Why allow communication with interplanetary spacecraft to be silent for stretches of time; only getting updates on certain days & times of the week?

Question

Communication between planetary bodies in space, e.g. from the moon, or Mars, takes a long time and has many shortcomings, like line of sight requirements.

Communication with interplanetary spacecraft is often silent for stretches of time; only getting updates on certain days & times of the week.

Why is this enough to be acceptable for deep space exploration?

What are the challenges with improving this? So far it does't seem like a priority, so there must be some reasons why this is okay, can it be expressed in an answer?

I was thinking about repeaters, or store-and-forward schemes, are these possible?

Answer 1

1. There are lots of spacecraft out there. Take a look at this tracking page for the Canberra site of the NASA Deep Space Network. The right side lists all of the interplanetary spacecraft that NASA communicates with. There are currently over 30 listed.

2. Some spacecraft have more interesting or urgent needs than others. Any spacecraft that is entering a new phase of its life cycle, taking pictures or measurements, driving around, or performing an experiment is going to have a lot more data to send to Earth, than one which is merely hurtling through space. That's why most of the DSN communication is from Mars. In contrast, our farthest operational spacecraft (the two Voyagers and New Horizons) aren't currently near anything interesting; an occasional status message and tracking of position and velocity is sufficient.

3. Power limitations. Spacecraft have limited electrical power available. This particularly an issue for those in the outer solar system, where sunlight is too dim for solar panels, and the radioisotope thermal generators which are used decline with age. In that case, the RTG charges the batteries for a few days, which provides enough power for a transmission.

4. There are a limited number of communication dishes. The DSN currently has only 13 operational dishes, among 3 sites (Canberra, Australia; Goldstone, California; and Madrid, Spain). Often some of the dishes are being used for other activities such as maintenance, training, testing, radio astronomy, and dish arrays.

5. Each DSN site covers only a portion of the sky. If you look at the DSN Now page, you'll see that the one or two DSN sites which happen to be facing Mars are usually very busy. (This is because of items #1 and #2 above.) So if your deep space probe happens to be in the same part of the sky as Mars, you're going to be competing with them for dish time, while the other DSN sites are facing the wrong way.

6. Thus, dish time is prioritized and scheduled among spacecraft. The left side of the tracking page linked above shows the schedule. There are hundreds of time slots reserved, planned by JPL months in advance.

Low priority spacecraft therefore are silent for extended periods of time.

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Our farthest functional spacecraft -- the Voyagers (VGR1, VGR2) and New Horizons (NHPC) -- communicate less frequently. However, I did catch a rare occurrence of all three of them on the DSN on Nov 21, 2020:

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You also mentioned relay networks. We've had questions before about them. Stations on Earth can be built more cheaply, with larger dishes and more power than those in space, and can be maintained/upgraded. Also, a relay spacecraft might be "in between" the Earth and a target today, but its orbit will eventually put it farther away than the Earth is to the target.

The one exception is the Mars Relay Network. However, this has more to do with the Mars day/night cycle than with other considerations. The downloading of data and pictures, mission planning, and the uploading of commands is often done during the Martian night; so that driving, performing experiments, and taking pictures happen during Martian daylight. (I read about this somewhere on mars.nasa.gov about a week ago, but I can't find the page now.) Since the nighttime side of Mars faces away from Earth, relay satellites in orbit around Mars are needed.

Answer 2

The Deep Space Network is an amazing resource developed over the years to manage the communications and navigation tracking of dozens of interplanetary missions. With large radio antennae built in locations carefully selected across Earth, the DSN schedules commmunication times for all active and quiescent spacecraft.

When a mission is arriving at and entering a dynamic phase of its mission when near-real-time data and commanding is important, it gets high priority relative to the other missions. When a mission is cruising for months with minor course adjustments (a.ka., quiescent), it gets periodic check-ins. Since all of these interplanetary missions, so far, have been robotic it is fine to not maintain constant communication with all of them.

There are efforts to build up commercial networks to provide additional capability.

Repeaters have not been necessary. At Mars NASA is using the orbiting missions to provide communication relays for the surface missions. This allows additional data downloads when the surface site is not in line-of-sight with the DSN antennae on Earth, as the orbiters come into view every hour as they race around Mars.

Answer 3

The communication bandwidth used between an Earth Station and a distance spacecraft is basically a narrowband to reduce power transmitted from the craft. Redundancy bits, for error correction is introduced and compression algorithms are applied along with the transmitted data.

Hence, the data transitted is serialized over a bandwidth. At Earth Station receiver end, these signals are forwarded corrected and the audio,video or temetery signals are reconstructed. Hence, there is a delay involved during reception.