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QUESTION:
How do they keep time in space?
ANSWER from Charles Whetsel on February 20, 1998:
With clocks, of course! Okay, well they're not exactly like the clocks on Earth (with minute hands and hour hands),
but they are very similar to the clocks inside computers that keep track of time. It is very important for all
of the robotic spacecraft that we send into space to know what time it is. That way we can tell them "At 11:30,
you'll be flying over a really neat canyon. Take a picture" or "At 9:15, the sun will come out, so you
should have plenty of electrical power to turn on the radio." Another factor in all of this is that during
the months when Mars is the furthest away from the Earth, the messages that we send to the spacecraft via radio
(travelling at the speed of light) take almost 20 minutes to get from earth to Mars. When the spacecraft sees the
message, and generates the acknowledgement message ("I got it"), it takes another 20 minutes for that
message to make it back to us controllers here on earth. This is what we call a "40 minute round-trip light-time
(RTLT)." This makes it very important to have a clock on the spacecraft for coordinating events, because if
we waited until 11:30 to tell the spacecraft to take a picture of the canyon "now", the message wouldn't
even get to the spacecraft until 11:50 when the canyon was long gone.
ANSWER from Rich Hogen on February 21, 1998: With clocks, of course! But if you think the answer ends there, think
again. There are three major pieces of the "time puzzle."
Puzzle Piece Number One: the spacecraft clock keeps ticking ------------------------------------------------------------
Spacecraft function like remote-control robots. For many or most actions performed by the spacecraft there are
humans on the ground designing,
building and transmitting commands for those actions. The spacecraft does not stop for nighttime, weekends, holidays
or any other calendar events. We
humans require food and sleep, so we have to rotate daily shifts to keep up with the spacecraft, but the question
of time goes further than scheduling!
Now we have to think about time zones and antenna pointing.
Puzzle Piece Number Two: no single place on Earth sees the spacecraft all the time ------------------------------------------------------------
The Earth is orbiting the sun, but it's also spinning. That's why the Sun, Moon, galaxies, nebulae, comets and
stars move across the sky in a day. If
the Earth were not rotating, the sun would take a year to move across the sky. At noon the Sun is directly overhead,
but it's only noon at one longitude at a time. Places east or west of your location on Earth see the sun at a different
position in the sky, so it's a different time of day there. If it's noon where you are, it's midnight on the opposite
side of the Earth. That's why there are "time zones", like "Pacific Standard Time" in California
and "Eastern Standard Time" (three hours later) in New York, and other time zones around the planet.
Other planets, like Mars, move slowly through the field of stars in our sky. When we send spacecraft to other planets
we have to use antennas spaced evenly around Earth if we want at least one of our antennas at all times to "see"
the spacecraft. But wait! That means the people operating the antennas are working at a different LOCAL TIME OF
DAY from each other. When it's noon in the western U.S.A. it's something like 4:00AM at the Deep Space Network
antennas in Australia, and it's about 8:00PM at the DSN antennas in Spain! How can we keep it straight?
Puzzle Piece Number Three: Greenwich Mean Time (GMT) or Universal Time (UT) -----------------------------------------------------------
Okay, so how do all these people on the spinning Earth and in different time zones effectively schedule and communicate
about timed events on the spacecraft? Fortunately the world has set a standard "day" that is determined
at the longitude of the Greenwich (pronounced gren-itch) Observatory in England. When it's noon in Greenwich (or
at any other location on that same longitude line -- look at a globe to find them) then it's 1200 hours GMT or
UT. Everyone on the planet involved in working with spacecraft uses UT time as a convention, and so do the spacecraft.
The space-tolerant "digital watch" on the spacecraft is set for UT time and everyone on Earth working
on the spacecraft's mission also uses UT, so all of our watches are synchronized. But wait! There's yet another
missing piece of the puzzle!
