jholt%Nosc@usiiden.UUCP (05/05/85)
From: <crash!usiiden!jholt@Nosc>
[forwarded from the InterComEx BBS (303) 367-1935 by joe holt -
thanx as always to Mark Felton, one of the sysops of this board
and the one responsible for these articles]
From: _Project Galileo: A Return to Jupiter_
[it should be noted that this is a NASA publication]
The great Italian astronomer Galileo Galilei shocked the world in
January 1610 when he announced the discovery of four satellites
oribiting Jupiter. His discovery provided proof to Nicolaus
Copernicus' theory that Earth and the other planets orbit the
Sun, and Earth is not the center of the universe. Galileo told
the story of his discovery:
"On the seventh day of January in the present year 1610, at
the first hour of the night, when I was viewing the heavenly
bodies with a telescope, Jupiter presented itself to me; and
because I had prepared a very excellent instrument for
myself, I perceived (as I had not before, on account of the
weakness of my previous instrument) that beside the planet
there were three starlets, small indeed, but very bright."
Galileo thought those "starlets" were just more of the fixed
stars that his telescope was allowing him to discover with
astounding regularity. But the next night he saw they had changed
position. The night after that was cloudy. Then on January 10, he
saw only two "starlets", the third having disappeared behind
Jupiter.
On the 11th:
"I had now decided beyond all question that there existed in
the heavens three stars wandering about Jupiter as do Mercury
and Venus about the Sun, and this became plainer than
daylight from observations on similar occasions that
followed."
On January 13, 1610, Galileo spotted the fourth satellite.
Although he nearly paid for his observatons and later writing
with his life, Galileo remained the most respected scientist of
his time. Today, those four satellites - Io, Europa, Ganymede,
and Callisto are called the Galilean satellites in his honor.
A NASA project to orbit Jupiter and send an instrumented Probe
into the giant planet's atmosphere is under way at the Jet
Propulsion Laboratory. The mission, called Project Galileo after
the Italian astronomer, will begin an in depth exploration of the
Jovian system: Jupiter, the Galilean satellites, and the giant,
invisible magnetosphere surrounding Jupiter. Scientists believe
that Jupiter is made of the original material from which stars
form, largely unmodified by nuclear processes. Close range
studies of Jupiter should provide important information about the
beginning and development of our solar system and provide new
insights into phenomena that directly relate to our understanding
of all the planets.
Project Galileo was originally scheduled for launch in early 1982
as the scientific successor to the Voyager mission to Jupiter.
The Galileo probe was designed to be attatched to the Orbiter,
and the combination spacecraft was scheduled to be launched from
an Earth orbiting Space Shuttle. The relative position of Earth,
Mars and Jupiter at that time makes it possible to send a heavy
spacecraft to Jupiter via Mars in a relatively short time.
Problems in the Space Shuttle development, however delayed the
Galileo launch until recently. But now two spacecrafts will make
the trip; an Orbiter spacecraft and an instrument Probe flying
aboard a Probe Carrier spacecraft.
The Orbiter will fly within a few hundred kilometers of the
surface of Mars. The Orbiter will use Mars' gravity and a long
burn of its own rocket motor to boost it the rest of the way to
Jupiter.
When the Orbiter arrives at Jupiter about one year before the
probe, it will photograph the region where the Probe will never
enter to ensure achievement of the original mission goals. As the
Orbiter reaches its closest approach to Jupiter, it will fire its
retrorocket engine for about 50 minutes to slow the spacecraft
and permit capture by the planet. Within a few hours of closest
approach to Jupiter, the Orbiter will fly past the volcanic
satellite, Io, for close scientific observations. Io's gravity
will further slow the spacecraft.
At that point the spacecraft will be orbiting Jupiter in an
elliptical path, ranging from more than 15 million Km (9.3
million mi) to 285,000 Km (178,000 mi) above Jupiter's cloud
tops. Thereafter, the orbit will change through a series of
elliptical paths to take the spacecraft to all regions of
Jupiter's environment. That will be accomplished by using the
gravity of the satellites to bend the orbit each time the
spacecraft comes close to one of them. Eventually the orbit will
be so altered that the spacecraft's closest approach to Jupiter
will be 900,000 Km (560,000 mi) above Jupiter's cloud tops.
During at least one orbit, the spacecraft will fly through and
study Jupiter's magnetotail - the portion of the magnetic region
directly opposite the Sun - to a distance of 150 times the radius
of Jupiter, more than 10 million Km (6.2 million mi) from the
planet. Observations of the magnetotail are not possible from
Earth or with flyby spacecraft because the spacecraft pass close
to Jupiter, and their trajectories are too strongly deflected to
reach that region.
The Orbiter will complete 11 orbits of Jupiter while making a
close flyby of one Galilean satellite - Io, Europa, Ganymede or
Callisto - on each orbit. The Orbiter, carrying 11 scientific
instruments and weighing 2660 Kg (5864 lb) at launch, will
transmit scientific and engineering data at rates up to 115K
bits per second.
