THE MAN from Mars dropped into Earth orbit yesterday, just in time. He had heard that the first Reagan budget was about to appear. What, he asked, was the focus of public attention?

There's intense speculation, we explained, about the size of the deficit and whether it could be held under $100 billion. The Martian consulted his notes. "Didn't you Americans once have a president who bent the budget into a pretzel to keep it under $100 billion?"

Yes, indeed--but that was the whole budget, not just the deficit. The memory is still fresh. Lyndon B. Johnson was a man who took large round numbers seriously. He spent the fall of 1964 gloomily predicting that the next budget couldn't possibly be kept under $100 billion. But when it came out in 1965, lo, the figure was $99.7 billion. The Martian wanted to know whether the number turned out to be accurate. We laughed, and explained that everyone knew it to be pretty fake from the beginning. In those days the trust funds, like Social Security, weren't counted in the totals--just as off-budget accounts aren't counted today. The real budget number for spending wasn't $99.7 billion but $127 billion. Even that turned out to be a low estimate. When all the bills were in, spending turned out to have been $135 billion.

"If everybody knew the number was bent," the man from Mars asked, "why did the president go to such lengths to produce it?" He keeps asking questions like that. You can tell that he's from Mars.

"It makes people feel better," we patiently explained. "That's why Mr. Reagan is going to struggle so hard, and so publicly, to keep his deficit figures under $100 billion. It's a matter of paying respect to the proprieties, like the medicine man doing the rain dance. You may not get any rain, but everybody finds it gratifying to know that the poor fellow is doing everything humanly possible."

If Mr. Reagan's estimated budget deficit for 1983 should be, say, $99.7 billion, the Martian asked, how should one take that number?

With caution, we advised. One should make a few rough corrections, with a blunt pencil. First, one should add in the off-budget spending--currently about $20 billion a year. Then one should deduct all the savings from legislation that obviously won't pass. Mr. Reagan, for example, is already backing off his own plan to withhold taxes on interest and dividends. Then one should correct for the excessively optimistic economic forecasts on which all the other arithmetic is based. In a day or two, the Congressional Budget Office will publish a report telling everybody what a more realistic estimate might be.

If the CBO is putting out a fairly reliable set of numbers, the Martian asked, why does the president put out different ones?

"That's politics," we said, "which would hardly interest a serious economist like you."

"I'm not an economist at all," the Martian indignantly exclaimed. "I'm an anthropologist. I'm writing a book on the tribal habits of the smaller planets."