Physics Education Publication Date Is interstellar travel to an exoplanet possible? Tanmay Singal1 and Ashok K. Singal2 1Department of Physics and Center for Field Theory and Particle Physics, Fudan University, Shanghai 200433, China. [email protected] 2Astronomy and Astrophysis Division, Physical Research Laboratory Navrangpura, Ahmedabad 380 009, India. [email protected] Submitted on xx-xxx-xxxx Abstract undertake such a voyage, with hopefully a positive outcome? What could be a possible scenario for such an adventure in a near In this article, we examine the possibility of or even distant future? And what could be interstellar travel to reach some exoplanet the reality of UFOs – Unidentified Flying orbiting around a star, beyond our Solar Objects – that get reported in the media system. Such travels have been in the realm from time to time? of science fiction for long. However, in the last 50 years or so, this question has gained further impetus in the mind of a man on the 1 Introduction street, after the interplanetary travel has become a reality. Of course the distances to In the last three decades many thousands of be covered to reach even some of the near- exoplanets, planets that orbit around stars est stars outside the Solar system could be beyond our Solar system, have been dis- arXiv:1308.4869v2 [physics.pop-ph] 6 Dec 2019 hundreds of thousands of time larger than covered. Many of them are in the habit- those encountered within the interplanetary able zone, possibly with some forms of life space. Consequently, the time and energy evolved on a fraction of them, and hope- requirements for such a travel could be fully, the existence of intelligent life on some immensely prohibitive. The questions we of them. Can we ever get in physical contact want to explore here are: What could be with the extra-terrestrials, assuming they the possible limitations, if any, for such are there? Radio communication over inter- interstellar travels, and could humans ever stellar distances is one possibility [1]. What Volume/Issue/Article Number 1 www.physedu.in Physics Education Publication Date about the possibility of humans ever visiting Ganymede or Titan, as the planets them- “them”? Or an even more pertinent ques- selves are all gaseous, lacking a solid surface tion first – Is an interstellar space travel to to make a landing. an exoplanet around a star beyond our So- lar system possible? This begs a question: Could man possi- bly ever travel to distant stars to visit some In last 50-60 years, the mankind, first exoplanets, perhaps in a habitable zone, time in its history, has not only ventured to possibly encounter some extraterrestrial into outer space, humans have successfully life? After all, a mere century back, a trip to stepped on the Moon, the first time ever on the Moon, culminating in a human landing another celestial body. Rover explorations on it, looked as much impossible and such of the surface of Mars have been made accounts in science fiction seemed to be just many times, probes have landed on Venus, a fig of imagination, as an interstellar travel and many other missions have been sent to to an exoplanet may appear now. Such other planets. The Galileo spacecraft that analogies though may have their own jus- entered orbit around Jupiter, made a num- tification grounds, but the fact remains that ber of close flybys to study Jupiter’s satel- the distances involved in interstellar travel lite Ganymede. In the Cassini-Huygens mis- are immensely larger. The nearest star out- sion, while Cassini orbited Saturn and stud- side the solar system (Proxima Centauri) is ies its rings before it plunged into Saturns at- as many times (∼ a hundred million times) mosphere, the Huygens probe successfully farther than the Moon, as the latter is com- landed on Saturn’s moon Titan. pared to distance between adjacent rooms In recent years, India too has sent two ((∼ 4 m) in a building. From a simple logic missions, Chandrayan-1 and Chandrayan- one could then expect that going to a star 2, to the Moon, and Mars Orbiter Mission will at least be as much more difficult than (MOM), India’s first interplanetary mission, going to the Moon as the going-to-the-Moon has successfully reached Mars. A third mis- was with respect to a walk just next door sion to the Moon is now being planned, and within an office building. Of course, the other interplanetary missions are in the off- shortness of human lifetime makes things ing. Perhaps in a decade or so, India may all the more difficult. With the maximum also achieve a human landing on the the speeds achieved so far by the spaceships Moon. After that one could imagine such within the solar system, it will require