NASA limits foreign contributions to U.S. planetary missions, Science Insider
"Today, at a meeting of asteroid and comet scientists in Washington, D.C., NASA officials explained some of the new rules for the next mission, to be selected in 2016. Among them was a stipulation that the principal investigator would not be allowed to recruit foreign instrument contributions in excess of one-third the value of the U.S. instruments on the payload, even though those contributions don't count against the $450 million cap."