September 7, 1985

After seven years as a lonely scientific sentinel in space, an American spacecraft, redirected from another mission, is fast approaching a small comet and is to fly through its luminous tail next week for the first probe of one of these mysterious wanderers of the solar system. It is not the celebrated Halley's comet that is to be intercepted and investigated first, but an obscure one named Giacobini-Zinner. The comet, too dim to be seen with the unaided eye, has swung around the Sun and is traveling outward to an intersection with the flight path of an equally obscure spacecraft, the International Cometary Explorer. The comet and the spacecraft were four million miles apart yesterday. The plan is for the spacecraft, traveling at 46,000 miles an hour, to pass within 6,200 miles of the comet's core at 7 A.M. Wednesday. The half-ton craft could spend as long as three hours plowing through the dust, gases and magnetic forces of the comet's tail.

President Reagan said today he was sorry that he ''carelessly gave the impression'' last month that racial segregation had been eliminated in public places in South Africa. ''I didn't intend to say that,'' Mr. Reagan said, referring to his remarks in a radio interview last month. The President's comments came as the White House weighed a series of compromises designed to prevent Congress from overriding an expected Presidential veto of legislation providing for sanctions against South Africa. A Senate vote is expected next week that would give final Congressional approval to a package of modest economic sanctions.

The nation's unemployment rate fell three-tenths of a percentage point in August, to 6.9 percent, the lowest level since President Reagan took office, the Labor Department reported today. The surprisingly large decline, which came after six consecutive months in which the overall jobless rate registered 7.2 percent, was widely interpreted as the clearest indication yet that a long-anticipated economic rebound might be at hand. At a brief televised White House news conference, Mr. Reagan hailed what he called ''the exciting news'' and declared that the 1985 economy, which expanded at a weak 1 percent annual rate in the first six months, ''is packing new power.'' Lowest Rate Since April 1980 The unemployment rate, which stood at 7.4 percent when Mr. Reagan took office in January 1981, rose to 10.7 percent in the 1982 recession. It last stood as low as 6.9 percent in April 1980.

The authorities here announced today that they were closing 454 schools and colleges for students of mixed race after days of violence in Cape Town's mixed-race areas. A Cabinet official said the Government could no longer guarantee the students' safety. The announcement came as Nelson Mandela, South Africa's most prominent political prisoner and a man regarded by many blacks as their true leader, was reported to have undergone medical tests by a urologist in a hospital in Cape Town. Mr. Mandela, who has been in prison for more than two decades, is serving a life sentence on charges of sabotage. A prison statement said Mr. Mandela, 67 years old, was returned to his cell at Pollsmoor Prison after being examined. The outcome of the test, the second of his urinary tract this year, was not immediately known.

Keith Hernandez of the Mets, admitting for the first time publicly that he has used cocaine, told a United States District Court today how the drug was ''a demon in me'' and how it is ''the devil on this earth.'' The star first baseman, the second player to testify in a trial of a man accused of drug dealing, said that he used cocaine for three years, from July 1980 to June 1983, and that he played one game in 1980 under the influence of the drug. ''That was the year I was crazy the second half, the time of my greatest use,'' he said. The statements came 24 hours after Lonnie Smith, an outfielder with the Kansas City Royals, testified that five players, including Mr. Hernandez and Joaquin Andujar, the St. Louis Cardinals' star pitcher, had used cocaine. Mr. Hernandez, 31 years old, discussing his cocaine use as a member of the Cardinals, said he had used the drug with Mr. Smith, Mr. Andujar, Bernie Carbo and Lary Sorensen. He said that there were other players involved but that he could not recall their names.