The five astronauts aboard the space shuttle Challenger spent another near-perfect day in space today, maneuvering the robot arm in the shuttle cargo bay flawlessly and taking photographs of parts of the Earth never before seen by men from space.

The nighttime launch last Tuesday of astronauts Richard H. Truly, Daniel C. Brandenstein, Dale A. Gardner, Dr. William E. Thornton and Guion Stewart Bluford II has brought them over regions of the Southern Hemisphere that crews before them have seen only at night. The astronauts have brought enough film to take 2,400 pictures of Earth, more than twice many as previous crews have taken.

Already, the astronauts have photographed an erupting volcano named Iliwerung in Indonesia and a second erupting volcano farther east on an island so small that nobody in space or on the ground could identify it. The crew has also taken the first pictures from space of the African desert, islands in the Indian Ocean, parts of the Amazon jungle and the Australian deserts as well as of a tropical storm named Ellen south of the island of Guam.

The timing of their flight also has given the crew the first nighttime glimpse of parts of the Northern Hemisphere. The crew reported seeing the lights of New Orleans and Mexico City for the first time from space.

Early today, Truly asked the Mission Control Center in Houston where they were. Told they were over northwestern Mexico, Truly replied, "Nice clear night, we see a lot of cities below us. The reason I had to ask you is because we have the lights turned off in the flight deck."

As the sun began to rise moments later, Truly said the air below was so clear they could "look straight down at large cities and a whole series of towns where we can see every street."

Truly told Mission Control they could see thunderstorms building up in Florida and when they looked north they could see the entire Eastern Seaboard up to North Carolina.

Once again, the astronauts communicated much of the time through a $100 million communications satellite named the Tracking and Data Relay Satellite hovering over the South Atlantic. Launched into orbit last April and bumped into its proper orbit by ground commands, the TDRS satellite is essential to the next flight of the space shuttle, which will carry the $1 billion spacelab built by European Space Agency.

"We believe we'll be ready for spacelab," TDRS Program Manager Robert O. Aller said today at the Johnson Space Center. "I wouldn't bet my pay check on 100 percent reliability but we're looking for a highly successful TDRS operation on the next mission."

Today, the astronauts communicated through the satellite secretly, using an encryption technique that is being tried for the first time. Not only will it allow secret communications on military shuttle missions, it will also allow commercial users to send proprietary messages to customers or branches around the world without fear of compromising them.

The astronauts dropped down about 50 miles in their orbit today to test a new lightweight insulation against deterioration at lower altitudes. Even in space, there could be just enough oxygen at the lower levels of space to degrade some materials like the insulation officials want to use on future shuttle craft.