How Long does it take to get to our solar system planets?
Those unfamiliar with orbital calculations might think that
the best way to reach a planet from Earth is to wait until it is as close as possible,
and then point the rocket in its direction and take off.
But no...This method would not work for several reasons.
The most obvious is that the planets, including Earth, are not motionless in space,
but move in almost circular orbits at speeds of tens of kilometers per second.
Therefore, instead of "aim and shoot," it is necessary to put the spacecraft on an elliptical
orbit (called a Hohmann transfer orbit) that connects Earth to where the planet will be by the
time the spacecraft reaches the planet's orbit. This, however, imposes definite launch dates.
The actual travel time will be given by half the time it takes to travel the entire Hohmann orbit,
which is a value easily calculated through Kepler's third law.
Let us take the example of Mars. The transfer orbit from Earth to Mars, an ellipse whose major
axis is equal to the sum of the semi-axes of the orbits of the two planets involved,
can be accomplished in less than 17 months, which means that the trip to Mars with this system
would take about 8.5 months. Note that a Hohmann transfer orbit only fires the rocket motor twice:
once to leave Earth's orbit and once to match the speed and go into orbit with the destination
Could we go faster? Sure, but we would have to carry around a lot of extra fuel,
which would also mean increasing the speed at launch: basically, a dog biting its own tail!
Beyond that, we have to consider that the duration of the trip also depends on the kind of encounter
we want to have with the planet. There is indeed a lot of difference between arrival with orbital
insertion and arrival with a fast flyover. In the former case, the departure speed will have to be
slowed down near arrival to facilitate orbital insertion. And this will mean carrying and burning
a lot of fuel and lengthening the travel time. In the second, on the other hand, the departure
speed can be preserved until the end, without slowing down. That is why, as we are about to see,
some missions to Jupiter have turned out to be very fast and others very slow.
The second discriminator in interplanetary flight is that between inner and outer planets. Strange
as it may seem, going toward the Sun by being attracted by its strong gravity is
not an advantage at all, It's fine only if you're content to do fast flyovers... but if you want to
orbit a planet like Mercury or Venus you have only two options: braking with retrorockets to
counteract the Sun's gravity or losing speed by doing flybys with neighboring planets.
In the former case, you will need a huge amount of fuel, and in the latter, the travel times and
distances will be stretched out of proportion. The same thing happens with the outer planets
but in reverse. Indeed, in this case, one must overcome the Sun's gravity, which tends to slow
down the probe, and flybys with other planets serve to acquire more energy and thus more speed.
These we have just laid out are the general principles of interplanetary navigation,
but what if we went and found out the times,
velocities and trajectories used to get to each of the planets in the solar system?
7 - Mercury. Distance from the sun:
58 million kilometers. Minimum distance from Earth: 91 million kilometers.
For the reasons already stated, of all the planets that make up our Solar System, it is perhaps the
most complicated to reach, despite being decidedly close to Earth. Even to reach this small planet,
in fact, it will be necessary to follow the Hohmann orbit, which in this case
will be about 310 million kilometers long. This distance could be covered in just 106 days,
but we would be flying over the planet at a speed of 58 km/s (more than 200,000 km per hour),
which would mean we would only have a few minutes to survey the case. We would be a bullet in space!
In 1974, Mariner 10 was the first probe to make the flyover but first slowed its speed
so that it would have more time for photographic shots... It still made the trip in 133 days.
Much slower were the probes that had as their target to imbed themselves in an orbit around
the planet, they were forced to try to lose speed by first encountering other planets.
Such as BepiColombo, which left in 2018 and will enter Mercury orbit in 2025,
after a fly-by of Earth, 2 of Venus, and 3 of Mercury. Performing gravitational slingshot
maneuvers on such occasions will reduce fuel consumption, making it lower overall than
that required for a direct route from Earth to Mercury, which is faster but more expensive.
6 - Venus. Distance from the Sun: 128 million kilometers,
minimum distance from Earth: 41 million kilometers
Despite its great proximity to Earth, the Hohmann orbit for Venus
also involves a very long route of a whopping 402 million kilometers--a distance that on
average a probe could travel in less than 150 days The first mission to be directed to Venus
was Venera 1 in 1961. The Soviet probe, launched at full power reached Venus in 97 days.
However, it performed a flyby at too high a speed and then got lost in orbit around the Sun.
The following year NASA's Mariner 2 probe also flew over the planet--but
more cautiously, taking 115 days. In 1990, NASA's Magellan entered
Venus orbit after a journey of a whopping 463 days, penalized by the use of a reduced
power engine following program cancellations after the 1986 Shuttle Challenger disaster
In 2006, ESA's Venus Express reached Venus orbit in 153 days.
using the main engine to reduce speed so it could be caught in Venusian gravity.
