The Parker Solar Probe is set to launch on August 11th 2018. It will perform 7 orbit-lowering gravity assists around Venus for ~6 years before reaching its final operational orbit.

It has been well documented that the very powerful Delta IV Heavy is required, such as here by ULA:

Due to the extremely high energy required for this mission, the Delta IV Heavy’s capability will be augmented by a powerful third stage provided by Northrop Grumman Innovation Systems.

And here on Spaceflightnow:

To do that, we need a really big rocket that can provide us with a high (escape velocity). The Delta 4-Heavy was the best we could get, but even that wasn’t sufficient. We still need a third stage to provide even more of a boost for us.

To me, this makes it sound like the margins for the energy budget will be very small.

So why is the launch window over a week wide?

The official site gives it as 11th-19th August and this presentation from JHUAPL gives it as 31st July - 19th August. Both sources appear to be recent (2018).

In those 8 days the Earth-Venus phase angle will have changed by ~4.9° which seems very significant considering the approach-dependent nature of gravity assists. If there were a larger energy budget available, dispersions from launch timing could be for corrected later in the mission, but this doesn't appear to be the case.

Has the narrowness of the energy budget margin simply been overstated and large corrections will be able to be made? Or am I missing some other consideration?

Note: General considerations for launch windows of interplanetary missions are useful, but in this case I am interested particularly in Parker Solar Probe's as-yet unique mission design.

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    $\begingroup$ It's only a week?! That is an extremely narrow launch period! Not "such a wide launch window" in any sense. Just looked it up. The Parker launch period is Aug 11-23. A one-week launch period would, in my experience, have an unacceptably high risk of getting delayed at least a year and a half for the next opportunity. At great expense. A 13-day launch period is just barely on the edge of acceptable. A three-week period is what is usually designed for. $\endgroup$
    – Mark Adler
    Commented Aug 10, 2018 at 22:01
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    $\begingroup$ @MarkAdler thank you for the perspective! So in other words, my interpretation of how tight the energy margins are is a bit off, but they will be tighter than on other missions, making the window a bit narrower than would be preferable considering other factors - delays, weather etc. ? $\endgroup$
    – Jack
    Commented Aug 11, 2018 at 8:01
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    $\begingroup$ Yes, they are apparently tighter than usual. Launches can and are delayed for many reasons, including range violations (boats where they shouldn't be), weather (fog, winds aloft), launch vehicle problems (I've seen nozzle delamination, bad batteries, cork insulation falling off), ground system problems (required tracking not available), and spacecraft problems. $\endgroup$
    – Mark Adler
    Commented Aug 11, 2018 at 16:20
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    $\begingroup$ And in fact the launch was scrubbed today due to a gaseous helium regulator alarm on the launch vehicle. $\endgroup$
    – Mark Adler
    Commented Aug 11, 2018 at 16:40
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    $\begingroup$ It launched! (Whew.) $\endgroup$
    – Mark Adler
    Commented Aug 12, 2018 at 15:19

3 Answers 3


There is some information in this paper although it does not directly address your question. It does mention that the first Venus flyby is at a relatively high altitude (about 2500 km) which may make it less sensitive. I also observe that, since all of the gravity assists are with Venus, once the first Venus flyby is completed, the absolute time doesn't matter. Subsequent flybys will all occur the same number of Venus years later than the first one, and will work whatever time the first one takes place.

I also note from the same paper that the target launch vehicle changed during the design phase, from an Atlas to a Delta-4. That change (and associated scaling up of the third stage) will have added a substantial amount of energy to the launch, so while it might have been just too much for the Atlas, there may be considerable flexibility with the Delta.

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    $\begingroup$ Your points about the high altitude subsequent assists are both great! I'll have a read of the paper. $\endgroup$
    – Jack
    Commented Aug 1, 2018 at 18:10

New launch schedule: Rescheduled to lift off at 3:31 a.m. EDT on Aug. 12, 2018.

Scrub Announcement Video: "Parker Solar Probe Launch Postponed".

There will be live coverage on the NASA Kenedy YouTube Channel.

Current live stream: "NASA Live: Official Stream of NASA TV's Media Channel".

To me, this makes it sound like the margins for the energy budget will be very small.

The margins will be small enough to not waste fuel, by carrying unnecessary fuel.

But there will be extra fuel to allow for error and to permit some wiggle room.

A Goldilocks fuel budget.

So why is the launch window over a week wide?

