by Matt Williams April 20, 2018 (universetoday.com)
• In two recent research papers, Harvard Professor Abraham Loeb and Sonneberg Observatory researcher Michael Hippke looked at the challenges that extra-terrestrials would face launching chemical rockets from large planets and also planets in a close orbit to its central sun. Both situations create a gravity well that would require an escape velocity impossible for chemical rocket propulsion.
• Our G-type Earth is unique in that it is relatively small and at such a distance from its yellow dwarf star that it will allow a rocket to escape from the Earth’s gravity well. Says Loeb, “By a fortunate coincidence, the escape speed from the orbit of the Earth around the Sun is at the limit of attainable speed by chemical rockets… But the habitable zone around fainter stars is closer in, making it much more challenging for chemical rockets to escape from the deeper gravitational pit there.”
• The most common star in the galaxy is the M-type red dwarf star, accounting for 75% of the stars in the Milky Way Galaxy. The red dwarfs are also the most likely stars to have rocky planets. The nearest star, Proxima Centauri, is such an M-type star. It has an Earth-sized planet, Proxima b, with a ‘habitable zone’ for possible life that is much closer to its’ fainter star than is the Earth to the Sun. “A civilization on Proxima b will find it difficult to escape from their location to interstellar space with chemical rockets,” says Loeb.
• Hippke suggests that planets that have more mass than the Earth, and therefore a higher surface gravity, along with flatter topography, shallow oceans, and a thicker atmosphere, may be ideal for biological life. But the higher surface gravity means that it will also have a higher escape velocity. The amount of propellant needed lift a rocket out of the planet’s gravity would make this method of propulsion impractical. This could have a serious effect on an alien civilization’s space travel. Explains Hippke, “On more massive planets, space flight would be exponentially more expensive. Such civilizations would not have satellite TV, a moon mission, or a Hubble Space Telescope. This should alter their way of development in certain ways we can now analyze in more detail.”
• Both Loeb and Hippke also noted that extra-terrestrial civilizations could address these challenges by adopting other methods of propulsion. Chemical propulsion may be something that few technologically-advanced species would adopt because it is simply not practical. Therefore, it would make sense to search for extraterrestrial signals associated with lightsails or nuclear engines near dwarf stars.
• “Civilizations from Super-Earths are much less likely to explore the stars,” reasons Hippke. “They would be (to some extent) “arrested” on their home planet, and make more use of lasers or radio telescopes for interstellar communication instead of sending probes or spaceships.”
• [Editor’s Note] On the other hand, advanced civilizations who have mastered anti-gravity propulsion, space-time wormholes, and space-time bubbles would consider chemical-fueled or even nuclear-fueled rockets to be primitive technology. The UFO’s we see darting all around the sky certainly aren’t rockets.
Since the beginning of the Space Age, humans have relied on chemical rockets to get into space. While this method is certainly effective, it is also very expensive and requires a considerable amount of resources. As we look to more efficient means of getting out into space, one has to wonder if similarly-advanced species on other planets (where conditions would be different) would rely on similar methods.
Harvard Professor Abraham Loeb and Michael Hippke, an independent researcher affiliated with the Sonneberg Observatory, both addressed this question in two recently–released papers. Whereas Prof. Loeb looks at the challenges extra-terrestrials would face launching rockets from Proxima b, Hippke considers whether aliens living on a Super-Earth would be able to get into space.
The papers, tiled “Interstellar Escape from Proxima b is Barely Possible with Chemical Rockets” and “Spaceflight from Super-Earths is difficult” recently appeared online, and were authored by Prof. Loeb and Hippke, respectively. Whereas Loeb addresses the challenges of chemical rockets escaping Proxima b, Hippke considers whether or not the same rockets would able to achieve escape velocity at all.
For the sake of his study, Loeb considered how we humans are fortunate enough to live on a planet that is well-suited for space launches. Essentially, if a rocket is to escape from the Earth’s surface and reach space, it needs to achieve an escape velocity of 11.186 km/s (40,270 km/h; 25,020 mph). Similarly, the escape velocity needed to get away from the location of the Earth around the Sun is about 42 km/s (151,200 km/h; 93,951 mph).
