[meteorite-list] Dawn Journal - February 28, 2013

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 1 Mar 2013 13:26:32 -0800 (PST)
Message-ID: <201303012126.r21LQWGi012526_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_02_28_13.asp

Dawn Journal
Dr. Marc Rayman
February 28, 2013

Dear Impordawnt Readers,

The indefatigable Dawn spacecraft is continuing to forge through the
main asteroid belt, gently thrusting with its ion propulsion system.
As it gradually changes its orbit around the sun, the distance to dwarf
planet Ceres slowly shrinks. The pertinacious probe will arrive there
in 2015 to explore the largest body between the sun and Neptune that
has not yet been glimpsed by a visitor from Earth. Meanwhile, Vesta,
the fascinating alien world Dawn revealed in 2011 and 2012, grows ever
more distant. The mini-planet it orbited and studied in such detail
now appears only as a pinpoint of light 15 times farther from Dawn than
the moon is from Earth.

Climbing through the solar system atop a column of blue-green xenon
ions, Dawn has a great deal of powered flight ahead in order to match
orbits with faraway Ceres. Nevertheless, it has shown quite admirably
that it is up to the task. The craft has spent more time thrusting and
has changed its orbit under its own power more than any other ship from
Earth. While most of the next two years will be devoted to still more
thrusting, the ambitious adventurer has already accomplished much more
than it has left to do. And now it is passing an interesting milestone
on its interplanetary trek.

With all of the thrusting Dawn has completed, it has now changed its
speed by 7.74 kilometers per second (17,300 mph), and the value grows as
the ion thrusting continues. For space
enthusiasts from Earth, that is a special speed, known as "orbital
velocity." Many satellites, including the International Space Station,
travel at about that velocity in their orbits. So does this mean that
Dawn has only now achieved the velocity necessary to orbit Earth? The
short answer is no. The longer answer constitutes the remainder of this
log.

We have discussed some of these principles before, but they are
counterintuitive and questions continue to arise. Rather than send our
readers on a trajectory through the history of these logs even more
complicated than Dawn's flight through the asteroid belt, we will
revisit a few of the ideas here. (After substantial introspection, your
correspondent granted and was granted permission to reuse not only past
text but also future text.)

While marking Dawn's progress in terms of its speed is a convenient
description of the effectiveness of its maneuvering, it is not truly a
measure of how fast it is moving. Rather, it is a measure of how fast it
would be moving under very special (and unrealistic) circumstances. To
understand this, we need to look at the nature of orbits in general and
Dawn's interplanetary trajectory in particular.

The overwhelming majority of craft humans have sent into space have
remained in the vicinity of Earth, accompanying that planet on its
annual revolutions around the sun. All satellites of Earth (including
the moon) remain bound to it by its gravity. (Similarly, Dawn spent much
of 2011 and 2012 as a satellite of distant Vesta, locked in the massive
body's gravitational grip.) As fast as satellites seem to travel
compared to terrestrial residents, from the larger solar system
perspective, their incessant circling of Earth means their paths through
space are not very different from Earth's itself. Consider the path of a
car racing around a long track. If a fly buzzes around inside the car,
to the driver it may seem to be moving fast, but if someone watching the
car from a distance plotted the fly's path, on average it would be
pretty much like the car's.

Everything on the planet and orbiting it travels around the sun at an
average of 30 kilometers per second (67,000 mph), completing one full
solar orbit every year. To undertake its interplanetary journey and
travel elsewhere in the solar system, Dawn needed to break free of
Earth's grasp, and that was accomplished
by the rocket that carried it to space more than five years ago. Dawn
and its erstwhile home went their separate ways, and the sun became the
natural reference for the spacecraft's position and speed on its voyage
in deep space.

Despite the enormous push the Delta II rocket delivered (with
affection!) to Dawn, the spacecraft still did not have nearly enough
energy to escape from the powerful sun. So, being a responsible resident
of the solar system, Dawn has remained faithfully in orbit around the
sun, just as Earth and the rest of the planets, asteroids, comets, and
other members of the star's entourage have.

