Some of the disadvantages of space travel include NASA spending money on projects when it is not aware how much commercial investment it can rely on, there are risks to human safety with each mission and certain orbits are very slow. In addition, changes to space travel funding may make it difficult for students to become astronauts.
Although NASA now partners with private firms for space travel funding, there is no guarantee that private companies can invest. While many show an interest, this may mean NASA begins working on projects which are then left without commercial funding. If only a small number of private firms show an investment, establishing contracts becomes inefficient. In addition, space travel is unsafe, with historic tragedies including the loss of Challenger in 1986 and Columbia in 2003. Further losses could be damaging in terms of human life and space travel's reputation, leading to restrictive regulations.
Logistically, some aspects of space travel are challenging. As ships can only carry limited amounts of fuel, they rely on planetary orbits and gravitational thrusts to complete missions. These transfers also limit the number of launches a ship can make. For example, the Earth-Mars transfer is only available once every 2 years, and the Earth-Jupiter transfer is available once every 13 months. Both are slow.
With the Federal government shifting space program funding to private sources, there are less opportunities for students to become astronauts. Usually they enter the program after a career in the airforce, but less federal funding means reduced chances of this happening.