# What's the reasonable "window" in days for a Mars-Earth Hohmann transfer?

As any popular science reader knows, when traveling between Earth and Mars, there's a Hohmann transfer opportunity about every two years (I believe it's every 26 months, specifically).

What I've never understood or seen mentioned, what is the reasonable realistic window for that - as actually used in practice by NASA, Rosaviakosmos, CNSA, ISRO, ESA, etc.

So. You're getting ready to do a Hohmann transfer (in either direction). Do you have only a matter of minutes? Hours? Days? A couple of months?

Are there some practical examples of past launches?

• Feb 2 '18 at 17:57
• Near-duplicate of space.stackexchange.com/questions/23093/… Feb 2 '18 at 17:58
• That question you answered is absolutely superb, Russell. You know, that discusses the factors in the window. This QA may be useful to Popular Scientists , particularly with the superb answer below, as it gives example real-life windows. Feb 2 '18 at 18:43
• There is a transfer window planner for Kerbal Space Program. While the specific numbers won't align, the idea is right and lets you explore a porkchop plot. Feb 2 '18 at 21:51

Techically a Hohmann transfer is a single instant, but as you mentioned, there is a window about the Hohmann that is nearly optimal.

Just to give you an idea, here is the launch window sizes for some upcoming/ recent missions

• InSight- 33 days
• Curiosity- 26 days
• Maven- 20 days
• MRO- 21 days

Of some note is this statement in the MRO link:

The launch period--the time period in which the orbiter could launch--lasted approximately three weeks (Aug. 10-30, 2005). Throughout the entire launch period, the mission had daily launch opportunities, known as "windows," of at least 30 minutes, the minimum needed for launch.

The bottom line is, there is usually a few weeks, each day of which during that period of time has a window of between 30 minutes- 2 hours where the launch can be performed.

• Do those numbers actually include the transfer window though? As I understand from another recent answer, interplanetary missions will typically spend some time in a parking orbit while they wait for the right moment Feb 2 '18 at 21:36
• See the curiosity link. They can spend some time, but not much, in orbit. Feb 2 '18 at 21:46
• More importantly, how were these window times calculated?
– Paul
Feb 3 '18 at 1:09
• That's a much more involved question... The short answer is, computers. Feb 3 '18 at 1:09

To complement @PearsonArtPhoto's information on upcoming missions, you can also look at the historical launch dates of Mars missions to get a sense of the launch windows actually used. While the launch window for a single mission isn't, logistically speaking, exactly the same thing as the span of a set of missions within a single 26-month opportunity cycle, they are closely related.

Four separate Mars-bound launches occurred over about 3-week spans in each of the 1971 and 1973 opportunities:

1971

• Mariner 8, 9 May 1971 (USA)
• Kosmos 419, 10 May 1971 (USSR)
• Mars 2, 19 May 1971 (USSR)
• Mars 3, 28 May 1971 (USSR)

1973

• Mars 4, 21 July 1973 (USSR)
• Mars 5, 25 July 1973 (USSR)
• Mars 6, 5 August 1973 (USSR)
• Mars 7, 9 August 1973 (USSR)

Later launches such as the November-December 1996 launches of MGS and Mars Pathfinder, and the December-January 1998-1999 launches of Mars Climate Orbiter and Mars Polar Lander spanned closer to 4 weeks, which may reflect general increases in performance margins for those missions.

• This must be the best QA on the whole site! :) Thanks again ... Feb 3 '18 at 14:14