Whether it is for a spacecraft or moon orbiting a planet, a planet or
Dawn orbiting the sun, the sun orbiting the Milky Way galaxy, or the
Milky Way galaxy orbiting the Virgo supercluster of galaxies (home to a
sizeable fraction of our readership), any orbit is the perfect balance
between the inward tug of gravity and the inexorable tendency of objects
to travel in a straight path. If you attach a weight to a string and
swing it around in a circle, the force you use to pull on the string
mimics the gravitational force the sun exerts on the bodies that orbit
it. The effort you expend in keeping the weight circling serves
constantly to redirect its path; if you let go of the string, the
weight's natural motion would carry it away in a straight line (ignoring
the effect of Earth's gravity).

The force of gravity diminishes with distance, so the sun's pull on a
nearby body is greater than on a more distant one. Therefore, to remain
in orbit, to balance the relentless tug of gravity, the closer object
must travel faster, fighting the stronger pull. The same effect applies
at Earth. Satellites that orbit very close (including, for example, the
International Space Station, around 400 kilometers, or 250 miles, from
the surface) must streak around the planet at about 7.7 kilometers per
second (17,000 mph) to keep from being pulled down. The moon, orbiting
almost 1000 times farther above, needs only to travel at about 1.0
kilometers per second (less than 2300 mph) to balance Earth's weaker
hold at that distance.

Notice that this means that for an astronaut to travel from the surface
of Earth to the International Space Station, it would be necessary to
accelerate to quite a high speed to rendezvous with the orbital
facility. But then once in orbit, to journey to the much more remote
moon, the astronaut's speed eventually would have to decline
dramatically. Perhaps speed tells an incomplete story in describing the
travels of a spacecraft, just as it does with another example of
countering gravity.

A person throwing a ball is not that different from a rocket launching a
satellite (although the former is usually somewhat less expensive and
often involves fewer toxic chemicals). Both represent struggles against
Earth's gravitational pull. To throw a ball higher, you have to give it
a harder push, imparting more energy to make it climb away from Earth,
but as soon as it leaves your hand, it begins slowing. For a harder
(faster) throw, it will take longer for Earth's gravity to stop the ball
and bring it back, so it will travel higher. But from the moment it
leaves your hand until it reaches the top of its arc, its speed
constantly dwindles as it gradually yields to Earth's tug. The
astronaut's trip from the space station to the moon would be
accomplished by starting with a high speed "throw" from the low starting
orbit, and then slowing down until reaching the moon.

The rocket that launched Dawn threw it hard enough to escape from Earth,
sending it well beyond the International Space Station and even the
moon. Dawn's maximum speed relative to Earth on launch day was so high
that Earth could not pull it back. As we saw in the explanation of the
launch profile, Dawn was propelled to 11.46 kilometers per second
(25,640 mph), well in excess of the space station's orbital speed given
three paragraphs above. But it has remained under the sun's control.

Now we can think of the general problem of flying elsewhere in space as
similar to climbing a hill. For terrestrial hikers, the rewards of
ascent come only after doing the work of pushing against Earth's gravity
to reach a higher elevation. Similarly, Dawn is climbing a solar system
hill with the sun at the bottom. It started part way up the hill at
Earth; and its first rewards were found at a higher elevation, where
Vesta, traveling around the sun at only about two thirds of Earth's
speed, revealed its fascinating secrets to the visiting ship. The ion
thrusting now is propelling it still higher up the hill toward Ceres,
which moves even more slowly to balance the still-weaker pull of the sun.

