INTRODUCTION
Reaching Pluto has proven to be a challenging endeavor, and the time it would take to travel there is a matter of interest. Many of us recall learning in school that Pluto was once classified as the ninth planet in our solar system. However, its status came into question after the discovery of several similarly sized objects in the outermost region of the solar system known as the Kuiper Belt, beginning in 1992. This led to a heated debate among astronomers. The debate intensified with the discovery of Eris, which is larger than Pluto. In 2006, the International Astronomical Union felt compelled to formally define the term “planet,” effectively leading to the exclusion of Pluto from the list of planets. The decision to redefine the term “planet” has sparked ongoing debate, and its correctness remains uncertain. In this article, we intend to explore the time it would take to reach Pluto, taking into account all potential options.
PLUTO FACTS AND COMPOSITION
After traveling for 9.5 years and covering a distance of 5 billion kilometers, New Horizons completely changed our understanding of Pluto in 2015. Previously considered a barren ice ball, it turns out that Pluto has a diameter of 2400 km and harbors a large rocky core similar to Earth. This core is covered by a frozen water mantle, potentially housing an underground ocean, and is further covered by layers of volatile ices like nitrogen, methane, and carbon monoxide. The sublimation and precipitation of these gases on the surface create a diverse topography with plains, craters, canyons, and even possibly ice volcanoes, making Pluto one of the most colorful worlds in the Solar System. New Horizons’ sensors unveiled breathtaking landscapes, with scientists suggesting that the variety of these terrains results from interactions between volatile ices and inert water ice, leading to intriguing cycles of evaporation and condensation. Unlike Earth, where only water condenses and evaporates, Pluto experiences interactions between three elements across its entire surface, adding complexity to its geological processes. One of the most fascinating features on Pluto is the geologically young heart-shaped plain called Sputnik Planum. Its complex topography, including irregularly contoured nitrogen ice cells and water ice hills floating in a frozen nitrogen sea, points to active processes slowly transforming the area. The atmosphere surrounding Pluto is equally remarkable, with a deep blue color, layers of haze, and unexpected cold and density. This affects the escape of the upper atmosphere into space and its interaction with the solar wind. Contrary to previous assumptions, Pluto’s atmospheric loss is more similar to Earth’s than to a comet’s tail, with methane being the primary gas escaping despite nitrogen dominating near the surface. The complex photochemistry of methane and organic compounds in the atmosphere leads to the formation of reddish soot particles known as tholins, creating low-altitude hazes and contributing to the surface’s brick-red color. These low hazes suggest that Pluto experiences daily weather changes, potentially forming complex cloud systems, resembling Earth more than Mars. Additionally, Pluto boasts an impressive satellite system, featuring Charon with a diameter of 1200 km and four smaller satellites with diameters ranging from about 40 km for Nix and Hydra to about 10 km for Styx and Kerberos.
ANOTHER ELEMENT ABOUT PLUTO DECLASSIFICATION
The moons of Pluto have highly anomalous rotation rates, unusual orientations, and icy surfaces with albedos and colors very different from those of Pluto and its major moon Charon. There is evidence that some of them are the result of the merger of two smaller bodies, and the craters present date their formation to at least 4 billion years ago, reinforcing the idea that they formed following a collision that produced the binary system Pluto-Charon. Speaking of this rare “double planet” configuration, so similar to the Earth-Moon system (with the two objects locked in a gravitational resonance, always showing the same face to each other), what would you think of a planet that has a satellite in the sky almost eight times the angular diameter of our Moon as seen from Earth? Would you really call that a “dwarf” planet? In short, folks, in those few minutes the probe took to fly over it, Pluto revealed itself to be such a complex world that it immediately made us think we absolutely have to go back soon! But how? New Horizons took more than nine years to get there… Is that a lot? Is that a little? To those unfamiliar with the distances and times that govern the cosmos, it may seem like an eternity… But can we do better or not?
HOW TO GET TO PLUTO?
You can’t answer above question without first understanding that the duration of a journey to such a distant celestial body depends on many variables dictated by initial choices, so there can’t be a single answer. But how did the New Horizons probe reach Pluto so quickly?
NEW HORIZONS PROBE
Launched on January 19, 2006, when Pluto was still considered a planet, New Horizons remains the fastest spacecraft to have left Earth due to its relatively small mass of about one ton and the powerful Atlas V launch vehicle that boosted it. When the rocket’s engine in the final stage of the launch vehicle shut down, New Horizons was traveling at 16.21 km/s (58,000 km/h or 36,000 mph) relative to Earth, allowing it to cross the Moon’s orbit just nine hours after launch, a feat that took Apollo missions three days to accomplish. Despite its impressive speed, the probe did not travel directly to its final destination, Pluto and its moons, but instead took a longer, indirect route through the solar system towards Jupiter during its first year in space. As New Horizons approached Jupiter, it began to accelerate due to the planet’s gravitational influence and its trajectory started to change as Jupiter pulled the probe towards itself, resulting in a sharp curve in the probe’s path when viewed from above. On February 28, 2007, the small probe made its closest approach to Jupiter at about 2 million kilometers before continuing on its path, now traveling about 4 km/s (14,400 km/h or 8,950 mph) faster than before the encounter with Jupiter.
