WASHINGTON — The Supreme Court gave a skeptical hearing Tuesday to 28 California Institute of Technology scientists who are challenging the government’s use of background checks to learn about their personal lives, including past drug use.

They won a ruling from the U.S. 9th Circuit Court of Appeals saying these open-ended, probing questions violated their constitutional right to privacy. The decision stressed they held “”low risk”” jobs at the Jet Propulsion Laboratory in Pasadena. Caltech runs JPL under a contract with the National Aeronautics and Space Administration.

Last year, however, then-Solicitor General Elena Kagan urged the high court to reverse the ruling. She said it threatened the government’s use of standard background forms that are filled out by millions of public employees as well as by a growing army of private contract workers.

Kagan did not participate in Tuesday’s argument, but all of her new colleagues — with the exception of Justice Sonia Sotomayor — sounded as though they would take her advice and uphold the use of background checks.

The justices spent much of the hour debating whether the Constitution puts any limits on the questions asked of prospective employees.

Justice Antonin Scalia said there was no such privacy right. “”I don’t see it anywhere in the Constitution,”” he told Dan Stormer, a Pasadena, Calif., lawyer who represented the Caltech employees.

But Chief Justice John G. Roberts Jr. and Justice Samuel A. Alito Jr. said they would not close the door to all such claims. “”Isn’t there some right to tell the government: That’s none of your business?”” Roberts commented.

Alito said he questioned whether the government could ask people to fill out forms revealing what they ate, what they read or whether they smoked cigarettes and to describe their sex lives. “”Is that OK?”” he pressed Neal Katyal, the acting U.S. solicitor general.

Probably not, Katyal agreed, but he urged the justices to rule that the government can ask open-ended questions of its employees and contract workers. He also suggested the justices avoid a broad pronouncement on whether the Constitution protects the privacy of personal information.

The justices are employers as well as jurists, and at several points, they noted they need to know about the backgrounds of their law clerks before hiring them. “”How do you do this except by asking open-ended questions?”” Alito commented.

When Stormer stressed that the Caltech scientists held “”low risk”” positions at the “”campus-like”” JPL, Roberts objected. “”How do you know if he’s a high risk or low risk? What if his neighbor says, ‘He likes to plant bombs’?””

Throughout the argument, Sotomayor said the court should confront the question of whether there are any limits on what government employers can ask of employees. “”What about your genetic makeup? Could you ask that?”” she said.

Katyal dodged the question by saying the court should avoid ruling on whether the Constitution sets an outer boundary for such inquiries. “”The real check is the political process,”” he said.

The justices will vote later this week and issue a ruling in several months in the case of NASA vs. Nelson.