To track the Red Planet’s spin rate, planetary scientists relied on the Rotation and Interior Structure Experiment (RISE) onboard NASA’s InSight lander. They found the planet’s rotation is accelerating by about 4 milliarcseconds per year — corresponding to a shortening of the length of the Martian day by a fraction of a millisecond per year.
The RISE instrument on the InSight lander is part of a long tradition of Mars landers using radio waves for science, including the twin Viking landers in the 1970s and the Pathfinder lander in the 1990s.
But none of those missions had the advantage of InSight’s advanced radio technology and upgrades to the antennas within NASA’s Deep Space Network on Earth.
Together, these enhancements provided data about five times more accurate than what was available for the Viking landers.
In the case of InSight, scientists would beam a radio signal to the lander using the Deep Space Network.
RISE would then reflect the signal back.
When scientists received the reflected signal, they would look for tiny changes in frequency caused by the Doppler shift.
Measuring the shift enabled researchers to determine how fast the planet rotates.
“What we’re looking for are variations that are just a few tens of centimeters over the course of a Martian year,” said RISE principal investigator Dr. Sebastien Le Maistre, a researcher at the Royal Observatory of Belgium.
“It takes a very long time and a lot of data to accumulate before we can even see these variations.”
Dr. Le Maistre and colleagues examined data from InSight’s first 900 Martian days — enough time to look for such variations.
They had their work cut out for them to eliminate sources of noise: water slows radio signals, so moisture in the Earth’s atmosphere can distort the signal coming back from Mars.
So can the solar wind, the electrons and protons flung into deep space from the Sun.
“It’s a historic experiment. We have spent a lot of time and energy…
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