Escape from Proxima b

Apr 24, 2018

By Abraham Loeb

Almost all space missions launched so far by our civilization have been based on chemical propulsion. The fundamental limitation here is easy to understand: a rocket is pushed forward by ejecting burnt fuel gases backwards through its exhaust. The characteristic composition and temperature of the burnt fuel set the exhaust speed to a typical value of a few kilometers per second. Momentum conservation implies that the terminal speed of the rocket is given by this exhaust speed times the natural logarithm of the ratio between the initial and final mass of the rocket.

To exceed the exhaust speed by some large factor requires an initial fuel mass that exceeds the final payload mass by the exponential of this factor. Since the required fuel mass grows exponentially with terminal speed, it is not practical for chemical rockets to exceed a terminal speed that is more than an order of magnitude larger than the exhaust speed, namely a few tens of kilometers per second. Indeed, this has been the speed limit of all spacecraft launched so far by NASA or other space agencies.

By a fortunate coincidence, the escape speed from the surface of the Earth, 11 kilometers per second, and the escape speed from the location of the Earth around the sun, 42 kilometers per second, are close to the speed limit attainable by chemical propulsion. This miracle allowed our civilization to design missions, such as Voyager 1 and 2 or New Horizons, that could escape from the solar system into interstellar space. But is this fortune shared by other civilizations on habitable planets outside the solar system?

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3 comments on “Escape from Proxima b

  • @OP – The fundamental limitation here is easy to understand: a rocket is pushed forward by ejecting burnt fuel gases backwards through its exhaust.

    This author seems to stuck in the mindset of historical chemical rockets!

    Since the required fuel mass grows exponentially with terminal speed, it is not practical for chemical rockets to exceed a terminal speed that is more than an order of magnitude larger than the exhaust speed, namely a few tens of kilometers per second.

    . . . . Which is why NOBODY is suggesting chemical rockets as the drive for inter stellar probes or ships!

    Indeed, this has been the speed limit of all spacecraft launched so far by NASA or other space agencies.

    This is utter nonsense! It totally ignores ion drives which have been in use for years since the 1960s and 1970s, and the proposed narrow beam laser directed energy pointed at solar sails!

    By a fortunate coincidence, the escape speed from the surface of the Earth, 11 kilometers per second, and the escape speed from the location of the Earth around the sun, 42 kilometers per second, are close to the speed limit attainable by chemical propulsion. This miracle allowed our civilization to design missions, such as Voyager 1 and 2 or New Horizons, that could escape from the solar system into interstellar space.

    Chemical rockets were VERY inadequate for these missions, and required gravity assists and long voyage times! However they opened up frontiers while more advanced systems were in development!

    https://en.wikipedia.org/wiki/Ion_thruster#Missions

    Ion thrusters can also be used for interplanetary and deep-space missions where acceleration rates are not crucial. Continuous thrust over a long interval can reach high velocities while consuming far less fuel than traditional chemical rockets.

    Among electric thrusters, ion thrusters have received the most serious commercial and academic consideration. Ion thrusters are seen as the best solution for these missions, as they require high change in velocity but do not require rapid acceleration.

    While it is reasonable to explain why chemical rockets are not suitable for interstellar travel (bulky fuel and too slow), The Scientific American, really should use space scientists with up-to-date knowledge, to write space articles on rocket propulsion!

    NASA developed the NSTAR ion engine for use in interplanetary science missions beginning in the late-1990s. It was space-tested in the highly successful space probe Deep Space 1, launched in 1998. This was the first use of electric propulsion as the interplanetary propulsion system on a science mission.

    The Japanese space agency’s Hayabusa launched in 2003 and successfully rendezvoused with the asteroid 25143 Itokawa and remained in close proximity for months to collect samples and information. It was powered by four xenon ion engines.

    The European Space Agency’s satellite SMART-1 launched in 2003 using a Snecma PPS-1350-G Hall thruster to get from GTO to lunar orbit.

    Dawn launched on September 27, 2007, to explore the asteroid Vesta and the dwarf planet Ceres. It used three Deep Space 1 heritage xenon ion thrusters (firing one at a time).

    Geoffrey A. Landis proposed to use a space laser source and ion thruster to propel an interstellar probe.

    @OP link – The escape speed from the location of Proxima b to interstellar space is about 65 kilometers per second.

    Just as nobody has plans to launch interstellar probes directly from Earth using chemical rockets, no alien would launch directly to an interstellar trajectory!
    They would only use high thrust rockets to achieve planetary orbit, and then use systems (possibly assembled in space) with high exhaust velocities and/or external energy inputs! (Ion drives, lasers, solar sails, fly-by gravity assists, refuelling staging outposts etc.) for interplanetary or interstellar travel!

    Launching a rocket from rest at that location requires the fuel-to-payload weight ratio to be larger than a few billions in order for the rocket to escape the gravitational pull of Proxima Centauri.

    Which is why it is ridiculous to use chemical propellants for any purpose other than short hops to planetary or close lunar orbits!



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  • Written by a theoretical physicist/astronomer, without the least knowledge of actual rocket engineering.

    He got paid for a strawman argument.

    SA continues to slum it. Next stop not Proxima b but National Geographic disgrace.

    New Scientist, it is complained, is getting a bit populist. It is, though, rather more reliable.



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  • phil rimmer #2
    Apr 25, 2018 at 7:07 am

    He got paid for a strawman argument.

    Do you think they would pay me for writing an article about why punting across the Atlantic will fail? 🙂

    SA continues to slum it. Next stop not Proxima b but National Geographic disgrace.

    I am reminded of this earlier loopy “science-mag” article which we discussed!

    https://www.richarddawkins.net/2015/07/nasa-is-seriously-considering-terraforming-part-of-the-moon-with-robots/



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