The U.S.-based company that pioneered the concept of space tourism is now offering future clients a little something extra.

Space Adventures, Limited says anyone who wants to pay the $20 million ticket for a trip to the International Space Station will get the opportunity to conduct a spacewalk, for an extra $15 million.

The person would step outside the ISS for a 90-minute sightseeing activity, tethered to the station and accompanied by a Russian cosmonaut.

Space Adventures has already sent three wealthy tourists to the orbital outpost aboard a Russian spacecraft since 2001, with a fourth (Japanese businessman Daisuke Enomoto) currently training for a flight in September. The private travelers spend six months training for the 10-day mission in Russia.

Spacewalking candidates will be required to undergo an extra month of training, and would spend an additional six to eight days aboard the I.S.S.

Some information for this report provided by AP.