What is the point of this app?
Space is vast. Even the Solar System, a minuscule part of the Universe, is vast compared to the sizes of the planets. The best way to truly appreciate the vastness of the Solar System is with a scale model that accurately represents the size of planets and distances between them. Many museums, planetariums and cities have set up just that—"Solar System Walks" where you can walk from the Sun to the outer planets. SpaceWalking brings that idea to the iPhone, so you can create a Solar System Walk wherever you are, at five different scales.
How does it work?
SpaceWalking combines a 3D scale model of the Solar System with location-based data from GPS (Global Positioning System) satellites to place the Solar System in your neighborhood. After you choose a scale and starting point, the app scales the model and locates it at your current latitude/longitude coordinates. You can toggle between a MapView, which overlays the Sun and planets on a map, and PlanetView, which shows your view in the virtual Solar System. As your GPS-based location changes, your location on the map and in the Solar System are updated, usually every few seconds.

When you're ready to start walking to the next planet, be sure to press the Back arrow to depart the zoomed-in view.
Got any tips for a good experience?
Give the GPS some time to lock in precisely on your location. On the "Position Planets" screen, watch and wait until the blue dot is accurately placed on your location.

Walk slowly among the inner planets to give the GPS signal enough time to track your motion.

Be sure to notice the real-world size of objects (shown when you Visit, then zoom in on objects) to appreciate the size and scale of the Solar System.

Look around in PlanetView as you walk, especially between Mercury and the Asteroid Belt, to spot asteroids between the planets. Also look for a robotic spacecraft out in the Kuiper Belt.

Check the Achievements (in Options) for some specific walking routes and destinations.

Try different scales. You probably won't want to walk the entire length of the Solar System at the 2000m scale, but a walk among the inner planets is spectacular when the Sun is 18 meters in diameter!
How accurate is the app?
In the scale model of the Solar System, all object sizes and distances are accurate to within a few kilometers. However, it is the accuracy of the GPS signal that affects how well the app works for you. With a weaker signal, planets may shift locations in the real world, and your movements may not be updated frequently enough for fluid movement through the Solar System. The accuracy of GPS signals is slowly improving, but currently GPS is not accurate enough to detect small movements (walking just a few yards/meters). For that reason, when you get close to an object in the virtual Solar System, a "Visit" button appears onscreen. Touch it to zoom in close to that object. Then you can touch the planet and any moons to zoom in for even closer inspection.
Does it work indoors?
Not very well. Buildings interfere with GPS signals, so you will generally have a much poorer experience indoors, as the planet locations will be imprecise. You may have trouble getting close enough to planets to get the "Visit" button. You may have similar problems when outdoors but surrounded by tall buildings.
Why can't I use my camera for the background view?
We originally intended to provide an option for just that, but in testing, we discovered problems with that. When viewing a 2D screen, the only cues about size and scale are the layering and overlapping of objects. With a camera providing the background, your mind will interpret all virtual objects as being smaller than the nearest object in the camera field. For example, if the Sun is 2 meters in diameter, but a table in the camera view is just 1 meter away, the Sun will appear (as your mind interprets the scene) to be much smaller. Since this app is all about size and scale, we decided not to offer an option for the camera view to reduce the chances of that kind of misinterpretation of the screen images.
Why are the measurements only in metric?
The metric system is the standard unit of measurement in science. The more we use metric units, the more familiar and adept we become with them. However, we may add an option for Imperial units in a future update.
Will it drain my battery when running in the background?
No, the app "sleeps" when not in use. However, if you want to quit the app completely, follow the standard procedure: Double-click the round button below the screen to display recent apps, then touch-hold the SpaceWalking app icon for a moment, then touch the small red minus sign button to quit the app.
Why isn't Pluto listed as a planet?
Since it's discovery in 1930, Pluto has always been an oddball—a tiny, rock and ice planet in the outer realm of the Solar System, far from the other rocky planets. The discovery of similar objects out there, in the Kuiper Belt, led to great debate among astronomers about Pluto's proper classification. In 2006, the International Astronomical Union (IAU) reclassified Pluto as a "dwarf planet"—one of several in the Kuiper Belt, along with Ceres, the largest asteroid in the Asteroid Belt. We have included Pluto and its moon Charon as our representatives of the Kuiper Belt, just as Ceres is our main representative of the Asteroid Belt.
Why is it tricky sometimes to approach Neptune and Pluto?
The Solar System is so vast, relative to the sizes of the planets, that it pushes the limits of our 3D technology, creating occasional imprecision in the GPS-based placements of Neptune and Pluto. They are definitely visitable; try standing still for a little while longer when Positioning Planets to ensure that they are placed accurately.
Why can't I log into my Twitter account?
A recent Twitter policy change has broken the Twitter login in the app. We are working on a fix and plan to publish an updated version of the app soon.
I can't find the New Horizons spacecraft.
The description for the New Horizons achievement says that spacecraft is near Uranus; in fact, it is near the Kuiper Belt (Pluto). We will fix that soon in an update. Sorry for the confusion!

Available on the App Store
SpaceWalking requires:
iPhone 3GS
iPhone 4