|
|
Planetary News: Deep Impact (2005)
Deep Impact Mission Timeline
By Emily Lakdawalla
June 30, 2005
Comet Tempel 1 and the Deep Impact spacecraft are playing a
game of astronomical chicken -- and the Deep Impact impactor is going to lose,
big time. But when the impactor is obliterated by Tempel 1 on July 4 at 05:52
UTC (22:52 on July 3, PDT), it will expose the innards of a comet to human
eyes for the first time.
The Deep Impact flyby spacecraft will have a ringside seat at the creation
of an impact crater on a comet. The images that it captures will give us new
insight into the mechanics of cratering, the structure and composition of
comets, and perhaps a better understanding of what ingredients were available
to make planets, moons, asteroids, and comets when our solar system was being
born.
The basic idea behind the Deep Impact mission is simple: send a spacecraft
to a comet; shoot a projectile at the comet to make a crater; and watch the
crater develop and see what's inside the comet. To make this work, Deep Impact
is actually two separate spacecraft. One, the flyby spacecraft, is a fairly
traditional, SUV-sized solar-powered craft with a High-Gain Antenna for communicating
with the Earth. It also carries two main science instruments, the High-Resolution
Imager and the Medium-Resolution Imager, for capturing images, animations,
and spectral information about the comet.
The second part of Deep Impact is the Impactor. This is a battery-powered
probe, weighing 370 kilograms (816 pounds), with its own camera and a computer
that will run an Auto-Navigation program to guide itself into a precise impact
with Tempel 1. The impactor will separate from the flyby craft 24 hours prior
to the encounter. The impactor will return images to the flyby spacecraft
until its demise.
In order to dig a deep hole on Tempel 1, mission planners needed a lot of
energy. Explosives would not work, for a wide variety of reasons. Instead,
the mission relies on good old-fashioned Newtonian mechanics to deliver the
necessary energy.
Deep Impact and Tempel 1 are both in orbit around the Sun, but they are in
two very different orbits. Deep Impact's orbit is a small, nearly circular
one with a perihelion (closest distance to the Sun) near Earth's orbit and
an aphelion (farthest distance from the Sun) near Mars's orbit. Tempel 1's
path is more elliptical. Its perihelion is near Mars's orbit, and its aphelion
is near Jupiter's orbit. Because Tempel 1 will be near its perihelion and
Deep Impact near its aphelion when the two bodies meet, Tempel 1's velocity
relative to the Sun will be far higher than Deep Impact's.
That difference in velocity is what will make the encounter a spectacular
event. The two bodies will meet at a relative velocity of 10.3 kilometers
per second (23,000 miles per hour). If you recall your high school physics,
you'll remember that the energy of such an encounter can be calculated using
a formula: the kinetic energy of an object is equal to half the mass times
the square of the velocity. So when the 370-kilogram Deep Impact impactor
yields up all its kinetic energy in a collision with Tempel 1, the amount
of energy available is 19 Gig joules, which is equivalent to the amount of
energy released by exploding 4.8 tons of TNT. Deep Impact needs no explosives
-- kinetic energy does all the work.
Here is a detailed timeline of the events that are set to take place as Deep
Impact meets Tempel 1.
Spacecraft
time (UTC)* |
Spacecraft
time (PDT)* |
Time with
respect
to impact |
Event |
| Jun 26 |
Jun 26 |
- 7d |
Continuous imaging
Days away from the encounter, the nucleus of the
comet will still only be a speck to the cameras of Deep Impact. The
images' value will be for navigation -- helping to set the parameters
for the last trajectory correction maneuver -- and also to watch for
jetting or other cometary behaviors. |
Jun 29
18:00 |
Jun 29
11:00 |
-4d 12h |
Optical navigation images acquired every 9 minutes |
July 3
00:07 |
July 2
17:07 |
-1d 06h |
Trajectory Correction maneuver
This is the last maneuver Deep Impact will do to
line itself up for a collision with Tempel 1 before releasing the impactor. |
July 3
01:57 |
July 2
18:57 |
-1d 04h |
Impactor's battery activated
Until now, the impactor has been relying upon power
from the flyby craft. |
July 3
06:07 |
July 2
23:07 |
-23h 45m |
Impactor release
The impactor is sent on its separate way with only a tiny push from
the flyby craft. The difference in velocity between the two spacecraft
is only 34.8 centimeters per second (0.78 miles per hour).
