The Personal Website of Mark W. Dawson

Manned Interplanetary Missions

Introduction

Interstellar space is vast, and even the space within our Solar System is vast, especially when compared to the space between the Earth and the Moon. As enamored as the American public is with the idea of interstellar space travel, they are enthralled by the possibility of humans traveling to other planets in the Solar System in the near term. However, sending humans to other planets in our Solar System presents challenges that must be overcome for a successful mission.

Distance

Current Fastest Propulsion Travel Times

Planetary probes are designed to explore various celestial bodies and often reach remarkable speeds during their missions. Some of our spacecraft probes have achieved high velocities by Earthly standards, but such speeds will be difficult for a manned spacecraft to reach. These speeds are not only a testament to engineering but also crucial for gathering data that enhances our understanding of solar phenomena and their effects on the solar system. Below are some of the fastest probes currently known:

Probe Name	Speed (mph)	Objective
Parker Solar Probe	430,000	Observations of the Sun's outer corona
Helios-B	149,000	Observations of the Sun's solar processes and the solar environment
Voyager 1	38,000	Flyby of Jupiter and Saturn, and study the interstellar space beyond the Sun's heliosphere
New Horizons	36,000	Flyby of Pluto and its moons

Achieving High Speeds

The Parker Solar Probe reaches its record speed through a combination of solar gravity assists in its trajectory toward the Sun. As it approaches perihelion, the Sun's strong gravitational pull accelerates the probe significantly, allowing it to achieve speeds far exceeding those of other spacecraft. Other spacecraft's high velocities were achieved through gravity assists from planetary flybys.  However, such flybys may be impractical for manned spacecraft.

Current Travel Times to the Planets

Travel times to the planets in our solar system can vary significantly based on the spacecraft's speed and the distance between Earth and the target planet at the time of launch. Below is a summary of estimated travel times to some of the major planets using current technology.

Closest Planets

Planet	Distance	Travel Time
Mercury	48-138 million miles	40-120 Days
Venus	24-162 million miles	20-270 Days
Mars	Closest: ~35 million miles Average: ~140 million miles Farthest: ~250 million miles	6 months 9 months 15 months

Outer Planets

Planet	Distance	Travel Time
Jupiter	365-601 million miles	3 to 6 years
Saturn	746 million-over 1 billion miles	6 to 7 years
Uranus	1.6-2.0 billion miles	8 to 12 years
Neptune	2.7-2.9 billion miles	10 to 12 years

These travel times are based on direct paths and optimal conditions, but actual missions often take longer due to factors such as spacecraft speed and orbital mechanics. It can be expected that spacecraft speed will increase over the next several decades, but not to a magnitude that would significantly change current travel times.

Traveling at 1% to 10% of the speed of light (about 18,600 miles per second) would significantly shorten travel time to the planets in our solar system compared with current spacecraft speeds. However, such speeds are not expected to be achieved for many decades, if not centuries.

Communications to Spacecraft

Communication time between Earth and a spacecraft varies based on the distance between Earth and the spacecraft. Thus, while a spacecraft is traveling, the communication delays change. In interplanetary travel, such delays affect spacecraft operations. It is impractical to communicate with Earth to receive advice and assistance in the event of an emergency, and even in non-emergency situations, such advice and assistance may not be practical due to the changing nature of the non-emergency situation. It should also be noted that effective communication requires two-way communication, which doubles the times in the following list:

• Earth to Sun: ~8 minutes 20 seconds one way
• Earth to Mercury: ~3 to 13 minutes one way depending on orbital positions
• Earth to Venus: ~3 to 6 minutes one way depending on orbital positions
• Earth to Mars (closest): ~3.1 minutes one way — occurs at opposition (once every ~26 months)
                          (average): ~12.5 minutes one way
                          (farthest): ~22.4 minutes one way — occurs at conjunction
• Earth to Jupiter: ~33–54 minutes one way depending on orbital positions
• Earth to Saturn: ~68–95 minutes one way depending on orbital positions
• Earth to Uranus: ~2.4–3.0 hours one way depending on orbital positions
• Earth to Neptune: ~4.0–4.4 hours one way depending on orbital positions

Key Challenges of Long-Duration Space Travel

Long-duration space travel to distant planets, such as Mars and beyond, poses several significant challenges that astronauts must overcome. These challenges can affect both the physical health of the crew and mission success.