There's a setup time to prepare for launching, it's easier to figure out a way to launch every day for a week than to be able to launch a dozen times in a year (which would not be possible for an interplanetary launch) since people would have to pack up and leave, then return early to get setup again - or not leave until it's launched, with a huge wait between windows (but that's not how interplanetary windows work, where you are chasing a moving target, as opposed to simply orbiting Earth).

Whether you leave at the start or end of the window on day one, or a different day, changes the route and possibly ends up putting you in a non-optimal position around the Sun, but still not a bad position, nor as disadvantageous as a further few weeks or months delay (and scrapping all prior calculations).

Sources/Proof of the above answer.

On NASA's Blog an article titled "Parker Solar Probe Launch Window Extended to August 23" (Aug 2 2018) they wrote:

"NASA and its mission partners have analyzed and approved an extended launch window for Parker Solar Probe until Aug. 23, 2018 (previously Aug. 19). The spacecraft is scheduled to launch no earlier than Aug. 11, 2018, at 3:48 a.m. with a window of 45 minutes.".

So day one the window is 45 minutes. If they can't launch within that window it's on to day two.

But on August 7 2018, in the article: "Launch Week Begins for Parker Solar Probe" they wrote:

"... scheduled for Saturday, Aug. 11, at 3:33 a.m. EDT, the opening of a 65-minute window.".

So now it's earlier, and longer. Their newest Blog entry: "Parker Solar Probe Proceeds Toward Launch Aug. 11" confirms the same date and time with no mention of the day's window.

On NASA's webpage: "Chapter 9 - From Earth to Venus" they write about the Magellan mission:

"Fortunately, a 64-minute launch window had been designed for May 4. After 59 anxiety-filled minutes, the winds dissipated and the clouds parted just enough for launch at 2:46:59 p.m., eastern daylight time (see Figure 9-2), only 5 minutes before the end of the launch window for that day. The shuttle slowly rose out of the billows of steam and accelerated toward the low clouds. It went briefly out of sight and then reappeared for a few seconds, framed in a blue window amid the clouds. It was truly picture perfect.

The Space Shuttle Atlantis compensated for the delay in launch by yaw steering into the correct orbit plane. After five revolutions around the Earth at an altitude of 296 kilometers (160 nautical miles), Magellan was slowly deployed from the shuttle (see Figure 9-3). Sixty minutes later, with the solar panels extended as shown in Figure 9-4, the IUS ignited its two SRMs in rapid succession and propelled the spacecraft on very nearly the precise trajectory to Venus. After firing its attitude-control thrusters for a small course correction, the IUS separated from Magellan and used its remaining fuel to move away from the spacecraft.

Magellan's Path to Venus

The original May 1988 launch period would have allowed Magellan to reach Venus 4 months later via a Type-I trajectory, meaning that from launch to destination, the spacecraft would have traveled less than 180 degrees around the Sun. There was a similar opportunity in the October 1989 launch period initially set aside for Magellan but sub-sequently assigned to the Galileo mission to avoid further delays in its launch.

However, the positions of Earth and Venus during the late-April to late-May 1989 launch period required a Type-IV trajectory (see Figure 9-5). This meant that the spacecraft would travel between 1-1/2 to 2 times around the Sun (slightly more than 540 degrees) and that it would arrive at Venus on August 10, 1990. While it dictated a longer cruise duration (15 months), the Type IV actually had the advantages of reductions in launch energy and Venus approach speed.


Back to the Drawing Board

Magellan's Type-IV trajectory and the resultant Venus arrival date brought about some changes in the basic mapping plan developed for the 1988 mission.

Superior conjunction (where the Sun is positioned between Venus and the Earth) will now occur during the primary mapping mission, instead of at the end. The result is that up to 18 days of mapping data will be lost around November 2, 1990, because radio interference from the Sun will make it impossible to communicate with the spacecraft. Fortunately, the missing data can be recovered in early July 1991, if the mission is extended for additional 243-day mapping cycles.

The trajectory also dictates an approach over the north pole; this will result in a mapping swath from north to south, the reverse of that planned for the 1988 mission.

So you can see how utilizing a different portion of the window can alter (but not ruin) the execution of the mission.

The Venus DRM mission, scheduled for April 30, 2021 on an Atlas V 551 L/V, uses modern calculations based on what we have learned since the Magellan mission; it would be more representative of what is to be done during the week starting Saturday, Aug. 11 2018, at 3:33 a.m. EDT.

We spin (at the equator) at close to 1000 miles per hour (1600 km/hr) and move through space around the Sun at a speed of 66,000 miles per hour (107,000 km/hr).

