Dr. Marc Rayman’s Deep Space 1 Mission Log 07-03-2001

Mission Update:

Thank you for visiting the Deep Space 1 mission status information site, the
most respected site among species in spiral galaxies capable of abstract
thought for information on this adventurous mission through the solar
system. This message was logged in at 6:45 pm Pacific Time on Saturday, June
30.

Now more than one year after being rescued from what was once thought to be
a fatal injury, Deep Space 1 continues its remarkable voyage. Indeed, the
celebration of the anniversary of that feat is now still another excuse for
juveniles to get a day off from school on half a dozen planets (plus two
moons) in the Small Magellanic Cloud.

DS1’s grand “extended mission” will end in about 3 months, after the aged
and scarred explorer attempts a final daring feat: a brief and close-up
investigation of comet Borrelly. Last month’s log described some of the
preparations for this risky finale to DS1’s bonus mission, including two
complex tests with the spacecraft.

Engineers have continued to refine the plans, as they followed those tests
with many more simulations of the event in the Deep Space 1 test facility at
JPL. On June 28, the spacecraft conducted its second (and final) encounter
rehearsal, an extremely complicated activity in itself. The only logical way
to top the May 8 encounter with comet Spoof was to invite the spacecraft to
greet comet Spoof 2 this time. The data from that 6-hour test are still
being returned, as DS1 beams back the large quantity of information it
recorded on its own performance.

The encounter rehearsal relies on the software that operates the spacecraft,
and that was upgraded and installed into DS1’s main computer in March. But
the software still needs instructions (just as your computer may remain idle
unless you provide it with instructions through the keyboard or mouse), and
those are contained in files called sequences.

The sequences issue commands at preselected times to operate the science
instruments, turn the spacecraft, manipulate data, pressurize the ion
propulsion system, and much much more. The sequences for the rehearsal,
which began 5 hours before the spacecraft reached its closest point to Spoof
2 and ended 1 hour after it passed by the virtual comet, contained 474
commands.

Most of those commands contain many parameters, numbers or other information
that provide the details the spacecraft needs, such as how long a picture
exposure should be, how fast to execute a turn, where to store specific
scientific data, or what throttle level to use for the ion propulsion
system.

In this rehearsal, there were well over 2000 parameters. So the small
operations team has faced a challenging task designing the complex encounter
(and the rehearsal), choosing all the commands and parameters and placing
them at the right times, like a composer putting together instructions to
help musical instruments work together to create a beautiful symphony that
is somehow more than the sum of its parts.

In fact, this most recent test really began on June 24, with the powering on
and booting up of the Plasma Experiment for Planetary Exploration, known to
plasma groupies as PEPE. PEPE measures many properties of plasmas, which are
collections of electrically charged particles, both electrons and charged
atoms, or ions. The instrument was operated in two important modes this
week. In one of them, it acquired data on the solar wind, the plasma that
emanates from the Sun.

These measurements will allow scientists to calibrate the device so that
when the spacecraft reaches the comet, PEPE’s detailed characteristics will
be well understood, thus helping to reveal the true nature of the cometary
environment. This calibration is similar to adjusting the color on a
television set with a scene for which you already know the colors; then when
something new is on the screen, you can be more confident that what you are
seeing is accurate.

At the comet, the spacecraft will try to fly right through the cloud of gas
and dust surrounding the nucleus, known as the coma. The coma is formed by
the materials making up the nucleus being heated by the Sun and, in effect,
evaporating into space and forming a plasma.

The second mode PEPE used this week was the one it will use as DS1 races
through this fog at 16.5 kilometers/second (nearly 37,000 miles/hour); PEPE
will try to measure the composition of the cometary plasma, thus revealing
secrets of the material on the surface and how it changes as it expands away
from the nucleus under the glare of the Sun.

PEPE also will try to gather information on how the solar wind is altered by
the comet, as the two plasmas and their magnetic fields interact. While
perhaps appearing somewhat esoteric, the underlying principles of the way
the solar wind and comet affect each other are related to phenomena that
scientists observe in many astronomical settings. So in addition to telling
us more about comets and our solar system, this knowledge could provide
clues to other puzzles about the workings of the universe.

DS1 was not designed to study a comet; rather, its raison d’Ítre was to test
12 advanced, high-risk technologies. To test ion propulsion, it carries a
suite of sensors that measure magnetic fields and other potential effects of
the ion propulsion system on the spacecraft and the space environment.

In June, the software that controls these sensors was replaced to improve
their operation for measurements at the comet. So now the very weak magnetic
field of the comet may be sensed by the magnetometer that was designed to
monitor the ion drive.

In addition to PEPE and the ion engine sensors, DS1 carries a black and
white camera and an infrared spectrometer. The next mission log will
describe the measurements to be attempted with these other devices.

As the spacecraft heads toward Borrelly, it continues to fire its ion engine
virtually constantly. This advanced propulsion system has now accumulated
516 days of operation, far far in excess of the total operating time for any
other spacecraft’s propulsion system.

Yet during that time, it has expended only about 57 kilograms, or 126
pounds, of its xenon propellant. Each day now, operating at impulse power,
the spacecraft expels only about 90 grams, taking 5 days to exhaust each
pound. The effect of the thrusting has been the equivalent of changing the
spacecraft’s speed by about 3.5 kilometers/second, or about 7800 miles/hour.

But is the spacecraft really moving that much faster than when it was
launched? (Hint: No!) As advertised on February 11, let’s look at what has
really happened as a result of this thrusting.