5 - Mars. Distance from the Sun:
228 million kilometers, minimum distance from Earth: 56 million kilometers
As we have already seen, with the technology we currently possess,
reaching Mars would require a journey lasting about 8 months,
which is necessary to cover the Hohmann orbital path, which for the red planet is even almost
600 million kilometers, i.e. a dozen times longer than the minimum distance to Earth!
The first missions, which only had to fly over the planet and quickly take a few photographs,
were launched on a direct path, obviously taking much less time. The first NASA probes,
Mariner 6 and 7, arrived at the flyby with Mars after 156 and 131 days, respectively.
The slower and heavier missions, such as the 1976 Viking missions, took twice as long,
about 300 days to enter orbit around the planet. The last major mission, the one that laid
the Perseverance rover on the sands of Mars, remained average, taking 203 days.
4 - Jupiter. Distance from the Sun:
778 million kilometers, minimum distance from Earth: 620 million kilometers
Even in the case of the giant of the Solar System, space missions have had very different timelines.
The first probe to reach Jupiter was Pioneer 10, which left on March 3, 1972, and arrived
at its destination on December 3, 1973, after 640 days of flight. Not even a two-year journey!
It was in fact a simple flyover, with the probe traveling the 1.273 billion kilometers
of Hohmann's orbit without ever slowing down. Later, in the late 1970s, the Voyager 1 and 2
probes also made a close pass of the planet, taking as little as 550 days of travel.
The Galileo probe, on the other hand, was the first to attempt to enter orbit, and it took
a whopping 2,242 days to do so! In fact, after leaving in 1989, it had to make two gravitational
flyovers of Earth and one of Venus to pick up considerable initial velocity and then decelerate
enough to succeed in entering a stationary orbit around Jupiter in 1995, after more than 6 years.
3 - Saturn. Distance from the Sun: 1427 million kilometers,
minimum distance from Earth: 1277 million kilometers
Pioneer 11 was the second space mission, after Pioneer 10,
to reach Jupiter and the first to fly over Saturn...
After Mariner 10, it was the second probe ever to make use of a gravity assist;
in fact, thanks to its encounter with Jupiter, it increased its speed and
succeeded in 1979 in reaching and passing Saturn. The journey lasted 2340 days, almost 6.5 years.
In November 1980 it was the turn of Voyager 1, also accelerated by the previous encounter
with Jupiter to fly over Saturn. And it succeeded in the record time of 1165 days,
or just over 3 years! In contrast, its twin Voyager 2 took just over 4 years.
In 1997 it was Cassini's turn to be launched toward Saturn.
Unlike the others, it availed itself of a very complicated orbit,
which led it to have gravity assist with Venus, Earth, and Jupiter. This enabled
it to make the journey in 2452 days (6.7 years) despite minimal fuel consumption.
2 - Uranus. Distance from the Sun: 2870 million kilometers,
minimum distance from Earth: 2720 million kilometers
So far, only one mission has reached the seventh planet in the solar system, Voyager 2, which took
only 8.5 years to get to fly over the gas giant. It should be noted that the expected time to
make the Earth-Uranus path following the Hohmann orbit is 16 years. Only
thanks to gravitational assists with Jupiter and Saturn, Voyager 2 was able to almost halve it.
1 - Neptune. Distance from the Sun:
4500 million kilometers, minimum distance from Earth: 4347 million kilometers
The last planet in the Solar System has also been reached so far by only one probe:
again the legendary Voyager 2, which after its encounter with Uranus flew over Neptune in 1989
taking 12.5 years to make the entire journey starting from Earth.
Without the gravitational assists, following Hohmann's orbit,
the journey would have taken more than 30 years instead!
Pluto. Distance from the Sun:
5900 million kilometers, minimum distance from Earth: 4300 million kilometers
Yes, okay, we all know that Pluto is no longer considered a planet,
but we can't leave it aside since the latest and deepest mission, New Horizons,
was dedicated precisely to its exploration... Launched in January 2006, with a speed of 58 536
kilometers per hour it was the fastest man-made object to leave Earth so far. So fast that it
took only 374 days to reach Jupiter, from which it had a gravity assist and an increase in speed.
Thanks to all this, despite Hohmann's orbit for Pluto predicted a travel time of 45 years,
the New Horizons arrived at the most famous of the dwarf planets in just 9.5 years!
This goes to show how space navigation is only a matter of acceleration and speed.
Of course, propulsion systems capable of getting us to even the most distant planets in just a
month's journey are already being studied for the near future. We just have to wait and trust!
For that matter, do you have anything better to do?