At this point, the comet's nucleus is not
much more than a speck to the cameras, only a few pixels across
to the High Resolution Imager. |
July 3
06:06 |
July 2
23:14 |
- 23h 38m |
First telemetry from impactor after release
Mission workers on Earth will be waiting eagerly
to hear that the impactor, on its own for the first time, is healthy. |
July 3
06:19 |
July 2
23:19 |
-23h 33m |
Flyby spacecraft deflection maneuver
The flyby spacecraft fires its engines for about
14 minutes to divert its path out of the way of Tempel 1. The burn
will slow the spacecraft's speed by 102 meters per second (227 miles
per hour). The reduction in speed is necessary in order that the flyby
craft can keep the impactor in view during the impact. Its course will
eventually take it through the comet's inner coma, a safe 500-kilometer
(310-mile) distance from the comet's nucleus, 14 minutes after impact. |
July 3
06:53 |
July 2
23:53 |
-22h 59m |
First image from flyby spacecraft after deflection maneuver**
After sending the impactor on its separate way,
the spacecraft will turn to point its two cameras at the comet, capturing
color images and spectra of the comet's coma. These data will help
scientists nail down the comet's rotation rate, among other things. |
July 3
06:58 |
July 2
23:58 |
-22h 54m |
First image of impactor by flyby spacecraft** |
July 3
07:52 |
July 3
00:52 |
-22h 00m |
First images from impactor; image pairs acquired every 2 hours
The first pair of images from the impactor will
be full frames, one with an exposure set to view the nucleus, and one
with an exposure set to view the much dimmer coma. |
July 3
17:52 |
July 3
10:52 |
-12h 00m |
Test run of impactor's final image sequence
Two minutes will be spent running the same imaging
sequence that the impactor will run in the final two minutes before
its encounter with Tempel 1 in order to verify that the sequence will
execute properly. It'll get only one chance! |
July 3
19:52 |
July 3
12:52 |
-10h 00m |
Impactor imaging resumes
Image pairs will be captured at the following times:
-10:00; -08:00; -07:00; -06:00; -05:00; -04:00; -03:00; -02:30; -02:00; -01:30;
-01:00 |
July 4
03:53 |
July 3
20:53 |
-01h 59m |
AutoNav system begins imaging
It is up to the impactor's own computer to observe
Tempel 1 and choose the best possible impact site. The best site would
be fully illuminated by the Sun and relatively smooth in order to reduce
the number of variables affecting the outcome of the impact. As part
of the autonav function, the impactor captures images of the comet
every 15 seconds with its own camera, sending them back to the flyby
spacecraft, which relays them on to Earth. The flyby craft is also
capturing AutoNav images every 15 seconds in order to improve its own
targeting of impact events. |
July 4
04:21 |
July 3
21:21 |
-01h 31m |
Impactor Targeting Maneuver 1
This is the first of three thruster firings that
the impactor performs to line itself up with Tempel 1. The impactor
carries four 22-Newton thrusters, which can fire in pulses varying
in length from 0.015 to 0.5 seconds. |
July 4
04:52 |
July 3
21:52 |
-01h 00m |
Pace of impactor imaging accelerates
As the impactor approaches Tempel 1, it will capture
images more and more rapidly until it is acquiring pictures every 0.7
seconds. |
July 4
05:17 |
July 3
22:17 |
-35m 00s |
Impactor Targeting Maneuver 2 |
July 4
05:39 |
July 3
22:39 |
-13m 00s |
Impactor Targeting Maneuver 3
This last maneuver sets the final course for the
impactor. The impactor is still capturing images and beaming them back
to the flyby spacecraft. |
July 4
05:52 |
July 3
22:52 |
-00m 10s |
Dust abrasion may end transmission of impactor images
The Impactor Targeting Sensor is now acquiring
images with resolutions as high as 1 meter per pixel. By the final
moments, cometary dust will have abraded the camera's optics, degrading
the quality of the images, and possibly ending transmission. |
July 4
05:52 |
July 3
22:52 |
-00m 02s |
Last possible impactor image
The final image that the impactor could capture
and transmit in its entirety to the flyby craft will be captured 2
seconds before impact. |
July 4
05:52 |
July 3
22:52 |
-0s |
IMPACT!
