For a precious few minutes this morning, the space shuttle Endeavor became part of the Internet. Since its record 16-day mission began last Thursday, several hundred thousand E-mailed questions have poured into NASA's "Astro-2" site on the World Wide Web, but only three have made it past the space agency's sorters so far. "Why did they schedule the liftoff for 2 a.m.?" asked Thomas Maier, Jr. of Decatur, Ga. "I like to watch and it is past my bedtime. I am 9 years old you see." Crew member Tammy Jernigan, up at 5 a.m. EST, replied that astronomy studies indicated that an early liftoff would speed their search for intergalactic helium. Huh?