This outlook summarizes the latest trends and achievements in orbital, interplanetary and interstellar propulsion and makes a preliminary outlook for the upcoming decades. While interplanetary propulsion is a reality, we are approaching an interstellar propulsion ability which may realize sometime in the next decades or centuries. Despite the progress, we are still many many decades away from realizing plans to reach the Oort cloud and the nearby stars. Nevertheless, technological progress is pushing the ability of space travel towards an incremental step from interplanetary exploration to interstellar exploration. Several technologies and concepts may enable this dramatic transition. Those are mainly having to do with the developing ability to harness and concentrate immense quantities of energy and the development of reactionless highly-energy efficient propulsion concepts.
Space exploration began at Earth's orbit at an extraordinary pace back inn the 20th century. In fact, the first probe to reach the Kármán line space boundary at 100km above Earth was a German V-2 rocket as early as 1944. This was soon followed by the US space program, intially relying on the same V-2 rocket technology. Then, the Soviet Union joined the space exploration effort, which turned into the notorious space race. The multi-staged rocket propulsion technology allowed to reach increasingly higher orbits and hence in 1950s, after passing to 1000km mark above Earth's ground level, it became possible to set orbital satellites. It started with the Soviet Sputnik-1 satellite in 1957 and was followed with multiple launches by both the USSR and the US. Earth's Moon was the next target and shortly also transformed into the interplanetary space exploration of the solar system. In 1959, the Soviet Luna-1 mission was the first probe to exit Earth's gravitational pull, and though missing its target - the Moon - it was the first human made probe to enter an orbit around the sun in the interplanetary space.
The 1960s brought incredible achievements in manned space exploration with the first man launched to space by USSR in 1961 and the first man setting foot on the Moon under US flag in 1969. However, during this decade the first unmanned probes were also able to reach Mars at 1.6 AU distance from the sun - starting with Soviet Mars-1 in 1962 and US Mariner-4 in 1965. In the 1970s, this came about the exploration of the outer solar system with US taking with lead by first sending Pioneer-10 towards Jupiter and Pioneer-11 towards Saturn. By the end of the decade the famous Voyager-1 and Voyager-2 also began their exploration missions. As this is being written, there are five interplanetary probes launched by the US since the 1970s, which are already on their way out of the solar system at speeds of about 2.5-3.0 AU/year. Voyager-1 is the furthest away at about 157 AU from the sun, while Pioneer-10 and Voyager-2 are both currently at about 133 AU from the sun. In addition, Pioneer-11 and the New Horizons missions are following with the first now at about 110 AU from the sun and the second at about 53 AU. From this year on, the Voyager-1 probe is due to continue leading at highest speed amond the five probes and should reach the inner boundary of the Oort cloud within some 300 years. All the probes will take tens of thousands of years to pass through the outer Oort cloud and reach true interstellar medium.
Figure 1. Spaceflight progress on the distance scale of an exponential chart - the first orbital phase (blue), the second interplanetary phase (orange) and the proposed missions to outer solar system (grey). Dashed lines indicate theoretical progression of orbital and interplanetary exploration at the similar exponential rate, which has not realized but did pave way to much of the modern science fiction.
As seen at the above exponential chart of orbital and interplanetary exploration, the current multi-stage rocket technology coupled with planetary gravitational assist is essentially a linear progress in terms of distance reachibility. In other words, our probes are limited to certain speeds while escaping from the solar system. As a result, currently launched probes are very limited in their reach and cannot even reach the Inner Oort cloud anytime in our lifetimes. However, considering the past incremental transition from orbital (blue dashline) to interplanetary phase (orange dashline), one might theorize that there could be a similar incremental step from interplanetary to an interstellar phase. Even more likely is a kind of intermeditate step (grey dashline) making it possible to reach the outer outskirts of the solar system, namely the Inner Oort and the Outer Oort regions. Such step would definately involve a technologal transition as descibed in the next paragraph.
We are still many decades away from realizing plans to reach nearby stars. Nevertheless, technological progress is pushing the ability of space travel towards an incremental step from interplanetary exploration to interstellar exploration. Several technologies and concepts may enable this dramatic transition. Those are mainly having to do with the developing ability to harness and concentrate immense quantities of energy such as nuclear driven propulsion and the development of reactionless highly-energy efficient propulsion concepts such as light sails and the recently revealed concept of electromagnetic sails. Some controversial methods relying on advanced physics are also explored in laboratories including propellant-less electromagnetic propulsion relying on solid-state switches, but this is still highly speculative to be realized at this point of time. Relativistic propulsion methods are an even more speculative field and at present are purely theoretical.
What near future holds for deep space exploration? Though Pioneer-10, Voyager-1 and Voyager-2 have already penetrated out of the Kuiper belt (30-50 AU from our sun) and out of the heliosphere (75-90 AU from our sun), those and similar probes are hardly able to reach the Inner Oort cloud within a human lifetime scale. However, there is a solid probability that unmanned missions could soon be sent to explore the outer reaches of our solar system on the verge of the interstellar space - namely the Inner and Outer Oort cloud. At least two forefront technologies are aiming to fix this - nuclear-electric rockets and solar sails may allow sending probes to the solar gravity lens point on the inner edge of the Oort cloud at 550 AU from the sun within a reasonable timeframe.
While the TAU mission to reach the distance of 1000 AU from the sun framed by NASA/JPL in the 1980s never realized, the Fast Outgoing Cyclopean Astronomical Lens (FOCAL) mission proposal is being formulated for this purpose utilizing available technologies and may take place sometime in 2040s or 2050s. On top of this, there is a hypothetical NASA proposal to send an unmanned probe to the Alpha Centauri star system in 2069, but it lacks any concrete plan and technology basis. Moreover, despite the populatity of the topic, the light sail technology proposed by Startlight and Breakthrough Starshot initiatives may allow reaching the Inner and Outer Oort cloud, but are hardly imaginable to be able to reach the near stars during the 21st century due to the immense energy demands. We shall keep track on those and other developments.
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