Mr. Andropov, meet Samantha Smith from 'Down East'

''The difference between communism and capitalism is probably that I have more freedom. Russian kids probably can't go out and catch rabbits or snakes like I can,'' quips Samantha Smith, impishly offering to catch a snake in her ample rural backyard.

But with a suddenly knitted brow, she amends her comparison of the United States and the Soviet Union with an economic example: ''Say you have a big popcorn popper, like at the movies, and you want to sell popcorn. You can't just sell it on the street [in Russia]; you probably have to get permission.''

The world, according to this 11-year-old, is seen in black and white. But as she leaves next week for a two-week tour of the Soviet Union at the invitation of Soviet President Yuri Andropov, she's headed for a lesson in the shades of gray that make up the world of international affairs.

Samantha's views are a combination of hard-headed realism and fresh-faced idealism. The latter causes many to wonder if she is being used as a Soviet propaganda tool and if her young concerns for peace can make a difference.

It is Samantha's childlike brand of absolutism, though, that plucked her out of the schoolyard and dropped her into the talk show and network news circuit.

She asked Mr. Andropov in a letter last year why two nations would keep bombs even though both claim they have no desire to start a war. ''Why,'' she asked, ''do you want to conquer the world?'' And, she asked, do Russian kids believe, as she does, that bombs are bad?

Although her fifth-grade scrawl was excerpted in an April Pravda article designed to show the respect the world has for the Soviet leader, Andropov did not answer her letter at first. So she wrote back to ask why her letter was used and why Andropov had not responded. Within days, the Soviet leader wrote a 500 -word telegram inviting her on a free trip to the USSR to see that ''everybody in the Soviet Union stands for peace and friendship among nations.'' Samantha and her parents leave for the Soviet Union next Thursday.

She wants to ''take a look around the Kremlin,'' see the Bolshoi Ballet and the circus, and visit Soviet youth. Most of all, she wants to see Andropov. But that hasn't been promised. The point of the trip, says Samantha's father, Arthur Smith, an English professor at the Augusta campus of the University of Maine, is for Samantha to pursue her questions.

His daughter agrees, saying, ''It doesn't make sense that we have bombs to protect ourselves if we aren't going to start a war. So why don't we get rid of the bombs?''

Although Mr. Smith says the family leans toward a belief in disarmament rather than a nuclear freeze, it appears that the family has no link with the antinuclear movement. Samantha says she merely read about the issue in a magazine.

It's hard, Mr. Smith explains, to tell special-interest groups that the only reason Samantha is going on the trip is for the narrow pursuit of an answer to her questions. Yet many hope that the child will take up special-interest causes with Andropov. Many, such as disarmament groups and those seeking the release of dissidents ''seem to think Samantha has strong influence with Soviet leadership, which she doesn't,'' he says.

Cynics, as well as those in the US government, bristle at the notion that Andropov's gesture will be viewed as merely an act of goodwill. It is believed that the whole trip will be used as propaganda both to promote the Soviets and their views and to contrast them positively against US ideas. Already, Andropov has suggested that Samantha's misconceptions about the arms race are a product of poor American education and propaganda.

Samantha holds no illusions about the way the Soviets may handle her trip. ''Of course they aren't going to show me the bad things. If Russians came to visit here, I wouldn't show them the ugly things.''

Would Andropov use her as a propaganda tool? ''No,'' she says, with blue eyes wide. ''He wouldn't do that.''

The trip won't just help the Soviets, says Mr. Smith, who often clarifies his daughter's remarks for the gaggle of reporters that seem ever-present these days.

''Samantha's questions are unflinching, and it will be as useful for presenting the American view as much as anything. When the Soviets get a load of Samantha they'll be impressed with the independence and expressiveness of a middle-class American child,'' says Mr. Smith of his daughter, who is catcher on a softball team, likes to play in the wooded thickets in her neighborhood, and has attended a Jewish synagogue and a number of churches.

Indeed, his daughter is a gangly-legged bundle of energy, given to breathless bursts of honesty that would make poker-faced diplomats cringe. Though Samantha is likely to represent a thoroughly American view, Mr. Smith and the US State Department stress that the whole trip is a private affair. Queried about what role the State Department might play in advising and debriefing the Smiths, neither the family nor the government would acknowledge any contact except in the issuance of passports.

Samantha says that in the weeks since the affair began, many of her misconceptions about the USSR have been corrected. She says, for example, she thought Hitler was a Czar and that concentration camps were the Soviets' doing.

Did she ever think her original letter would cause her to think this much? ''No,'' she says, ''I thought I'd just get a form letter, just like I did from the Queen'' - of England, of course, to whom she also wrote.