Interplanetary spaceflight: Difference between revisions

{{short description|Crewed or uncrewed travel between stars or planets}}[[File:Solar System Missions.png|thumb|All [[List of active Solar System probes|active Solar System space probes]] in 2024 (and a list of upcoming ones)|310x310px]]

Remotely guided [[space probe]]s have flown by all of the observed [[planet]]s of the Solar System from [[Mercury (planet)|Mercury]] to [[Neptune]], with the ''[[New Horizons]]'' probe having flown by the [[dwarf planet]] [[Pluto]] and the [[Dawn (spacecraft)|''Dawn'' spacecraft]] currently orbiting the dwarf planet [[Ceres (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.<ref>{{cite web|title=NASA Spacecraft Embarks on Historic Journey Into Interstellar Space|website=[[Jet Propulsion Laboratory]]|url=http://www.jpl.nasa.gov/news/news.php?release=2013-277|access-date=20 February 2014|archive-date=20 October 2019|archive-url=https://web.archive.org/web/20191020191620/https://www.jpl.nasa.gov/news/news.php?release=2013-277|url-status=live}}</ref>

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]] ([[Venera 7]], 1970), [[Mars]] ([[Mariner 9]], 1971), [[Jupiter]] (''[[Galileo (spacecraft)|Galileo]]'', 1995), [[Saturn]] (''[[Cassini/Huygens]]'', 2004), and most recently [[Mercury (planet)|Mercury]] (''[[MESSENGER]]'', March 2011), and have returned data about these bodies and their [[natural satellite]]s.

[[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.<ref>{{cite web |last1=Hatfield |first1=Miles |title=Parker Solar Probe Captures Visible Light Images of Venus' Surface |url=https://www.nasa.gov/feature/goddard/2022/sun/parker-solar-probe-captures-its-first-images-of-venus-surface-in-visible-light-confirmed |website=NASA |access-date=29 April 2022 |date=9 February 2022 |archive-date=14 April 2022 |archive-url=https://web.archive.org/web/20220414155959/https://www.nasa.gov/feature/goddard/2022/sun/parker-solar-probe-captures-its-first-images-of-venus-surface-in-visible-light-confirmed/ |url-status=live }}</ref><ref name="Geophysical Research Letters 2022">{{cite journal | journal=Geophysical Research Letters | last1=Wood | first1=B. E. | last2=Hess | first2=P. | last3=Lustig-Yaeger | first3=J. | last4=Gallagher | first4=B. | last5=Korwan | first5=D. | last6=Rich | first6=N. | last7=Stenborg | first7=G. | last8=Thernisien | first8=A. | last9=Qadri | first9=S. N. | last10=Santiago | first10=F. | last11=Peralta | first11=J. | last12=Arney | first12=G. N. | last13=Izenberg | first13=N. R. | last14=Vourlidas | first14=A. | last15=Linton | first15=M. G. | last16=Howard | first16=R. A. | last17= Raouafi | first17=N. E. | doi=10.1029/2021GL096302 | date=9 February 2022 | title=Parker Solar Probe Imaging of the Night Side of Venus | volume=49 | issue=3| pages=e2021GL096302 | pmid=35864851 | pmc=9286398 | bibcode=2022GeoRL..4996302W }}</ref> Possibly representing the illusive [[ashen light]].<ref name="n420">{{cite web | title=Nightside observations by the Parker Solar Probe: implications for the reality of the Ashen Light – British Astronomical Association | website=British Astronomical Association – Supporting amateur astronomers since 1890 | date=2024-05-21 | url=https://britastro.org/journal_contents_ite/nightside-observations-by-the-parker-solar-probe-implications-for-the-reality-of-the-ashen-light | access-date=2025-03-14}}</ref>]]