Tempel 1 and the impactor try to occupy the same
space at the same time. Tempel 1 wins. The impactor is obliterated
as the kinetic energy of the impact is released in a blast of light
and heat. Telescopes back on Earth, including land-bound ones like
Mauna Kea, Palomar, and Kitt Peak, and spaceborne ones like Hubble,
SWAS, and XMM-Newton, watch for the impact flash. The flyby spacecraft
also continues to watch, staying pointed on Tempel 1, as a crater opens
and a spray of debris, called ejecta, flies out of the crater.
At the moment of impact, the High Resolution Imager has a resolution
of 17 meters per pixel. The images will be panchromatic (black and
white) so that they may be captured as rapidly as possible, several
per second. Once returned to Earth, the sequence of high-speed images
can eventually be assembled into an animation of the impact.
Over the next few hundred seconds, a crater is dug, and a cone-shaped
curtain of ejecta spreads out from the crater. How wide and deep
the crater becomes, and at how steep an angle the ejecta flies, depend
on the properties of the comet, which aren't yet known.
The spacecraft also captures infrared spectra at high speed, searching
for the pristine materials that the impact excavates, and mapping
out the distribution of different materials within the crater.
|
July 4
05:59 |
July 3
22:59 |
+07m 00s |
First image from flyby spacecraft after impact
Although the impact has already happened, the flyby
spacecraft has not yet passed its closest approach distance. Four minutes
after the impact, the pictures captured by the High Resolution Imager
will have a resolution of 12 meters per pixel, and will continue to
a highest resolution of 3 meters per pixel. Color imaging takes longer
than the rapid black and white imaging performed right around impact,
but will allow scientists to map in detail the composition of the interior
of the crater. The crater may still be forming on Tempel 1, and ejecta
is still flying in all directions. |
July 4
06:05 |
July 3
23:05 |
+13m 00s |
Flyby spacecraft turns to shield attitude
The spacecraft now approaches the inner coma and
faces the hazard of dust impacting the sensitive optics and electronics.
To prevent any damage, the spacecraft must turn to a direction that
will shield the instruments from dust impacts. During this period,
the spacecraft continues returning data to Earth. |
July 4
06:06 |
July 3
23:06 |
+14m 10s |
Flyby spacecraft closest approach to Tempel 1
The spacecraft passes within 500 kilometers (300
kilometers) of the comet's nucleus, but the cameras are still protectively
pointed away. |
July 4
06:51 |
July 3
23:51 |
+59m 00s |
Flyby spacecraft turns to Tempel 1 and resumes imaging
Safely away from the dusty coma, the flyby spacecraft
turns to look back at Tempel 1. The turn will take 9 minutes to complete.
The impact site is no longer visible, but the cloud of ejecta is
scattered into the space around the comet. The spacecraft captures
color images and spectra of the ejecta. As the distance between Tempel
1 and Deep Impact increases, the rate of observations will decrease. |
July 5
05:52 |
July 4
22:52 |
+01d 00m |
End of encounter phase
One day after the encounter, Tempel 1 has diminished
to a speck again, and the mission is essentially over. However, the
flyby spacecraft may continue to play back data for up to two more
days, if bad weather or other glitches on Earth prevents the reception
of data earlier. A few more backwards-looking observations will be
performed during this time in order to monitor changes in the comet's
activity and look for any large chunks of comet that may have been
launched into orbit around the comet nucleus. |
| * Note these times are "Earth received time," that
is, the time of the event plus the one-way light time between the spacecraft
and the Earth, which will be approximately 8 minutes at the time of
the mission. |
| ** Events marked with this symbol were listed in a press
kit issued by JPL on June 9, but not in a version issued June 28. Thus
these event times should be considered approximate only. |
|
|
[ what is this? ]