Interplanetary spaceflight
Interplanetary spaceflight or interplanetary travel is spaceflight between bodies within a single planetary system. Spaceflights become interplanetary by accelerating spacecrafts beyond orbital speed, reaching escape velocity relative to Earth at 11.2 km/s, entering heliocentric orbit, possibly accelerating further, often by performing gravity assist flybys at Earth and other planets. Most of today's spaceflight remains Earth bound, with much less being interplanetary, all of which performed by uncrewed spacecrafts, and only just a few spaceflights having accelerated beyond, to system escape velocity, eventually performing interstellar spaceflight.
Uncrewed space probes have flown to all the observed planets in the Solar System as well as to dwarf planets Pluto and Ceres, and several asteroids. Orbiters and landers return more information than fly-by missions. Crewed flights have landed on the Moon and have been planned, from time to time, for Mars, Venus and Mercury. While many scientists appreciate the knowledge value that uncrewed flights provide, the value of crewed missions is more controversial. Science fiction writers propose a number of benefits, including the mining of asteroids, access to solar power, and room for colonization in the event of an Earth catastrophe.
A number of techniques have been developed to make interplanetary flights more economical. Advances in computing and theoretical science have already improved some techniques, while new proposals may lead to improvements in speed, fuel economy, and safety. Travel techniques must take into consideration the velocity changes necessary to travel from one body to another in the Solar System. For orbital flights, an additional adjustment must be made to match the orbital speed of the destination body. Other developments are designed to improve rocket launching and propulsion, as well as the use of non-traditional sources of energy. Using extraterrestrial resources for energy, oxygen, and water would reduce costs and improve life support systems.
Any crewed interplanetary flight must include certain design requirements. Life support systems must be capable of supporting human lives for extended periods of time. Preventative measures are needed to reduce exposure to radiation and ensure optimum reliability.
Current achievements in interplanetary travel
Remotely guided space probes have flown by all of the observed planets of the Solar System from Mercury to Neptune, with the New Horizons probe having flown by the dwarf planet Pluto and the Dawn spacecraft currently orbiting the dwarf planet Ceres. The most distant spacecraft, Voyager 1 and Voyager 2 have left the Solar System as of 8 December 2018 while Pioneer 10, Pioneer 11, and New Horizons are on course to leave it.In general, planetary orbiters and landers return much more detailed and comprehensive information than fly-by missions. Space probes have been placed into orbit around all the five planets known to the ancients: The first being Venus, Mars, Jupiter, Saturn, and most recently Mercury, and have returned data about these bodies and their natural satellites.
File:OSIRIS-REX SamCam TAGSAM Event 2020-10-20 small.gif|thumb|upright=0.8|OSIRIS-REx collecting a sample from asteroid 101955 Bennu
—
The NEAR Shoemaker mission in 2000 orbited the large near-Earth asteroid 433 Eros, and was even successfully landed there, though it had not been designed with this maneuver in mind. The Japanese ion-drive spacecraft Hayabusa in 2005 also orbited the small near-Earth asteroid 25143 Itokawa, landing on it briefly and returning grains of its surface material to Earth. Another ion-drive mission, Dawn, has orbited the large asteroid Vesta and later moved on to the dwarf planet Ceres, arriving in March 2015.
File:Wispr 4thflyby.gif|thumb|WISPR of the Parker Solar Probe took this visible light footage of the nightside of Venus in 2021, showing the hot faintly glowing surface, and its Aphrodite Terra as large dark patch, through the clouds, which prohibit such observations on the dayside when they are illuminated. Possibly representing the illusive ashen light.
Remotely controlled landers such as Viking, Pathfinder and the two Mars Exploration Rovers have landed on the surface of Mars and several Venera and Vega spacecraft have landed on the surface of Venus, with the latter deploying balloons to the planet's atmosphere. The Huygens probe successfully landed on Saturn's moon, Titan.
No crewed missions have been sent to any planet of the Solar System. NASA's Apollo program, however, landed twelve people on the Moon and returned them to Earth. The American Vision for Space Exploration, originally introduced by U.S. President George W. Bush and put into practice through the Constellation program, had as a long-term goal to eventually send human astronauts to Mars. However, on February 1, 2010, President Barack Obama proposed cancelling the program in Fiscal Year 2011. An earlier project which received some significant planning by NASA included a crewed fly-by of Venus in the Manned Venus Flyby mission, but was cancelled when the Apollo Applications Program was terminated due to NASA budget cuts in the late 1960s.