about manned trips to Mars. Other countries are 80,000 years on a one-way journey to this also planning such expeditions in near fu- nearest star. Thus it may not look possi- ture. As for the Jovian planets like Jupiter ble to reach other stars within a human life- or Saturn, manned missions if any, will have time, although on a theoretical basis theory to have bases on one of their satellites, e.g. of relativity could allow one to do so. For Volume/Issue/Article Number 2 www.physedu.in Physics Education Publication Date instance, a spaceship accelerating continu- dream only. ously with a convenient value of g, that is In this article, we ignore the techni- the acceleration that we are used to on the cal aspects of the mission as technology surface of the Earth, could travel to the most is bound to improve rapidly over time. distant parts of the universe within a human Further, we assume 100% efficiency of the lifetime, without violating the speed-limit of rocket engine in converting fuel energy into c, the speed of light. In principle, interstellar kinetic energy of the exhaust, something travel may thus appear possible. that might not really be possible. We carry forth the possibility of our endeavour with- However, energies involved in such an out delving into many other equally im- endeavour would make it next to impossi- portant issues such as the long term effects ble. In a spaceship the fuel needed for the of cosmic radiation on the health of space later parts of the journey has to be carried travellers and their requirements for food, aboard and thus also needs to be acceler- medical and other life-sustaining needs. We ated till it is utilized. Therefore the initial consider mainly the minimum basics of the mass at the start of such a voyage is expo- travel, which are distance, time and energy. nentially larger than the final payload. With conventional chemical fuel such an arduous journey will need a fuel-mass of a whole 2 The story so far galaxy. Even within the best possible sce- nario, where almost 100% of mass is con- Till date there have been five spacecrafts that verted into energy (in a typical thermonu- have crossed the threshold of escape veloc- clear reaction only about 0.7% of mass is ity from the solar system and four of them converted into energy), one would require are already headed towards the interstellar initial mass to be millions of times the mass space. of the final payload and the energy required Pioneer 10 was launched in 1972, flew may be worth hundreds of years of total en- past Jupiter in 1973 and became the first ergy consumption of the whole world. If spacecraft to achieve escape velocity from we imagine that the energy is beamed from the solar system. The contact was lost in power plants on the Earth to the spaceship, January 2003 and is heading in the direc- it will again require many hundred million tion of Aldebaran in Taurus. Pioneer 11 was megawatts of power throughout the dura- launched in 1973, flew past Jupiter in 1974 tion of such a trip, which might last for a and Saturn in 1979. The contact was lost in very long time. It therefore looks that at November 1995. The spacecraft is headed most we might travel to other planets within toward the constellation of Aquila. our solar system but the distant stars will Pioneer 10, as well as Pioneer 11, carry ever remain within the realm of a distant gold-anodized aluminium plaques in case Volume/Issue/Article Number 3 www.physedu.in Physics Education Publication Date most mysterious part of the whole message for them [2]. Voyager 1 was launched in September 1977, flew past Jupiter and Saturn, made a close approach to Saturn’s moon Titan and is now at a distance of about 145 astronomical unit (au), where one au (=1.5 × 108 km) is the average distance of the Earth from the Sun. Voyager 2 was launched in August 1977, Figure 1: The message, featuring human flew past Jupiter, Saturn, Uranus, Neptune figures along with several coded-symbols and is now at a distance of about 125 au. inscribed on the gold-anodized aluminium Both probes are already past heliopause, the plaques, carried aboard Pioneer and Voy- region where the solar wind interacts with ager spacecrafts. the interstellar medium at distances around 120 au from the Sun. Voyagers are thus either spacecraft is ever found by intelli- presently exploring the boundary between gent life-forms from another planetary sys- the Sun’s influence and interstellar space, tem. The plaques feature the human fig- where nothing from the Earth has flown be- ures along with several coded-symbols that fore, and are expected to return valuable are designed to provide information about data, hopefully, for another decade.