EARTHLINGS CHEER MARS LANDING AT PASADENA FESTIVAL.

Byline: Eric Wahlgren Daily News Staff Writer

Sure, Nathaniel Costa thought the chance to build a robot out of Legos was something else.

And meeting famed astronaut Buzz Aldrin in person was pretty cool, the 13-year-old Altadena resident said.

But like many of the estimated 8,000 space fans, trekkies, sci-fi devotees and others who flocked to Planetfest '97 on Friday, one thing did leave Nathaniel truly dumbstruck.

It was the sight of the first live images in about two decades of the surface of Mars that were being beamed back to Earth and displayed on a 25-foot video screen at the Pasadena Convention Center.

``I'm in awe,'' said the La Salle High School student as black-and-white photos of the rocky Martian surface flashed onto the screen like some high-tech, intergalactic slide show.

The Mars lander Pathfinder touched down Friday, providing earthlings with striking new pictures of the Red Planet which will be broadcast at the space festival throughout the weekend.

``It's just exciting to be back on Mars,'' said Melissa Strait, a chemistry professor at Michigan's Alma College and an expert in meteorites. ``We haven't seen it for 20 years.''

The dramatic broadcast capped a day of speeches and lectures by astronauts and space scientists during the first day of Planetfest '97, which also features nearly 200 exhibits and activities for children and adults alike.

In addition to making motorized robots out of Legos, children Friday built small rockets out of film canisters, checked out what a manned spaceship to Mars might look like, and browsed through dozens of other booths.

``I want to be an astronaut,'' said 11-year-old Heather Doornbos, in a statement echoed by youngsters throughout the day. ``I really like everything about space.''

Suzanne Gibson, who helped build the model of a human-carrying craft that could be bound for Mars, said her booth was mobbed by children all day long.

``We have no problem holding a child's interest,'' Gibson said. ``Unfortunately, many adults though have lost their sense of wonder. When we ask a child if they want to go to Mars, they say, `How about now?'''

But Cindy Jalife, the event's organizer, said she believes adults have also been fascinated by NASA's new wave of Mars explorations, fueled by speculation that a primitive form of life might have existed there.

``I'm sure there are a lot of people out there hoping they find life,'' said Jalife, an official with the national Planetary Society, which sponsored the event.

Heather said she was hoping so much to see Martians on the Red Planet that her eyes played tricks on her when the snapshots from Pathfinder appeared on the screen.

``I thought I saw an alien, but I realized it was a rock,'' said Heather, who is from Rancho Palos Verdes.

Despite the let-down, Heather's mother, Ann Doornbos, called the landing on Mars no small feat.

``It's something you'll be able to tell your grandkids about,'' said Ann Doornbos, a middle school science teacher.

Planetfest '97 continues today from 10 a.m. to 5 p.m. and Sunday from 10 a.m. to 4 p.m. at the Pasadena Convention Center, 300 E. Green St. Tickets are $25 for adults and $11 for children ages 3 to 17. For more information, call [phone].

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Photo: Eventgoers applaud Pathfinder's touchdown on Mars on Friday at Planetfest '97 in Pasadena.

John Lazar/Special to the Daily News