Hazard	Description
Space Radiation	Astronauts are exposed to high levels of cosmic radiation, which can damage DNA and increase cancer risk.
Space Particles	Empty space is not completely empty; it contains sparse particles of matter ranging in size from microscopic to large rocks. A collision with these particles could cause extensive damage and even destroy the spacecraft.
Isolation and Confinement	Extended periods in a confined space can cause psychological stress and behavioral issues among crew members. The monotony and isolation of space travel can also affect mental health, necessitating careful crew selection and support strategies.
Microgravity Effects	Prolonged exposure to microgravity leads to muscle atrophy, bone density loss, and potential vision problems. Long missions can also lead to weakened immune systems and other health issues due to the harsh space environment, as I have examined in my article on “Human Impacts of Long-Duration Space Travel”
Maintaining Ecosystem	Ensuring a safe, habitable environment inside the spacecraft is crucial and involves temperature control, air quality, and waste management.
Maintaining Spacecraft	On a long-duration trip, equipment failures aboard the spacecraft can be expected. Spare parts and tools for repair must be stored aboard the spacecraft, increasing the spacecraft's mass and, thus, the fuel required for propulsion.

Addressing these challenges is essential for the success of future missions to distant planets, and ongoing research aims to develop effective solutions. In addition, the following problems need to be addressed.

Energy

Energy is essential; without sufficient energy for spacecraft operations, a spacecraft cannot function. This energy must be generated and stored aboard the spacecraft and be available at all times without failure. Thus, redundant energy generators may be required. Conventional nuclear fission reactors would fit this bill, but they have issues and concerns that must be addressed for successful long-term operation aboard a spacecraft. Energy storage also requires large-capacity, highly efficient batteries, which have issues and concerns that must be addressed.

A radioisotope thermoelectric generator (RTG) is a type of nuclear battery that converts heat from the natural decay of plutonium-238 (Pu-238) into electricity. This process uses thermocouples, which generate an electric current when there is a temperature difference between two junctions made of different metals. NASA is developing next-generation RTGs, such as the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), to improve efficiency and power output for future unmanned missions. For manned, long-duration space travel, the question is whether RTGs can provide sufficient power for spacecraft operations.

Solar power for outer-planet journeys is impractical because the distance to the Sun would be too great to generate sufficient energy to power the spacecraft.

Propulsion

Propulsion for a manned spacecraft requires significant fuel. Not only must you have fuel to propel the spacecraft to its destination, but you must also have fuel to decelerate it near its destination. You must also have fuel to accelerate the spacecraft back to Earth and to decelerate it as it approaches Earth. And you must have fuel to accelerate and decelerate the fuel that the spacecraft requires to complete its round-trip journey. This is quite a bit of fuel for even a modest spacecraft, and the greater the spacecraft's mass, the greater the non-linear increase in fuel required.

Disposable fuel tanks that are jettisoned when no fuel remains are a potential solution, but they add mass to the spacecraft, which may require additional fuel for the mission. It may also be possible to send unmanned refueling spacecraft to the destination, which would reduce fuel requirements for the outbound portion of the mission.

Consumables

The air they breathe, the water they drink, and the food they eat are consumables that must be stored on board for the duration of the round trip. While recycling air and water is helpful, it is not sufficient to meet the needs of a round trip. Recycling also requires equipment and materials to achieve it, as well as the energy to operate the equipment.

Again, it may also be possible to send unmanned resupply spacecraft to the destination, which would reduce storage requirements for consumables during the outbound portion of the mission and reduce the spacecraft's mass and the fuel required for propulsion.

Logistics

While it may be possible to send unmanned refueling and resupply spacecraft ahead of the manned spacecraft, such resupply spacecraft also pose issues and concerns. Because space is vast, it may be very difficult to navigate and to rendezvous between the manned spacecraft and the resupply spacecraft during the journey. Rendezvous between spacecraft during the journey also requires additional fuel to accomplish, perhaps for both spacecraft. Given the speeds of both spacecraft and the navigational precision required for rendezvous, it is possible that both spacecraft would have difficulty joining up. Consequently, the only practical solution may be for refueling and resupply to occur at the destination. And if one or more of these refueling and resupply spacecraft should fail to reach its destination, it would put a successful return to Earth in jeopardy.

