The crew of the space shuttle Discovery placed in orbit a communications satellite for the Arab world today and also began preparations for the first "Star Wars" experiment by astronauts.

As it crosses the Hawaiian Islands Wednesday on its 37th orbit of Earth, Discovery will have an eight-inch mirror positioned in a side hatch window that Air Force engineers on the island of Maui will attempt to strike with a laser beam. Moving at 17,500 mph, the spaceplane will have its nose pitched down and its port side tilted toward Maui as it speeds southeast across the Pacific.

If the test works, the laser beam will be reflected back to the Air Force installation on Maui -- evidence that laser weapons could bounce their beams off bigger orbiting mirrors to strike and destroy hostile satellites. This is part of the mission of the Reagan administration's Strategic Defense Initiative, or "Star Wars" plan.

"Things look good for Wednesday's test. No cloud cover is being forecast for Maui," Air Force Capt. Martin Hauser said from Houston's Johnson Space Center. "We have a six-minute window in which we can conduct the test, which itself will last about one minute." The test is set for 2:30 p.m. EDT.

Earlier today, Discovery's crew launched an Arabsat satellite out of the shuttle's cargo bay for a consortium of 22 Arab countries that includes the Palestine Liberation Organization. The PLO has a 0.2 percent ownership in the satellite. The Arabsat is built with coding safeguards so that none of the owners can use it to eavesdrop on the others or stop any one country from broadcasting to other countries.

Saudi Arabian Prince Sultan Salman Saud, one of Discovery's seven crew members and the first Arab in space, watched the satellite deployment, saying, "Very, very good job. It never looked better. As a matter of fact it looked much better than the Morelos deployment."

Morelos is the Mexican satellite deployed Monday. The crew has one more satellite launch, on Wednesday -- a Telstar-3 satellite for American Telephone & Telegraph Co.

No sooner had the Saudi prince spoken than he was congratulated in Arabic by astronaut Michael M. Mullane from Mission Control Center in Houston.

A little later the crew beamed back a television broadcast showing Discovery's cargo bay with only one satellite left.

At a news conference at Johnson Space Center, Dr. Ali Mashat, director-general of Arabsat, said the new satellite will serve communications needs of all Arab countries from Morocco in the west to Saudi Arabia in the east, a distance of more than 2,500 miles.

Mashat said Arabsat joins a sister satellite that was placed in orbit more than a year ago by the European Space Agency's unmanned Ariane rocket, which is competing for satellite payloads with the U.S. shuttle. Mashat said it cost the Arab world $23.75 million to use Ariane and $20.5 million to use the shuttle. Of course, he pointed out, Ariane could not have carried Prince Saud into orbit to make history.

"We have one more satellite which we are going to put in storage in case there is a failure of one or the other of the two we have, or our communications demands grow to the point where we have to use the third," Mashat said. "I do not know if we will use Ariane or the shuttle the next time. That question is up to our entire membership."

Mashat confirmed that Arabsat's membership includes the PLO, which is dedicated to destruction of Israel. A "fact sheet" issued by Arabsat is emblazoned with a map of the Arab world in which Israel goes unidentified.

"The members of Arabsat include Palestine," Mashat said in reply to a question. "The official representative of Palestine is the PLO. Membership shares in Arabsat range from 50 percent down to two tenths of 1 percent. That's Palestine; they have the smallest share."