This sequence of radar images from May 29 shows asteroid 1998 QE2 and its moon, seen as a bright spot. Watch on YouTube





When astronomers analyzed radar readings to create their first maps of 1998 QE2, the big asteroid that's due to sail past Earth on Friday, they were surprised to find that it has a moon twice as big as an ocean liner.

1998 QE2 itself is way bigger: The latest readings from NASA's Deep Space Network antenna in Goldstone, Calif., are consistent with earlier estimates that the asteroid is about 1.7 miles (2.7 kilometers wide). But the moon is hefty as well. Astronomers estimate its diameter at 2,000 feet (600 meters). That's big enough to wipe out an area of the size of Virginia if it were to strike land.

Fortunately, neither space rock will come any closer than 3.6 million miles (5.8 million kilometers), or about 15 times the distance between Earth and the moon, during a flyby that reaches its climax at 4:59 p.m. ET Friday.

Even though Earth is in no danger, the close encounter is stirring up interest because it gives astronomers a rare opportunity to see an asteroid up close. Such observations could help NASA plan for efforts as the Osiris-Rex mission, which will bring back a sample from the asteroid Bennu in 2023; and an even more ambitious mission to corral an asteroid by the mid-2020s.

Goldstone's first radar observations of 1998 QE2 were made on Wednesday evening, producing images with a resolution of about 250 feet (75 meters) per pixel. The pictures show that QE2 has a rotation period of less than four hours, and is marked by several dark surface features that are suggestive of large craters. In an image advisory, NASA's Jet Propulsion Laboratory says that the image resolution will get better as more radar readings become available, from Goldstone as well as the Arecibo Observatory in Puerto Rico.

More about the close encounter:

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.