(female narrator) Days that are 24 hours and years of 365 days are standard units of time on Earth. But on the planet closest to the sun, time takes on a new meaning.

(Describer) Mercury is shown.

The sun's pull on Mercury has slowed down this small planet's rotation, so one full day-- that's noon to noon-- takes about 176 Earth days. A day on Mercury would last 4,224 hours. That's a long time to be in school. What else do we know about Mercury? It's the closest planet to our sun, and like Earth, it's terrestrial. It's temperature ranges from a scorching 427 degrees Celsius, when the sun shines on the surface, to a frigid -173 degrees Celsius on its night side. In many ways, the surface of Mercury is like that of our moon-- heavily cratered and old. But we have even more questions about Mercury. Let's talk with Peter Bedini from the Johns Hopkins Applied Physics Laboratory to find out what makes Mercury so mysterious. There are several things that confuse us about the planet. It is very dense. We don't know why. It doesn't have an atmosphere. There are so few particles in the environment

that it's called an exosphere, and it's wispy and moves a lot-- very dynamic. Mercury has a dynamic magnetic field, like Earth but smaller, and that's very unusual for a planet of that age. You'd think that that magnetic field would be gone already. There are signs that there was volcanism on the planet. There are many mysteries about the planet. Mercury's position so close to the sun makes it difficult to study. Not even Hubble, our most powerful space telescope, can look at Mercury without permanently damaging its optics. So, how can we learn more about this unusual planet? Only one NASA spacecraft, Mariner 10, has ever visited Mercury. Mariner 10 was programmed to fly by Mercury three times and take images of the heavily cratered surface. Unfortunately, the spacecraft essentially saw the same side of the planet on each flyby. Scientists and engineers spent the next decades developing new techniques in designing a spacecraft that could survive the extreme conditions of Mercury. NASA's MESSENGER mission is the result of their work. MESSENGER, or the Mercury Surface, Space Environment, Geochemistry, and Ranging is the first mission to Mercury in over 30 years. MESSENGER completed three flybys of Mercury, capturing stunning, never-before-seen imagery of the surface before inserting itself into orbit around the planet. A neat navigational technique that MESSENGER used on the way to Mercury was the use of solar radiation pressure, or solar sailing, to guide the probe's trajectory.

(Bedini) We realized that by changing the angles of solar panels, or tilting the body and exposing different surfaces to the sun, we could actually affect what path the spacecraft takes and tweak the trajectory a bit. The biggest savings that gives you is that you don't risk operating your thrusters, which is risky with a spacecraft. By sailing on solar pressure like a sailboat in a wind, we could direct the spacecraft where we wanted it to be. But why bother studying Mercury? How will knowing more about this planet affect us? A family portrait, one of MESSENGER's newest contributions, helps us answer that question. The 34-image mosaic of the solar system gives scientists a snapshot of neighboring planets from Mercury's perspective. These images help us understand that Earth is a part of a planetary family formed by common processes.

(Bedini) I think of the solar system as a large laboratory that has in it many planetary bodies very similar to Earth's. Mercury is one of the more elusive ones. We're going to fill in that gap. It's an extreme case of the terrestrial planets. It's closest to the sun, it has the magnetic field, high density, and by learning how it evolved, and knowing what we know about the other terrestrial planets, we can learn a lot more about how Earth itself came to be. By gathering data about Mercury, we'll be able to gain insight into our own history. With the MESSENGER spacecraft helping us unveil Mercury's mysteries, who knows what astronomers will figure out before the next day on Mercury goes by.