Meanwhile, the Probe will be launched one month after the
Orbiter, in March 1984, and will be transported to Jupiter on a
special Probe Carrier spacecraft. Traveling on a long trajectory
that does not pass Mars, the Probe and its Carrier will reach
Jupiter one year later than the Orbiter in the summer of 1987.
After being released from the Probe-Carrier spacecraft, the Probe
will descend toward Jupiter's thick atmosphere.
Scientists want the instrument-laden Probe to enter Jupiter's
light colored Equatorial Zone, between 1 and 5.5 degrees north or
south latitude. They believe the topmost clouds of that portion
of Jupiter's atmosphere consist primarily of ammonia. By entering
at that location, the Probe should be able to measure Jupiter's
important cloud layers.
As the Probe strikes the upper layers of Jupiter's atmosphere, it
will slow so rapidly that it will feel the effects of 400 times
Earth's gravity. Once the strongest deceleration forces have
passed, the Probe will deploy a parachute. The decent module will
begin to take atmospheric measurements and transmit its findings
to the Probe Carrier spacecraft for relay back to Earth. 40
minutes after entry, scientists expect the Probe to reach an
atmospheric density of about 10 bars (10 X the atmospheric
pressure at Earth's surface), below what are believed to be
Jupiter's lowest water clouds.
At the end of 60 minutes, the Probe will have penetrated 15 to 20
Earth atmospheres. Below that, increasing temperature and
pressure and weakening radio signals will eventually bring the
Probe mission to an end.
Meanwhile, the Probe Carrier spacecraft will monitor signals from
the Probe, pricking up scientific information and relaying it to
Earth. The data also will be recorded on the Probe Carrier for
later playback if needed.
Once the Probe's work is done, the mission operations emphasis
will revert to the Orbiter. The primary mission is scheduled to
end about 20 months after Probe arrival at Jupiter.
The Galileo Orbiter will incorporate a new dual spin design. Part
of the spacecraft will be three axis stabilized so the camera and
some other instruments can be accurately and steadily pointed.
The other portion will spin so its instruments can sweep out
space to make their measurements. Since Jupiter is too far from
the Sun for solar cells, the Orbiter will use radioisotope
thermoelectric generators similar to those flown on the two
Voyager spacecrafts.
Jupiter is vastly different from Earth, Mercury, Venus and Mars.
While these terrestrial planets are mostly rock, Jupiter's major
constituents are hydrogen and helium, in about the same ratio as
the Sun.
Jupiter is the noisiest source of radio signals, except the Sun,
in our sky. Its magnetic field - the largest in our solar system
- would reach from Earth to Venus. Jupiter may change gradually
from a gaseous hydrogen helium to a liquid metalic hydrogen. The
tops of the clouds - all that can be seen of the planet - are
wracked by huge storms that appear to well up from deep within
Jupiter's interior.
The four Galilean satellites differ from each other in much the
same way as the planets differ with distance from the Sun. Io has
been subjected to a gravitational tug of war that has resulted in
at least eight large, active volcanoes; Europa appears to be
rocky with an ice crust. Ganymede and Callisto, while different
from each other in significant ways, both consist mostly of
water.
The Orbiter is designed to:
Inspect the surfaces of the satellites (the camera may see
details as small as 30-100 meters across) to gain information
about their composition, present state, and geological
history.
Make comprehensive observations of Jupiter's weather.
Study the magnetosphere- its size and shape and how it
changes, how particles enter and leave it, and how Jupiter's
satellites affect it.
The Probe is designed to:
Determine the temperature, pressure, density, and composition
of the various levels of Jupiter's atmosphere down to a level
at which pressure is about 10 X that at sea level on Earth,
perhaps 129 Km (80 mi) below the cloud tops.
Measure and compare the flows of energy through the
atmosphere, inward from the Sun and outward from Jupiter's
interior.
Even before the time of Galileo, people have been interested in
Jupiter, our largest planet. More than 475 scientists - including
90 from 10 foreign countries - submitted proposals in Project
Galileo; 115 scientists were selected to form the Galileo science
team. Project Galileo will be the U.S.' fifth mission to Jupiter;
predecessors include Pioneer 10 & 11 and Voyager 1 & 2.
The Jet Propulsion Laboratory is the overall management center
for Project Galileo. The Orbiter's rocket propulsion system will
be provided by the Federal Repulblic of Germany. NASA Ames
Research Center will develop the Probe and Probe Carrier
spacecraft. Radio signals from the two spacecraft will be
received on Earth by JPL's Deep Space Network.
