Interstellar space travel

Schematic representation of the Milky Way system.

The interstellar neighborhood.

Interstellar space travel includes all space travel in interstellar space , u. a. with the aim of reaching another star system. The challenges lie in overcoming the long distances, the resulting long travel time and having to carry your own energy source. So far, only theoretical concepts exist for manned interstellar space travel.

conditions

The path from the Sun to Alpha Centauri (distances on a logarithmic scale).

The main obstacle to interstellar space travel is the immense distances that have to be overcome. As the outermost planet, Neptune orbits the sun at a distance of 30  AU , but it is 120 AU to the beginning of the interstellar space at the heliopause and 4.2  light years (268,000 AU) to the next star .

The space probes Voyager 1 and Voyager 2 are the only terrestrial spacecraft to have reached the heliopause functionally and took 35 and 41 years respectively. It would take them a hundred thousand years to get to the nearest stars at this speed.

speed

In order to be able to reach the target within a reasonable time (i.e. within decades or centuries), the spacecraft would have to travel at an appreciable fraction of the speed of light. In addition, there is the problem of being able to brake the speed again in a relatively short time in order to have sufficient time to observe the target object or even to take the return flight.

shielding

The space between the stars is filled with the interstellar medium - gas, dust, radiation. At very high speeds, even collisions with the smallest dust particles can cause major damage, as can radiation. This requires protective systems. In addition, the interstellar gas can have a braking effect.

Energy source

Interstellar spacecraft need their own energy sources. From approx. 10 AU the sunlight is so weak that solar modules are unsuitable for generating energy. Previous probes into the outer solar system used radionuclide batteries .

autonomy

An interstellar spacecraft would have to function autonomously in order to be able to approach and investigate the target location without help from Earth, since signals from Earth to the spaceship would take several years.

longevity

Another problem is the lifespan of the systems . The electronics are particularly affected. Due to the still young technology branch (beginning around the 1960s) there are still numerous open questions about the service life of electronic components / systems.

Possible drives

→ Main article : Space propulsion

If the object is to reach the closest star within a manageable period of time (~ half a century), the object must accelerate to an approximately relativistic speed (e.g. ~ 0.1c) within a short period of time and, if so, decelerate again. The challenge here can be illustrated with the Ziolkowski equation :

${\displaystyle \Delta v\ =v_{\text{e}}\ln {\frac {m_{0}}{m_{1}}}}$

With

${\displaystyle v_{\text{e}}=I_{\text{sp}}\cdot g_{0}}$,   Take-off mass and payload. ${\displaystyle {m_{0}}=}$ ${\displaystyle {m_{1}}=}$

In order to obtain a high speed change ( ), a high effective outflow speed ( ) of the reaction gas or a high specific impulse ( ) (engine code) is required. Furthermore, a lot of fuel has to be converted ( ) in order to generate the required energy. Therefore a high thrust is necessary, which generates the necessary acceleration energy within a "short" period. ${\displaystyle \Delta v}$ ${\displaystyle v_{\text{e}}}$ ${\displaystyle I_{sp}}$ ${\displaystyle m_{0}/m_{1}}$

Based on this consideration, two engine categories can therefore be excluded: Chemical engines (see also) have a high thrust, but because of the use of chemical energy , the efficiency ( ) of these engines is very low. Electric drives are highly efficient, but the fuel emissions are rather low due to the use of electrical charges and their repulsion with one another. ${\displaystyle I_{sp}}$

In some concepts, the nuclear pulse drive is therefore mainly favored, which from today's point of view would be feasible. Antimatter propulsion could also hold promise in the distant future. Because of the energy required to accelerate the fuel masses, some scientists prefer the fuelless drive, which accelerates the object by applying force through external fields (see, inter alia, Breakthrough Propulsion Physics Project ). A possible question here is whether the gravitational fields of the neighboring star systems could have an influence on the flight path of an object. An ESA study showed that a multi-body problem in interstellar space is negligible, which means that only the sphere of influence of a star system is of importance. I.e. an object can take a position in interstellar space, outside of the spheres of influence, without being moved significantly from the position by the gravitational forces of the star systems.

Concepts for unmanned interstellar space travel

The knowledge regarding interstellar space and the heliosphere is currently still low, so that the first interstellar missions serve to explore these areas first. Some missions, such as that of the IBEX probe, can provide initial findings from the earth, but only a probe on site can analyze the nature of the space (matter distribution, magnetic fields etc.) and confirm or refute the current models.

