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An answer to this question states that the expected downlink rate for New Horizons at Pluto encounter will be about 1,000 bits/sec. Understanding the issues of distant transmitters and data rates as posed in this question, my question is about actual numbers. Are the values stated for New Horizons "as good as it gets", or is it just the result of a limited mission budget? Could a modest increase in mission expenditure offer a meaningful increase in downlink data rate, or would even a modest increase in performance require a substantial increase in on-board equipment, and therefore mission cost?

Also: data is still being downlinked from Voyager 1 despite its much greater distance from Earth. How does Voyager's power supply and transmitter differ from New Horizons in so far as it affects data rates?

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    $\begingroup$ Define "practical". With a bigger antenna and/or more transmitter power, you can increase the data rate. Though for a mission like this, why would you? The data from a short flyby is all stored on board, and sent back over a long period of time after the flyby. I might ask instead if they could have made the mission cheaper with the same data return, and accepted a longer period of time to return all of the data collected. $\endgroup$
    – Mark Adler
    Jun 1, 2015 at 5:20
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    $\begingroup$ By the way, two questions should be posted as two questions. $\endgroup$
    – Mark Adler
    Jun 1, 2015 at 5:21
  • $\begingroup$ Please define "modest". $\endgroup$
    – Deer Hunter
    Jun 1, 2015 at 7:13
  • $\begingroup$ @MarkAdler Maybe this is the start of yet more questions, but at some point, would on-board storage capacity not become an issue, along with considerations like stability of the storage medium due to radiation effects? Also, New Horizons is only doing a brief encounter, but what about missions with longer-term science objectives and the prospect of accumulating science data on a continuous basis faster than it can be downloaded? $\endgroup$
    – Anthony X
    Jun 2, 2015 at 2:01

1 Answer 1

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There are two ways to increase transmission speed. Both rely on increasing the power of the radio transmission.
1. Diameter: antenna gain increases with the square of the antenna diameter.
2. Transmission power: RF power increases with the log of the amplifier input power.

When you increase the antenna diameter, you run into limitations of the launcher: the fairing must be large enough to accommodate the antenna. Alternatively, you can build a folding antenna: the Russian radio telescope satellite Spektr-R has an antenna diameter of 10 m. But this hasn't been done a lot. It makes the spacecraft a lot more complicated and adds failure points.

Increasing transmitter power also has issues. New Horizons and Voyager are powered by RTGs, which are very expensive. A 2005 report estimated the cost of a New Horizons-class RTG at $60-90 million. The main driver is the cost of producing Plutonium-238.

So mission budget plays a large role. Antenna diameter feeds into the launch weight and fairing size, RTG power goes directly to cost. New Horizons used the biggest rocket available at the time apart from the Shuttle, Voyager benefited from multiple gravity assists. Increasing launch weight would have resulted in a slower launch and longer travel times, which also add to the cost.

Let's compare our 2 data points:

Voyager:
- 470 W of power available at launch,
- launch weight 721 kg,
- 3.7 m main antenna.

New Horizons:
- 213 W of power available at launch,
- launch weight 478 kg,
- 2.1 m main antenna.

Let's say you could increase transmission speed by a factor of 50 (go from a 2 m antenna to 10 m, and double the transmitter power). From Pluto, that's still only 25 kbit/s. This would shorten the download phase from 1.5 years to 2 weeks, but doesn't fundamentally change the equation: the encounter data still starts to arrive long after the encounter.
Also your spacecraft now weighs 4 tons (WAG based on the weight of Spektr-R) and needs approximately a Saturn V to launch at sufficient speed. That money would easily pay for a year of DSN time.

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  • $\begingroup$ Well there goes a long day of research ;) Brilliant answer, I just couldn't come up with the numbers. $\endgroup$
    – Vedant Chandra
    Jun 1, 2015 at 10:23

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