Puzzle Piece Number Four: The Speed Limit of Radiation -----------------------------------------------------------
We communicate with the spacecraft by sending and receiving electromagnetic radiation that is specially modulated
to carry information. Light is electromagnetic radiation. The speed of light is the speed limit for all electromagnetic
radiation. It takes about eight minutes for the light from the Sun to reach the Earth, because of the distance
between the Earth and the Sun. So when a spacecraft is very far from Earth it can take minutes or hours for the
electromagnetic radiation to travel between Earth and the spacecraft. Right now the "One-Way Light Time"
(OWLT) for signals traveling between Mars
Global Surveyor and Earth is about forty minutes. So when something happens on the spacecraft, we don't even find
out about it until forty minutes after
the fact! We have to plan ahead. If the spacecraft is scheduled to turn away from Earth for a propulsive manuever
at "time X", we have to send commands more than one-way light time BEFORE that, no later than "time
X minus OWLT". And then we only see the effect of the turn one-way light time AFTER it actually starts to
happen on the spacecraft. You get the idea. Scheduling and timing can become very complicated. So, it all starts
with clocks but gets complicated by the speed of light, the spinning, moving Earth, human requirements and the
spacecraft mission activities themselves.
I hope this helps answer your question.
Rich Hogen
Lockheed Martin Astronautics
Mars Global Surveyor Operations
QUESTION:
Is there sound in space? For example, if there is an explosion in space, could it be heard elsewhere? Is there
enough gas in space to transmit sound?
ANSWER from Bob Haberle on November 7, 1997:
While there are molecules in space, they are separated by such vast distances that they rarely hit each other.
Collisions between molecules is required for the transmission of sound waves. No collisions - no transmission.
Light waves, however, and radiation in general, can propagate in space. That is why we can see the stars and planets.
But if they make noise, we wouldn't be able to hear it.
QUESTION:
Why does NASA work so hard at descovering new things in the universe and very little on earth?
ANSWER from Donna Shirley on October 20, 1997:
NASA does a lot of work on the earth. In fact, NASA spends twice as much on Mission to Planet Earth as it does
on space science (non-earth science). You
can see some of JPL's earth work at http://www.jpl.nasa.gov/earth/ or look at NASA MTPE home page at http://www.hq.nasa.gov/office/mtpe/
QUESTION:
From prior questions you have answered it sounds as though DSN tracking works on a sort of "time share"
basis (3 contacts/day, 3 contacts/week, etc). Is this because DSN tracks many other missions also?
ANSWER from Leif Harcke:
For the first 30 days of the mission, the Deep Space Network will track Mars Pathfinder 24 hours per day, 7 days
per week. After January 4, 1997, that
tracking time will be reduced to 3 passes per day, 3 days per week as you noted. The Deep Space Network has one
antenna at each of 3 complexes (Goldstone, California; Canberra, Australia; Madrid, Spain) that is capable of transmitting
at the 7 gigahertz radio frequency that commands Mars Pathfinder. Two other projects, Mars Global Surveyor and
Near Earth Asteroid Rendezvous also require the use of this antenna for commanding their spacecraft. Scheduling
conflicts are not a problem currently, as the three missions are at very different places in the sky. Conflicts
will increase as the Mars Pathfinder and Mars Global Surveyor orbits converge on Mars in the summer of 1997.
--Leif Harcke, Telecommunications Systems Analyst
QUESTION:
While watching pre-launch pictures, technicians appear dressed in isolation
suits. Why?
ANSWER from Rob Manning:
Technicians and engineers that work in the vicinity of the lander must wear what we call "bunny" suits
(it was a joke name originally, but many years ago the name caught on). These are clean head-to-toe garments that
prevent dirt and biological contamination of the lander by the workers. At other times when the hydrazine fuel
was being loaded into Pathfinder's fuel tanks, some workers had to wear "SCAPE" suits. These suits are
also head-to-toe, but they also provide self-contained breathing equipment which is strapped to their backs. In
fact they look a lot like space suits. Hydrazine, in addition to being highly flammable, is an extremely caustic
and dangerous liquid. These suits are designed to protect the workers in the unlikely event of a hydrazine leak.
