I am concerned by the tone of Jessica Tuchman Mathews's op-ed piece last week, ''The Mars Extravaganza'' {Oct. 5}. I am not going to debate whether sending Americans to the Moon and Mars is wise or affordable under current budget restraints. I am not going to deny that there are numerous challenges facing our environment (pollution, deforestation, extinctions, etc.), as Mrs. Mathews points out. But Mrs. Mathews falls into the environmentalist trap of asking, ''Why should we spend all that money on space when there are so many problems here on Earth?''

The right question to ask is, ''How can we best spend money to solve these problems here on Earth?'' The surprising answer is: in space. Only through space-based observations can we understand what's happening to this planet. More important, only through space-based industry can we halt and reverse the trends threatening our environment.

Are messy industrial processes threatening groundwater supplies? Move the industries to orbit and send down only the finished goods. Is open-pit mining erasing huge tracts of wilderness? Go get an asteroid, which contains far more nickel, iron and other metals than humanity has mined to date. Are burning fossil fuels polluting the atmosphere and contributing to CO

buildup? Put solar power stations in orbit and beam down limitless quantities of safe, clean, unpolluting energy. Are Third World children dying from disease for lack of medicine? Build a pharmaceutical factory in the microgravity of orbit, where we can make life-saving drugs for a tiny fraction of the cost of Earth-based processes.

These activities, and hundreds more, do not require a trip to Mars, but they cannot be carried out by machines. Only the intelligence and flexibility of men and women in orbit can break the grip of Earth's gravity and bring the bounty of space to all mankind.

America knows how to carry out these activities quickly, safely and economically. So do Japan, the Soviet Union and the Europeans. But we are hobbled by NASA, a bureaucracy beholden to its unreliable and obsolete Shuttle, its bloated Space Station Freedom and a host of other constituencies. If private industry were encouraged to begin the commercial and profitable use of space without the 1,001 regulations enforced by our government, then we could see astonishing gains in space technology -- and in the benefits of space for the first, second and third worlds -- by the end of the decade.

A vigorous and independent space program could be the best friend of the entire environmental movement. I encourage Mrs. Mathews to explore its potential benefits for the problems she deplores; she shouldn't throw out this baby industry with NASA's dirty bath water. STEPHEN FLEMING Sterling