InSight’s Landing Site: Elysium Planitia
InSight's landing site is Elysium Planitia, a flat, smooth plain selected not for its surface features, but for safety considerations. InSight's purpose is to study the interior of Mars, not the surface. Thus, in the selection of a landing site, what's on the surface mattered less on this mission than for previous rover missions focused on the geology.
Planitia is Latin for a flat surface, geometric plane or flatness or a plain. Elysium is from the ancient Greek name for an afterlife paradise, usually referred to in English as the Elysian Fields.
The landing site lies in the western portion of Elysium Planitia, centered at about 4.5 degrees north latitude and 135.9 degrees east longitude. This is just 373 miles (600 kilometers) from Curiosity’s landing site, Gale Crater.
The landing ellipse is about 81 miles (130 kilometers) long, generally west to east, and about 17 miles (27 kilometers) wide, covering the area within which the spacecraft has about a 99 percent chance of landing when targeted for the center of the ellipse.
InSight takes 360-panoramic images in all directions at this landing site. Scientists expect a flat surface, no hills nearby and few large rocks in view. That is based on high-resolution images taken from orbit as part of thorough evaluations for selecting the site.
Selecting Where to Land
Several workshops took place in 2013, 2014 and 2015, to evaluate 22 candidate landing ellipses and then four finalists. All 22 of those sites are in Elysium, which is one of only three areas on Mars that meet two of InSight's needs.
One requirement is that the landing site must be close enough to the equator. This ensures that the lander's solar array can provide adequate power at all times of the year and the lander can stay warm: between 5 degrees north latitude and 3 degrees north latitude.
The site's elevation must be low enough to have sufficient atmosphere above the site for a safe landing, because the spacecraft relies on the atmosphere for deceleration during descent.
The High-Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter plays an important role in evaluating candidate landing sites on Mars. HiRISE took about 150 images of candidate InSight landing sites, covering nearly all of the ground of the final site.
Landing Successfully on Mars
InSight's scientific success and safe landing depends on landing in a relatively flat area. It also depends on landing in an area where rocks are few in number. A slope too steep could foil the robotic arm's access to a sufficiently large work area. A sufficiently steep slope in the wrong direction could jeopardize how much power the solar arrays can produce. A large enough rock at the landing site could block one of the solar arrays from opening.
The site evaluation also considered the subsurface structure. For mission success, InSight’s heat flow probe must be able to penetrate the ground in the lander's workspace. The probe was designed to burrow into soil (not rock) to a depth 10 to 16 feet (three to five meters). The Thermal Imaging System (THEMIS) on NASA's Mars Odyssey orbiter provided key evidence that the chosen landing site is suitable for burrowing. THEMIS observations can show how quickly the ground cools at night or warms up in sunlight. Solid rock changes temperature more slowly than softer ground.