Going to Venus en route to Mars?

[lpetrich]

At first sight, that seems absurd, since Venus is on the opposite side of the Earth's orbit from Mars, closer to the Sun instead of farther from it.

Making the case for slingshotting past Venus on the way to Mars

The researchers suggest that a mission that includes a Venus stopover would provide benefits both to NASA and the astronauts aboard such craft. They note that stopping by Venus would greatly reduce fuel costs because the spacecraft could use Venus's gravity as a springboard to Mars. They also note that taking the Venus to Mars route would allow the spacecraft to return to Earth sooner should something go wrong. They also note that a Venus flyby would allow for a more hands-on approach to studying Venus—astronauts could control research drones in real time. Without such a presence, those controlling drones have to contend with the time delay as messages travel between Earth and Venus, which can take anywhere from five to 28 minutes. Perhaps most importantly, the researchers note, the window of opportunity would be greatly reduced. Earth and Mars only ever align for space travel every 26 months, which means astronauts on such a craft would have to spend more than a year on or near Mars. With opposition missions, the window is reduced to just 19 months.

That is, Earth-Mars 2 yr 2 mo and Venus-Mars 1 yr 7 mo.

Noting
[2006.04900] Human Assisted Science at Venus: Venus Exploration in the New Human Spaceflight Age
Has all the details.

 

[hyzer]

Better that routing through Atlanta . . .

 

[lpetrich]

Better that routing through Atlanta . . .

???

 

[James Brown]

This is not new. NASA has favored opposition missions to Mars for decades. But to my mind it has lots of additional risks for few, if any benefits.

Robert Zubrin covered this in his 1996 book, The Case For Mars. The problems with an opposition mission:

1. Oppo missions have larger delta-V requirements. It takes a lot of fuel to accelerate to Venus speeds. More fuel means bigger engines and less useful payload.

2. More delta-V means longer engine burns, increasing the risk of engine failure.

3. Throwing a larger spacecraft with larger engines at Mars means less reliance on Martian gravity captures and aerobraking for slowing down into Martian orbit. Thus they'll need even more fuel to slam on the brakes.

4. An opposition mission does shorten mission time. But it increases the amount of time astronauts are in interplanetary space. I've seen estimates that for a 19-month oppo mission, crews would spend as little as a month on the Martian surface. A nearly two-year commitment for a flags-and-footprints mission. If the weather is bad when they arrive, they might not land at all.

5. Engineers argue that a shorter trip duration minimizes crew exposure to radiation and the effects of zero gravity. But an oppo mission increases those risks. Longer time in zero gravity, plus more intense radiation and twice the solar heating as they zip around Venus. (Oppo missions are derided as Venus Fry-Bys.)

6. Longer durations in space mean a higher reliance on the ship's life support. Going to Mars is risky, but astronauts would be safer on Mars than they would in space. Plus you need to pack more food, air, and water for the crew, whereas on Mars, the CO₂ atmosphere can be converted to oxygen and water using Victorian-era chemistry.

7. At the end of the oppo mission, the spacecraft hits the Earth's atmosphere at a higher speed compared to the conjunction mission, further increasing the risks of hardware failure, or a trajectory miscalculation resulting in either a spacecraft burnup or skipping into interplanetary space.

I liken the opposition mission plan to that of a family that decides to fly to Hawaii for Christmas vacation, spending ten days in transit flying from one airport to another, with half a day at the beach, weather permitting. Put simply, the opposition mission plan is just plain silly. It maximizes both cost and risk, and minimizes mission science return. It is favored only by those who wish to present a piloted Mars mission as a pipe dream, or who wish to increase the mission's technical difficulty so as to justify funding some new propulsion system they are pushing.

--Robert Zubrin, The Case for Mars, p. 82

 

[Jimmy Higgins]

Better that routing through Atlanta . . .

???

Airline hub. Sometimes you can fly from Cleveland to Atlanta... to go to LA or Boston. And there is no slingshot that makes it quicker.

 

[Shadowy Man]

At first sight, that seems absurd, since Venus is on the opposite side of the Earth's orbit from Mars, closer to the Sun instead of farther from it.

Not absurd at all. NASA has been doing Venus flybys for a long time. Galileo, on its way to Jupiter, passed by Venus in 1990.

 

[Loren Pechtel]

This is not new. NASA has favored opposition missions to Mars for decades. But to my mind it has lots of additional risks for few, if any benefits.

Robert Zubrin covered this in his 1996 book, The Case For Mars. The problems with an opposition mission:

1. Oppo missions have larger delta-V requirements. It takes a lot of fuel to accelerate to Venus speeds. More fuel means bigger engines and less useful payload.

Huh? Venus injection is .28km/sec, Mars injection is .39km/sec. (Assuming it's done in low Earth orbit, numbers are in addition to Earth escape. Ion engines need more delta-v.)

4. An opposition mission does shorten mission time. But it increases the amount of time astronauts are in interplanetary space. I've seen estimates that for a 19-month oppo mission, crews would spend as little as a month on the Martian surface. A nearly two-year commitment for a flags-and-footprints mission. If the weather is bad when they arrive, they might not land at all.

