Good journalism, perhaps by definition, has hope built into it. Tonight's Bill Moyers documentary on CBS paints no pretty picture, but it ambitiously illuminates a problem it convinces us is crucial, and it's one of the bravest things you're ever likely to see on an American television network.

"The Vanishing Family -- Crisis in Black America," a 90-minute "CBS Reports" at 9 on Channel 9, studies what Moyers -- and appropriate cited authorities -- see as a radical, ruinous restructuring of the black family unit. The key element he sums up in a simple sentence: "Most black children are now growing up without their fathers."

Moyers recites what he calls the "fearsome statistics": that nearly 60 percent of black children born now are born out of wedlock; that "black teen-agers have the highest pregnancy rate in the industrial world"; that nearly half of the black children in America are "being raised in poverty"; that now the rule as often as not is the single-parent black home in which the father is the absent parent.

But fearsome statistics are poor television, and what makes "Vanishing Family" hit home with tremendous impact and a stirring sense of importance are the people Moyers talks to in the urban ghetto of Newark who bring the numbers to dispiriting life, who populate a new disembodied culture in which "mothers are children, fathers don't count, and the street is the strongest school."

Walking through the university that is the Newark inner city, Moyers says, "You won't find in these neighborhoods the prime-time family of Bill Cosby."

A young woman named Clorinda, who has an angelic maternal smile, tells Moyers of her first child, born when she was 15. "I thought it'd be fun when I had it," she says. Now 17, she has changed her mind. The father, 18, doesn't live with or support the family. He tells Moyers, "I spend most of my time listening to the radio."

Alice, who is six months pregnant with her third child when Moyers first encounters her, lines up with other single mothers for her $385 monthly welfare check, but what she tells Moyers would warm the heart of a Reagan Republican: "I don't like welfare because it makes me lazy . . . I like to work." Jobs are hard to find, and she has children to look after. Moyers asks her boyfriend, an engaging fellow named Timothy, 26, why he doesn't use birth control. He says, "Girls don't like them things."

Later, intruding to good purpose on an intimate moment, the camera watches as Alice's third child is born. Timothy, present in the delivery room, shouts, "I'm the king!" One of the most heartbreaking sights on the program is a series of drawings done by Timothy that show him possessed of a genuine talent he is unable to put to productive use.

In another moving sequence, Moyers travels with members of one family to North Carolina to touch some unpaved earth, at a reunion of cousins and uncles and in-laws. Some of the traditions that the city has helped destroy still exist here: the strong influence of the church, and reverence for the family as an institution. But to at least one of the young men, the place seems dull, and he longs to return to the instant gratifications of the city.

Not everything in the program is discouraging. Back in Newark, Moyers finds passionate community leaders and activists who are trying to guide and inspire children born into fatherless, impoverished homes. Newark police Lt. Shahid Jackson was once a street kid himself; now he works with the Police Athletic League to give kids outlets for their energy and the authority and discipline missing from their lives.

One quartet of boys has formed a rap group whimsically named the Educated Three. They're not here to do the "Super Bowl Shuffle." They sing about the kinds of problems Moyers is agonizing about. Moyers asks one of them if a guy can get killed on the streets. He says, "Sure. See how you blink your eye? Like that." There's poetry here, but it is mainly the poetry of casual despair. There can be stopgap measures, but the cycle remains unbroken: children born to children who drop out of school, then out of the mainstream, then into a kind of angry anticulture.

The point of this report is to make the general clear from the particular; what the collapse of the family unit in the ghetto portends could be, says Moyers, "a new outlaw society . . . a subsociety where people will annihilate themselves." Newark social worker Carolyn Wallace, who operates a community center with her husband, laments, "We are destroying ourselves."

Although CBS network programmers have given "The Vanishing Family" a classically undesirable time slot, CBS News is lending the program unusually emphatic support. A "Face the Nation" broadcast Sunday is keyed to themes in the Moyers program. The subject is "Alternatives to Welfare Dependency." On the "Nightwatch" program next week, the Moyers documentary will be repeated, in two parts (Tuesday and Wednesday at 2:30 a.m., Channel 9), accompanied by a panel discussion of the program's implications.

Tonight's broadcast will be followed by a panel discussion as well, with the participants to include the Rev. Jesse Jackson and Harvard economist Glenn Loury, who in a December speech at the National Press Club here warned of "the profound alienation of the ghetto poor from mainstream American life."

CBS News insiders will bluntly admit that one reason for the panel discussions is anxiety that "The Vanishing Family" will be attacked by some black leaders. Others who view the program may see it as a new manifestation of white liberal guilt in action, perhaps even a remonstrance to urban blacks (Dan Rather didn't help the CBS case when, with incredible clumsiness, he prefaced a "CBS Evening News" item on a new Urban League study of the Reagan administration by saying it applied to "our fellow Americans who happen to be black").

Greater emphasis might well have been placed in the Moyers program on the role chronic unemployment plays in the ghetto life cycle he describes. But that is probably a small quibble when considering a documentary that took guts as well as smarts to put together, and one that fills you with passion; something must be done. "The Vanishing Family" is as essential as network news programs get. It's why Edward R. Murrow put CBS News on the map in the first place. It's why we've left the tubes and the wires and the transistors in the television set and didn't turn it into a planter long, long ago.