As Prof. Loeb told Universe Today via email: “Chemical propulsion requires a fuel mass that grows exponentially with terminal speed. By a fortunate coincidence the escape speed from the orbit of the Earth around the Sun is at the limit of attainable speed by chemical rockets. But the habitable zone around fainter stars is closer in, making it much more challenging for chemical rockets to escape from the deeper gravitational pit there.”
As Loeb indicates in his essay, the escape speed scales as the square root of the stellar mass over the distance from the star, which implies that the escape speed from the habitable zone scales inversely with stellar mass to the power of one quarter. For planets like Earth, orbiting within the habitable zone of a G-type (yellow dwarf) star like our Sun, this works out quite well.
Unfortunately, this does not work well for terrestrial planets that orbit lower-mass M-type (red dwarf) stars. These stars are the most common type in the Universe, accounting for 75% of stars in the Milky Way Galaxy alone. In addition, recent exoplanet surveys have discovered a plethora of rocky planets orbiting red dwarf stars systems, with some scientists venturing that they are the most likely place to find potentially-habitable rocky planets.
Using the nearest star to our own as an example (Proxima Centauri), Loeb explains how a rocket using chemical propellant would have a much harder time achieving escape velocity from a planet located within its habitable zone.
“The nearest star to the Sun, Proxima Centauri, is an example for a faint star with only 12% of the mass of the Sun,” he said. “A couple of years ago, it was discovered that this star has an Earth-size planet, Proxima b, in its habitable zone, which is 20 times closer than the separation of the Earth from the Sun. At that location, the escape speed is 50% larger than from the orbit of the Earth around the Sun. A civilization on Proxima b will find it difficult to escape from their location to interstellar space with chemical rockets.”
Hippke’s paper, on the other hand, begins by considering that Earth may in fact not be the most habitable type of planet in our Universe. For instance, planets that are more massive than Earth would have higher surface gravity, which means they would be able to hold onto a thicker atmosphere, which would provide greater shielding against harmful cosmic rays and solar radiation.
In addition, a planet with higher gravity would have a flatter topography, resulting in archipelagos instead of continents and shallower oceans – an ideal situation where biodiversity is concerned. However, when it comes to rocket launches, increased surface gravity would also mean a higher escape velocity. As Hippke indicated in his study: “Rockets suffer from the Tsiolkovsky (1903) equation : if a rocket carries its own fuel, the ratio of total rocket mass versus final velocity is an exponential function, making high speeds (or heavy payloads) increasingly expensive.”
For comparison, Hippke uses Kepler-20 b, a Super-Earth located 950 light years away that is 1.6 times Earth’s radius and 9.7 times it mass. Whereas escape velocity from Earth is roughly 11 km/s, a rocket attempting to leave a Super-Earth similar to Kepler-20 b would need to achieve an escape velocity of ~27.1 km/s. As a result, a single-stage rocket on Kepler-20 b would have to burn 104 times as much fuel as a rocket on Earth to get into orbit.
To put it into perspective, Hippke considers specific payloads being launched from Earth. “To lift a more useful payload of 6.2 t as required for the James Webb Space Telescope on Kepler-20 b, the fuel mass would increase to 55,000 t, about the mass of the largest ocean battleships,” he writes. “For a classical Apollo moon mission (45 t), the rocket would need to be considerably larger, ~400,000 t.”
While Hippke’s analysis concludes that chemical rockets would still allow for escape velocities on Super-Earths up to 10 Earth masses, the amount of propellant needed makes this method impractical. As Hippke pointed out, this could have a serious effect on an alien civilization’s development.
“I am surprised to see how close we as humans are to end up on a planet which is still reasonably lightweight to conduct space flight,” he said. “Other civilizations, if they exist, might not be as lucky. On more massive planets, space flight would be exponentially more expensive. Such civilizations would not have satellite TV, a moon mission, or a Hubble Space Telescope. This should alter their way of development in certain ways we can now analyze in more detail.”
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