The chairman of the board of trustees of the Hirshhorn Museum and Sculpture Garden, J. Tomilson Hill, has resigned, the Hirshhorn announced this week.

Hill, 63, president and chief executive of Blackstone Alternative Asset Management and a vice chairman of the Blackstone Group, joined the board in May 1999 and became chairman in 2004.

“After eight years as chairman, I decided that it was the right time to step down,” Hill said in a statement. “It is my great hope that the Hirshhorn will continue its commitment to organizing world-class exhibitions, supporting emerging artists and adding great works to the collection. With a talented and diverse group of curators, I know the Hirshhorn will thrive well into the future.”

Hill “has devoted countless hours to the museum, advising the director and its senior staff members, as well as other leaders at the Smithsonian. Bringing to the task a deep appreciation for contemporary art, he has helped shape the Hirshhorn into a museum that serves as a platform for inspiration and understanding,” the Hirshhorn said in the announcement.

There was no reason cited for the departure, and Hirshhorn spokesman Chris Wailoo declined to elaborate. Hill is the second board member to resign in a year, following Barbara Levine.

The resignation comes at a time of high-stakes fundraising for the Bloomberg Balloon, named after Bloomberg LP, which donated more than $1 million to the project.

Sources close to the project cite a growing dissatisfaction with its pace and the amount of time and resources being devoted to it. The inflatable structure, informally known as the Bubble since its 2009 announcement, is to protrude from the top and base of the museum and provide temporary space to house performances and lectures. Initially scheduled to open in the fall, the timeline has been pushed back to the spring. Wailoo said that fundraising is ongoing and that “we’re still planning on” a 2013 opening.

The board’s vice chairman, Constance R. Caplan, becomes acting chair in accordance with trustee bylaws.