Stage I: Exploration of interstellar space

One of the first drafts, in addition to the first interstellar exploration mission (precursor mission) (1977), which only provided for penetration into interstellar space in order to be able to carry out experiments, was the TAU (Thousand Astronomical Units) mission. This design by NASA / JPL (1980s) should cover up to 1000 AU with technology already tested  . An ion drive with xenon as fuel and a radionuclide battery as energy source was provided as the drive system . The mission should last 50 years. NASA continued a similar concept, but for interplanetary research missions , in 2003 with the Prometheus project and the JIMO, which has since been canceled . The RTG energy source in combination with an ion drive is a common concept (see inter alia), but other proposals also exist.

These are based on the ongoing development in the field of lightweight satellite construction and solar sail technology. One of these proposals includes a 250 kg light probe that is supposed to reach a distance of 200 AU within 15 years using a solar sail with a radius of about 200 m and some gravity assist maneuvers . The solar sail should be pushed off after the acceleration phase of approx. 5 AU (see also for further concept).

The goals of such a mission are to:

1. Exploration of the interstellar medium, its origin and the formation of matter in the galaxy.
2. Exploration of the heliosphere and its interaction with the interstellar medium.
3. Research into fundamental astronomical processes in the heliosphere and the interstellar medium.
4. Determination of fundamental properties of the universe.

Another benefit in answering these questions can be to find a solution for using the interstellar medium for the propulsion system or the energy supply. Should such a possibility exist, the cost of an interstellar spaceship to the nearest star system could be significantly reduced.

Outer Solar System Missions: Pioneer 10 , Pioneer 11 , Voyager 1 and Voyager 2 .

Realized missions

Voyager 1 (August 2012) and Voyager 2 (November 2018) were the only probes to have reached interstellar space. Pioneer 10 and 11 are similarly distant, but contact with them was lost long before that. Also, New Horizons will no longer be functional from lack of energy long before it reaches the heliopause. None of the probes mentioned are interstellar probes in the strict sense, as they were not originally built for an interstellar journey. Their main aim was to study parts of the solar system. Accordingly, their devices were not designed for long journeys.

Stage II: Exploring other star systems

Artist's impression of an Orion spaceship from the NASA design phase

While only a few 100 AU have to be covered for a trip to interstellar space, a trip to other star systems involves covering a distance of several 100,000 AU (1  ly ≈ 63,000 AU). This change in order of magnitude can be seen primarily in the selection of the drive system. The main propulsion system in the Orion , Daedalus , Longshot and Icarus projects is the nuclear pulse propulsion system . Of all the proposed systems, this is considered to be the most technically feasible. Further topics that were and are dealt with in the projects are protective mechanisms against radiation and microparticles, artificially intelligent systems and mission processes. In addition to these studies, there are other suggestions regarding missions to other star systems.

Another idea is to send small probes (~ 50 kg) to neighboring star systems, which reproduce themselves at the target location, set up communication receivers and transmitters and prepare for possible colonization by humans. The nanorobots receive the raw materials on site using in-situ technology. The advantage of this mission is the lower energy expenditure to shoot such a probe, in contrast to a fully functional probe (e.g. Cassini-Huygens with 5,364 kg), to another star system.

On April 12, 2016, Yuri Milner and Stephen Hawking presented the Breakthrough Starshot project, funded by Milner with $ 100 million , which aims to develop a concept to accelerate nanosatellites using laser beams and to send them to Alpha Centauri, from where they send back images should.

Concepts for manned interstellar space travel

NASA illustration of two O'Neill cylinders .

The goal of manned interstellar space travel will be the exploration and colonization of alien solar systems. While there are already some publications on unmanned missions, only a few exist for manned missions. One of these publications is the Wayland Report, which was written based on the Icarus study (unmanned). The Wayland Report deals with a generation ship.

The same framework conditions apply to manned interstellar space travel as to unmanned interstellar space travel. In addition, there are further challenges due to the “human” payload: humans have a limited lifespan and they need an environment that enables life.

Travel while awake

If an interstellar journey is to take place within a human lifespan, a relativistic speed must be achieved. The energy expenditure would be immense. In order to accelerate a manned spaceship to "only" 10% of the speed of light, amounts of energy would be necessary that are in the order of magnitude of the annual world energy demand.