Relative to the local standard of rest, our Sun and the Earth are moving at about 43,000 miles per hour (70,000 km/hr) roughly in the direction of the bright star Vega in the constellation of Lyra. [Source: AstroSociety.org - "How Fast Are You Moving When You Are Sitting Still?"].

The Earth rotates 360° in 23 hours, 56 minutes, and 4 seconds (1,436.06667 minutes), so the 65 minute window represents 360 / (1 436.06667 / 65) = 16.2945081 degrees - quite a wide range of trajectories.

Venus travels around the Sun at an average speed of 78,341 miles per hour or 126,077 kilometers per hour in its orbit around the Sun.

See Wikipedia's webpage: "Hohmann Transfer Orbit - Application to interplanetary travel", "Bi-Elliptic Transfer", and "Interplanetary Transport Network" for information about "Gravity Assist":

"With any Hohmann transfer, the alignment of the two planets in their orbits is crucial – the destination planet and the spacecraft must arrive at the same point in their respective orbits around the Sun at the same time. This requirement for alignment gives rise to the concept of launch windows.".

A demonstration of the orbits of Earth and Venus around the Sun is offered in the YouTube video "Earth Venus Tango Round the Sun" and is explained at the Wikipedia page "Venus - The Pentagram of Venus":

"The pentagram of Venus is the path that Venus makes as observed from Earth. Successive inferior conjunctions of Venus repeat very near a 13:8 orbital resonance (Earth orbits 8 times for every 13 orbits of Venus), shifting 144° upon sequential inferior conjunctions. The resonance 13:8 ratio is approximate. 8/13 is approximately 0.615385 while Venus orbits the Sun in 0.615187 years.".

Because of the resonance period, due to the orbital speed versus the distance from the Sun, the planets Earth and Venus remain relatively near each other for a longer period than if the ratio were 13:1 - still, precise calculations are especially important over extremely long periods of time.


You are correct in your assumption that larger launch windows require higher fuel margins on the rocket. Therefore, the main reason for such a large launch window is not because they just want to, but because it allows for some flexibility in the actual launch point. Should they have to abort the countdown and need to recycle, they need some time (usually hours to days) until they can attempt another launch.

Besides technical difficulites, there can be secondary factors delaying a launch. Unfavourable weather conditions, boats or planes within restricted areas and even natural desasters like wildfires can cause delays.

On normal launches, delays is not that much of an issue, the next launch opportunity might be within days. For interplanetary launches like SPP (Solar Parker Probe) however, the next Launch opportunity might appear years later. This would cost a lot of money and is fairly impractical. This means that they have to make sure that they can hit the launch window.

To ensure that they hit the launch window, they try to make it as large as possible so they have time for troubleshooting or waiting for range availability if needed.

  • $\begingroup$ Thank you, but I'm not convinced this answers my question. For sure, in the general case, launches will use the maximum window available for the reasons you give. However, I'm asking how the orbital mechanics of the mission allow for such a wide window, not why the launch providers would like to have one. Following this reasoning alone, all launches would have indefinite windows to allow for unforeseen delays. This clearly isn't the case though, especially when trying to target a specific intercept for a gravity assist. $\endgroup$
    – Jack
    Commented Aug 1, 2018 at 13:13
  • $\begingroup$ Ah, ok! About the orbital mechanics: There is basially one optimal flight path with one very specific launch date. If you launch earlier or later, you can spend more dV (difference in velocity, 'fuel') to compensate by flying a different path with the same endresult but a higher fuel cost. That alternative path is basically less efficient but can compensate for earlier/later launches. You basically have to 'catch up' or 'wait' for the constellation to fall in place while you're on the way instead of beeing on the ground. Hope this helps $\endgroup$
    – DaGroove
    Commented Aug 1, 2018 at 16:27
  • $\begingroup$ Thank you again, but this is still the general case. As mentioned in the question, it doesn’t appear that the mission will have much margin for compensation of a non-optimal launch time. Possibly I’m simply misinterpreting how tight the margins are... $\endgroup$
    – Jack
    Commented Aug 1, 2018 at 17:57
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    $\begingroup$ @Jack you're thinking about it the wrong way round. The question isn't "the spacecraft is 685 kg, how long is the launch window?", it's "what's the biggest I can make the spacecraft and still have it compatible with a 8 day long launch window?" $\endgroup$
    – djr
    Commented Aug 1, 2018 at 21:02
  • $\begingroup$ @djr you're correct and that is what I would expect the process to be. I'm interested in the rationale behind deciding on a launch window (for these reasons), building a spacecraft of the appropriate mass yet still declaring that the energy margins are incredible tight. $\endgroup$
    – Jack
    Commented Aug 2, 2018 at 9:08

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