1. M. Hart, Quarterly J. Astonomical Soc. 16, 128 (1975) reprinted in Extraterrestrials, Where are They? ed. by B. Zuckerman, M. Hart, 2nd edn. (Cambridge University Press, Cambridge, 1995), p. 1–8
2. http://voyager.jpl.nasa.gov/mission/fastfacts.html
3. e.g. C. Singer, Extraterrestrials, Where are They? ed. by B. Zuckerman, M. Hart, 2nd edn. (Cambridge University Press, Cambridge, 1995), pp. 70–85
4. For recent work by NASA to develop faster means of space transportation see for example American Institute of Physics conference proceedings, p. 813 (2006)
5. http://www.heavens-above.com/solar-escape.asp