The journey would take centuries, but the researchers have calculated how much the velocity of selected objects would need to change to put them on collision courses with Mars.

Mars is cold, its surface is dry and its atmosphere is more than 150 times thinner than Earth’s. If humans ever wanted to make the planet more habitable, they would need to raise its temperature and increase atmospheric pressure, among other changes.

One possible approach would be to supply Mars with vast amounts of water and gases, increasing the density of its atmosphere.

‘It would be necessary to create an atmosphere capable of persisting on Mars for decades or centuries’, explains Ryszard Gabryszewski, PhD, from the Space Research Centre of the Polish Academy of Sciences.

One potential source of water and other volatile compounds is the population of trans-Neptunian objects — bodies that orbit far beyond Neptune.

In a study published in Icarus, scientists from the Space Research Centre of the Polish Academy of Sciences calculated whether selected trans-Neptunian objects could be redirected towards Mars and how much their velocity would need to change to put them on the required trajectories.

‘A vast reservoir of icy bodies extends beyond Neptune's orbit, in the Kuiper Belt, and in an even more distant region called the Scattered Disk. There are millions of objects rich in water, carbon monoxide, nitrogen, ammonia, and other volatiles. These are precisely the compounds that would be most needed during a hypothetical Mars atmospheric pressure increase’, Gabryszewski explains.

The challenge is the enormous distance between these objects and Mars.

Mars orbits at an average distance of 1.5 astronomical units from the Sun, while typical Kuiper Belt objects are more than 30 astronomical units from the star.

Sending such distant objects towards Mars would therefore involve enormous distances and extremely long transfer times.

The researchers tested several scenarios and found that selected objects from the Kuiper Belt or Scattered Disk could be directed towards Mars with a relatively low change in velocity, known as delta-v.

An icy body about 1 kilometre in diameter could have a mass of hundreds of millions or even billions of tonnes.

Such an object could not simply be steered onto a new trajectory with a short manoeuvre of the kind used by spacecraft. Instead, the researchers considered applying a very small force over an extremely long period.

A weak acceleration maintained for decades or centuries could gradually alter an object's orbit and eventually put it on a course towards Mars.

In the future, such a sustained weak force could potentially be produced by technologies such as controlled evaporation of material from an object's surface.

‘If the goal were to significantly transform the Martian atmosphere, the most effective approach would be to use a small number of very large objects, with diameters in the hundreds of kilometres. Using kilometre-sized bodies, however, would require a significantly larger number of them to be directed towards Mars’, Gabryszewski explains.

The effects could be particularly pronounced in some of Mars’ largest depressions, including Hellas Planitia.

The floor of the vast basin lies about 7 kilometres below the planet’s mean elevation, meaning atmospheric pressure there is naturally higher than in most other regions of Mars.

‘Imagine directing several small, volatile-rich bodies into this area. Their collisions could temporarily increase the local content of water vapour and other gases over several decades or even longer, although the further course of the situation would depend on atmospheric circulation and thermal conditions’, Gabryszewski says.

Among the scenarios analysed, one particularly effective method involved using weak thrust to gradually change the shape and orbit of a trans-Neptunian object so that its trajectory would cross Mars’ orbit at the right location and time.

Under this scenario, the journey to Mars would take approximately 380 years.

Another scenario combined weak thrust with a gravitational assist from Neptune.

Spacecraft can use planetary flybys to change their speed and trajectory without consuming additional fuel. A small trans-Neptunian object could similarly use the gravitational field of a planet to alter its course.

In this scenario, Neptune acted as a natural gravitational assist, changing the object's direction and speed and helping send it towards the inner Solar System.

The transfer time, however, increased to approximately 540 years.

Ewelina Krajczyńska-Wujec (PAP)

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