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.<ref>{{cite book |last1=Rees |first1=Martin |author-link1=Martin Rees |last2=Goldsmith |first2=Donald |date=2022 |title=The End of Astronauts: Why Robots Are the Future of Exploration |url=https://books.google.com/books?id=uKZcEAAAQBAJ |publisher=Belknap Press |isbn=978-0674257726}}</ref> 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.<ref>{{cite journal | url=http://zuserver2.star.ucl.ac.uk/~iac/spaceflight.html | title=The Scientific Case for Human Spaceflight | last=Crawford | first=I.A. | date=1998 | journal=Astronomy and Geophysics | pages=14–17 | access-date=2007-04-07 | archive-date=2019-04-06 | archive-url=https://web.archive.org/web/20190406172932/http://zuserver2.star.ucl.ac.uk/~iac/spaceflight.html | url-status=live }}</ref>   

There are also non-scientific motives for human spaceflight, such as adventure or the belief that humans have a spiritually fated destiny in space.<ref>{{cite journal | url=https://asmedigitalcollection.asme.org/memagazineselect/article-abstract/126/11/37/379040/The-Urge-to-ExploreIt-Brought-the-First-Creatures?redirectedFrom=fulltext | title=The Urge to Explore | last1=Aldrin | first1=Buzz | last2=Wachhorst | first2=Wyn | date=2004 | journal=Mechanical Engineering | volume=126 | issue=11 | pages=37–38 | access-date=2024-06-22 }}</ref><ref>{{cite journal | url=https://www.sciencedirect.com/science/article/abs/pii/S009457651631222X | title=Myth-free space advocacy part I--The myth of innate exploratory and migratory urges | last=Schwartz | first=James | date=2017 | journal=Acta Astronautica | pages=450–460 | access-date=2024-06-22 }}</ref>

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.<ref>{{cite web | url=http://ssi.org/?page_id=2 | title=A Space Roadmap: Mine the Sky, Defend the Earth, Settle the Universe | last=Valentine | first=L | date=2002 | publisher=Space Studies Institute, Princeton | url-status=dead | archive-url=https://web.archive.org/web/20070223030553/http://ssi.org/?page_id=2 | archive-date=2007-02-23 }}</ref>

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 (in accordance with [[Kepler's laws of planetary motion|Kepler's Third Law]]). 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.<ref name=curtisbook>{{cite book |last=Curtis |first=Howard |date=2005 |title=Orbital Mechanics for Engineering Students |edition=1st |publisher=Elsevier Butterworth-Heinemann |isbn=978-0750661690 |page=[https://archive.org/details/orbitalmechanics00curt/page/n273 257]|title-link=Orbital Mechanics for Engineering Students }}</ref> Then, if additionally the spacecraft wishes to enter into orbit around the destination planet (instead of just flying by it), it must match the planet's orbital speed around the Sun, usually requiring another large velocity change.

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 (also known as a [[delta-v]]), in this case an increase, of about 3.8&nbsp;km/s. Then, after intercepting Mars, it must change its speed by another 2.3&nbsp;km/s in order to match Mars' orbital speed around the Sun and enter an orbit around it.<ref name=marsdeltavs>{{cite web|title=Rockets and Space Transportation |url=http://www.pma.caltech.edu/~chirata/deltav.html |access-date=June 1, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20070701211813/http://www.pma.caltech.edu/~chirata/deltav.html |archive-date=July 1, 2007 }}</ref> For comparison, launching a spacecraft into low Earth orbit requires a change in speed of about 9.5&nbsp;km/s.

This maneuver can only change an object's velocity relative to a third, uninvolved object, – possibly the “centre of mass” or the Sun. There is no change in the velocities of the two objects involved in the maneuver relative to each other.  The Sun cannot be used in a gravitational slingshot because it is stationary compared to rest of the Solar System, which orbits the Sun.  It may be used to send a spaceship or probe into the galaxy because the Sun revolves around the center of the [[Milky Way]].

