WonderQuest:  On the web since 1997...      

“Brainless” sea stars have brains, nights everywhere average 12 hours, astronauts dodge rads

How does a sea star function without a brain? How does it coordinate its arm movements and sense light with the "eyespots" if there are no brain cells to process or use the information? Kevin, Penryn, California

The doughboy sea star eats corals in the Indo-Pacific Ocean. Photo courtesy of Ronald L. Shimek, Reefkeeping.com.

A sea star (starfish) manages nicely without a control center made up of white and gray matter, housed in a head — what we think of as a brain. Their entire nervous system acts like a distributed brain. So, it has a brain — just not like ours.

Clusters of nerves (ganglia) ring its center like a spider web. Nerves branch off from this central system and run radially out the arms.

The branches of these nerves coordinate arm movements. In some species and individuals, one arm almost always takes the lead when the sea star walks. More often, though, the arms take turns. This exotic creature moves by taking in seawater and channeling the water through a system of canals ringing its center. It changes water pressure to push out its feet and move.

Its eyespots, located at the ends of the arms, sense light. Changes in light intensity may, for example, cause different nerve firing rates, which can induce a sleep-like state. Some stars shun light; others seek it.

Minute clam odor traces can excite the nervous system, which then launches an attack on the prey. Some star species pry open clam shells (a slit as tiny as the diameter of a blood-cell is enough). It then injects its stomach (inside out) into the opened home to eat the hapless clam. The stomach releases digestive juices and absorbs the resulting soup.

Further Reading:

Reefkeeping: A spineless column by Ronald L. Shimek

Nights everywhere average 12 hours

How long is the average length of nights in my area? I need the information to compute the cost of security lights per year.  Ken, Paducah, Kentucky

The length of night in Paducah, Kentucky or any other place on Earth, averaged over the whole year, is simply 12 hours. This is so because Earth tilts toward the Sun during half the year and then away the other half, wherever you are. The figure won’t be exactly 12 hours because the atmosphere bends the Sun’s rays when the Sun is low. But it will be close enough for calculating the cost of security lights.

Night begins with a rising full moon.  Photo courtesy of John Bortniak, NOAA.

"The dense lower layers of the Earth’s atmosphere has the effect of making the Sun seem somewhat higher than it actually is," says Robert Massey, astronomer at the Royal Observatory, Greenwich. The effect can hasten sunrise a few minutes and delay sunsets by a few minutes.

So, night lasts just under 12 hours, averaged over the year.

Further Reading:

USA Today: Earth’s tilt by Jack Williams

USA Today: Higher appearing Sun, Why day is longer than night at the equinox by Jack Williams

U.S. Naval Observatory: Table of sunrise, sunset at any location for one year

HM Nautical Almanac Office: Sunrises and sunsets for any location for the year or any part of the year.

Astronauts dodge rads

How did astronauts in the Apollo 14 mission survive being exposed to high radiation doses in a short time? 

The astronauts who went to the Moon on Apollo 14 accumulated about 11.4 millisieverts (1,140 millirem) in their nine-day mission. A round trip to Mars will expose astronauts to 1300 mSv over two and a half years. The maximum permissible exposure per year, though, is just 50 mSv. How do the astronauts survive? Paul, Los Angeles, California

November 1969.  Al Bean walking on the moon and absorbing radiation from space. Photo courtesy of NASA Johnson.

What a good question. "Maximum permissible exposure" means different things for different people. It takes, however, the same amount of exposure to kill a person whether she’s on the ground or in space. A single dose of 4,000 mSv without treatment will do it.

The Apollo 14 astronauts received far less than the lethal single dose and therefore survived.

The annual dose limits vary — depending on what you do and what damage you’re willing to accept. The general public’s annual limit is: 4 mSv. X-ray technicians have a of limit 50 mSv, the figure mentioned in your question.

Astronauts have the highest annual dose limits. The National Council on Radiation Protection and Measurements (NCRP) sets these limits for all missions except "exploratory circumstances in space", such as a mission to Mars. According to the NCRP, astronauts can take up to 500 mSv in their blood-forming organs (such as bone marrow), 2000 mSv in eyes, and 3000 mSv in skin.

We worry about damage. Different dosages cause different damages: 1000 mSv (single dose) for radiation sickness, 2500 for female sterility, 3500 for male sterility, and 4000 mSv for death (without treatment). So, the dosage that the Apollo 14 astronauts experienced (11.4 mSv) was relatively safe.

Mars is a different proposition. To protect those spacefarers, the idea is take cover! We’re devising a network of radiation sensors in the inner solar system to warn our people of impending space storms so they can dodge the rads.

By the way, back in August 1972, the lucky astronauts of Apollo 16 and 17 narrowly escaped death. Between those two missions, one of the largest solar flares ever recorded sent hundreds of billions of electron volts of energy hurtling our way. If the flare had occurred during a mission — even inside their shielded spacecraft, the astronauts would have absorbed lethal doses in 10 hours. Instead, they were between missions on Earth. The atmosphere saved them (and us, for that matter).

Further Reading:

Windows to the Universe, University of Michigan: Astronauts and space radiation

Space.com: Mars Odyssey shows intense but manageable radiation risk

Space.com: How miniature radiation detectors will keep astronauts safe in deep space

(Answered Feb. 20, 2004; updated April 18, 2008)