Martin Marietta Corp., the top aerospace firm in the area, received its first major instrument contract from NASA's Goddard Space Flight Center in Maryland last week.

Under the $8.5 million contract, Martin Marietta will design, build and test a gamma-ray instrument to be used for exploration on Mars.

The spectrometer will be mounted on the Mars Observer Mission, scheduled by NASA for launch in 1991. The contract will be handled by Martin Marietta's Denver Aerospace Center in Denver.

While in orbit, the instrument will measure gamma rays emitted from the surface of Mars. Gamma rays, which are produced by cosmic rays hitting the planet and natural radiation in the soil, are a precise measure of the composition of the soil. Scientists will use data collected during the mission to determine the planet's surface composition.

The Mars mission is planned to enter Mars' orbit in 1991 and to circle the planet for two years.

"There were attempts to develop a gamma-ray instrument for the Apollo XV and Apollo XVI, and we know the Russians are working on a similar concept," said a spokesman from the space center.

"But this is the most sophisticated attempt so far. The project will lead to a new generation of instruments essential for future planetary projects," the spokesman said.

Martin Marietta is not a newcomer to Mars exploration. The company built the two Viking spacecraft that landed on Mars in 1976.

During that mission, an X-ray spectrometer was used to analyze the planet's surface. The instrument collected data at two sites.

The gamma-ray spectrometer, said Dr. Benton Clark, senior researcher at Martin Marietta Denver Center, will tell more.

Clark, who has worked on the Mars mission for 15 years, said the spectrometer is vital to selecting a site for a manned mission.

"If you want to send a man to Mars, you don't want him to land in an area where the same surface stretches on and on. You try to select an area that is warm and wet to reduce the amount of water transported , with a variety of landscapes to explore," Clark said.

Mars has the diversity. The landscape ranges from canyons the size of the United States to volcanos more than twice the size of Mount Everest. But it lacks bodies of water. Among other things, scientists hope to find permafrost, frozen ice crystals in the soil, that could lead them to the wet, warm climate conducive to a manned mission.

"This project is a logical step in the sequence of exploration," Clark said. "Putting man on Mars is inevitable, it's just a matter of who's going to do it first."

Martin Marietta, under contract to Orbital Sciences Corp. of Vienna, also is building the Transfer Orbit Stage that will be used as the upper stage booster for the scheduled Mars mission.

The Alexandria biotech firm, Interleukin-2 Inc., has announced that more than half of the terminally ill cancer patients being treated with its interleukin product continue to survive.

The company was founded in 1983 by Maximilian de Clara to produce a human protein (interleukin-2) as a treatment for cancer. Clinical studies began at St. Thomas' Hospital Medical School in London in 1983. Under Dudley C. Dumonde, chairman of the department of immunology at St. Thomas', 40 cancer patients have been treated with the protein. Twenty-seven patients continue to survive.

The product is a chain of amino acids that regulates and stimulates certain activities of the body's immune system. The company believes the product can be used to strengthen the immune system of patients undergoing chemotherapy and can assist the body's defenses against some forms of cancer.

According to the company, a clinical trial exemption certificate in the United Kingdom is expected "shortly." Interleukin-2 then intends to establish a manufacturing facility in Britain that will be capable of producing enough of its product to treat up to 20,000 patients a year by the end of 1987.

"In line with the results achieved at St. Thomas' and our manufacturing plans in the U.K., by the end of 1987, we expect to post an operating profit on the sale of our clinical trial material," De Clara said.

C3 Inc., the manufacturer of mini- and microcomputers in Reston, received its third Air Force contract this year, bringing future revenue on existing contracts to $228 million.

The latest $8.5 million contract calls for the installation and maintenance of 1,226 hand-held computer terminals that will be used by the Air Force to keep track of inventory and cargo at 78 bases around the world.

The portable terminals, called Source Data Capture Terminal Systems, transmit information by radio frequency to a central Honeywell Inc. computer system. C3 will integrate software and provide training for the system.

Earlier this year, C3 obtained contracts to provide 150 advanced graphics terminals for the Strategic Air Command and an automatic digital network for the Air Force.

Systematics General Corp. of Sterling, Va., received a contract from a division of Gould Inc. of Florida to develop and manufacture security equipment to be used by the U.S. government.

The equipment, referred to as Tempest equipment, is used by the U.S. Security Program to keep electronic equipment from sending signals that could be detected by someone unauthorized for top-secret information.

SGC will develop and manufacture at least 50 Tempest equipment racks, printers and related equipment. The equipment will be used as a support system to Gould's Tempest-qualified computer system and will hold various electronic devices.

Gould has an option to purchase up to 100 more of each Tempest unit, bringing total potential contract value to $4.5 million.

Investors Savings & Loan Association of Richmond said the shareholders of Citizens Savings & Loan, also of Richmond, approved the merger of Citizens into Investors.

Approximately 97 percent of Citizen's shares were voted in favor of the merger, which is expected to be completed once federal and state regulatory approvals are obtained.

Investors said it hopes to complete the merger by July. Investors will pay $18.25 a share for each of the 1.11 million Citizen common shares outstanding.

The Federal National Mortgage Association will offer $500 million in 9 percent coupon mortgage-backed securities backed by FHA-insured and VA-guaranteed seven- to eight-year seasoned loans for the corporations mortage portfolio.

The offering, to be placed June 3, is a part of the $10 billion sale of home mortgages announced by Fannie Mae April 30.

The securities will be available to the public in minimum amounts of $25,000 through an 18-member nationwide selling group of securities dealers. The securities will have an issue date of June 1 and settlement will be on June 23. The securities will be issued in book-entry form through the Federal Reserve Bank of New York.