The diary of Meriwether Lewis, 21 September 1805
鈥淚 find myself growing weak for the want of food and most of the men complain
of a similar deficiency, and have fallen off very much.鈥�
The diary of William Clark, 26 September 1805
鈥渟everal men bad, Capt Lewis sick I gave Pukes Salts &c. to several, I am
a little unwell. hot day our axes are Small & badly calculated to build
Canoes of the large Pine.鈥�
WHEN Meriwether Lewis and William Clark sailed and paddled their way up the
Missouri River with a team of 30 soldiers, their goals were to find a river that
ran into the Pacific Ocean and to catalogue the strange plants and animals they
encountered. As the first white men to explore the American northwest, they
carried all the supplies they needed, from weapons for hunting and tools for
boat building to soap and writing paper, as well as trinkets to barter with the
natives.
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Life was hard. At times close to starvation, the team sometimes
survived only on hand-outs from the native Americans. Their equipment and
techniques were sometimes unsuited to the tasks they had to undertake. They
eventually learnt from a chief called Twisted Hair how to use fire rather than
axes to hollow out canoes, for example. And for most of their
two-and-a-half-year journey there was no way to send messages home. On their
return, they found that they had been given up for dead.
A similar problem exists today but for a rather different type of explorer.
Like Lewis and Clark, robotic space probes navigate a wilderness devoid of the
infrastructure we take for granted. There are no communications networks and no
standard navigation systems. Once they leave the Earth, spacecraft are on their
own.
That, however, is set to change. NASA is preparing to wire up the Solar
System by creating a network of spacecraft that will provide the essential
communications and navigation infrastructure for future explorers, be they
robots or human. The first planet to get hooked up will be Mars.
At NASA鈥檚 Jet Propulsion Laboratory (JPL) in Pasadena, California, a team of
scientists and engineers is planning to surround the planet with a constellation
of at least six navigation satellites that will allow robots鈥攐r
humans鈥攐n the ground to pinpoint their position. The satellites will also
act as communications relays, picking up low-power signals from the surface and
boosting them back to Earth.
Eventually, the system should allow almost constant communications between
the two planets at a data rate capable of sustaining high resolution video. The
first of these multipurpose spacecraft is due to be launched in 2003, so the JPL
team is racing against the clock to design them and to persuade the US
government to stump up the money to build them. The team aims to prove that
satellites weighing just 200 kilograms and costing as little as $50
million each can do the trick.
Of course, the Mars Network will be only the first stage. NASA hopes to
extend the network to other planets and eventually the entire Solar System. In
future, explorers wanting to phone home will simply tap into this
鈥渋nterplanetary internet鈥� to communicate anywhere in the Solar System.
Without such an infrastructure, interplanetary exploration is a difficult
business. Take, for example, NASA鈥檚 Mars Pathfinder mission, which bounced onto
the surface of the Red Planet on 4 July 1997. It carried solar panels for power,
communications equipment and data-storage systems鈥攅verything needed to
carry out the few experiments it had brought 150 million kilometres from
Earth.
By any standards Pathfinder was a successful mission but it also showed the
limits of operating alone so far from home. Steering the spacecraft to its
landing spot was tough. During the journey, controllers calculated the
spacecraft鈥檚 position by comparing the time and frequency shifts of signals
reaching different dishes on Earth. But once it was in the Martian atmosphere,
Pathfinder鈥檚 rate and direction of descent were difficult to predict because of
the winds and changes in the density of the atmosphere. Incredibly, the
spacecraft came to rest just 20 kilometres from its target. But scientists will
have to do better if they want a spacecraft to land on the lip of a canyon or
near another craft.
The dusty Martian atmosphere also limited the amount of sunlight available
for generating the power needed to send signals to Earth. Communications were
further limited by the 11 minutes it took signals to travel to Earth. Whenever
the mission鈥檚 rover, Sojourner, bumped into a rock, it waited for further
instructions 鈥攐ften for an entire day. 鈥淎 mission lives for 90 days. Every
step takes one of them. It鈥檚 a nightmare,鈥� says Dan McCleese, chief scientist at
JPL鈥檚 Mars exploration office.
To make matters worse, data trickled back from Pathfinder at a rate of 8
kilobits per second, a fraction of the speed the average computer modem is
capable of. 鈥淭his $150 million mission barely fills up your hard drive,鈥�
laments Chad Edwards, a physicist and head of the telecommunications and mission
operations technology office at JPL. NASA鈥檚 constellation of satellites around
Mars is being designed to tackle all these problems and more.
Setting up a communications link with Mars is a complex problem, however.
NASA would like the Mars Network to be compatible with the Earth-bound Internet,
but there will have to be major differences in the way the systems will works.
