“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.
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,
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.
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
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
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
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).
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)