THE PROGRAM of radical economic reform being prepared in Moscow marks the end of perestroika. That grand idea of Mikhail Gorbachev's was to "restructure" socialism. But the new "500 Days" program abandons socialism, replacing it with a market system: capitalism. It means the further and final discrediting of the Communist Party as a fount of wisdom, source of power and instrument of value to the society. The party is now a museum exhibit, relic of a boundless catastrophe. Soviet politics are becoming increasingly diverse, decentralized, democratic -- and disorderly. The economy is to be directed at the republic level and by individual/consumer andfactory/industry initiative.

Mr. Gorbachev came late to the realization that half-measures were pointless -- so late as to enable others, first among them erstwhile loyalist Boris Yeltsin, to seize his early mandate of renewal in both the political and economic realms. Irony abounds but perhaps not tragedy. The Soviet Union has two charismatic leaders -- Mr. Gorbachev, who is charismatic abroad and who remains at the helm of foreign policy, and Mr. Yeltsin, who is charismatic at home and who is increasingly providing the leadership for foot-to-the-floor domestic reform.

The West is sometimes so thrilled to see its ideas catching on that it forgets how inhospitable Soviet (and Russian) traditions and current conditions are to them. The Soviet Union is inflicting upon itself the sort of wrenching changes that victors force on countries that have lost a war. Mr. Gorbachev hesitated not simply because he lacked imagination; he shrank from the very real inflation, unemployment, disorder and even disintegration that full reform may bring. The Yeltsin camp saw the down side, but judged the risks of halfheartedness to be greater.

The new reforms transform the American debate on ''helping Gorbachev.'' Those terms are irrelevant. What is important is that the Soviet Union is meeting the two broad conditions of generous American aid. President Bush remarked in Helsinki that Soviet policy in the Gulf (and by extension Soviet foreign policy overall) compel Americans to be ''as forthcoming as we possibly can'' regarding the economy. The reforms, as they bite, satisfy the equally basic requirement that aid help build and lock in a new system, not prop up the old. The United States is in no position to "write out large checks," said Mr. Bush, but otherwise there are "many ways" to help. It's time to spell them out.