December 18, 2018

InSight tweaks trajectory to home in on Mars landing site


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Artist’s concept of the InSight spacecraft in its interplanetary cruise configuration. Credit: NASA/JPL-Caltech

On the eve of its blazing entry into the Martian atmosphere, NASA’s InSight spacecraft briefly pulsed its control jets Sunday to target a safe, flat landing zone near the planet’s equator — the last in a series of trajectory correction maneuvers since the mission’s launch in May.

Still attached to its cruise stage, which ferried the lander from Earth to Mars, InSight fired its thrusters at 4:47 p.m. EST (1:47 p.m. PST; 2147 GMT) Sunday to steer the spacecraft closer to an ideal landing site in Elysium Planitia, a broad, flat equatorial plain selected by mission managers because of its relative safety.

InSight’s landing zone is free of large boulders, craters, and other topographic hazards, according to surveys based on high-resolution imagery from the Mars Reconnaissance Orbiter, a satellite in orbit around Mars.

Officials ordered the trajectory correction maneuver in a meeting Sunday morning after an assessment of InSight’s course toward Mars. The previous course correction Nov. 18 moved InSight’s landing ellipse — a football-shaped area in which the spacecraft could conceivably touch down — to the mission’s planned landing spot.

But the ellipse did not have the landing target dead-center, and it included some areas with rougher terrain, where a landing might be more risky, according to Tom Hoffman, InSight’s project manager at NASA’s Jet Propulsion Laboratory in Pasadena, California.

Tom Hoffman, InSight’s project manager, motions to a region officials hope the lander will avoid due to rougher terrain. Credit: NASA TV/Spaceflight Now

In a press conference Sunday, Hoffman said InSight’s final course correction was expected to be “just a very small burn, it’s only a few centimeters per second.”

“It’s almost a breath of air out of your mouth,” he said. “We hope that we’re going to move about 11 miles (17 kilometers) from where we are today to that red X.”

Mission planners set aside propellant and time for InSight to conduct the trajectory correction maneuver Sunday, but officials were not sure it would be required until a meeting Sunday morning, according to Hoffman.

“If we go farther northwest than where we’re currently showing, we get into a region that we’re not as comfortable landing in, which is the reason we had a very exciting … 6 o’clock meeting this morning,” Hoffman said.

“We listened to all the different inputs, and the final decision was to go ahead and do the TCM (trajectory correction maneuver), let’s move ourselves back to that red X and be exactly where we really want to land, from both a safety standpoint, as well as making sure that we have the right location for our science instruments.”

Some InSight officials have joked that Elysium Planitia is the biggest parking lot on Mars, notable for its expanse and featureless terrain, not for the rugged geology often found at other Mars landing sites.

“Where we’re going to land is a place called Elysium Planitia, which very roughly translated means ‘heavenly plain,” Hoffman told reporters Monday. “And, indeed, it is is a very heavenly plain, and it is very plain, but it is actually perfect. It’s safe. It’s great, not only to land, it’s a great place to do the science that we want to do.”

A final opportunity to uplink commands to InSight is available to ground controllers around two hours before the spacecraft enters the Martian atmosphere at 2:47 p.m. EST (1947 GMT) Monday. Engineers may elect to send up minor adjustments to the algorithm that will guide InSight through the atmosphere to its landing site.

The lander will jettison its cruise stage, which ferried InSight from Earth to Mars following the mission’s May 5 launch aboard an Atlas 5 rocket, a few minutes before slamming into the outermost layers of the Martian atmosphere. The cruise module will burn up during its plunge toward Mars.

InSight’s heat shield will take the brunt of the force to slow InSight from its initial entry velocity of 12,300 mph (5.5 kilometers per second). Cocooned in a flying saucer-shaped shell, the lander will slow down using aerodynamic friction, building up temperatures on the outer skin of the craft’s heat shield as high as 2,700 degrees Fahrenheit (1,500 degrees Celsius).

Artist’s illustration of the InSight spacecraft descending under parachute. Credit: NASA/JPL-Caltech

A supersonic parachute will then unfurl — shot out of the top of the entry vehicle like a mortar shell — at a velocity of around 861 mph (385 meters per second), and at an altitude of approximately 36,400 feet (11,100 meters) above ground level.

Fifteen seconds later, InSight will release its heat shield and extend its three landing legs, each fitted with a trigger sensor to detect touchdown.

But InSight’s parachute is not enough to slow the spacecraft enough for a soft landing. The Martian atmosphere — less than one percent the thickness of Earth’s — is far too thin to allow landers to safely reach the surface using parachutes alone.

InSight will next activate a landing radar to measure the spacecraft’s altitude and descent rate, and after about two minutes under the parachute, the probe will release from its backshell at a speed of around 134 mph (60 meters per second) and an altitude of approximately 3,600 feet (1,100 meters).

One second after backshell separation, the lander will begin firing 12 braking rockets to slow its descent velocity and null out horizontal movements. The spacecraft will also rotate to ensure it is in the correct orientation for touchdown, with its solar arrays extending east and west from the deck, and the robotic arm’s work area on the south side of the lander.

Around 164 feet (50 meters) above the surface, InSight will transition to a constant velocity mode. Around 15 seconds later, InSight will reach the surface at a velocity of around 5 mph (2.24 meters per second), and immediately switch off its liquid-fueled thrusters.

Once on the surface, InSight will unfurl two fan-like solar arrays to begin generating power. Cameras on the stationary lander, built by Lockheed Martin, will survey its surroundings before engineers beam up commands for InSight to use its robotic arm to place a pair of science instruments on the nearby surface.

One of the sensors is a French-built seismometer encased in a vacuum enclosure and a wind shield. It will detect tremors — marsquakes — caused by geologic activity deep inside the Martian crust.

Another instrument, developed in Germany, will hammer up to 16 feet (5 meters) below the Martian surface and measure the heat coming from the planet’s interior.

The instrument deployments could take two or three months to complete as scientists and engineers on Earth methodically plan the sequence based on the conditions at InSight’s unexplored landing site.

“It’s taken more than a decade to bring InSight from a concept to a spacecraft approaching Mars — and even longer since I was first inspired to try to undertake this kind of mission,” said Bruce Banerdt of JPL, InSight’s principal investigator. “But even after landing, we’ll need to be patient for the science to begin.”

The lander will stand around three feet (90 centimeters) tall on the Martian surface, with some uncertainty because InSight’s legs could compress into the soil after touchdown. With its solar panels unfolded, the lander will span around 19.7 feet (6 meters) wide.

Data collected by InSight should reveal information about Mars’s internal structure, including the size and make-up of its core. Scientists hope InSight’s discoveries will tell them about how rocky planets like Mars and Earth formed.

Mars has a geologic record dating back billions of years, a record that has been largely erased on Earth by plate tectonics and vigorous geologic activity that refreshes the crust.

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Follow Stephen Clark on Twitter: @StephenClark1.


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