On Earth, computers communicate over the Internet using a set of rules called
TCP/IP, or Transmission Control Protocol/Internet Protocol. Information is
broken into packets, and each packet is labelled with a destination address so
that it can be routed around the network. Once the packets reach their
destination, they are reassembled.
There is also a system of error correction: the sending computer checks that
the data have been received and sends them again if they have not. 鈥淭he protocol
is constantly saying: `Did you get that message? No. Here it is again鈥�,鈥� says
Adrian Hooke, an electrical engineer at JPL who is in charge of standardising
the operations of NASA鈥檚 interplanetary missions.
On Earth, computers have the luxury of communicating with each other almost
instantly. But the round trip time for signals from Earth to Mars can be
anything from 15 to 50 minutes depending on the two planets鈥� relative positions
around the Sun. With delays this big, a TCP/IP network would grind to a
halt.
Coming up with a protocol that can handle such delays is the biggest
challenge for the team designing the new system. It will also have to be
extremely robust because data streams crossing the interplanetary divide are
often obscured or corrupted by background noise. 鈥淲e have big garbage-collecting
antennas on Earth that pull data out of the mud,鈥� says Hooke, referring to
NASA鈥檚 Deep Space Network, an array of 70-metre-wide satellite dishes in
California, Spain and Australia that track space probes.
Nevertheless, transmission errors are common and the protocol must be able to
cope. 鈥淩ight now, communications is an ad hoc process. We invent those
techniques over and over for each mission,鈥� says Hooke. 鈥淥n recent missions,
we鈥檝e adopted the brute force method of sending the file twice.鈥� This relies on
the hope that the same data wouldn鈥檛 be lost in both transmissions. On Earth, a
computer compares both transmissions and pieces together the complete
broadcast.
Although this strategy works well, Hooke and his team have decided against it
for the Mars Network. Sending everything twice is just not efficient, he
complains. Instead, Hooke is developing a protocol that monitors which data
packets have arrived and which have not and asks for any missing pieces to be
sent again. Crucially, the system is designed to keep running regardless of
whether or not all the data packets have turned up. Missing packets can be sent
again hours or even days later.
Data trickle
Another problem with TCP/IP is that the labels, or headers, added to packets
use up precious space. Each address header takes up 500 bits, a greater
proportion of the transmissions than scientists would like. So Hooke has
developed a streamlined protocol that requires just 48 bits per header, which
should take up less than 2 per cent of each broadcast.
What he sacrifices with this new scheme is compatibility with the
Internet鈥攁 price worth paying, in his view, because of the extra data that
could be squeezed into every broadcast. With a gateway on the ground to act as
an interpreter between the two, however, scientists will be able to access the
Mars Network via the Internet.
NASA could also allow occasional access from the outside. The idea would be
to inspire the next generation of scientists by allowing schoolchildren to
control a Mars rover from their classrooms, download weather reports or point
cameras. But security will be a concern. 鈥淲hatever we do for public interaction
would be tightly controlled,鈥� says Hooke.
NASA will probably protect the Mars Network with firewalls similar to those
that already guard its systems around the world. These should prevent hackers
taking over spacecraft or releasing viruses into the interplanetary
internet.
The spacecraft鈥檚 other role is to provide a navigation network. Here,
travellers can navigate using the Global Positioning System, a constellation of
24 satellites. The satellites are arranged so that for any point on the Earth鈥檚
surface, at least four are always above the horizon. 鈥淓ach spacecraft is
constantly sending out a message saying `It鈥檚 such and such a time and this is
where I am鈥�,鈥� says Edwards. GPS receivers work out their position by comparing
the arrival times of signals from three satellites with a reference time signal
from a fourth satellite. The system is accurate to within 100 metres and, with
modifications, can be made accurate to within centimetres.
NASA cannot afford to send 24 satellites into Mars orbit and, to begin with,
it will have to make do with just one. This will have to be used for both
navigation and communication. Combining these functions in one satellite will
save a lot of money, but there are disadvantages, too. For a start, the best
orbits for navigation and communications satellites are not the same. The GPS
satellites orbit Earth at 19 000 kilometres, allowing each one to cover a large
patch of the surface. Mars Network satellites orbiting the Red Planet at a
similar height would also almost always be in sight of Earth, allowing more or
less continuous communications between the planets.
But all this must be balanced against the power available on Mars itself.
NASA wants to equip its surface vehicles with small low-power transmitters which
would help reduce the weight of the spacecraft and thus the cost of Mars
missions. But these signals could only be picked up by communications satellites
in low orbits. And in such an orbit, Earth would be hidden behind Mars for much
of the time.
