Scientists studying Venus say NASA has a bias against the planet when it come to scientific missions and infrequent visits are hindering their research.

Venus researchers voiced their frustrations at the annual meeting of NASA's Venus Exploration Analysis Group near Washington this week, the journal Nature reported.

There hasn't been a U.S. mission to Venus since the Magellan probe radar-mapped its surface in the early 1990s, they noted, and NASA recently rejected a number of Venus research proposals.

NASA has denied any bias.

"There were just better proposals" for other Solar System targets, Jim Green, director of planetary science at NASA, said.

Venus researchers were upset.

"A lot of us are dismayed," says David Grinspoon, astrobiology curator at the Denver Museum of Nature and Science, a co-investigator on several of the proposals.

Research grants allocated to Venus studies have made up just 2 percent of NASA's planetary-science funding since 2005.

Without new missions supplying data for research, funding for Venus studies has dwindled, leading to fewer students entering the field, Grinspoon said -- which mean a smaller group of researchers to lobby for missions.

"Because of this feedback loop, the community has shrunk," he said.