Posts Tagged ‘space science’

Slice of History: Anechoic Chamber

Wednesday, June 8th, 2011

By Julie Cooper

Each month in “Slice of History” we’ll be featuring a historical photo from the JPL Archives. See more historical photos and explore the JPL Archives at

Anechoic Chamber
Anechoic Chamber — Photograph Number 383-5765Ac

This aerodynamic noise facility, also called an anechoic chamber, was used to study the noise generating mechanisms in supersonic and subsonic jets in the early 1970s. It was housed at NASA’s Jet Propulsion Laboratory in building 57 (which no longer exists) located next door to the wind tunnel that was in building 79 at the time. The large round opening in the wall is an exhaust silencer inlet. Standing next to it is Paul Massier, co-author of a technical report about this chamber. On the right is a support structure for microphones. Fiberglass wedge blocks cover the ceiling and walls, which were mostly reinforced concrete. Spaces were left open to allow for observation windows and instruments to record test data. There were also openings in the walls that allowed air to flow into the chamber to replace the air forced out during tests.

This post was written for “Historical Photo of the Month,” a blog by Julie Cooper of JPL’s Library and Archives Group.

Slice of History: Transition Pipe

Friday, March 4th, 2011

By Julie Cooper

Each month in “Slice of History” we’ll be featuring a historical photo from the JPL Archives. See more historical photos and explore the JPL Archives at

Transition Pipe
Transition Pipe — Photograph Number 327-287A

This test setup was part of an investigation in 1954 of the stability of laminar pipe flow with respect to disturbances of different frequencies and amplitudes. A disturbance generator was developed using vibrating aluminum reeds and instruments measured how a small amplitude disturbance in the air flow changed as it propagated down the 115–foot length of a 2” aluminum pipe. It appears to be located in the concrete channel that was used in the 1940s as a hydrodynamic tank with a rocket-propelled towing car (the “Hydrobomb”). At the end of the room you can see metal rungs that were used to climb down into the channel when the water was drained.

This post was written for “Historical Photo of the Month,” a blog by Julie Cooper of JPL’s Library and Archives Group.

Red, Red Moon and Other Lunar Eclipse Phenomena

Monday, December 20th, 2010

By Dr. David Diner

Total Lunar EclipseTiny airborne particles, or aerosols, can affect the appearance of the moon during a total lunar eclipse, sometimes giving it a reddish hue. Copyright Ian Sharp

A lunar eclipse occurs when the Earth is positioned between the sun and the moon. Although the Moon passes through the Earth’s shadow, the lunar disk remains partially illuminated by sunlight that is refracted and scattered by the Earth’s atmosphere.

Refraction is the bending of light that occurs when the rays pass through media of different densities (our atmosphere is more dense near the surface and less dense higher up). Scattering of sunlight by molecules of air also deflects the light into different directions, and this occurs with much greater efficiency at shorter (bluer) wavelengths, which is why the daylight sky appears blue. As we view the sun near sunrise or sunset the light traverses a longer path through the atmosphere than at midday, and when the air is relatively clear, the absence of shorter wavelengths causes the solar disk to appear orange.

Tiny airborne particles, also known as aerosols, also scatter sunlight. The relative efficiency of the scattering at different wavelengths depends on the size and composition of the particles. Pollution and dust in the lower atmosphere tends to subdue the color of the rising or setting sun, whereas fine smoke particles or tiny aerosols lofted to high altitudes during a major volcanic eruption can deepen the color to an intense shade of red.

If you were standing on the Moon’s surface during a lunar eclipse, you would see the Sun setting and rising behind the Earth, and you’d observe the refracted and scattered solar rays as they pass through the atmosphere surrounding our planet. Viewed from the Earth, these rays “fill in” the Earth’s shadow cast upon the lunar surface, imparting the Moon’s disk with a faint orange or reddish glow. Just as we sometimes observe sunrises and sunsets with different shades of orange, pink or red due to the presence of different types of aerosols, the color of the eclipsed lunar disk is also affected by the types of particles that are present in the Earth’s atmosphere at the time the eclipse occurs.

Lunar Eclipse, the Moon’s Interior, and the Holy GRAIL

Wednesday, December 15th, 2010

By Sami Asmar

Earth's moon

In addition to the awesome views they offer, lunar eclipses have always provided scientific clues about the moon’s shape, location and even surface composition. Although there will continue to be opportunities for observers to examine and reflect on fundamental concepts about the moon, such as its origin and interior structure, more modern tools are aiding these observations.

When it comes to understanding what a moon or a planet is made of remotely — short of touching it or placing seismometers on its surface or probes below the surface — classical physics comes to the rescue. By measuring the magnetic and gravitational forces that are generated on the inside and manifested on the outside of a planet or moon, we can learn volumes about the structure of its interior.

A spacecraft in the proximity of the moon can detect these forces. In the case of gravity, the mass of the moon will pull on the spacecraft due to gravitational attraction. If the spacecraft is transmitting a stable radio signal at the time, its frequency will shift by an amount exactly proportional to the forces pulling on the spacecraft.

This is how we weigh the moon and go further by measuring the detailed distribution of the densities of mountains and valleys as well as features below the moon’s surface. This collection of information is called the gravity field.

In the past, this has lead to the discovery mascons on the moon, or hidden, sub-surface concentrations of mass not obvious in images or topography. If not accounted for, mascons can complicate the navigation of future landed missions. A mission, human or robotic, attempting to land on the moon would need to have a detailed knowledge of the gravity field in order to navigate the landing process safely. If a spacecraft sensed gravitational pull higher than planned, it could jeopardize the mission.

GRAIL spacecraft

The Gravity Recovery and Interior Laboratory (GRAIL) mission, scheduled to launch in September, is comprised of twin spacecraft flying in formation with radio links between them to measure the moon’s gravity field globally. This is because a single spacecraft with a link to Earth would be obstructed when the spacecraft goes behind the moon, leaving us with no measurement for nearly half of the moon, since the moon’s far side never faces the Earth. The GRAIL technique may also reveal if the Moon has a core with a fluid layer.

So as you go out to watch the lunar eclipse on the night of Dec. 20, think about how much we’ve learned about the moon so far and what more we can learn through missions like GRAIL. Even at a close distance from Earth, the moon remains a mystery waiting be uncovered.