I do agree it's longer in space, less time on Mars. I don't know the times.

7. At the end of the oppo mission, the spacecraft hits the Earth's atmosphere at a higher speed compared to the conjunction mission, further increasing the risks of hardware failure, or a trajectory miscalculation resulting in either a spacecraft burnup or skipping into interplanetary space.

I think he must be assuming playing pool with Venus on the way back, also. That doesn't make sense.

 

[James Brown]

Huh? Venus injection is .28km/sec, Mars injection is .39km/sec. (Assuming it's done in low Earth orbit, numbers are in addition to Earth escape. Ion engines need more delta-v.)

I'm weak in orbital mechanics, so I'm not understanding what you're suggesting. Do you mean that it's easier to reach Venus than it is Mars? By 'injection', are you referring to putting a craft into orbit around a body?

I see that Earth's velocity around the sun is ~30 km/sec. Mars' velocity is ~24 km/sec. Venus' velocity is ~35 km/sec. As I understand it, when a spacecraft leaves Earth's orbit in the same direction, it's jumping off a fast-moving platform, so the engine's speed of, say, 3 km/sec is combined with Earth's velocity to total 33 km/sec. Climbing out of the Sun's gravity well toward Mars slows the spacecraft down to about 21 km/sec, just 3 km/sec slower than Mars itself, which is slow enough to enter orbit.

But I don't follow the logic of increasing the velocity from Earth to Venus, adding 5 km/sec just to get there, zip around the planet, then head off toward Mars. The idea of using that time to control research drones at Venus in real-time seems like a stretch. The Galileo spacecraft used Venus for a slingshot and was gone in about a day.

I think he must be assuming playing pool with Venus on the way back, also. That doesn't make sense.

I think you're right. Zubrin analyzes the then-vogue idea of using a Venus flyby on the return voyage, whereas the OP link is discussing using Venus on the initial voyage. No mention that I see about the return voyage there. But it still feels like additional risks for little-to-no gain to me.

Some more stats here.

 

[barbos]

https://marspedia.org/Mars_mission_duration

 

[Loren Pechtel]

Huh? Venus injection is .28km/sec, Mars injection is .39km/sec. (Assuming it's done in low Earth orbit, numbers are in addition to Earth escape. Ion engines need more delta-v.)

I'm weak in orbital mechanics, so I'm not understanding what you're suggesting. Do you mean that it's easier to reach Venus than it is Mars? By 'injection', are you referring to putting a craft into orbit around a body?

No. I'm talking about the burn needed to send a rocket to the body. Here's a map of what it takes to get around the solar system (Earth-centric, it doesn't look at voyages between other bodies.):

https://www.reddit.com/r/space/comments/1ktjfi/deltav_map_of_the_solar_system/

The image needs to be viewed at full size to be able to read it.

I see that Earth's velocity around the sun is ~30 km/sec. Mars' velocity is ~24 km/sec. Venus' velocity is ~35 km/sec. As I understand it, when a spacecraft leaves Earth's orbit in the same direction, it's jumping off a fast-moving platform, so the engine's speed of, say, 3 km/sec is combined with Earth's velocity to total 33 km/sec. Climbing out of the Sun's gravity well toward Mars slows the spacecraft down to about 21 km/sec, just 3 km/sec slower than Mars itself, which is slow enough to enter orbit.

This is true if you're poking along. However, you can save a fair amount of fuel courtesy of the Oberth effect--when you do your burn deep in a gravity well you get more from it. (Basically, kinetic energy goes at the square of velocity, but rockets don't care. Borrow some velocity from something massive, burn, return the velocity and you carry away extra energy.)

But I don't follow the logic of increasing the velocity from Earth to Venus, adding 5 km/sec just to get there, zip around the planet, then head off toward Mars. The idea of using that time to control research drones at Venus in real-time seems like a stretch. The Galileo spacecraft used Venus for a slingshot and was gone in about a day.

Yeah, you're not going to get much science time.

I think he must be assuming playing pool with Venus on the way back, also. That doesn't make sense.

I think you're right. Zubrin analyzes the then-vogue idea of using a Venus flyby on the return voyage, whereas the OP link is discussing using Venus on the initial voyage. No mention that I see about the return voyage there. But it still feels like additional risks for little-to-no gain to me.

Some more stats here.

The gain is a shortened total mission time--less time for the shit to hit the fan. I see no other meaningful benefit.

 

[fromderinside]

Stop the silliness.

There's a much more profitable target and method with no human constraint on getting there and getting back here.

Robot rockets to the asteroid belt for exploiting stuff we can already see is there. A lot more mass in the Asteroid belt than there is in Mars. There are groups of products available there from every part of our merry little trip around the sun.

Hell! There's even water if we wish.

Build a base on the moon. Launch from there. Return product to the moon. Manufacture whatever is needed from what we get and then send more or less finished products, uncontaminated by atmosphere back to earth for further processing and exploitation. It's much easier to make suitable environments on the moon for humans if any are needed.

If you're still in to cost management you can use earth as the means for slinging to asteroid belt.

happy daze.