Reasons for interplanetary travel
The costs and risk of interplanetary travel receive a lot of publicity—spectacular examples include the malfunctions or complete failures of probes without a human crew, such as Mars 96, Deep Space 2, and Beagle 2.Many astronomers, geologists and biologists believe that exploration of the Solar System provides knowledge that could not be gained by observations from Earth's surface or from orbit around Earth. However, they disagree about whether human-crewed missions justify their cost and risk. Critics of human spaceflight argue that robotic probes are more cost-effective, producing more scientific knowledge per dollar spent; robots do not need costly life-support systems, can be sent on one-way missions, and are becoming more capable as artificial intelligence advances. Others argue that either astronauts or spacefaring scientists, advised by Earth-based scientists, can respond more flexibly and intelligently to new or unexpected features of whatever region they are exploring.
Some members of the general public mainly value space activities for whatever tangible benefits they may deliver to themselves or to the human race as a whole. So far the only benefits of this type have been "spin-off" technologies which were developed for space missions and then were found to be at least as useful in other activities. However, public support, at least in the US, remains higher for basic scientific research than for human space flight; a 2023 survey found that Americans rate basic research as their third-highest priority for NASA, after monitoring Earth-endangering asteroids and understanding climate change. Support for scientific research is about four times higher than for human flight to the Moon or Mars.
Besides spinoffs, other practical motivations for interplanetary travel are more speculative. But science fiction writers have a fairly good track record in predicting future technologies—for example geosynchronous communications satellites and many aspects of computer technology.
Many science fiction stories feature detailed descriptions of how people could extract minerals from asteroids and energy from sources including orbital solar panels and the very strong magnetic field of Jupiter. Some claim that such techniques may be the only way to provide rising standards of living without being stopped by pollution or by depletion of Earth's resources.
There are also non-scientific motives for human spaceflight, such as adventure or the belief that humans have a spiritually fated destiny in space.
Finally, establishing completely self-sufficient colonies in other parts of the Solar System could, if feasible, prevent the human species from being exterminated by several possible events. One of these possible events is an asteroid impact like the one which may have resulted in the Cretaceous–Paleogene extinction event. Although various Spaceguard projects monitor the Solar System for objects that might come dangerously close to Earth, current asteroid deflection strategies are crude and untested. To make the task more difficult, carbonaceous chondrites are rather sooty and therefore very hard to detect. Although carbonaceous chondrites are thought to be rare, some are very large and the suspected "dinosaur-killer" may have been a carbonaceous chondrite.
Some scientists, including members of the Space Studies Institute, argue that the vast majority of mankind eventually will live in space and will benefit from doing so.
Economical travel techniques
One of the main challenges in interplanetary travel is producing the very large velocity changes necessary to travel from one body to another in the Solar System.Due to the Sun's gravitational pull, a spacecraft moving farther from the Sun will slow down, while a spacecraft moving closer will speed up. Also, since any two planets are at different distances from the Sun, the planet from which the spacecraft starts is moving around the Sun at a different speed than the planet to which the spacecraft is travelling. Because of these facts, a spacecraft desiring to transfer to a planet closer to the Sun must decrease its speed with respect to the Sun by a large amount in order to intercept it, while a spacecraft traveling to a planet farther out from the Sun must increase its speed substantially. Then, if additionally the spacecraft wishes to enter into orbit around the destination planet, it must match the planet's orbital speed around the Sun, usually requiring another large velocity change.
Simply doing this by brute force – accelerating in the shortest route to the destination and then matching the planet's speed – would require an extremely large amount of fuel. And the fuel required for producing these velocity changes has to be launched along with the payload, and therefore even more fuel is needed to put both the spacecraft and the fuel required for its interplanetary journey into orbit. Thus, several techniques have been devised to reduce the fuel requirements of interplanetary travel.
As an example of the velocity changes involved, a spacecraft travelling from low Earth orbit to Mars using a simple trajectory must first undergo a change in speed, in this case an increase, of about 3.8 km/s. Then, after intercepting Mars, it must change its speed by another 2.3 km/s in order to match Mars' orbital speed around the Sun and enter an orbit around it. For comparison, launching a spacecraft into low Earth orbit requires a change in speed of about 9.5 km/s.