--Rob Manning
QUESTION:
What criteria is used to choose names for the space crafts?
ANSWER from John Moreau on July 24, 1997:
Traditionally, spacecraft have been named for famous explorers and discoverers. But frequently names reflect some
other significant contribution to history. Certainly though, spacecraft names have tended to be biased towards
famous male personages. Therefore it was the right time for the Mars Pathfinder Rover to be named after a famous
woman. The name Sojourner was picked from a long list of suggested names sent in by schoolchildren around the country.
The "backup" rover, which is at JPL in the Mars test bed is named Marie Currie, after a famous female
scientist.
QUESTION:
Is there any noticable favorable change on Capital Hill for NASA money due to the Pathfinder's success?
ANSWER from Mark Adler on July 21, 1997:
It's very hard to tell what is cause or effect on Capitol Hill, but NASA's budget does not appear to be facing
any significant challenges as it is currently going through approval.
Mark Adler
QUESTION:
How do I see some of the other missions done in the past at NASA or other information about NASA?
ANSWER from Smart Filter on July 17, 1997:
Try this URL:
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-spacecraft.html#voyager Most of the page is a photo gallery,
but at the bottom are links to sites
with information about past (and one future) NASA missions. There is also a wealth of general information on the
NASA homepage at: http://www.gsfc.nasa.gov/NASA_homepage.html Just click on the box that interests you, and enjoy
learning all kinds of new stuff!
QUESTION:
By knowing how long it takes for signals to go to different locations in space, how can this help with the study
of the Solar System?
ANSWER from Steve Wall on January 31, 1997:
That's a really good question. When you're curious about the way something works, the first thing to think about
is what ways you have of finding out
about it - what you can see, or smell, or measure that has to do with it. We use the time it takes for signals
to travel in several different ways. Most directly, the time tells us how far away objects are. If the signals
come from an object (like the light or radio signals from stars, or the radio signals from our spacecraft), we
also can use the way that signal changes to tell us what direction it comes from. We can tell how far away something
is because we know how fast signals travel and we can use the time it takes to tell us the distance. We can tell
the direction by listening to the pitch, or frequency, of the signal to tell if the object
is coming closer or going farther away. Once we know how far away things are and where they are, we can start to
think about how the positions of things could tell us what they are made of and where they might have come from.
We have been watching the way objects move in the sky for a long time now, and we can even predict where they will
move in the future most of the time. That's how we know when eclipses will happen, when we will see a comet, and
even when tides will come. The more of how things move we understand, the more we can start to guess what forces
make them move, and what has made them become the way they are.
Steve Wall
QUESTION:
Which material will the Mars spaceship be made of?
ANSWER from Mark Adler on June 7, 1997:
In general most of the structure of spacecraft and spaceships is made of aluminum and/or carbon composites. This
is to make them as light as possible. Bicycles also need to be as light as possible, which is why you will find
the better bicycle frames also made from aluminum or carbon composites.
Other special materials are also used on spacecraft in certain places. For example, where they may get very hot
from entry or from burning rocket fuel there are materials that can take very great heat and protect the underlying
surface.
mark
QUESTION:
What is the space suit made of?
ANSWER from the Internet:
(http://www.jsc.nasa.gov/pao/factsheets/wardrobe.html) The suit comprises several layers including a polyurethane-coated
nylon pressure bladder, a polyester structural restraint layer with folded and pleated joints (for mobility), and
a woven Kevlar, Teflon, and Dacron anti-abrasion outer layer.