Planetary Environment

The magnetospheres around the gas giant planets (Jupiter, Saturn, Uranus, and Neptune) are filled with charged particles. In astronomy and planetary science, a magnetosphere is the region of space surrounding an astronomical object, such as a planet, where charged particles are influenced by that object's magnetic field. It is created by a celestial body with an active interior dynamo. These charged particles pose a health hazard to the crew and may also affect the spacecraft's electronic and electrical systems. Thus, the spacecraft must not approach these planets too closely, lest these charged particles damage the crew and the spacecraft.

The moons orbiting these planets also pose significant challenges to exploration, including high radiation exposure for the crew from the magnetosphere. These conditions create operational challenges because radiation can damage spacecraft systems and disrupt mission activities. Each moon has different environmental conditions that require advanced landing and return technologies, which would need to be designed and developed, as current technologies are insufficient for this task. These issues complicate missions aimed at understanding the moons' geology and make manned exploration of these moons highly unlikely.

The problems of the Martian environment, as discussed in my article “Can We Go to Mars?”, are serious and must be addressed. In particular, Martian dust containing perchlorates, silica, and heavy metals causes respiratory issues, thyroid problems, and chronic diseases, all of which must be mitigated before any Mars mission is undertaken.

Venus has an extremely thick atmosphere composed mostly of carbon dioxide, with clouds of sulfuric acid, resulting in surface temperatures around 872 degrees Fahrenheit and atmospheric pressure about 93 times that of Earth. These harsh conditions make it one of the most inhospitable environments in the solar system and render any manned exploration of Venus impossible.

Mercury has a very thin atmosphere composed mainly of oxygen, sodium, hydrogen, and helium, which cannot retain heat, leading to extreme temperature fluctuations. Daytime temperatures can soar to 800°F, while nighttime temperatures can plummet to -290°F, making the planet inhospitable. Mercury also experiences intense solar radiation due to its proximity to the Sun, making it dangerous for the crew and the spacecraft unless Mercury is used as a shield against the Sun’s radiation. Consequently, manned exploration of Mercury is highly unlikely.

These environmental problems raise the question of why we would send humans into these environments, which could kill or seriously harm the crew and possibly disable the spacecraft.

Economics

The economics of designing, building, and operating an interplanetary spacecraft, as well as resupplying and refueling spacecraft, are daunting. At this time, it is impossible to estimate the costs because a potentially suitable design that addresses all the challenges must be developed before a cost estimate can be made. There is also the issue of the new technologies and engineering that would be necessary for such a voyage, which cannot be estimated until they are designed and developed. We can, however, expect that such missions will be very expensive. Thus, a cost-benefit analysis for manned spacecraft to these targets may not be favorable to undertaking these interplanetary missions.

Conclusions

While the thrill of undertaking these interplanetary missions is highly satisfying, the potential for crew members to die or become ill, and the possible crippling or destruction of the spacecraft during these missions, would be crushing. Consequently, sober thought must be given to these manned interplanetary missions before we commit the resources to accomplish them.

Please note that I wrote this Article with the help of my AI Search Assistance, as I am not sufficiently knowledgeable about this topic to write it myself. Many of the AI Search Assistance results are based on a Wikipedia article and other sources on these topics, which I have hyperlinked within this Article. I have also merged and edited the different AI Search Assistance results for organizational purposes and to improve readability, clarity, and understandability.

Disclaimer

Please note that my Science Articles disclaimer applies to this article, as it does to all of my science articles. These science articles are for the purposes of general scientific information that is understandable to a layperson, so that they can grasp the concepts of the topic being discussed. As such, they are neither as accurate nor as thorough as academics, scientists, engineers, and science writers strive to achieve. My hope is that these science articles will be informative and interesting, and that they will pique the reader's interest in exploring these topics in further detail via the hyperlinks that I have embedded into these articles.