."
Don't be surprised then if upcoming missions to Mars or the outer Solar System are performed with greater flexibility, and on a tighter budget.
Further Reading: arXiv Astrophysics