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.

In a frightening moment aboard the research ship Knorr, the scientists realized that the robot vehicle Argo had struck one of the Titanic's smokestacks on the bottom of the North Atlantic. ''It scared the heck out of me,'' Dr. Robert D. Ballard, leader of the undersea exploration group that found the Titanic, said yesterday. He feared that either the high-technology robot, which has provided the first pictures of the luxury liner since she sank 73 years ago, had been damaged or that the Titanic had sustained new damage. Evidently, both escaped serious harm.

A twin-engine DC-9 veered to the right moments after takeoff here this afternoon, nosed into a nearby woods and exploded in an orange fireball, killing all 31 people on board. The plane, Midwest Express Flight 105, was en route to Atlanta from Madison and Milwaukee's Billy Mitchell Field. It was carrying 26 passengers, a crew of four and one other company employee. According to early reports, the pilot radioed the tower on takeoff that he had an emergency on board but did not have time to say what it was.

President Reagan, who has threatened to veto protectionist legislation, said today that he would ask Congress soon to work with him to devise a ''freer and fairer trading system,'' but he declined to go into detail. In his radio talk Saturday, the President is expected to announce a plan to end what the Government regards as unfair trade practices by Japan, the European Economic Community, South Korea and Brazil, according to Administration and Congressional sources. Such practices discourage and sometimes block imports from the United States and other countries. If the trade practices are not stopped, the officials said, the President could restrict American imports from these countries. Revised Policy Statement A senior official also reported that the Administration was preparing a revised statement of its trade policy that would emphasize ''fair trade,'' a phrase that has been used by organized labor and other advocates of protectionism.

A delegation of United States Senators returned today from a visit to Moscow and reported ''movement'' in the Soviet position on arms reduction, which they attributed to the ''flexibility'' of Mikhail S. Gorbachev, the Soviet leader. The Senators said they got the impression that Mr. Gorbachev was anxious for success at his meeting with President Reagan, scheduled for November in Geneva.

It is a measure of a new volatility in West German politics that the hopes of the opposition Social Democratic Party for winning the 1987 general elections ride on the shoulders of one man. Johannes Rau is a populist, a blurry centrist, a conciliator, an American-style pragmatist in a party famous for tearing itself apart over ideological subtleties. Like the man he will probably run against, Chancellor Helmut Kohl, he is an outsider to Bonn, a politician who draws vitality from a crowd, an orator gifted at anecdote and not shy about appealing to gut emotions. In German political argot, the 54-year-old Mr. Rau, Premier of North Rhine-Westphalia, is a Hoffnungstrager, a man who inspires hope - in the electorate and, to its mild astonishment, in his own party. Last May he drew a stunning 52 percent of the popular vote in state elections; the Social Democrats will shortly ask him to pull off a similar miracle in the entire Federal Republic.

A commuter train derailed in eastern France today after the engineer apparently failed to observe speed limits, and two people were hurt, the state-owned railway company said. A company spokesman said the locomotive and three cars had left the rails near the town of Noveant with more than 50 people aboard.

When Israel established its ''security zone'' in southern Lebanon three months ago, many Israeli policy makers were dubious about how long it could be maintained. Some Israeli officials, fearing continued strife between Israeli-supported militiamen and their Shiite Moslem adversaries, were so pessimistic about the security zone's chances that they spoke of letting ''nature take its course'' and allowing the mainstream Shiite militia, Amal, to gradually assume control of the area. But officials now say those days are over - as is the debate in the Israeli military establishment over the security zone. Israeli military officials in southern Lebanon and Tel Aviv now say the security strip has proven its worth, even though there have been car bombings almost every week. 'Fulfilling Its Function' ''After three months in operation we can say that the security zone is fulfilling its function - even better than some of its original proponents expected,'' said Uri Lubrani, the coordinator of Lebanese affairs in the Israeli Ministry of Defense. ''We intend to maintain it for the foreseeable future.''