Lighting at Vesta
FREQUENTLY ASKED QUESTIONS:
How far does Dawn travel on its trip from the Earth to Vesta? From Vesta to Ceres?
The trip from Earth to Vesta is about 2 billion miles or 3.5 billion km. The trip from Vesta to Ceres is about 1 billion miles or 1.6 billion km.
Why does it take so long to get to Vesta and then from Vesta to Ceres? The travel time scheduled appears to be significantly longer than would be required by a Hohmann transfer orbit.
The reason that Dawn has a longer trip time than might be required by conventional means is that Dawn uses an ion propulsion system which precludes achieving a Hohmann transfer orbit. Hohmann transfer orbits are the most propellant-efficient means of moving between two circular coplanar orbits. Hohmann transfers are certainly not the fastest route between orbits; however, they are used frequently because most missions are tightly constrained in mass, so propellant is a very precious resource.
To accomplish a Hohmann transfer, two propulsive maneuvers are required. The first one breaks the spacecraft out of the initial
orbit and puts it in an orbit that intersects the desired final
orbit. The spacecraft is then in the Hohmann transfer orbit, which
is an ellipse tangent to both circular orbits. After the spacecraft
has coasted to the point that connects the transfer orbit to the
desired final orbit, it fires its engine a second time, now to
circularize its orbit, thus matching the target orbit.
Dawn's ion propulsion system is far more efficient than a
chemical propulsion system would be, but it produces much less
thrust. In other words, it takes significantly less xenon propellant
for Dawn to change its velocity by a given amount than it would if
it used chemical propellants, but it also takes longer. Ultimately
ion propulsion can allow a spacecraft to achieve a higher speed than
one with chemical propulsion could. (Ion propulsion provides what I
always like to call acceleration with patience.)
Dawn cannot provide a sufficiently large acceleration to follow a
Hohmann transfer orbit -- the thrust is simply too gentle. As a
result, after receiving its initial boost out of Earth orbit from the
Delta rocket, it spirals away from the Sun until it reaches Vesta's
orbit. It thrusts most of the way to Vesta, very gradually adding
energy to its orbit around the Sun, rather than beginning with a huge
burn and coasting to Vesta. This gentle reshaping of the orbit, in
contrast to the more abrupt changes typical of chemical propulsion,
is a characteristic of ion propulsion and other so-called low-thrust
propulsion systems, such as solar sails. As low-thrust propulsion is
just beginning to be used for reaching destinations, some of our
standard conceptions for how spacecraft move around the solar system
may need to be revised.
An example might help illustrate the difference between using chemical and ion propulsion. The engine on a conventional
interplanetary spacecraft may burn roughly 300 kilograms of
propellants in around 20 minutes of operation, achieving a velocity
change of perhaps 1000 meters/second. At its maximum thrust,
Dawn's ion engine can expend only about 0.25 kg of xenon per day,
changing the spacecraft's velocity by 10 m/s. To achieve that 1000
m/s thus would require only 25 kg of xenon -- a tremendous savings given
the high cost of launching spacecraft from Earth -- but it would take
100 days. As the spacecraft recedes from the Sun, its solar arrays
produce less power, so it operates at a lower throttle level, using
still less propellant and taking still longer to achieve these
Dawn will carry enough propellant to change its speed by more than 10 kilometers/s (or about 6 miles per second) over the course of the mission, far more than any spacecraft's propulsion system has ever accomplished, but it will require an accumulated thrust time of more than 6 years. Although it will take Dawn longer to go from Earth to Vesta and from
Vesta to Ceres with ion propulsion than it would with chemical
propulsion, the longer trip time is well worth it. Dawn will use a
significantly less expensive rocket than it would if it had to carry
the much more massive propellants required for a conventional
chemical propulsion system. In fact, Dawn simply would be
unaffordable without ion propulsion. Now however your tax dollars
and mine can be used to accomplish a broad and exciting program of
solar system exploration, including the acquisition of a wonderfully
rich set of science data at Vesta and Ceres.
Answer provided by Marc Rayman, Dawn Chief Engineer.
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Last updated March 17, 2008.