Apollo Algorithm Perseveres Yet Again to Guide Landing on Mars

For the second time since 2012, NASA’s Johnson Space Center has played an important role in landing a rover on Mars.

The Apollo program entry guidance procedures that once steered astronauts safely down to Earth following Moon missions were successfully used on NASA’s Mars Science Laboratory (MSL) mission, enabling the Curiosity rover to reach the Gale Crater landing site with unmatched precision. It did so again aboard NASA’s Mars 2020 Perseverance rover on Feb. 18, 2021.

Mars 2020 marks NASA’s eighth successful Mars landing since the first of the twin Viking missions settled onto the planet in 1976. With each touchdown, the accuracy of the targeting has improved. Though most of Perseverance’s spacecraft and rover design mimics Curiosity, Mars 2020 uses several new techniques to refine the landing point. One is a Range-Trigger, which uses downrange-based information to control the timing of parachute opening. The second is Terrain Relative Navigation that employs a Lander Vision System (LVS). With LVS, Perseverance takes and processes images of the ground during descent, allowing the spacecraft to evaluate safe landing areas and steer toward them — much like Neil Armstrong did during the exciting Apollo 11 touchdown on the Moon.

Success in Simplicity
While NASA makes it look deceptively easy, landing spacecraft on Mars poses considerable challenges. Since 1971, only the United States has found success thus far — experiencing the loss of one lander in the last 30 years. Prior to the failure of the 1999 Polar Lander mission, NASA began to prepare for a subsequent mission, initially called the “01 Lander.” Mars mission planners began to look for a guidance solution to improve landing accuracies. That search led to a “fly-off” competition among five guidance candidates.

Two were developed by Johnson: one by the Engineering Directorate for shuttle landings, and another by the Flight Operations Directorate and the Draper Laboratory of Cambridge, Massachusetts, for Apollo.

Claude Graves, who died in 2006 and was considered the father of shuttle entry guidance within Johnson, was among those involved in the competition. Through his experiences with Apollo and his collaborations with others throughout NASA, Graves ensured the lunar mission entry algorithm was among those competing in simulations.

The Apollo algorithm, championed by Graves and implemented by Johnson’s Gilbert Carman, had just 300 lines of code and was consistently in the top one or two performers in almost every category. It was doing as well and, sometimes, better than others that had 10,000 lines of code.

Though “01 Lander” planning was suspended after the Polar Lander loss, already-developed “01” hardware was subsequently resurrected as the Phoenix Mars Lander, which successfully touched down on Martian soil in 2008.

Good Then, Still Great Now
Echoes of Apollo — specifically with the Apollo-derived algorithm — continued in 2000 as conceptual work for MSL resumed.

Precision landing using Apollo guidance code in flight software was proven with Curiosity’s touchdown, for which the algorithm predicted the actual landing point to about two kilometers — much more accurate than previous Mars ballistic-type entries. Perseverance would replicate the same approach, but in the midst of a very strange Earthly obstacle: COVID-19.

As if landing on Mars wasn’t hard enough, the Johnson team overcame the added complexity of planning and executing mission operations largely in a remote environment during the pandemic. Normally, all pre-landing simulations would be carried out on-site at the Jet Propulsion Laboratory (JPL). Instead, due to social distancing and other safety restrictions, Flight Dynamics and Entry Guidance teammates Lynn McGrew and Brian Killeen supported many of those tests either from home or Johnson’s Mission Control Center backroom areas. 

“The entire team deserves praise for succeeding in an immensely difficult mission in the face of many forms of adversity,” Killeen noted during his tenure on the mission. “Remote support from launch to landing required technical competence, communication skills and drive … and a lot of web meeting patience.”

As Perseverance closed in on the Red Planet, McGrew and Killeen were seated in a mission control support room for the much-publicized “Seven Minutes of Terror” that described the harrowing and complex sequence of events occurring as the spacecraft traveled down to the surface.

The drama began as the spacecraft reached the top of the Martian atmosphere while traveling at 13,200 mph, and only ended when the sky crane lowered the rover to the region known as Jezero Crater. Jubilation reverberated from that control room — and beyond.

Due to the strict social-distancing protocols in place, the team was spread out across several mission-support areas and could not celebrate as closely as previous missions had done, but that did not dampen the excitement felt by all involved.

“The rover’s name, ‘Perseverance,’ couldn’t have been more appropriate,” McGrew said. “It embodies the attitude we’ve held to ensure this endeavor is a success.” 

The Apollo heritage guided-entry algorithm activated six minutes before touchdown. For nearly three minutes, the software raised and lowered the rover’s altitude and steered through three sweeping bank reversals, with small thruster firings to keep it on course.

“It was amazing to watch, learn from, and work alongside some of the brightest minds in space exploration,” Killeen said. “To watch our hard effort translate into putting a rover on Mars was indescribable!”

McGrew added, “No matter how many times I’ve seen EDL [Entry, Descent, and Landing] happen on Mars, it never ceases to be a nail-biting, thrilling, and proud experience. It’s a ride I’m happy to go on again and again.”

From left: Lynn McGrew, Gavin Mendeck (via Microsoft Teams on the laptop), and Brian Killeen. Image courtesy of Lynn McGrew.

Meet the Team

Lynn McGrew
Guidance and Flight Dynamics Engineer

A 17-year veteran of Mars-lander missions, McGrew first worked as a trajectory analyst on the Spirit and Opportunity rovers and the Phoenix lander while at JPL, and then came to work at Johnson in 2008 (first as an International Space Station flight controller, and later transitioning to work the MSL mission in 2010 on Entry Guidance). After Curiosity’s successful landing in 2012, McGrew then assumed the Entry Guidance lead for the Mars 2020 rover mission in 2013, where she is responsible for the analysis and testing of the Apollo guidance algorithm in Mars entry simulations, as well as the overall guidance performance and operations support of the vehicle.

Brian Killeen
Guidance and Trajectory Analyst

Killeen joined NASA full-time in 2014 as an International Space Station flight controller. In 2018, he transitioned to the Engineering Directorate as a trajectory analyst, where he began his role on Mars 2020 and Artemis I. On the Perseverance team, he is responsible for generation of onboard guidance parameters, analysis of guidance performance, and operations support of the vehicle.

Gavin Mendeck
Flight Dynamics Engineer

Mendeck started at Johnson in 2000, focusing on EDL throughout most of his career. He worked Curiosity’s entry guidance from concept to postflight, spanning 13 years. Mendeck has also supported Orion Guidance, Navigation, and Control (GNC) and systems engineering; the Columbia accident investigation; public entry risk assessment for shuttle; uncrewed sample returns; Commercial Crew insight for GNC; aerosciences; and parachutes. He leads the flight software team for SPLICE, a multi-center NASA project advancing the state-of-the-art in precision safe landings for future missions.