Why can't we watch live as NASA's latest rover descends and lands on Mars?
NASA’s Perseverance rover is off to the “red planet” on the Mars 2020 spacecraft, successfully launching from Cape Canaveral on Thursday.
After its seven-month, 314 million mile journey it will land in Mars’ Jezero crater in February 18, 2021, but only after what its scientists call “seven minutes of terror” as the parachute unfurls and then the rover is lowered to the surface of Mars on a tether from its descent stage.
Why can’t we watch those “seven minutes of terror” live on TV? Or get live streaming of Perseverance’s first views of Mars as soon as it opens it eyes?
“We probably could do it today, but definitely not in HD,” said Stephen Towne, Directorate Chief Technologist for the Interplanetary Network at the NASA Jet Propulsion Laboratory, to me via email earlier this week.
Here’s why live TV from Mars is difficult, impossible in high definition (HD), and how NASA has incredible plans to change that.
Using lasers.
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First, we need to get straight the definition of “live TV” from another planet.
What does ‘live from Mars’ mean?
We’re talking about something broadcast and sent back to us on Earth without first being stored. “We think of seeing something live as seeing it in real-time as it is actually happening with no taped delay,” said Towne. He explained that although there is very little transmission delay in Earth-based systems, there is a little delay between the Moon and Earth.
However, we would still regard the live broadcasts of the Apollo astronauts on the Moon as live. Live broadcasts from Mars are similarly possible, but with the same caveat. “There will be a time delay in the reception of the live broadcast due to the distance between Mars and Earth,” said Towne, citing one inarguable factor—fundamental physics.
Orbital mechanics is everything here; Mars is currently getting closer to Earth, but by the time Perseverance lands it will be getting further from Earth. At its closest point, there’s a 4 minute delay for radio signals to travel from Mars to Earth. At the most distant point, its 24 minutes. When Perseverance lands, it will be about 14 minutes.
NASA's Mars 2020 rover will store rock and soil samples in sealed tubes on the planet's surface for ... [+]
Is Perseverance up to it?
It’s not for a lack of cameras. The Perseverance rover and other parts of the Mars 2020 spacecraft feature an astonishing 23 cameras. Together they’ll send us images of Mars in breathtaking detail, but the landing?
All we’ll get is a pieced-together “first person” version of the spacecraft’s descent after the fact, albeit in HD.
Perseverance will also be able to send data directly into NASA’s Deep Space Network (DSN) antennae on Earth. However, at between 80 and 3,125 bits-per-second (b/s) to a 34m antenna or 800 and 15,625 b/s to a DSN 70m antenna, that’s not going to cut it for HDTV.
What do HDTV broadcasts require?
HD (1080p) broadcasts require upwards of 8 megabits-per-second (Mb/s) and 4K Ultra High Definition (UHD) over 57 Mb/s. So broadcasting TV pictures directly from Perseverance is a complete non-starter.
However, that’s not the primary way of getting data back to Earth. Perseverance will be able to send data from the surface to a Mars orbiter at a maximum of 2 Mb/s, which will then really it to Earth. That’s about the same bandwidth required for a stable stream from Netflix.
“Once it is on the orbiter—such as the Mars Reconnaissance Orbiter—it can be sent to Earth at 500 kilobits-per-second (kb/s) up to 3 to 4 Mb/s depending upon distance between Mars and Earth,” said Towne.
This artist's concept shows what Deep Space Station-23, a new antenna dish capable of supporting ... [+]
Can we broadcast live TV from Mars?
So, yes, we can—but the quality would be very basic. It’s not just about data rates, but the system used. For example, a rover can send back images, but pointing an antenna with any significant gain at Earth during the descent, with enough battery power for the transmitter, would be hard to achieve.
You’d also end up with a bits-per-second data rate, so the quality would be terrible.
A better—but still tricky—plan would be to have a rover send its video signal to an orbiter above it that can relay the images back to Earth. “It has a similar rover antenna pointing challenges [but it’s a] better plan and has a chance of getting reasonable data rates, though HD is still challenging,” said Towne.
A third option would be to have an orbiter with a huge camera that can be trained on the rover as it lands and simultaneously send that signal back to Earth.
For the latter two options to work, there would need to be a clear line of sight to Earth, but that’s possible. There’s another bonus too. “When we land on Mars, it is usually when Mars is at close range, so that helps,” said Towne.
However, NASA has something up its sleeve—its Deep Space Optical Communications (DSOC) technology.
Antenna dishes at NASA's Deep Space Network complex in Goldstone, California, photographed on Feb. ... [+]
What is DSOC?
It’s about upgrading from radio to optical communications, so using data encoded in photons and beamed over laser light. That vastly increases the data rate. “It’s a very significant step in demonstrating the viability of optical communication at Mars distances,” said Towne. NASA has demonstrated optical communication of up to 622 Mb/s from the Moon, but Mars at close range is over 150 times farther away from Earth, which makes communicating from Mars 22,500 times harder.
“With the DSOC flight terminal and the 5m Hale Telescope on Palomar Mountain as a receiver, we expect to get on the order of 50 Mb/s at Mars close range equivalent,” said Towne. “With a larger ground telescope, say 10m diameter, we could support 200 Mb/s.”
That data rate will reduce as Mars gets farther away from Earth—and DSOC is, for now, too big for a Mars rover—but it’s a critical first step. “The point is that moving to optical gives us the potential for much higher data rates,” said Towne.
Perseverance’s “seven minutes of terror” will have to be endured alone this time, but in future Martian rovers, landers and—in time—crewed spacecraft will be able to broadcast live from the red planet.
Wishing you clear skies and wide eyes.
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