NEW HORIZONS APPROACH JUPITER
Surprisingly, the source of this speed increase is quite unexpected. Essentially, the speed was taken from Jupiter. The reduction in speed for Jupiter is directly related to the difference in size between the massive planet and the spacecraft, causing Jupiter to slow down by approximately a millionth of a trillionth of a millimeter per second! This clever maneuver resulted in cutting four years from New Horizons’ travel time to Pluto in one swift move. There was a compelling reason to expedite the journey: scientists aimed for New Horizons to reach Pluto before its tenuous atmosphere dissipated and froze as the planet moved away from the Sun. However, the high speed made it impossible for the probe to enter orbit around Pluto. New Horizons zoomed past Pluto without stopping, effectively passing between the dwarf planet and its largest moon, Charon. No available propulsion system could have decelerated the probe sufficiently to achieve orbit around Pluto. Hence, it is evident that the duration of the journey to Pluto is influenced by various factors, including the crucial factor of launch speed. Additionally, the type of propulsion utilized, the nature of the mission (flyby or orbital insertion, automated or crewed mission), and the position of Pluto along its highly eccentric orbit also play a role.
POSITION & ORBIT OF PLUTO
Let’s examine these factors one by one, okay? Pluto’s distance from Earth varies. Pluto has a highly eccentric orbit, meaning its distance from Earth ranges from 4.28 billion kilometers (28.58 AU) at perihelion to 7.52 billion kilometers (50.3 AU) at aphelion. An impressive difference, isn’t it? This means that to minimize travel time, missions should be scheduled when the distance is near its minimum. This is exactly what happened with New Horizons, which took advantage of the period when Pluto was near its perihelion. This is a significant limitation for current propulsion technologies, as the optimal launch window repeats at intervals of about 120 years. If New Horizons had launched when Pluto was at aphelion, it would have taken twice as long! Types of Propulsion Current Chemical Propulsion Chemical propulsion is currently the most widely used technology for space travel. It uses the chemical reaction of fuels and oxidizers to generate thrust. As we’ve seen, using chemical propulsion and gravitational assists, the journey to Pluto can take between 9 and 20 years, depending on its orbital position. Chemical propulsion is well-tested and reliable, but its efficiency is limited, requiring large amounts of fuel and offering relatively long travel times. Ionic and Solar Propulsion (Near-Future Technologies) Ionic propulsion uses electricity to ionize and accelerate a propellant (usually noble gases like xenon) to very high speeds. Solar propulsion, on the other hand, uses the pressure of sunlight on thin sails. These technologies could reduce the travel time to about 7-9 years, depending on Pluto’s distance from Earth at the time of launch. Ionic propulsion, for example, was successfully used in the Dawn mission to Vesta and Ceres. These systems are much more fuel-efficient than chemical propulsion but provide less thrust, making them ideal for long-duration, unmanned missions.
FUTURISTIC (NUCLEAR AND ANTIMATTER) PROPULSION
Nuclear propulsion uses nuclear reactors to generate heat and thrust, while antimatter propulsion exploits the reaction between matter and antimatter, releasing enormous amounts of energy. With nuclear propulsion, the journey could be reduced to about 3-5 years, while antimatter propulsion could theoretically reduce travel times to a few months. These technologies offer enormous potential for reducing travel times but are still in experimental development and pose significant technical and safety challenges.
TYPES OF MISSIONS
Automated Flyby Mission: An automated flyby mission involves a close pass of a probe without entering orbit around the planet. As demonstrated by the New Horizons mission, using chemical propulsion and gravitational assists, a flyby mission to Pluto can take about 9-10 years. Flyby missions allow for close-up data collection without the probe having to slow down for orbital insertion, but the downside is that they offer only a brief observation period. The flyby of Pluto by New Horizons lasted only three minutes.
Automated Orbital Mission: This mission involves the probe entering orbit around Pluto for extended studies. Using chemical propulsion, the duration could be similar to that of a flyby. It offers a prolonged period of observation and data collection but requires more fuel and more complex maneuvers for orbital insertion, as well as significant deceleration in the final stretch. The estimated travel time would be 12-15 years.
Crewed Mission: A crewed mission to Pluto would represent one of the greatest challenges in space exploration. With current technology, a crewed mission (including orbital insertion) would take about 15-20 years. The spacecraft would have to be a large vessel where astronauts could live for years, making it extremely heavy and requiring an enormous amount of fuel. There is no possibility of this happening without a propulsion system capable of reducing the travel time to just a few months. To conclude, Planet or Dwarf Planet? Pluto doesn’t care: however we decide to classify it, it will continue to travel silently along the edge of the Solar System. If we want to try sending another spacecraft to its vicinity, perhaps equipped with a rover to explore its surface, we will need to find a more powerful energy source as soon as possible. And, above all, we should keep in mind that the longer we wait, the farther Pluto moves away, greatly increasing travel times. All in all, the simplest solution would be to send another New Horizons right away, this time to enter orbit. If launched in about ten years, it would arrive around 2045, when Pluto would still be relatively close, at 39.5 AU, or 5.9 billion kilometers.