Sen. Robert C. Byrd (D-W.Va.) struck a note of optimism for U.S.-Soviet relations as he arrived here today, despite clouds raised by the Reagan administration's recent decision to test an antisatellite weapon and its charge that Soviet agents have used a potentially hazardous chemical powder to track the movement of U.S. diplomatic personnel.

Byrd, heading a bipartisan delegation of eight senators, said he is carrying a letter from Reagan to Soviet leader Mikhail Gorbachev, but he would not disclose its contents. He said at a press conference at Moscow's airport that he hopes the talks planned with Gorbachev and other Soviet officials will contribute to "an improved and constructive dialogue" between the two countries.

Sen. Strom Thurmond (R-S.C.), a veteran Senate hawk usually critical of Kremlin policies, echoed Byrd's upbeat note. "There have been differences in the past between the Soviet Union and the United States," Thurmond said, "but these differences can be resolved."

But Georgi Arbatov, head of the Soviet Union's U.S.A. and Canada Institute, appearing alongside the senators, seemed skeptical. Asked to assess the state of U.S.-Soviet relations, he responded flatly, "Very bad."

"The problem is to get the treaties and ratify them," Arbatov said, referring to the unratified SALT II treaty and, by implication, to negotiations on nuclear arms under way in Geneva. "At present we don't have anything."

The senators' optimistic statements stood in contrast to rising tensions between Moscow and Washington that flared up in mid-August. The United States announced then that it would test an antisatellite weapon. Moscow said that was a step toward implementation of Reagan's "Star Wars" program, which the Soviet government opposes.

Tensions rose further last week when Washington charged that Soviet agents have used a possibly harmful powder to track the movements of U.S. diplomatic personnel.

Byrd played down the ASAT and "spy dust" controversies, saying, "We are here to help Soviet leaders . . . better understand the desire for peace on the part of the American people."

Earlier this week, U.S. Agriculture Secretary John R. Block visited the Soviet capital and told reporters that his trip was part of an effort to improve relations between Washington and Moscow before the summit meeting between Reagan and Gorbachev in Geneva in November.

With Byrd and Thurmond are Sens. Claiborne Pell (D-R.I.), Sam Nunn (D-Ga.), Dennis DeConcini (D-Ariz.), Paul S. Sarbanes (D-Md.), John Warner (R-Va.) and George Mitchell (D-Maine). In addition to meeting with Gorbachev Tuesday, they will talk with members of the Supreme Soviet and Defense Ministry officials.