Sleeper ships

The crew of this spaceship concept is put into a so-called cryosleep , a kind of “artificial hibernation ” after departure from Earth , and is woken up again on arrival at the destination. The advantage of this concept is that large-scale food production and entertainment facilities can be dispensed with. However, the effects of such cryosleep, if it were technically possible, are still unknown in humans. This concept is fairly common in science fiction, including a. in the films / series Alien , Avatar , Demolition Man , Futurama , Pandorum , Passengers , Prometheus - Dark Characters , Star Trek and Interstellar .

Generation ships

→ Main article : Generational spaceship

The name of this spaceship concept comes from the fact that new generations are born and grow up on the spaceship during the journey to another star system. The generation ships are self-sufficient habitats, i. H. For example, food cultivation, drinking water and oxygen recycling should be enabled on board the ship. The O'Neill colonies or the Bernal sphere are models of thought for such habitats in near-Earth space or in our solar system . A pilot project for this was the (largely unsuccessful) experiment Biosphere 2 , in which an attempt was made to keep a closed system in ecological balance. A key question in this concept that has hardly been answered so far is the size and composition of the crew. The crew size is decisive for the total mass of the spaceship, as a person must be assigned a certain resource requirement (space, food, etc.).

On May 23, 2007 a scientific paper was published under the direction of Arturo Casadevall, which deals with mushrooms that convert radioactivity (probably by means of melanin ) into energy that can be used by their organism. It is conceivable that such mushrooms could be used to produce food for astronauts during space flights. Everywhere in space there is more background radiation than light that can be used by plants.

Embryo transport

This type of spaceship would send frozen human embryos on their way. A few years before, on or after arriving at the destination, these would be thawed, bred and raised by robots. This form of transport would, if possible, be the most effective form, since no complex habitat structures would have to be carried for a journey of several decades. The local resources could then be used on site so that robots could build the required habitats. Apart from technical challenges, this method is controversial from an ethical point of view. At the present time it is unclear whether, and if so, how growing up without adult human role models changes the psyche of children. Robots would have to be constructed that can reproduce human upbringing. Last but not least, an artificial uterus would have to be developed in which the embryo could grow.

Destinations

The stellar neighborhood has some interesting destinations. The following table lists the star systems from the list of the closest stars , which are close to us and therefore easier to reach and for which there are indications of planets. In addition, the next single and double systems of the respective spectral classes are listed.

Star system	Distance ( Lj )	dec	annotation
Alpha Centauri	4.3	−61 °	Triple system of one star each of class G, K and M, in which models allow the existence of terrestrial planets . The red dwarf Proxima Centauri according to current state of knowledge has a terrestrial Exoplaneten: Proxima Centauri b ; whether Alpha Centauri B has planets is unclear. The system is currently approaching the solar system and will have reached its closest approach in 28,400 years at 2.97 ly.
Barnard's arrow star	6.0	+ 5 °	Red dwarf and destination of the Daedalus project . The existence of an exoplanet has been discussed for a long time, but evidence has not yet been provided. Favorable destination, as it is only 5 ° above the ecliptic and is currently approaching the solar system (closest approximation with 3.74 ly in 9800 years).
Luhman 16	6.6	−53 °	Double system of two brown dwarfs
WISE 0855-0714	7.5	−7 °	Brown dwarf
Sirius	8.6	−17 °	Relatively young binary star system in which the mass of Sirius A is twice that of the Sun and in which the companion Sirius B is a white dwarf . Comparatively cheap destination, since it is only 17 ° below the ecliptic and is currently approaching the solar system (closest approach in 64,000 years with 7.86 ly).
Luyten 726-8	8.7	+ 18 °	Double system of two red dwarfs . Currently moving away from the Sun (closest approximation 7.2 ly 28,700 years ago).
Ross 154	9.7	+ 24 °	Red dwarf , is currently approaching the solar system and, with its closest approximation to 6.39 ly in 157,000 years, is a cheap target, as it could be reached at a speed of only 12.2 km / s relative to the solar system (for comparison: Voyager 1 has a speed of 17 km / s).
Epsilon Eridani	10.5	−9 °	Young star system in which a disk of dust was detected, with a distance analogous to the Kuiper belt . According to the theory of planet formation, the star system may have evolved terrestrial planets . However, this has not yet been proven.
Tau Ceti	11.9	−16 °	Currently the second closest sun-like star after Alpha Centauri . Only 1.6 ly away from YZ Ceti. Since both stars have planets, this would be a good destination for a dual mission. Is currently approaching the solar system and will have reached the closest approximation in 43,000 years with 10.6 ly.
YZ Ceti	12.1	−17 °	Red dwarf , currently only 1.6 ly away from Tau Ceti and would therefore offer itself as a successor target.
Wolf 1061	14.0	−13 °	Red dwarf , has three planets, including one potentially habitable, Wolf 1061c .
Peter 445	17.6	+ 79 °	Red dwarf , is currently approaching the solar system and will be passed by the Voyager 1 spacecraft at a distance of 1.6 ly in about 40,000 years (closest approach to the sun in 46,000 years with 3.45 ly). Will be the closest star to the Sun for around 8000 years in just under 45,000 years.

