The search is back on for a spacecraft that disappeared during a landing attempt nearly six years ago. And there are hints that the probe might have been found.
Mars Polar Lander was headed for a touchdown near the planet's south pole Dec. 3, 1999. But the spacecraft never reported home.
An investigation of the mishap concluded that the most probable cause of the failure was the generation of spurious signals when the craft's legs were deployed during descent.
Those bogus signals are thought to have given a false indication that the spacecraft had actually touched down. That, in turn, resulted in a premature shutdown of Mars Polar Lander's descent engines, with the craft falling through martian sky to destruction somewhere in the south pole region.
Making matters worse, Mars Polar Lander was designed under NASA's the "faster, better, cheaper" program that aimed to build highly focused projects for relatively small sums. The probe provided no entry, descent and landing telemetry data, so there was no way to know whether the lander reached its terminal descent propulsion phase. If it did reach this landing stage, it is almost certain that early engine shutdown occurred, an investigative report concluded.
But nobody really knows what happened.
Following the loss of Mars Polar Lander, NASA and the then National Imagery and Mapping Agency (NIMA) - now the National Geospatial-Intelligence Agency -- blended research talents to search for the missing spacecraft.
A study team used high resolution imagery taken by NASA's Mars Global Surveyor (MGS) spacecraft, now in orbit around the red planet, in the hopes of spotting the lander and related gear, including a protective aeroshell, heat shield and parachute.
But doing so proved challenging, to say the least. One of the principal difficulties in finding the lander using images from the MGS is that the Mars Polar Lander is only somewhat larger -- about six and a half feet across -- than the smallest objects the orbiter's camera could see on the surface of Mars at that time.
Despite the problems, in an initial analysis, NIMA researchers reviewed and assessed features seen in several images that they believe could be indicative of the lander and its protective aeroshell.
But an alternative view presented by NASA was that these features could be noise introduced by the camera system.
New observational tool
Fast forward to 2005. Enter, once again, the Mars Global Surveyor and its Mars Orbiter Camera (MOC).
MOC is operated daily at Malin Space Science Systems (MSSS) in San Diego, California.
At the March meeting of the Lunar and Planetary Science Conference in League City, Texas, how the Mars Global Surveyor's MOC is being used to obtain far-higher resolution imagery than originally designed was reviewed.
The tactic is called "cPROTO", short for compensated Pitch and Roll Targeted Observation, noted Michael Malin and Ken Edgett of MSSS in a research paper presented at the scientific gathering.
To acquire a cPROTO image, the entire MGS spacecraft is moved in pitch and roll directions, the team reported. "The 'c'in 'cPROTO' is for planetary motion compensation. While MGS is pitching, rolling, and moving along its orbit, Mars is rotating underneath it. The pitch and roll are timed to account for the rotation of Mars, as well as the desired image resolution and target location," the research paper explains.
Whereabouts of landers
However, uncertainty in the motion of MGS can cause specific targets to be missed by miles. Several tries are often needed to hit the intended target. Furthermore, other limitations mean that only certain parts of Mars are accessible to cPROTO imaging at any specific time during the year.
In addition, PROTO images have been taken - a method using no compensation for Mars rotation. Both techniques have proven workable, with images produced not only ideal for studying martian landforms and geologic materials - they can also reveal the whereabouts of Mars landers.
The Mars Exploration Rovers - Spirit and Opportunity -- can be seen from orbit.
Other PROTO and cPROTO efforts imaged the locales of the Viking and Mars Pathfinder landing sites.
Attempts were also made to image candidate British-built Beagle 2 landing locations. That craft also failed to phone home in late 2003. A small impact crater was detected, "unusual for that part of Isidis Planitia," a feature that also had a dark dune in it, Malin and Edgett said in their paper.
Additionally, the two researchers added that cPROTO images of candidate locations for the Mars Polar Lander are scheduled to be attempted this year.
Lost and found?
Word is that the MSSS team is hot on the trail of the missing Mars Polar Lander. Insiders suggest that MGS in cPROTO mode is to be trained on a suspected Mars spot next month. But the observation will again be challenging.
Meanwhile, a "next issue" teaser in the just issued June Sky and Telescope magazine has spotlighted the look for the lost-to-Mars mission: "Hidden in Plain Sight - The Mars Orbiter Camera, using a special technique to boost image resolution, may have found NASA's missing-in-action spacecraft, Mars Polar Lander."
What really happened to Mars Polar Lander may soon, quite literally, be resolved.
This article is part of SPACE.com's weekly Mystery Monday series.
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Leonard David is an award-winning space journalist who has been reporting on space activities for more than 50 years. Currently writing as Space.com's Space Insider Columnist among his other projects, Leonard has authored numerous books on space exploration, Mars missions and more, with his latest being "Moon Rush: The New Space Race" published in 2019 by National Geographic. He also wrote "Mars: Our Future on the Red Planet" released in 2016 by National Geographic. Leonard has served as a correspondent for SpaceNews, Scientific American and Aerospace America for the AIAA. He was received many awards, including the first Ordway Award for Sustained Excellence in Spaceflight History in 2015 at the AAS Wernher von Braun Memorial Symposium. You can find out Leonard's latest project at his website and on Twitter.