| Above: simulation of Mars Exploration Rover spacecraft descending through Mars'
| Above: simulation of retro rockets in backshell firing seconds before Mars Exploration
Rover lands on Mars.
| Above: simulation of Mars Exploration Rover bouncing on Mars inside its protective
covering of airbags.
On the phone,
tones can signal a connection. On paper, they can add shape and dimension. On
Mars, they can do both. This is why members of the Mars Exploration Rover Entry
Descent and Landing team at NASA's Jet Propulsion Laboratory will be on the lookout
for a series of tones during the mission's landings in January 2004.
"Tones are a simple
way to send information directly to Earth about what the rover thinks it is doing
as it enters the Martian atmosphere and prepares to land on Mars," said
Mars Exploration Rover Entry, Descent and Landing Telecommunications Lead Dr.
After the Mars Climate
Orbiter and Mars Polar Lander missions were lost in 1999, mission engineers began
looking at ways to strengthen communication during future landings. The Mars
Exploration Rover team plans to use tones in conjunction with other methods to
assess the state of the rovers shortly before, during and after landing. These
tones cannot be heard, but they can be detected by special equipment located
at NASA's Deep Space Network.
The Mars Exploration Rover
mission uses a radio called the Small Deep Space Transponder, which can generate
tones at up to 256 different frequencies-more than enough to cover the possible
states of the spacecraft, engineers said. Using data analysis equipment specially
designed by a JPL team for the Mars Exploration Rover mission, the tones are
presented as colored bars displayed according to frequency and received time.
In addition, the detection software decodes the meaning of the tones and displays
the names of various events associated with the tones. However, due to the extreme
conditions of heat and speed during landing, there is no guarantee the tones
will be detected-even if the mission is going exactly as planned.
The process of entry, descent
and landing on Mars is no walk in the park. It entails getting a 827-kilogram
(nearly a ton) spacecraft, entering the martian atmosphere at 19,300 kilometers
(12,000 miles) per hour, to safely slow to a stop on the surface in six nail-biting
minutes. Complicating matters is the martian surface, which is plagued with unpredictable
winds and obstacles: massive impact craters, cliffs, cracks and jagged boulders.
During the first four minutes
of descent, friction with the atmosphere slows the spacecraft to 1,600 kilometers
(1,000 miles) per hour. With only 100 seconds left, and at the altitude a commercial
airliner typically flies, a parachute opens to further slow the spacecraft to
321 kilometers per hour (200 miles per hour). With 6 seconds left, and at 91
meters (100 yards) above ground, the retro rockets fire to bring the spacecraft
to zero velocity. Seconds later, the lander freefalls from a height of about
four stories, cocooned in airbags to cushion the hard blow as it hits the ground
at 48 kilometers (30 miles) per hour, or more if it is windy. The lander then
bounces approximately 30 times from as high as a four-story building and down
to a rolling stop.
If the tones show up, they
will keep controllers from being locked in limbo, unable to learn the status
of the rovers until after the complete landing. While many events will occur
during the landing process, engineers hope about 15 of those events to be signaled
to Earth. Each rover is programmed to transmit a tone every 10 seconds to tell
engineers on Earth about its progress. The first tones sent by the spacecraft
will signal its deceleration as it enters the Martian atmosphere. Engineers could
also receive a tone after important events such as parachute deploy, heat shield
jettison and lander separation. Fault tones are also sent if the spacecraft thinks
one of its subsystems is performing unusually. Once the lander has touched down,
it will send five tones every 30 seconds to keep engineers informed about its
As with any form of radio
communication, many factors-- such as spacecraft antenna motion and atmospheric
conditions-- can affect the quality and reliability of radio signals or render
them undetectable. This is also true for the tones, which are also difficult
to detect because of the spacecraft's motion during descent.
"We engineers all
understand that because these signals are so difficult to detect, an absence
of them during or after landing does not necessarily mean that the rover has
had a bad day," Estabrook said.
In addition to the tones,
rover engineers will strengthen their chances of receiving information about
the spacecraft's descent onto Mars by waiting for information that has been relayed
from the Mars Exploration Rover to the Mars Global Surveyor spacecraft. The rovers
have been equipped with Ultra High Frequency radios, which they will use to communicate
with the orbiter as they descend onto the Martian surface. These radios can transmit
8,000 times more information than the tones. However, due to the unknown geometry
between the two spacecraft, engineers cannot guarantee that they will receive
this data. Within an hour of each rover's landing, the data should be at available
"The Mars Exploration
Rover team has gone to great lengths to improve the chances of receiving data
from the spacecraft during this period of great activity," Estabrook said. "But
we must be prepared to wait until at least the next morning, when the rover communicates
with us, to get detailed information on spacecraft health and find out what exactly
happened during descent."
Mars Exploration Rover Mission
NASA's Jet Propulsion Laboratory