Editing Interstellar travel (section)

=== Required energy ===
A significant factor contributing to the difficulty is the energy that must be supplied to obtain a reasonable travel time. A lower bound for the required energy is the [[kinetic energy]] <math>K = \tfrac{1}{2}mv^2</math> where <math>m</math> is the final mass. If [[deceleration]] on arrival is desired and cannot be achieved by any means other than the engines of the ship, then the lower bound for the required energy is doubled to <math>mv^2</math>.

The velocity for a crewed round trip of a few decades to even the nearest star is several thousand times greater than those of present space vehicles. This means that due to the <math>v^2</math> term in the kinetic energy formula, millions of times as much energy is required. Accelerating one ton to one-tenth of the speed of light requires at least {{convert|450|PJ|J TWh|sigfig=3|abbr=off|disp=or}}<ref>{{cite journal|last1=Zirnstein|first1=E.J|title=Simulating the Compton-Getting Effect for Hydrogen Flux Measurements: Implications for IBEX-Hi and -Lo Observations |journal=Astrophysical Journal|date=2013|volume=778|issue=2|pages=112–127|doi=10.1088/0004-637x/778/2/112|bibcode=2013ApJ...778..112Z|doi-access=free}}</ref> ([[world energy consumption]] 2008 was 143,851&nbsp;terawatt-hours),<ref>{{Cite book|title=Outer Solar System : prospective energy and material resources|last1=Badescu|first1=Viorel|last2=Zacny|first2=Kris|isbn=9783319738451|location=Cham, Switzerland|oclc=1033673323|date = 2018-04-28}}</ref> without factoring in efficiency of the propulsion mechanism. This energy has to be generated onboard from stored fuel, harvested from the interstellar medium, or projected over immense distances.