If Dawn had been in zero-gravity and not been obligated to obey the laws
of orbital motion, the thrusting to date would have accelerated it by
the 7.74 kilometers per second (17,300 mph) mentioned near the
beginning. Instead of making the spacecraft go faster, however, that
work was designed to climb the solar system hill. If Dawn had been
targeted to a destination closer to the sun than Earth, the same amount
of thrusting would have helped it speed up to descend the hill, dropping
into a lower solar orbit, where it would have to zip around the
gravitational master of the solar system faster than Earth.

To orbit a body that orbits the sun, a spacecraft has to match its
target's solar orbit. Except in science fiction, no spacecraft in
history other than Dawn has been designed to orbit two different
destinations around the sun. Without its ion propulsion system, this
mission would be quite impossible. Tighter orbits require greater
velocity in order to counterbalance the stronger pull of gravity.
Mercury and Venus orbit the sun faster than Earth. Mars moves around the
sun more slowly than Earth, and all residents of the more distant main
asteroid belt (including Dawn) revolve at an even more leisurely pace.

Because spacecraft wind up at different speeds relative to the sun,
their final velocity is not as important in their design and operation
as is the amount by which they /change/ their velocity after being
released from the rocket. Because of these complexities, rocket
scientists generally put all spacecraft on a level playing field (or, in
this case, a zero-gravity field free of the complications of the physics
of orbits) by using the change of velocity as a measure of the
spacecraft's maneuvering capability.

Dawn has slowed down tremendously since it departed Earth, but what is
noteworthy is the amount by which it has propulsively changed its speed.
If it had begun at a starting line with all other spacecraft on that
simplified playing field, by now it would be racing along at 7.74
kilometers per second (17,300 mph), far faster than any other
spacecraft. By the end of its mission, it would be flying at an
extraordinary 11 kilometers per second (24,600 mph).

Most satellites in low Earth orbit hardly change their speed at all,
relying instead on the momentum imparted to them by the rockets that
took them into space. As you can see by comparing the numbers above, a
rocket to Earth orbit delivers about the same speed that Dawn has
achieved already, and the rocket that sent the probe on its
interplanetary course provides roughly the same speed that Dawn will
attain over the coming years. (Of course, Dawn and the rocket have
different objectives. For example, our spacecraft did not have to plow
through Earth's atmosphere under its own thunderous power. Rockets do.
Nevertheless, the more petite Dawn is gracefully accomplishing its
unique space mission without the burden of enormous propellant tanks and
multiple stages.)

Having changed its speed by the same amount needed to go from the
surface of Earth to Earth orbit is only a coincidence. Dawn's rocket
gave it an even larger boost. But for maneuvering after launch, this
spaceship is in a class by itself.

Each spacecraft is designed for a specific mission. As no other
spacecraft has attempted a mission like Dawn's, no other spacecraft has
needed such an exceptional capability to change its own speed. (Some
others have used gravitational boosts from planets to change their speed
by more than Dawn. That is not a reflection of the spacecraft's
capability, however, but rather the particular trajectory it follows.)
Together, all the probes humankind has dispatched on interplanetary
journeys have helped provide us with new perspectives and new insights
on the nature of the solar system, including its origin and evolution.
And the people who are interested in them cannot help but be in awe of
the daunting challenges, the remarkable engineering, the vast distances,
the inspiring adventures, the thrilling sights, and the amazing new
knowledge. With its extraordinary ion propulsion system, Dawn is making
exciting contributions to this grand endeavor. It has already conducted
a richly detailed exploration of one exotic world and, as it thrusts
with its ion propulsion system to climb the solar system hill to
another, it looks forward to more treasures on its ambitious expedition.

Dawn is 5.8 million kilometers (3.6 million miles) from Vesta and 56
million kilometers (35 million miles) from Ceres. It is also 2.28 AU
(341 million kilometers or 212 million miles) from Earth, or 910 times
as far as the moon and 2.30 times as far as the sun today. Radio
signals, traveling at the universal limit of the speed of light, take 38
minutes to make the round trip.
Received on Fri 01 Mar 2013 04:26:32 PM PST