Drip, drop
February/March 2025
Q: It’s supposed to rain heavily all day. You decide to pass the time binging “Manifest.” You have a choice of watching via a Ka-band satellite link, Ku-band or X-band, now temporarily available through a special government program. Explain the physics of rain drops and radio waves that define the best choice.
Send a response of up to 250 words to aeropuzzler@aerospaceamerica.org. By responding, you are committing that the thoughts and words are your own and were not created with the aid of artificial intelligence. DEADLINE: 12 p.m. Eastern March 5.
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FROM THE JANUARY ISSUE:
We asked you whether the two spacecraft that carried out NASA’s Gravity Recovery and Interior Laboratory mission had to have the same mass to measure variations in lunar gravity. Here is the winning response and a note from scientist Michael Watkins, who co-conceived the concept behind GRAIL.
WINNER: They do not need to have the same mass to do their job. Each spacecraft’s response to variations in lunar gravity arises from the acceleration each spacecraft experiences in consequence of lunar gravity. The equation for the acceleration of a spacecraft due to a celestial body’s gravity does not contain the mass of the spacecraft itself because it divides out.**
Brent W. Barbee, AIAA senior member
Gaithersburg, Maryland
Brent is the lead planetary defense applications scientist at NASA’s Goddard Space Flight Center in Maryland, and an adjunct faculty member at the University of Maryland.
**The reference to “dividing out” is another way of saying that the mass of the two satellites was very small compared to the mass of the moon, so that the effect of gravity on each was essentially the same regardless of the precise mass of each. Aside from gravity, though, the satellite’s motions were also affected by non-gravitational forces like solar radiation pressure, which is sensitive to the area to mass ratio. And of course, it is generally less expensive to build two nearly identical spacecraft than two very different ones. So, in the end, the spacecraft were close in mass, but not really due to gravitational effects. — Michael Watkins