This means that the satellites would have to be able to receive signals from
the ground and wait for Earth to come into view before sending them back. At
best, such a system would allow communications with Earth for about a fifth of
the time. According to this plan, the first satellites will circle the Martian
equator every two hours. They would be able to handle communications only from a
lander or rover near the equator. Yet scientists are fascinated by the planet鈥檚
poles because the ice they harbour might contain signs of life. So later, other
satellites will fly perpendicular to the equator, in polar orbits.
No one knows exactly how to make the trade-off between picking up signals and
sending them back to Earth. Engineers are currently studying orbits at altitudes
of between 400 and 1000 kilometres. 鈥淚 think we have a lot more to do to
understand how to balance the communications and navigation needs,鈥� admits
Edwards.
Any satellite constellation would also differ in another big way from Earth鈥檚
GPS. Hand-held GPS receivers are passive devices鈥攖hey merely listen out
for signals from the satellites in orbit. 鈥淲e want to build systems to work in
two directions. That way a single satellite can provide much more information,鈥�
says Edwards.
The idea is that the ground station will trade signals with the satellite and
measure the time of the round trip. This gives an exact measure of distance
without having to rely on precise synchronisation of clocks. The clocks on GPS
satellites are constantly monitored from the ground and corrected for tiny
errors, but this will not be possible on Mars. 鈥淎 two-way system can measure
distance even if their clocks are not in sync,鈥� says Edwards.
Of course, a single distance measurement will not reveal the position on the
ground. So a Mars rover might have to wait for a satellite to pass overhead
three times at different elevations in the sky. It could then use these signals
to fix its position within a kilometre or so, as if there had been three
satellites overhead simultaneously. While far from perfect, one navigation
satellite would be better than none, says Carl Pilcher, NASA鈥檚 director for
Solar System exploration in Washington DC.
Martian maps
If all goes to plan, the Mars Network will become the communications and
navigation backbone that makes the future exploration of the Red Planet
possible. First off will be better maps of Mars. When photographing a planet,
one of the problems is to match the pictures from orbit with their exact
locations on the surface鈥攆rom space, one crater on a flat plane can look
identical to another a few hundred kilometres away. When mapping a planet,
scientists use points on the surface whose position is known relative to each
other to create a 鈥渃ontrol net鈥�. Any directions are then plotted relative to
this net. But this map is only as accurate as the measurements of the control
points. 鈥淗aving the navigation network up there means you can have much more
accurate maps of the planet,鈥� says Pilcher.
Then there is the mission to bring a Mars rock back to Earth. In 2003, a NASA
rover will scoop up 500 grams of soil at a carefully chosen location. An
accurate landing will be crucial. It will carry the sample back to an ascent
vehicle, which will blast the sample into orbit inside a grapefruit-sized
canister. In 2005, a French robotic probe will rendezvous with the canister and
carry it back to Earth. 鈥淒oing all that where you can only talk to these things
once or twice a day would be very, very difficult,鈥� Edwards says.
And as the exploration of Mars gains pace, the demand for better
communications will be greater still. NASA鈥檚 engineers envision the Mars
equivalent of geostationary communications satellites like those that broadcast
live news and sporting events on Earth. These 鈥淢ars Sats鈥� would fly circular
orbits along the Mars equator exactly once a day so that from the Martian
surface they would appear to hover overhead. These orbits are called
areostationary after Ares, the Greek god of Mars. And since they would fly at an
altitude of 17 000 kilometres or so, Mars would rarely block their view of Earth
and they could stay in contact with Earth almost all of the time. Two teams of
engineers at JPL are even thinking of using lasers to communicate between the
two planets because of the greater amount of data they can carry. These kinds of
laser links are already being tested on Earth.
Then there is the goal of sending humans to Mars. 鈥淚f we have to carry
everything we need with us, we鈥檒l never get there,鈥� says Edwards. So NASA鈥檚 plan
is to send an advance guard of robots that will produce fuel, drinking water and
breathable air using the natural resources on Mars. Any human explorers will
have to be able to find this base easily. Were they to land even 100 kilometres
away without some way to navigate the rugged terrain, they would die a slow,
agonising death. 鈥淐learly they鈥檝e got to land next to their infrastructure,鈥�
says Edwards.
If all goes to plan, the next generation of pioneers鈥攖he humans who
explore and colonise Mars鈥攚ill have more help on their epic journey than
Lewis and Clark. With the help of the Mars Network, NASA might even be able to
plan the return celebration in advance.
From the diary of William Clark, 23 September 1806
鈥渨e came in Sight of the little french Village called Charriton (Charrette)
the men raised a Shout and Sprung upon their ores and we soon landed opposit to
the Village. our party requested to be permited to fire off their Guns which was
alowed & they discharged 3 rounds with a harty cheer鈥very person,
both French and americans seem to express great pleasure at our return, and
acknowledged themselves much astonished in seeing us return. they informed us
that we were supposed to have been lost long since, and were entirely given out
by every person &c.鈥�
