An international team of researchers studying seismic data collected by NASA's Insight spacecraft has used the data to calculate the size of Mars' core. The group plans to discuss their findings at this year's Lunar and Planetary Science Conference, which will be held virtually due to the pandemic. As a prelude to the conference, team member Simon Stähler has made available a prerecorded presentation for those interested. The team intends to submit their results to a peer-reviewed journal in the near future.

Up until now, the only celestial bodies that have had their cores measured were Earth and the moon. To make such measurements, scientists have used seismic data from sensors that detect the sounds made by quakes. Eager to do the same for Mars, NASA sent Insight to the red planet—it landed near the planet's equator back in 2018 and began listening for Marsquakes soon thereafter. To date, sensors aboard the craft have captured seismic data for approximately 500 quakes.

You can spot "GEOSat' satellites in far-flung orbits… if you know exactly where and when to look.

Watch the sky long enough, and you're bound to see one.

Seasoned observers are familiar with seeing satellites in low Earth orbit, as these modern artificial sky apparitions lit by sunlight grace the dawn or dusk sky. Occasionally, you might even see a flare from a passing satellite, as a reflective solar panel catches the last rays of sunlight passing overhead.

But look closely along either side of the celestial equator (the imaginary line that the Earth's equator traces on the sky) at certain times of the year, and you might just see the ghostly flare of a distant GEOSat (geosynchronous satellite) as it briefly brightens into visibility and fades away.

Right around equinox in March or September is a good time to try and spy satellites in GEO as they reach near 100% illumination opposite to the sun, before entering the Earth's shadow and winking out.