Dr. Jo Pitesky: Welcome to NASA's PlanetQuest Podcast, episode 5. I'm Dr. Jo Pitesky.

(Melodramatic 1933 horror movie music).)

Invisible Man: Stop where you are. You don't know what you're doing.

Bobbie: I know what I'm doing all right.

1st Person in Mob: Get a'hold of him!

2nd Person in Mob: Lock him up!

Invisible Man: All right you fools � you've brought it on yourselves! � You're crazy to know who I am, aren't you! All right, I'll show you! I'll show you who I am, and what I am! HA HA HA HA!

Bobbie: He's all eaten away!

(Music fades out under next)

Pitesky: Once he stripped off his clothing and bandages, H.G. Wells' fictional character, the Invisible Man could no longer be seen. But sometimes you could tell he was there because of his effect on things you could see, like his footsteps in the snow or rain glistening on his skin.

Using a similar principle, a team led by Andrew Gould of Ohio State University will use NASA's SIM PlanetQuest mission to hunt for the unseen constituents of the universe, objects that are too faint or too small to be detected directly. Scientists know they're out there, they just can't see them.

Andrew Gould: With SIM we'll be able to see the masses of these object and we'll be able to distinguish between black holes, brown dwarfs, ordinary stars and neutron stars and figure out what's what. And this will be the first time we'll systematically be able to take a census of what's in our galaxy based on mass, and not light. So people have no trouble going off and seeing luminous objects with telescopes but we've never really taken a census of things we can't see. So taking a census of things we can't see is a basic goal of this project.

Pitesky: To study such elusive quarry, Gould's team will rely on the "gravitational lens" effect, predicted by Albert Einstein. Just as a glass lens can bend light, a massive object distorts space, making light passing nearby appear to bend. If Earth, the lens and a distant background star or galaxy happen to move into exact alignment, astronomers on Earth -- with a sufficiently powerful telescope -- would see the luminous background source as a ring around the lensing object. What's more, the image brightens during the lensing event since light that normally would miss the Earth is redirected toward it.

Gould: When this idea was proposed by Einstein in 1936 he immediately dismissed the possibility of ever detecting it � but with modern detectors we are able to monitor millions of stars so now thousands of these microlensing events have been detected so far.

So the interesting thing about them is that we're detecting something we can't see. So that's really cool. On the other hand, we actually have very little information about the things that we're detecting. SIM is going to be in solar orbit. So we're looking at this microlensing event from over here and SIM is looking from over here. So it's not going to see the same microlensing event we see on Earth and � by combining these two pieces of information together, we can get the mass of these objects.

Pitesky: Using the SIM PlanetQuest space telescope, scientists hope to make the first real "sighting" of dark matter, the mysterious stuff thought to constitute 90 percent of the galaxy. They also will investigate how much of the galaxy is composed of ordinary stars and how much consists of more exotic objects that can't be directly imaged.

Gould: For me the most exciting thing is being able to measure stuff that we can't see. People have been looking at stuff we can see for a long time, so it's great to take a look at stuff we can't see.

Pitesky: H.G. Wells would probably agree. (Horror music) For NASA's PlanetQuest, I'm Dr. Jo Pitesky.