@ARTICLE{topputo2013approximate,
  author = {Topputo, F. and Bernelli-Zazzera, F},
  title = {{Approximate Solutions to Nonlinear Optimal Control Problems in Astrodynamics}},
  journal = {ISRN Aerospace Engineering},
  year = {2013},
  volume = {2013},
  pages = {1--7},
  number = {Article ID 950912},
  doi = {10.1155/2013/950912}
}