Computers did not exist when [[Hohmann transfer orbit]]s were first proposed (1925) and were slow, expensive and unreliable when [[Gravity assist|gravitational slingshots]] were developed (1959). Recent advances in [[computing]] have made it possible to exploit many more features of the gravity fields of astronomical bodies and thus calculate even [[Low energy transfer|lower-cost trajectories]].<ref>{{cite web | url=https://www.discovermagazine.com/the-sciences/gravitys-rim | title=Gravity's Rim | publisher=discovermagazine.com | access-date=2023-04-12 | archive-date=2019-10-22 | archive-url=https://web.archive.org/web/20191022002414/http://discovermagazine.com/1994/sep/gravitysrim419 | url-status=live }}</ref><ref>{{cite book |last=Belbruno |first=E. |date=2004 |url=http://www.pupress.princeton.edu/titles/7687.html |title=Capture Dynamics and Chaotic Motions in Celestial Mechanics: With the Construction of Low Energy Transfers |publisher=Princeton University Press |isbn=9780691094809 |access-date=2007-04-07 |archive-url=https://web.archive.org/web/20141202074354/http://www.pupress.princeton.edu/titles/7687.html |archive-date=2014-12-02 |url-status=dead }}</ref> Paths have been calculated which link the [[Lagrange points]] of the various planets into the so-called [[Interplanetary Transport Network]]. Such "fuzzy orbits" use significantly less energy than Hohmann transfers but are much, much slower. They aren't practical for human crewed missions because they generally take years or decades, but may be useful for high-volume transport of low-value [[commodity|commodities]] if humanity develops a [[space-based economy]].

Aerobraking converts the spacecraft's [[kinetic energy]] into heat, so it requires a [[heatshield]] to prevent the craft from burning up. As a result, aerobraking is only helpful in cases where the fuel needed to transport the heatshield to the planet is less than the fuel that would be required to brake an unshielded craft by firing its engines. This can be addressed by creating heatshields from material available near the target.<ref>{{Cite web |url=https://www.nasa.gov/pdf/744615main_2011-Hogue-Final-Report.pdf |title=NASA.gov |access-date=2016-05-13 |archive-date=2016-06-02 |archive-url=https://web.archive.org/web/20160602012046/https://www.nasa.gov/pdf/744615main_2011-Hogue-Final-Report.pdf |url-status=dead }}</ref>

Fusion rockets are considered to be a likely source of interplanetary transport for a [[planetary civilization]].<ref>{{Cite web|title=The Physics of Interstellar Travel : Official Website of Dr. Michio Kaku|url=https://mkaku.org/home/articles/the-physics-of-interstellar-travel/|access-date=2021-09-27|archive-date=2019-07-08|archive-url=https://web.archive.org/web/20190708003829/http://mkaku.org/home/?page_id=250|url-status=live}}</ref>

Solar sails rely on the fact that light reflected from a surface exerts pressure on the surface. The [[radiation pressure]] is small and decreases by the square of the distance from the Sun, but unlike rockets, solar sails require no fuel. Although the thrust is small, it continues as long as the Sun shines and the sail is deployed.<ref>{{cite web | url=http://www.ugcs.caltech.edu/~diedrich/cgi/search.cgi?solar+sail | title=Abstracts of NASA articles on solar sails | url-status=dead | archive-url=https://web.archive.org/web/20080311000832/http://www.ugcs.caltech.edu/~diedrich/cgi/search.cgi?solar+sail | archive-date=2008-03-11 }}</ref>

It is possible to put stations or spacecraft on orbits that cycle between different planets, for example a [[Mars cycler]] would synchronously cycle between Mars and Earth, with very little propellant usage to maintain the trajectory. Cyclers are conceptually a good idea, because massive radiation shields, life support and other equipment only need to be put onto the cycler trajectory once. A cycler could combine several roles: habitat (for example it could spin to produce an "artificial gravity" effect), or a mothership (providing life support for the crews of smaller spacecraft which hitch a ride on it).<ref>{{cite magazine | url=http://www.popularmechanics.com/science/air_space/2076326.html?page=1 | title=Buzz Aldrin's Roadmap To Mars | date=2005 | last1=Aldrin | first1=B | last2=Noland | first2=D | magazine=Popular Mechanics | url-status=dead | archive-url=https://web.archive.org/web/20061211195430/http://www.popularmechanics.com/science/air_space/2076326.html?page=1 | archive-date=2006-12-11 }}</ref> Cyclers could also possibly make excellent cargo ships for resupply of a colony.

