Solar Walk 2 (for iPad)

The Solar Walk 2 ($2.99) iPad app is a new iteration of Vito Technology's Solar Walk, which depicts our solar system and its worlds. Like the original, Solar Walk 2 provides beautiful 3D representations of the Sun, Moon, and planets. It adds some cool, new features, but much of the new content—including a number of comets, asteroids, and dwarf planets—is only available through an in-app purchase. Unfortunately, 10 objects that were free in the original Solar Walk are now only available if you spring for premium access. But even without that purchase, Solar Walk 2 is worth it, and among the better solar system apps we have seen.

In addition to Solar Walk 2, Vito Technology also makes Star Walk 2 and the Editors' Choice Dino Walk: Continental Drift. Solar Walk 2 can be used on an iPhone, Apple Watch, iPad, or iPod touch. I tested it using an iPad Air 2 running iOS 9.2.

Design and Features
When you open Solar Walk 2, you see a wide-field view of our solar system, which quickly zooms in on the Sun, displaying it as an active star with prominences and corona surrounding a mottled yellow-orange disk. As is typical of astronomy programs, the center of the screen is open to show space scenes, leaving the perimeter for icons and to display information. To navigate, you pan by swiping in any direction, zoom out by pinching the screen, or zoom in by stretching it.

At the top of the screen is a bar, with the name of the current object being displayed at the center, the name of the previous object (with a back arrow) to the left, and three buttons identified by icons in the right corner: information ("i"), objects (a miniature solar system), and a menu (bulleted list).

Tapping the information icon takes you to a page of useful infographics about the object that's currently in view. For the Sun, for example, there are diagrams comparing its radius and mass with Earth, one comparing a person's weight on the sun (if one could actually stand on the sun and not vaporize) and Earth, and one comparing the rotation period of the Sun and Earth. You can access additional content about whatever object you're examining by choosing from a set of icons running down the left margin. You can read a description of the object (several paragraphs), or open its Wikipedia page from within the app. A calendar page highlights important dates related to the object, such as spacecraft flybys and launches, astronomical events, such as planetary conjunctions and oppositions, and (in the case of the Sun) solstices and equinoxes. You can see the cross section of the Sun or a planet by tapping an "internal structure" icon.

From the Menu icon, you can share a screenshot from within the app (via email, IM, Twitter, Facebook, or Flickr), run a tutorial on its operation, rate the app, or get information on other Vito Technology apps. You can also access a Settings menu, which lets you turn various features on and off. Among them are visual effects, mission effects, captions, and music.

At the right edge of the screen is a slider that controls the passage of time within the app. By default, it is set to the current time, but by moving the slider up or down, you can speed up, slow down, or reverse time. Time is less critical in Solar Walk 2 than it would be in a planetarium app, but there are several instances in which the slider is useful. If you set your view for the Sun and zoom out far enough, for instance, you can see the entire solar system. You can then set it in motion by speeding up the time.

New Worlds, for a Price
Solar Walk 2 significantly expands the number of objects the app is able to depict compared with the original Solar Walk, adding four dwarf planets, more than 20 asteroids, and more than a dozen comets, but there is a price to pay for them. None of the comets or asteroids is accessible without an in-app purchase, and the only dwarf planet the basic app includes is Pluto. Solar Walk 2 displays the same objects as the free Planets iPad app, except the latter app lacks Pluto. In addition, Solar Walk 2 presents more information about these worlds, plus infographics.

The comets, asteroids, and other dwarf planets, as well as additional content discussed below, are only available by buying "premium access" through an in-app purchase. A single purchase unlocks all of the app's premium content, and its price depends on how long you unlock it. Prices range from 99 cents for a month's access to $9.99 for unlimited access. In sheer number of objects, this is actually a step backwards from the original Solar Walk, which boasted Pluto, six dwarf planets and asteroids, and four comets in its basic version. With all its content unlocked, Solar Walk 2 is priced the same as Solar System, an exquisitely produced app that includes fewer asteroids and doesn't render individual comets, but provides more detailed information (and in many cases, shows images as well) of the Sun, the planets, and their satellites

Also included with premium access are space missions (flybys of planets by space probes), which were absent from the original Solar Walk. They are accessible through a flyby list, which opens when you tap a space-probe icon, or from the calendar page. When you launch a flyby, the app's time is reset to the date of the encounter. You can view a 3D representation of the spacecraft approach, pass, and then recede from the planet. By manipulating the time slider at the screen's right-hand edge, you can speed up or slow down the encounter, or view it in reverse.

Conclusion
The Solar Walk 2 iPad app provides enhancements, such as infographics, over the original Solar Walk. It offers many more comets, asteroids, and dwarf planets than the original, but only if you access Solar Walk 2's extra content through an in-app purchase. The full app with premium content unlocked is priced similarly to the gorgeous Solar System app. Solar System has a more appealing design than Solar Walk and easier navigation than Solar Walk 2, and provides more detailed information about some objects such as the Sun and the moons of various planets, though it renders fewer asteroids and no comets. The basic version of Solar Walk 2 is a good choice as a solar system app, but it's a shame that some content that came with the original Solar Walk is now only available if you invest in premium access.