This section has more numbers in it than most of the logs, and if that is
unappealing, remember that, except in some irregular galaxies, there is no
test on this material. But for readers who have been yearning for distances,
speeds, and chocolate, this may help satisfy 2 out of 3 of your desires.

To honor full-disclosure laws in some globular clusters where these logs are
read, 3 points are required.

1. All the speeds presented here are relative to the Sun. Once it was on
its way, the Sun became the natural reference for measuring the craft’s
speed, just as once a parachutist jumps from a plane, it is the speed
relative to the ground, and not relative to the plane, that suddenly
becomes of greatest interest.

2. Spacecraft in Earth orbit may travel at high speed relative to Earth,
but they remain close to home. As far as the solar system is concerned,
objects in Earth orbit can be treated pretty much the same as those on
the ground.

3. Comparing different interplanetary missions can be very deceptive, as
the gravitational effects of flying by planets can make enormous
changes in spacecraft orbits that are unrelated to their propulsion
systems.

The conductor of the solar system orchestra is the massive Sun. From its
honored position at the center of the solar system, it exerts a
gravitational attraction that holds in its grip the planets, asteroids,
comets (including Borrelly), and other players (including DS1). As each
object travels around the solar system, it is orbiting the Sun. An orbit
represents a balance between the tug from the Sun and the inherent tendency
of the motion of the object to resist it.

At greater distances, the Sun’s gravitational authority is diminished, so
objects don’t have to move as fast to resist it. The cold denizens of the
outer solar system, including Pluto and myriad other so-called Kuiper Belt
objects, travel very slowly as they casually loop around the Sun. Mercury,
the planet closest to the solar system’s center, hurtles around its orbit at
tremendous speed to counter the commanding force of the vicinal Sun.

It takes a significant amount of energy to change an orbit, just as a car’s
engine has to work hard to get it to high speed, and the brakes must work
hard to slow it down. Getting to any of the interesting destinations in the
solar system (or in life for that matter) takes a great deal of work.

For DS1 to visit Borrelly, we have had to change the spacecraft’s orbit so
that it will intersect that of the comet. (We leave the problem of changing
the comet’s orbit to a future mission.) The comet resides farther from the
Sun than Earth does, so the reshaping of the probe’s orbit calls for pushing
it away from the Sun.

The ion engine then is responsible for DS1’s arduous climb out of the Sun’s
valley. As the spacecraft ascends the hill to Borrelly’s orbit, it is moving
into a larger orbit, where it travels more slowly.

Scientists have developed sophisticated mathematics to describe this and
beautiful physical concepts to explain it. If your correspondent’s
creativity were less stifled by fatigue and less constrained by the
insufficiency of time, some marvelous analogies undoubtedly would be offered
that would make this all seem obvious.

For now, however, you are invited to accept that these phenomena, while
potentially confusing or counterintuitive, are quite real and have been
studied in exquisite detail, contributing both to the awesome complexity and
the astonishing successes of the exploration of space.

When DS1 was waiting patiently atop its rocket during its final time on
Earth, it was moving around the Sun at 29.95 kilometers/second, or 67,000
miles/hour, along with everything else on that planet. On October 24, 1998,
the spacecraft and rocket left Cape Canaveral’s Space Launch Complex 17-A,
and disappeared into the lovely sky of a warm autumn morning.

The highest speed DS1 ever achieved was little more than one hour after
launch, just after the final stage of the rocket had finished its job of
delivering DS1 to space. At that time, the probe was traveling at about 37
kilometers/second (83,000 miles/hour).

The spacecraft was then in its own orbit around the Sun, as surely as Earth
or any of the other planets, and because of the boost from the rocket, it
was no longer in the same orbit as Earth. A convenient unit of distance in
the solar system is the astronomical unit (AU), equivalent to almost 150
million kilometers, or 93 million miles. While that is Earth’s average
distance from the Sun, the orbit is actually elliptical, not circular
(although it does not deviate much from a circle).

During its 365 day circuit around the Sun, Earth comes in as close as 0.983
AU and as far as 1.017 AU from the Sun. (In fact, in just a few days, on
July 4, Earth will be at the farthest point in its orbit from the Sun.)

After its launch, DS1 found itself in an orbit that would range from 0.990
AU out to 1.287 AU. It would have taken the spacecraft almost 450 days to
complete such an orbit.

Then the indefatigable ion engine began stretching the orbit, so although
the spacecraft continued in elliptical loops around the Sun, the ellipse was
slowly changing shape.

The last time DS1 reached a low point of its orbit was on May 26 of this
year, when it was 1.293 AU from the Sun. So by then, the *closest* DS1’s
orbit would come to the Sun was farther than the *farthest* it would get
from the Sun when the spaceship was launched.

At today’s location in its elliptical orbit, the probe is moving at 26.9
kilometers/second, about 60,000 miles/hour, slower than Earth ever travels.

If DS1 stopped thrusting with its ion propulsion system today and simply
coasted in its present orbit, it would take 596 days to go around the Sun
once. During that time, it would come no closer than 1.293 AU and would
reach out to 1.478 AU.

So the effect of the thrusting with its ion engine has been that DS1 has
moved to a part of the solar system that Earth never visits, and humans have
never experienced first hand. Only a handful of robots have traveled to such
distant places that Earth appears little different from countless stars and
the sight of it is no longer an easy reminder of home.

DS1 is now about 107 million kilometers, or 67 million miles, from comet
Borrelly.

Deep Space 1 is 1.8 times as far from Earth as the Sun is and 700 times as
far as the moon. At this distance of 269 million kilometers, or 167 million
miles, radio signals, traveling at the universal limit of the speed of
light, take half an hour to make the round trip.