The EMU comprises the spacesuit assembly, the primary life support system (PLSS), the display and control module,
and several other crew items designed for spacewalks and emergency life support. The EMU accommodates a variety
of interchangeable systems that interconnect easily and securely in singlehanded operation for either normal or
emergency use. When preparing to work in space, the astronaut goes into the airlock of the space shuttle orbiter
and puts on the following parts of the EMU:
* A urine-collection device that receives and stores urine for transfer later to the orbiter waste management system.
* A liquid cooling and ventilation garment, a one-piece mesh suit made of spandex, zippered for front entry,
and weighing 6.5 pounds dry. The garment
has water-cooling tubes running through it to keep the wearer comfortable during active work periods.
* An in-suit drink bag containing 21 ounces of potable water, the "Snoopy Cap," or communications carrier
assembly, with headphones and microphones
for two-way communications and caution-and-warning tones, and a biomedical instrumentation subsystem.
To put on the spacesuit, the astronaut first dons the lower torso assembly and then rises into the top section
of the two-piece EMU spacesuit hanging
on the wall of the airlock. The upper torso of the spacesuit is a hard-shell fiberglass structure that contains
the primary life support system and the
display control module. Connections between the two parts must be aligned to enable circulation of water and gas
into the liquid cooling ventilation
garment and return. Then, the gloves are added and last to be donned is the extravehicular visor and helmet assembly,
which provides protection from
micrometeoroids and from solar ultraviolet and infrared radiation. Bearings in the shoulder, arm, wrist, and waist
joints allow the crewmember freedom
of movement. Bending, leaning, and twisting motions of the torso can all be done with relative ease. All fabric-to-hardware
connections are made with either mechanical joints or adhesive bonding. Materials used in the construction of the
suit are selected to prevent fungus or bacteria growth; however, the suit must be
cleaned and dried after flight use. The entire suit assembly is rated with a minimum 8-year life expectancy. The
nominal operating atmospheric pressure in the suit is 4.3 psid.
QUESTION:
Why do they call the suit you put on before you enter the cleanroom a "Bunnysuit"?
ANSWER from Smartfilter on February 2, 1997:
Because it makes you look like a bunny all dressed in white from our head coverings to our shoe coverings.
QUESTION:
Is there a planet called the X planet?
ANSWER from Jack Farmer on November 20, 1996:
Planet X was first suggested on the basis of anomalies in the orbits of the outer planets (Neptune, I think), but
the interest has since died down as the anomalies have been shown to be measurement errors. As far as I know there
is no evidence for a planet X.
QUESTION:
Was there ever life on Venus? If so, how did it die out.
ANSWER from Jack Farmer on November 20, 1996:
There is no evidence of an ancient rock record on Venus today. In fact, the surface is quite young and has been
completely resurfaced by volcanic eruptions. But the atmosphere of Venus bears clues of an earlier water-rich environment
which may have made life possible. The evidence is in the ratios of hydrogen isotopes (isotopes are different species
of the same element that differ only in the number of neutrons in the nucleus). This affects the atomic mass. The
hydrogen in Venus' atmosphere is enriched in the heavy isotope of hydrogen. This suggests that a whole lot of hydrogen
and water were lost to space, and further implies there was once water there! Since liquid water is the key to
life, it is possible that there was once life on Venus. If life did develop on Venus it probably died out because
of the increasing temperatures that happened because of the Greenhouse effect on Venus. The sun was less luminous
early in the history of the solar system and has gradually warmed up. As that happened, Venus became a hot-house
and lost its water and life, if it was ever there.
QUESTION:
How many minutes does it take to get to space?
ANSWER from Pieter Kallemeyn on March 24, 1997:
Not long. The boundary between "Air" and "Space" is generally considered to be 50 miles above
the surface of Earth. At that altitude, the effects of the Earth's atmosphere are too small to be noticed, and
anyone travelling at that height needs a spacecraft-like chamber or spacesuit. Current launch vehicles are very
fast, and can achieve that altitude in less than 10 minutes.
- Pieter Kallemeyn
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