See also : List of the next extrasolar systems

Others

In 1993, SETI researchers suggested looking for propulsion and energy signatures of spaceships of extraterrestrial, technical civilizations.

Another possibility is to set up a communication network in order to get in contact with another possibly existing civilization or to find an extraterrestrial communication network (theory / speculation). Due to their speed, electromagnetic waves are well suited for communication and can also be used for one-way information transmission.

NASA- Marshall , JPL and AIAA carried out theoretical investigations in 1999 to use annihilation of antimatter and nuclear fusion for propulsion of future space vehicles. At the beginning of 2011, DARPA and NASA-Ames started the 100 Year Starship project. In this research and evaluation program, the possibilities and challenges of manned, interstellar long-term flights are explored and strategies are designed. In September 2011, the 100-Year Starship Symposium took place in Orlando (Florida) , at which the necessary technologies, implementation, organization and financing of such a project were presented and discussed in more detail. In 2012, the former astronaut Mae Carol Jemison took over the management of the project. Funding is provided by the Defense Advanced Research Projects Agency and NASA. A public symposium has been held in Houston each year since 2011 .

Some private, not-for-profit research initiatives, such as B. the Tau Zero Foundation , Icarus Interstellar and the Institute for Interstellar Studies , are also concerned with researching new technologies and possibilities for future interstellar space flights. In May 2013 a symposium with Freeman Dyson , Paul Davies , Gregory and James Benford, Jill Tarter , Robert Zubrin , Neal Stephenson , Geoffrey A. Landis took place in San Diego , in August a congress in Dallas, u. a. with Friedwardt Winterberg , David Messerschmitt and Marc Millis.

In 2017, NASA announced that it would continue to research unconventional approaches as part of the NIAC program (NASA Innovative Advanced Concepts) .

literature

Books

• Paul Gilster: Centauri dreams - imagining and planning interstellar exploration. Springer, New York 2004, ISBN 0-387-00436-X
• Gregory L. Matloff: Deep-space probes - to the outer solar system and beyond. Springer, Berlin 2005, ISBN 3-540-24772-6
• Grigor A. Gurzadyan: Space dynamics. Taylor & Francis, London 2002, ISBN 0-415-28202-0
• Eugene F. Mallove, Gregory L. Matloff: The starflight handbook - a pioneer's guide to interstellar travel. Wiley, New York 1989, ISBN 0-471-61912-4
• Kelvin F. Long: Deep space propulsion - a roadmap to interstellar flight. Springer, New York, NY 2012, ISBN 978-1-4614-0606-8 .
• Roger D. Launius, et al .: Interstellar Flight and the Human Future in Space. Pp. 162-190 in: Robots in space - technology, evolution, and interplanetary travel. The Johns Hopkins University Press, Baltimore 2012, ISBN 0-8018-8708-9 .

items

• Claudio Maccone: SETI, extrasolar planets search and interstellar flight - When are they going to merge? Acta Astronautica 64, pp. 724-734, 2009, doi: 10.1016 / j.actaastro.2008.11.006
• Robert H. Frisbee: Limits of Interstellar Fligh Technology. in Marc G. Millis (et al.): Frontiers of Propulsion Science. American Inst. Of Aeronautics & Astronautics, Reston 2009, ISBN 1-56347-956-7 , pp. 31-126
• Pharis E. Williams: Superluminal Space Craft. American Institute of Physics, Volume 1103, Melville 2009, pp. 352-358, bibcode : 2009AIPC.1103..352W
• William B. Scott: To the Stars. Aviation Week & Space Technology, March 1, 2004, pp. 50–52, pdf zpower.com, accessed on August 22, 2012
• IA Crawford: Interstellar Travel: A Review for Astronomers. Quarterly Journal of the Royal Astronomical Society, Vol. 31, pp. 377-400, 1990, bibcode : 1990QJRAS..31..377C