[more current-newsish topics will be forwarded in the future]doug@escher.UUCP (Douglas J Freyburger) (05/09/85)
> The Orbiter will fly within a few hundred kilometers of the > surface of Mars. The Orbiter will use Mars' gravity and a long > burn of its own rocket motor to boost it the rest of the way to > Jupiter. This manuever and the fly-by of an asteriod near its projected flight path were still up in the air last I heard. By the way, the advantage of the burn close to Mars is that the kinetic energy difference in relation to the sun is gained by the space craft. Losing the mass inside the gravity well gains that much energy from Mars's gravitational field. I went over the equations on that several years ago before I believed it. Don't remember it exactly anymore. > The Orbiter will complete 11 orbits of Jupiter while making a > close flyby of one Galilean satellite - Io, Europa, Ganymede or > Callisto - on each orbit. The only time it will get close to Io is on the first pass. There is too much radiation that deep in, so most of the mission is being kept farther out. Still, the craft will get more than enough Rads to fry any of us. DOUG@JPL-VLSI, ...trwrb!escher!doug, etc. Douglas J Freyburger
jcp@brl-tgr.ARPA (Joe Pistritto <jcp>) (05/10/85)
Ok, I've heard lots of times about the 'gravity assist' maneuver used to sometimes dramatically increase the speed of a spacecraft by flying close to a bigger mass. Now the question is, how does this work? I understand that you would gain energy by dropping weight when inside a gravity well (and having the velocity vector pointed so that you could make it out, now lighter and requiring less energy), but is that the ONLY cause of the acceleration? I thought that this effect was also due to rotating the velocity vector of the spacecraft. I've also heard that the maneuver works best when you get closest to the object being slingshot off of. Is this true? (it would seem so) -JCP-
@S1-A.ARPA:host.MIT-MC.ARPA (05/10/85)
From: Ross Finlayson <rsf@Pescadero> The way I've always understood this is as follows: If the gravity assisting planet (Mars say) were \stationary/ (wrt. some fixed reference point), then after the gravity assist, the spacecraft's velocity (wrt. the reference point) would be unchanged, except for direction. In practice, however, the planet is \itself/ in motion (around the Sun), so some of the planet's kinetic energy (and thus velocity) is transferred to the spacecraft. That is, gravity assist works because the planet effectively "drags along" the spacecraft, not just because the planet is "sitting there". Please correct me if this is wrong. Ross.
henry@utzoo.UUCP (Henry Spencer) (05/12/85)
> Ok, I've heard lots of times about the 'gravity assist' > maneuver used to sometimes dramatically increase the speed of > a spacecraft by flying close to a bigger mass. Now the question > is, how does this work? I understand that you would gain energy > by dropping weight when inside a gravity well (and having the > velocity vector pointed so that you could make it out, now lighter > and requiring less energy), but is that the ONLY cause of the > acceleration? I thought that this effect was also due to rotating > the velocity vector of the spacecraft. There are two separate types of maneuver here. One, classically called a "gravity-well" maneuver, flies very close to a large mass so that an engine burn can be made at the lowest possible potential energy. The other, often called a "gravity-boost" maneuver, is a three-body maneuver (the Sun is usually the third body) using gravity alone to transfer some momentum from a large body to the spacecraft. The gravity-well maneuver exploits the difference between momentum and kinetic energy to give the spacecraft higher final velocity. How much velocity you gain from an engine burn is a matter of momentum; how much you gain or lose to a gravity field is a matter of energy. Since energy is proportional to the *square* of velocity, and leaving a gravity field subtracts a fixed amount of energy, the higher the velocity with which you start to leave a gravity field, the less velocity you lose to gravity. So an engine burn's effect is magnified if you fly into a handy gravity field before making it. The scales remain balanced: you carry the fuel down with you and don't bring it back up, so it loses energy. The gravity-boost maneuver exploits relative motions. With respect to Jupiter, flying past Jupiter will only rotate your velocity vector, and cannot change its magnitude. The key is the words "with respect to Jupiter". If what you care about is velocity with respect to something else, and Jupiter is moving with respect to that something else, then you can gain or lose velocity. In the limiting case, an extremely tight hyperbola whips you around an almost 180-degree turn with respect to Jupiter. You leave heading almost exactly back along your approach path. So Jupiter-relative Vdepart = -Vapproach. If your approach was, say, along Jupiter's orbit in the reverse direction, then viewed from the Sun you were moving at Vapproach-Vjupiter on the way in, and on the way out you're at Vdepart-Vjupiter = -Vapproach-Vjupiter = -(Vapproach+Vjupiter). So your Sun-relative velocity vector has been flipped 180 degrees (hence the minus sign), but has also had 2*Vjupiter added to it. There is nothing magic about it, since Jupiter has lost the same amount of Sun-relative momentum you've gained. But considering the relative masses, a given amount of momentum matters a lot more to you than to Jupiter! It is, of course, possible to combine the two effects. > I've also heard that the maneuver works best when you get > closest to the object being slingshot off of. Is this true? > (it would seem so) Both types of maneuver work best if you get as close as you can. The gravity-well maneuver works best if you convert as much potential energy as possible into kinetic energy first. The gravity-boost maneuver's results depend on how far your velocity vector rotates with respect to the body you're flying past; the closer the approach, the sharper the turn. Of course, you can only fly so close without hitting something. There was some interest in arranging Voyager 2's Neptune flyby to send it on to Pluto, but somebody calculated the approach distance and it came out to be several hundred kilometers *below* the cloud tops... -- Henry Spencer @ U of Toronto Zoology {allegra,ihnp4,linus,decvax}!utzoo!henry