A space elevator is a theoretical structure that would transport material from a planet's surface into orbit.<ref>{{cite web|url=http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html |title=The Space Elevator Comes Closer to Reality |last=David |first=D |publisher=space.com |date=2002 |url-status=dead |archive-url=https://web.archive.org/web/20101104104658/http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html |archive-date=2010-11-04 }}</ref> The idea is that, once the expensive job of building the elevator is complete, an indefinite number of loads can be transported into orbit at minimal cost. Even the simplest designs avoid the [[vicious circle]] of rocket launches from the surface, wherein the fuel needed to travel the last 10% of the distance into orbit must be lifted all the way from the surface, requiring even more fuel, and so on. More sophisticated space elevator designs reduce the energy cost per trip by using [[counterweight]]s, and the most ambitious schemes aim to balance loads going up and down and thus make the energy cost close to zero. Space elevators have also sometimes been referred to as "[[Space elevator|beanstalks]]", "space bridges", "space lifts", "space ladders" and "orbital towers".<ref>{{Cite journal|last=Edwards|first=Bradley C.|date=2004|title=A Space Elevator Based Exploration Strategy|journal=AIP Conference Proceedings|volume=699|pages=854–862|doi=10.1063/1.1649650|bibcode=2004AIPC..699..854E}}</ref>

In October 2015, the [[NASA Office of Inspector General]] issued a [[Effect of spaceflight on the human body|health hazards report]] related to [[human spaceflight]], including a [[human mission to Mars]].<ref name="AP-20151029">{{cite news |last=Dunn |first=Marcia |title=Report: NASA needs better handle on health hazards for Mars |url=http://apnews.excite.com/article/20151029/us-sci-space-travel-health-6dfd5b2c76.html |date=October 29, 2015 |work=[[AP News]] |access-date=October 30, 2015 |archive-date=January 30, 2019 |archive-url=https://web.archive.org/web/20190130041700/http://apnews.excite.com/article/20151029/us-sci-space-travel-health-6dfd5b2c76.html |url-status=live }}</ref><ref name="NASA-20151029oig">{{cite web |author=Staff |title=NASA's Efforts to Manage Health and Human Performance Risks for Space Exploration (IG-16-003) |url=https://oig.nasa.gov/audits/reports/FY16/IG-16-003.pdf |date=October 29, 2015 |work=[[NASA]] |access-date=October 29, 2015 |archive-date=October 9, 2022 |archive-url=https://ghostarchive.org/archive/20221009/https://oig.nasa.gov/audits/reports/FY16/IG-16-003.pdf |url-status=dead }}</ref>

Once a vehicle leaves [[low Earth orbit]] and the protection of Earth's magnetosphere, it enters the [[Van Allen radiation belt]], a region of high [[Ionizing radiation|radiation]]. Beyond the Van Allen belts, radiation levels generally decrease, but can fluctuate over time.<ref>{{cite web |title=Radiation Belts -- Fun Facts |url=https://www.nasa.gov/mission_pages/rbsp/mission/fun-facts.html |website=NASA |date=18 March 2015 |access-date=19 October 2021 |archive-date=3 November 2021 |archive-url=https://web.archive.org/web/20211103015724/https://www.nasa.gov/mission_pages/rbsp/mission/fun-facts.html |url-status=dead }}</ref> These high energy [[cosmic rays]] pose a [[Health threat from cosmic rays|health threat]]. Even the minimum levels of radiation during these fluctuations is comparable to the current annual limit for astronauts in low-Earth orbit.<ref>{{cite journal |last1=Mewaldt |title=The Cosmic Ray Radiation Dose in Interplanetary Space – Present Day and Worst-Case Evaluations |journal=International Cosmic Ray Conference |date=2005 |volume=2 |issue=29 |page=433 |bibcode=2005ICRC....2..433M |url=http://www.srl.caltech.edu/ACE/ASC/DATA/bibliography/ICRC2005/usa-mewaldt-RA-abs1-sh35-oral.pdf |access-date=19 October 2021 |archive-date=4 March 2016 |archive-url=https://web.archive.org/web/20160304053415/http://www.srl.caltech.edu/ACE/ASC/DATA/bibliography/ICRC2005/usa-mewaldt-RA-abs1-sh35-oral.pdf |url-status=live }}</ref>