Individual evidence

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5. ↑ daviddarling.info: Interstellar Precursor Mission. Retrieved June 5, 2011 . 
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8. ^ Nock, KT, "TAU - A mission to a thousand astronomical units," 19th AIAA / DGLR / JSASS International Electric Propulsion Conference, Colorado Springs, CO, AIAA-87-1049, May 11-13, 1987
9. ↑ Gruntman, Mike et al .: Innovative Explorer Mission to Interstellar Space. (PDF; 217 kB) 2006, accessed on June 5, 2011 (English). 
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12. ↑ Robert F. Wimmer-Schweingruber et al .: Interstellar heliospheric probe / heliospheric boundary explorer mission — a mission to the outermost boundaries of the solar system. (PDF; 1.2 MB) 2009, archived from the original on January 17, 2013 ; accessed on June 5, 2011 . 
13. ↑ NASA Institute for Advanced Concepts: A Realistic Interstellar Explorer - Phase I Final Report. (PDF; 452 kB) NASA, May 31, 1999, accessed June 30, 2011 (English). 
14. ^ G. Genta: Propulsion for Interstellar Space Exploration. (PDF) September 2000, accessed June 6, 2011 (English). 
15. ↑ Ashworth, Stephen: The Wayland Report: Sketches of a Manned Starship. (PDF; 1.2 MB) May 21, 2010, accessed on August 7, 2011 (English). 
16. ↑ Ekaterina Dadachova, Ruth A. Bryan u. a .: Ionizing Radiation Changes the Electronic Properties of Melanin and Enhances the Growth of Melanized Fungi. In: PLoS ONE. 2, 2007, p. E457, doi: 10.1371 / journal.pone.0000457 .
17. ↑ Michael D. Papagiannis: The Search for Extraterrestrial Technologies in our Solar System , bibcode : 1995ASPC ... 74..425P .
18. ^ R. Zubrin: Detection of Extraterrestrial Civilizations via the Spectral Signature of Advanced Interstellar Spacecraft , bibcode : 1995ASPC ... 74..487Z .
19. ^ Jones, Antonia J .: Self-replicating probes for galactic exploration. (PDF; 4.1 MB) 1991, accessed on August 7, 2011 (English). 
20. ↑ Timothy Ferris: Interstellar Spaceflight. Can We Travel to Other Stars? (PDF; 76 kB) Scientific American, 1999, accessed on May 22, 2011 (English). 
21. ^ Reaching for the Stars science.nasa.gov
22. ↑ Far Out Space Propulsion Conference Blasts Off science.nasa.gov, accessed November 7, 2011
23. ↑ 100-Year Starship Study Strategic Planning Workshop Held ( Memento from July 22, 2013 in the Internet Archive ) darpa.mil; DARPA, NASA team on '100-Year Starship' project theregister.co.uk; NASA's 100-Year Starship Project Sets Sights on Interstellar Travel space.com; Retrieved March 27, 2011
24. ↑ Not Such a Stretch to Reach for the Stars The New York Times, October 17, 2011
25. ↑ 100-Year Starship Symposium 100yss.org, accessed November 5, 2011
26. ^ To Infinity and Beyond at DARPA's 100-Year Starship Symposium popularmechanics.com, September 30, 2011; Radio show The Space Show, November 15, 2011 ( Memento of March 15, 2012 in the Internet Archive ) - Marc Millis, Paul Gilster on the DARPA project, accessed on November 22, 2011
27. ↑ New project lays the basis for Interstellar travel derstandard.at; Starship dreamers launch 100-year mission with DARPA grant washingtonpost.com, accessed June 19, 2012
28. ↑ 100YSS 2012 Public Symposium ( Memento from June 9, 2012 in the Internet Archive ); 2013 Public Symposium 100yss.org, accessed June 8, 2013
29. ^ Tau Zero: The Steps Ahead , centauri-dreams.org
30. ↑ Icarus Interstellar
31. ^ Institute for Interstellar Studies , i4is.org; The Institute for Interstellar Studies , centauri-dreams.org, accessed October 13, 2012
32. ^ Videos of Starship Century Symposium Part 1 starshipcentury.com, accessed June 18, 2013
33. ^ Icarus Interstellar Congress Schedule ( Memento from August 20, 2013 in the Internet Archive ) icarusinterstellar.org
34. ↑ NASA Invest's in 22 Visionary exploration Concepts nasa.gov; NASA Innovative Advanced Concepts (NIAC) Program accessed on May 4, 2017
35. ^ Mach Effects for In Space Propulsion: Interstellar Mission nasa.gov; NASA Awards Grant to Tau Zero tauzero.aero