In addition, [[coronal mass ejections]] from the [[Sun]] are highly dangerous, and are fatal within a very short timescale to humans unless they are protected by massive shielding.<ref>[http://www.popularmechanics.com/science/air_space/2076326.html?page=6 popularmechanics.com] {{webarchive|url=https://web.archive.org/web/20070814163410/http://www.popularmechanics.com/science/air_space/2076326.html?page=6 |date=2007-08-14 }}</ref><ref>{{cite journal | doi=10.1016/S1350-4487(99)00063-3 | pmid=11543148 | volume=30 | issue=3 | title=Shielding from solar particle event exposures in deep space | journal=Radiation Measurements | pages=361–382| year=1999 | last1=Wilson | first1=John W | last2=Cucinotta | first2=F.A | last3=Shinn | first3=J.L | last4=Simonsen | first4=L.C | last5=Dubey | first5=R.R | last6=Jordan | first6=W.R | last7=Jones | first7=T.D | last8=Chang | first8=C.K | last9=Kim | first9=M.Y | bibcode=1999RadM...30..361W }}</ref><ref>{{Cite web |url=http://www.nature.com/embor/journal/v4/n11/full/embor7400016.html |title=nature.com/embor/journal |access-date=2007-05-20 |archive-date=2010-08-21 |archive-url=https://web.archive.org/web/20100821050623/http://www.nature.com/embor/journal/v4/n11/full/embor7400016.html |url-status=live }}</ref><ref>{{Cite web |url=http://www.islandone.org/Settlements/MagShield.html |title=islandone.org/Settlements |access-date=2007-05-20 |archive-date=2016-04-05 |archive-url=https://web.archive.org/web/20160405143918/http://www.islandone.org/Settlements/MagShield.html |url-status=dead }}</ref><ref>{{Cite web |url=http://iss.jaxa.jp/iss/kibo/develop_status_09_e.html |title=iss.jaxa.jp/iss/kibo |access-date=2007-05-20 |archive-date=2016-12-18 |archive-url=https://web.archive.org/web/20161218171852/http://iss.jaxa.jp/iss/kibo/develop_status_09_e.html |url-status=live }}</ref><ref>{{Cite web |url=http://yarchive.net/space/spacecraft/debris_shield.html |title=yarchive.net/space/spacecraft |access-date=2007-05-20 |archive-date=2016-03-08 |archive-url=https://web.archive.org/web/20160308005359/http://yarchive.net/space/spacecraft/debris_shield.html |url-status=live }}</ref><ref>[http://uplink.space.com/showflat.php?Board=sciastro&Number=44199 uplink.space.com] {{webarchive|url=https://web.archive.org/web/20040328165528/http://uplink.space.com/showflat.php?Board=sciastro |date=2004-03-28 }}</ref>

Any major failure to a spacecraft en route is likely to be fatal, and even a minor one could have dangerous results if not repaired quickly, something difficult to accomplish in open space. The crew of the [[Apollo 13]] mission survived despite an explosion caused by a faulty oxygen tank (1970).<ref>{{Cite news |last=Chang |first=Kenneth |date=2020-04-13 |title=Apollo 13’s Astronauts Survived Disaster 50 Years Ago. Could It Happen Again? |url=https://www.nytimes.com/2020/04/13/science/apollo-13-anniversary.html |access-date=2025-03-15 |work=The New York Times |language=en-US |issn=0362-4331}}</ref>

*{{cite book| last =Seedhouse| first =Erik| title =Interplanetary Outpost: The Human and Technological Challenges of Exploring the Outer Planets| publisher =Springer| date =2012| location =New York| page =288| isbn =978-1441997470}}